data communication &fieldbus systems
TRANSCRIPT
Data Communication &
Fieldbus Systems
Conducted for
OMV Aktiengesellschaft
Dates
03 - 07 December 2012
By
David Olson Senior Consultant
Data C
omm
unications &Fieldbus S
ystems:
PLC
& S
CA
DA
Technologies 3-7 D
ecember 2012, B
ucurești
Data C
omm
unications & Fieldbus S
ystems
1
INTR
OD
UC
TION
This programm
etakes an in-depth look at the divisive field of data com
munications and Fieldbus
systems.
The increasing trend towards digital com
munications w
ithin the industrial plant environment has
brought in its wake a m
yriad of problems concerned w
ith interconnectivity and differing standards offered
bya
wide
varietyofcom
petingvendors.Forthe
user,ithasbecom
eprogressively
more
offered by a wide variety of com
peting vendors. For the user, it has become progressively m
ore difficult to differentiate betw
een the proffered options.
►W
HO
SHO
ULD
ATTEN
D?
Professionals involved in designing, selecting, sizing, specifying, installing, testing, operating and
maintaining data com
munication and Fieldbus
systems.
Any professional needing to get to grips w
ith the ever expanding and complex field of data
comm
unications and Fieldbussystem
s:
•Automation E
ngineers•C
hemical E
ngineers•C
onsulting Engineers
•Operations E
ngineers•P
rocess Engineers
•Process Operators
f•Electrical E
ngineers•E
lectricians•Industrial IT specialists•Installation and m
aintenance techniciansI
tt
tid
Ct
lEi
•Production professionals•Project professionals•System
Integrators•O
ther professionals who w
ant a better understanding fth
bjt
tt•Instrum
entation and Control Engineers
•Maintenance E
ngineersof the subject m
atter
Data C
omm
unications & Fieldbus S
ystems
2
INTR
OD
UC
TION
►IN
TRO
DU
CTIO
No
This workshop is designed to provide engineers and technicians w
ith an overview of m
odern di gital com
munication standards -starting at the basic R
S232 standard right through to
gg
gg
Profibus/Foundation Fieldbus
and beyond.
►PER
SON
AL IM
PAC
To
Of
ll
tifthi
kh
dl
tillb
blt
oO
n successful completion of this w
orkshop delegates will be able to:
•Appreciate the basic principles of data comm
unications•U
nderstand the essential differences between asynchronous
and synchronous transmission
•Apply modbus
in a practical manner and m
ake use of troubleshooting techniques•D
ifferentiate between m
odbusand m
odbusplus
y•Apply basic faultfinding techniques covering rs232 up to rs485•U
nderstand the importance of the iso
osimodel
•Understand the basis of cyclic redundancy
•Recognise
the different aspects of cable selection•Apply
differentshieldingtechnologies
p•G
ain an insight into the essential differences between asibus
and devicenet•M
ake an informed appraisal of the differences betw
een profibus
and foundation fieldbus•U
nderstandthe
basicsofethernet
•Apply different shielding technologies•U
nderstand the application of fibreoptics
•Appreciate the value of hart in both calibration, data collection and diagnostics•R
ecognisethe w
ide range of fieldbusoptions
•Understand the basics of ethernet
•Appreciate the application of ethernetin the industrial environm
ent•U
nderstand tcp/ipand its associated protocols
•Differentiate betw
een net and sub-net masking
ff
•Distinguish the functionality of ports and sockets
Data C
omm
unications & Fieldbus S
ystems
3
INTR
OD
UC
TION
►O
RG
AN
ISATIO
NA
LIM
PAC
To
Following the training and developm
ent experience provided by this workshop, participants w
ill return to their or ganizations equipped w
ith new skills and know
ledge that will enable them
to g
qpp
gevaluate the im
pact of new and existing technology in respect of your plant's com
munication
strategy and apply a rationalized comm
unication philosophy based on the best available practices.
oBy
leveragingthese
skillsyourenterprise
canexpectan
improvem
entinoverallproductivity
oBy leveraging these skills, your enterprise can expect an im
provement in overall productivity
through the reduction in downtim
es resultant from inappropriate data com
munications strategy
and through the ability to apply, install, and troubleshoot a modern digital com
munication
system.
►TR
AIN
ING
METH
OD
OLO
GY
►TR
AIN
ING
METH
OD
OLO
GY
oD
esigned for both novice and experienced engineers and technicians, this workshop starts at
fundamental principles and progresses through basic coding and form
atting systems, error
detection, and protocols through to advanced Fieldbusand device netw
orking concepts.o
Throughout the workshop, participants w
ill learn through active participation using exercises, questionnaires, and practical troubleshooting using protocol analysis and covering:R
S 232, R
S 485, M
odbus, Netw
orking, TCP
/IP.
Data C
omm
unications & Fieldbus S
ystems
4
INTR
OD
UC
TION
►C
OM
PETENC
IES EMPH
ASIZED
oU
nderstanding of the major technologies used in m
odern data comm
unication and fieldbussystem
so
Select and specify different types of com
munication strategy according to the specific
it
dli
tirequirem
ent and applicationo
The experience of exchanging ideas, problems and solutions w
ith other delegates from different
backgrounds and processes o
A renewed or refreshed ability to m
ake an informed decision regarding their com
munication
yg
grequirem
ents according to their process, plant or operation
Data C
omm
unications & Fieldbus S
ystems
5
SP
EA
KE
R P
RO
FILE
•M
r. David C
. Olson is
a seniorconsultantwith G
LOM
AC
S, specializing
in IT t
Nt
kiIT
itT
li
tid
il
managem
ent, Netw
orking, IT security, Telecomm
unicationsand w
irelesssystem
s.•
David has over28 years
of professionalexperiencein IT system
st
di
iith
ibilitif
llh
ft
managem
entand engineering, with
responsibilitiesforallphases
of systemdesgn
and development, delivery
of servicesand netw
orkarchitecture.
•P
rior tojoining G
LOM
AC
S, D
avid worked
with
theU
.S. departm
entof f
defenseand as a seniorsystem
sengineeerand projectm
anager with
Digital E
quipmentC
orporation.•
Mr. O
lson holdsdegrees
in Electricalengineering
and Informations
systems
managem
ent, and currentlylectures
in a E.U
. sponsoredtechnology
programin S
pain.•
David has presented
overthree
hundredsem
inars togovernm
entclientsand private
concernsin N
orth Am
erica, Europe, A
sia and theA
rabianG
ulf.
Data C
omm
unications & Fieldbus S
ystems
6
TAB
LE O
F CO
NTE
NTS
►D
AY O
NE
Pg. 10I
td
tit
dt
iti
►D
AY TW
O
pg.100ISO
OSI
dl
Introduction to data comm
unications•
Overview
•
Modern Instrum
entation and Control
S ystems
ISO O
SI model
Com
munications m
edia•
Cabling
•S
hieldingy
•"S
mart" Instrum
entation •
Basic principles
•D
ata comm
unications standards
Shielding
•G
rounding •
Fibreoptics
Error detection•
RS
-232 interface standard •
Faultfinding •
RS
-422 interface standard •
RS-485
interfacestandard
•C
hecksum
•C
yclic Redundancy
LAN
Standards•
TopologiesR
S-485 interface standard •
Interface Converters
•Topologies
•P
rotocols •
CS
MA
/ CD
(Ethernet)
•Token
Ring
Token Ring
•Token B
us
Data C
omm
unications & Fieldbus S
ystems
7
TAB
LE O
F CO
NTE
NTS
(continued)►
DA
Y THR
EEPg. 184
Sd
dPLC
´►
DA
Y FOU
R
Pg. 259C
AN
Bus
/DeviceN
etScada
and PLCs
Modbus•
Fundamentals
•Fram
eform
at
CA
NB
us/D
eviceNet
•Fram
e format
•A
rbitration •
Fragmentation
•E
rrorreportingFram
e format
•Function coding
•E
xception reports •
Troubleshooting
Error reporting
•D
eviceNetm
edia layer •
Netw
ork configuration Profibus
•FM
SH
AR
T•FSK
•C
abling and configuration •
Com
mands
•FM
S
•D
P physical layer
•P
A physical layer
•Foundation Fieldbus•
Com
parisonbetw
eenP
rofibusP
Aand
FF•
Com
mands
•A
ctuator Sensor Interface (ASI)
•P
hysical layer •
Modulation
principles
•C
omparison betw
een Profibus
PA
and FF •
Publisher/subscriber m
odel •
Physical layer
•Fram
e structure •
Lik
tih
dl
Modulation principles
•C
omponents
•Link active scheduler
•Function block
•D
evice descriptions Safety and R
eliability
Data C
omm
unications & Fieldbus S
ystems
8
TAB
LE O
F CO
NTE
NTS
(continued)
►D
AY FIVE
Pg. 405Eth
t►
Appendix
Pg. 464•
Gl
fTi
lEthernet
•E
thernet frame
•M
edia Access C
ontrol •
Collision
detection
•G
lossary of Terminology
•S
uggestions for further reading
Collision detection
•100B
ase-T repeater rules •
Gigabit E
thernet 1000Base-T
•G
igabit Ethernet M
AC
Layer Internet layer protocols
•Internet P
rotocol Version 4 (IP
v4) H
ost to Host (Transport) layer protocols
•Transm
issionC
ontrolProtocol
•Transm
ission Control P
rotocol •
Ports and S
ockets •
Sliding w
indows
•E
stablishinga
connectionE
stablishing a connection •
User D
atagram P
rotocol (UD
P)
•M
odbusover TC
P
Planning ScadaProjects
Data C
omm
unications & Fieldbus S
ystems
9
Day
One
Day O
ne
►►IN
TRO
DU
CTIO
N TO
DATA C
OM
MU
NIC
ATION
S
Overview
Oe
e
•M
odern Instrumentation and C
ontrol System
s•
"Sm
art"Instrumentation
Sm
art Instrumentation
•B
asic principles•
Data com
munications standards
•R
S232
interfacestandard
•R
S-232 interface standard
•Faultfinding
•R
S-422 interface standard
RS
48i
fd
d•
RS
-485 interface standard•
Interface Converters
Data C
omm
unications & Fieldbus S
ystems
10
K-069
SC
AD
A S
YS
TE
MS C
OM
E IN
MA
NY S
HA
PE
S A
ND S
IZE
S
There is a rem
arkable variety in the size, scope and complexity of S
CA
DA system
s.
The sm
allest, simplest S
CA
DA im
plementations consist of nothing m
ore than a local hardwired
controller with a prim
itive HM
I -- and probably the largest and most com
plex are the ones found in the pow
er control centers that manage the N
orth Am
erican power grids, w
ith hundreds of pieces of equipm
ent in just the control center.
Because there is such a w
ide scale of applications, there is a similarly huge variety of com
ponents and system
s that are used in the industry.
When one com
bines this observation with other differences -- traditions w
ithin certain industries, environm
ental issues (e.g. risk of explosion in some applications m
ake wireless technology
troublesome), vendor bias, etc. all lead to further differences in S
CA
DA system
configurations and solutions.
Consequently, the subject of S
CA
DA technology covers an unusual breadth of technologies and
best practices -- and approaches that are ideal and appropriate to one application are absurdly inappropriate in others.
It is a fascinating field!
Data C
omm
unications & Fieldbus S
ystems
11
SU
MM
AR
Y F
EA
TU
RE
S
SC
AD
A and Industrial C
omputing applications m
ake use of technology that is in many w
ays the sam
e as business Information T
echnology, with som
e very important differences. T
he main
distinguishing features of this class of service are:
Prio
rity on
reliable o
peratio
n
A key design feature is the system
atic use of redundancy in areas of systems activity that are
either particularly mission-critical or particularly fragile.
Redundancy m
easures often complicate the operation and adm
inistration of systems (e
.g.
managin
g d
uplica
te, in
consiste
nt d
ata
and d
esig
nin
g fa
il-safe
hand-o
ver tra
nsfe
r system
s).
Prio
rity on
timely o
peratio
n
Typically, process-related com
puter systems are required to coordinate process-w
ide operations w
ith careful attention to time.
Tim
e sensitivity is a two-fold issue:
It is important that a com
mon m
easure of absolute time be distributed, or synchronized
across the physical system.
Relative tim
e coordination is more critical, w
here the relative timing of com
plex sequences m
ust be carefully managed.D
ata Com
munications &
Fieldbus System
s12
SU
MM
AR
Y F
EA
TU
RE
S, cont.
Generally cautious approach to
new technology
The inform
ation technology (IT) industry has a long tradition of em
bracing imm
ature technology -- som
etimes to great efficiency benefit and often at great peril in term
s of system
stability and service continuity.
The scale of industrial applications tends not to challenge the perform
ance envelope as much
as business applications.
As a result, this kind of perform
ance-hungry approach to systems design is far less im
portant than assured, reliable, resilient and m
aintainable systems.
For industrial com
puting applications, these are far more im
portant design objectives -- the concept of bleeding edge technology has no place in core industrial applications.
Generally less dem
anding capacity concerns
Historically, real-tim
e data flow volum
es have been low com
pared to business data networks
(business data networks typically carry data volum
es at hundreds of megabit-per-second
rates and industrial networks often operate at kilobit-per-second rates).
This generalization is rapidly becom
ing less valid as increasingly sophisticated value-added services are m
ixed with raw
, real-time data stream
s in industrial applications.
Data C
omm
unications & Fieldbus S
ystems
13
K-016
PR
IMA
RY A
ND S
EC
ON
DA
RY A
UT
OM
AT
ION
The autom
ation and control resources that are typically used fall into two broad categories:
Prim
ary Control
At this level process variables are m
easured and operational set-points are defined, typically using closed-loop control system
s, and typically over physically small dim
ensions (local control).
This is the classical application of P
rogramm
able Logic Controller (P
LC) and in perhaps a little
less imm
ediate and local sense, Distributed C
ontrol System
(DC
S) technology.
Secondary C
ontrol
This level is typically associated w
ith less time-critical data gathering activities (com
monly
collected over plant-wide or system
-wide scale) , and m
ost operation controls are open-loop, involving hum
an operator judgment.
SC
AD
A system
s are typically associated with this role, and are com
monly supported by, or
integrated with the follow
ing 'back-end' systems:
Process m
odeling and optimization system
sB
atch managem
ent and scheduling systems
Com
puter Integrated Manufacturing (C
IM) system
sA
rtificial Intelligence (AI) system
s
Data C
omm
unications & Fieldbus S
ystems
14
RE
MO
TE
TE
RM
INA
LU
NIT
K-007
�������
MTU
CO
MP
UT
ER
SY
ST
EM
TELE
PH
ON
EM
OD
EM
TELE
PH
ON
EM
OD
EM
LINE
DR
IVE
RLIN
ED
RIV
ER
INS
TR
UM
EN
TIN
TE
RF
AC
ES
PR
OT
OT
YP
E S
CA
DA S
YS
TE
M
CO
MM
UN
ICA
TIO
NS
EQ
UIP
ME
NT
INT
ER
FA
CE
S
RE
MO
TE
TE
RM
INA
LU
NIT
RE
MO
TE
TE
RM
INA
LU
NIT
WIR
ELE
SS
LINK
WIR
ED
LINK
S
The netw
ork infrastructure, com
prising LAN
, W
AN and other
components.
The M
aster Term
inal U
nit (MT
U) consists
of at least three elem
ents:
MT
U com
puterC
omm
unications interfacesH
uman M
achine Interface (H
MI)
The R
emote
Term
inal Units
(RT
Us) and
instrument
interfaces.
HM
I
Data C
omm
unications & Fieldbus S
ystems
15
The P
rogramm
able Logic Controller (P
LC
) is a class of device that evolved from traditional
relay-based local control systems.
Classical features and applications are:
A control-oriented operating regim
e that is based on deriving a closed-loop local control function based on B
oolean logical conditions applied to input conditions -- a relationship that is com
monly expressed as a relay ladder diagram
.
Applications are typically stand-alone, w
here the PLC
functions as an autonomous
controller of some local activity, such as burner control or as a w
aste water pum
p control.
PLC
s have been enhanced in several ways, m
aking them m
uch more capable and easily
used:
Addition of hum
an interface features, supporting graphical displays, event logging and trend reporting.
Support for personal com
puter (PC
) software developm
ent/testing/downloading, m
aking application developm
ent a greatly simplified task.
Netw
orked connectivity linking multiple P
LCs, H
uman-M
achine Interfaces (HM
Is) and PC
s -- creating capability for supporting greatly enhanced system
scope and capacity.
PR
OG
RA
MM
AB
LE LO
GIC
CO
NT
RO
LLER
S
Data C
omm
unications & Fieldbus S
ystems
16
The D
istributed Control S
ystem (D
CS
) is an integrated hardware/softw
are system concept
that is designed to efficiently perform data acquisition and control functions.
The original concept, and the class of application from
which current generation D
CS
products have evolved w
as:
A digital program
mable system
optimized for perform
ing closed loop control -- and replacing earlier analog controllers for this class of application.
A system
concept using digital microprocessor technology, w
hich adds reliability, functionality and convenience features largely unavailable w
ith earlier analog technology.
Major deficiencies that preexisting analog control technology had w
as stability problems due
to sensitivity to the environment (vo
ltage, te
mpera
ture
, mech
anica
l) and difficulty in easily im
plementing non-linear control functions.
DC
S products are designed to be m
odular, multi-functional system
s characterized by:
a distributed architecture, with local control m
odules, human interface consoles, (typ
ically)
proprietary backbone network.
support a reasonably user-friendly programm
ing language and applications development
capability, such that end-user modifications to system
software becom
e feasible.
DIS
TR
IBU
TE
D C
ON
TR
OL S
YS
TE
MS
(DC
S)
Data C
omm
unications & Fieldbus S
ystems
17
PR
OT
OT
YP
E D
CS
SY
ST
EM
CO
NF
IGU
RA
TIO
N
LOC
AL
CO
NT
RO
LLER
MO
DU
LE
FIE
LDIN
ST
RU
ME
NT
INT
ER
FA
CE
S
NE
TW
OR
KS
EG
ME
NT
IST
YP
ICA
LLYR
ED
UN
DA
NT
LOC
AL
CO
NT
RO
LLER
MO
DU
LE
LOC
AL
CO
NT
RO
LLER
MO
DU
LE
OP
ER
AT
OR
CO
NS
OLE
S
Data C
omm
unications & Fieldbus S
ystems
18
K-012
TH
E R
OLE
OF M
INIC
OM
PU
TE
RS
For m
any years, minicom
puters were the com
puting platform of preference to designers of
SC
AD
A and related system
s. Reasons for this preference w
ere:
they were physically sm
all, manageable and inexpensive (com
pared with m
ainframe
alternatives)they w
ere comparatively insensitive to environm
ental factors (temperature, pow
er regulation, etc.)their architectural sim
plicity made fabrication of custom
ized interfaces feasible
The general lack of standards and relative im
maturity of the com
puter industry (during the time
when m
inicomputers w
ere nominated for this role) led to a tradition of highly proprietary,
remarkably inflexible system
s in industrial computing roles.
The need for a real-tim
e operating system forced the industry to reject the m
ore user-friendly operating system
s associated with typical business applications -- w
ith a net result that proprietary solutions rem
ained the industry norm for m
uch longer than was the case in the
business systems w
orld.
The generally increasing com
plexity of industrial computing applications and the com
pelling need for end-user organizations to be able to m
anage, maintain and expand their system
s has led to a strong trend in the industry to reject traditional proprietary, custom
configurations that used to be com
mon in favor of open technology solutions.
Data C
omm
unications & Fieldbus S
ystems
19
K-013
TH
E P
ER
SO
NA
L CO
MP
UT
ER A
S
AN IN
DU
ST
RIA
L CO
MP
UT
ING R
ES
OU
RC
E
The personal com
puter has become a universal com
puting resource -- one that has compelling
advantages in most all com
puter applications:
It is the global computing platform
of choice, making it the com
mon denom
inator in any industrial application that requires com
puter functionality.
The price/perform
ance features are unbeatable, even in ruggedized packages.
Num
erous developments are very beneficial to industrial applications (redundant disk
systems [R
AID
], multiprocessor configurations, som
e fault-tolerant configurations, open-architecture LIN
UX operating system
s ...)
Hardw
are performance and softw
are stability continue to improve.
Offsetting disadvantages (as applied to industrial com
puting) include:
Most developm
ents in PC and related technologies are oriented tow
ard business applications (and therefore typically not ideally suited to industrial applications).
It is not an inherently fault-tolerant or rugged architecture, making im
provements in these
areas piecemeal and therefore lacking in overall system
integrity.
Regardless of argum
ents pro and con, the PC w
ill undoubtedly have an ever more im
portant role in industrial com
puting applications.
Data C
omm
unications & Fieldbus S
ystems
20
D-329
TH
E U
BIQ
UIT
OU
S P
ER
SO
NA
L CO
MP
UT
ER
Most organizations -- indeed the w
hole world -- have effectively standardized on Intel-com
patible personal com
puters (running a variant of Microsoft's W
indows -- or increasingly, LIN
UX
).
This does not deny the legitim
ate and relatively stable niche market share enjoyed by A
pple M
acintosh systems -- but m
arket share realities dictate that new products (hardw
are and softw
are) are typically available for Intel systems first, and thus M
ac systems seem
to be doomed
to a second-tier role in the industry.
The utilization levels associated w
ith personal computers are typically low
(because procurem
ent decisions are based on human user convenience and efficiency, rather than
utilization expectations) -- with the net result that there is typically a m
assive amount of
underutilized capacity on most P
Cs used for business or personal applications. M
ore CP
U
cycles are consumed w
ith running screensavers than any other application!
New
networked com
puting applications sometim
es justify the deployment of these system
s, but by and large they are assum
ed to be available as a no cost resource whenever a new
client/server or w
eb-based application is being planned.
Recent corporate experience in the U
nited States show
s that desktop computer system
s are replaced (on average) every three years, and notebook com
puters every two years.
Data C
omm
unications & Fieldbus S
ystems
21
Num
erous vendors have developed ruggedized PC packages for diverse applications -- designed to
help the rather fragile conventional "office oriented" PC to survive in the environm
entally harsh w
orld of industrial applications.
Factors that threaten conventional P
C's in harsh environm
ents include:
RU
GG
ED
IZE
D P
C P
AC
KA
GIN
G
RU
GG
ED
IZED P
C P
AC
KA
GIN
G E
XA
MP
LE(w
ww
.acuraembedded.com
)
Heat
Vibration
Hum
idityD
ust and dirtC
austic atmosphere
Pow
er system stability and reliability
Som
e configuration issues that are im
portant in this context are:
Choice of operating system
; typically a W
indows variant or
Linux.
Choice of m
ass storage technology; typically a rugged hard drive or flash drive.
Data C
omm
unications & Fieldbus S
ystems
22
The roles of S
CA
DA
, PLC
and DC
S technology have changed w
ith the progressive developm
ent of technology:
SU
MM
AR
Y; S
CA
DA
, PLC
AN
D D
CS
RO
LES
DC
S
PLC
SC
AD
A
TR
AD
ITIO
NA
L RO
LES
EX
PA
ND
ING
RO
LES
Replacem
ent for discrete analog local closed loop control system
sD
igital technology offers flexibility in developm
ent and modification
Discrete logical closed loop
control applications traditionally served by relay system
sD
igital technology benefits
Open loop data collection and
control functions supported by hum
an operatorT
ypically involves off-site network
Increased sophistication of the hum
an interface and modular
features encroach on SC
AD
AM
any features similar to P
LC
PLC
systems frequently serve as
high-end RT
Us in S
CA
DA
app'sP
LC features and extensions
encroach on role of DC
S
SC
AD
A system
s integrate with
DC
S and P
LC system
sLow
end SC
AD
A applications lose
out to DC
S and P
LC
Data C
omm
unications & Fieldbus S
ystems
23
EN
TE
RP
RIS
E, C
ON
TR
OL, S
EN
SO
RA
ND D
EV
ICE N
ET
WO
RK
S
EN
TE
RP
RIS
EN
ET
WO
RK
S(IT A
pplications)
SE
NS
OR N
ET
WO
RK
S(M
anfacturing Autom
ation --generally binary on-off condition sensors
and controls -- so-called discrete I/O)
DE
VIC
E N
ET
WO
RK
S(P
rocess Autom
ation --generally analog variable sensors
and proportional controls)
SC
AD
AN
ET
WO
RK
(Control A
pplications)
RT
U/P
LCR
TU
/DC
S
MT
U (A
CT
S A
S A
GA
TE
WA
Y T
O T
HE
EN
TE
RP
RIS
E
NE
TW
OR
K
HM
I/MM
IH
MI/M
MI
NO
TE
: (1) R
arely will all four netw
ork types exist in one site
(2) Traditional distinctions
between these classes of
network are becom
ing less clear
Data C
omm
unications & Fieldbus S
ystems
24
TH
E S
HIF
T FR
OM A C
OM
PU
TE
R-C
EN
TR
ICT
O N
ET
WO
RK
-CE
NT
RIC P
AR
AD
IGM
NE
TWO
RK
CO
MP
UTE
R
US
ER
CO
MP
UTE
R
NE
TWO
RK
US
ER
Tw
o key observations apply here:
The netw
ork has become the strategic heart of our com
puting environment.
The com
puter is much closer (and therefore m
uch less threatening) to the user than previously.
Data C
omm
unications & Fieldbus S
ystems
25
AA
MO
DE
MM
OD
EM
BA
SIC
EQ
UIP
ME
NT TE
RM
INO
LOG
Y
DT
E
DC
ED
CE
D-003
DT
E: -
DC
E: -
Da
ta T
erm
ina
l Eq
uip
me
nt (D
TE
) consists of whatever apparatus physically
terminates the circuit (d
um
b te
rmin
al, p
erso
nal co
mpute
r, main
fram
e, e
tc.).
Data C
om
mu
nicatio
ns E
qu
ipm
ent (D
CE
) consists of equipment w
hich interfaces the D
TE
to the comm
unications facilities (DC
Es a
re typ
ically a
nalo
g m
odem
s or d
igita
l line
drive
rs).
Note that packaging decisions m
ay blur the distinction between these tw
o functions (e
.g. a
n in
tern
al m
odem
mounte
d in
side to
the P
C ca
bin
et).
CO
MM
UN
ICA
TION
FAC
ILITIES
A
DT
E
Data C
omm
unications & Fieldbus S
ystems
26
SIM
PLE
X, H
ALF D
UP
LEX
AN
D FU
LL D
UP
LEX
MO
DE
S O
F OP
ER
ATIO
N
SIM
PL
EX
OP
ER
AT
ION
is a one-way service,
such as television broadcasting or pocket pager service. It is not possible to com
municate in the reverse direction w
ith sim
plex, even for purposes of acknowledging
message receipt.
HA
LF
-DU
PL
EX
OP
ER
AT
ION
provides for com
munication in either direction, but not
simultaneously. The reversal of direction m
ay be sufficiently fast so as to create the illusion of full duplex operation.
FU
LL
-DU
PL
EX
OP
ER
AT
ION
provides for sim
ultaneous transmission in both directions.
Thus, neither direction of comm
unication is forced to w
ait for activity to complete in the
reverse direction, as is often the case with
half-duplex.
Operates like a one-w
ay street.
Operates like a street capable of
working in either direction, but
alternating, perhaps as controlled by a flag-person.
Operates like a typical city street,
or divided highway, w
ith no interaction betw
een opposing traffic (th
at is, th
ere
is ne
ver a
ny
req
uire
me
nt to
wa
it for, o
r be
slo
we
d d
ow
n b
y op
po
sing
traffic)
D-007
D
ata Com
munications &
Fieldbus System
s27
SY
NC
HR
ON
IZA
TIO
N
There are three dim
ensions to the synchronization, or timing problem
:
1. BIT
SY
NC
HR
ON
IZA
TIO
N: B
it synchronization is accomplished w
ith electronic circuitry (in e
ithe
r D
CE
's or D
TE
's). Bit synchronization is dependent on som
e prior knowledge of digital bit patterns, and
predictability of transitions between binary 1 and 0 values.
2. CH
AR
AC
TE
R S
YN
CH
RO
NIZ
AT
ION
: Character synchronization involves assuring all bits from
one character (o
r da
ta o
ctet) rem
ain properly associated with the byte or octet to w
hich they belong.
3. ME
SS
AG
E S
YN
CH
RO
NIZ
AT
ION
: Message synchronization involves unam
biguously m
arking (an
d o
bse
rving
) the beginning and end of a message, w
hich will typically involve m
ultiple segm
ents or packets.
10
11
01
10
10
IN O
RD
ER
FO
R U
SE
FU
L CL
OC
K R
EC
OV
ER
Y T
OF
UN
CT
ION
, TH
ER
E M
US
T B
E G
UA
RA
NT
EE
S O
FS
UF
FIC
IEN
TLY
FR
EQ
UE
NT
TR
AN
SIT
ION
S
(between 1 and 0) IN
TH
E D
AT
A
01
00
00
11
01
00
00
11
01
00
00
11
TH
E B
YT
E (O
ctet) BO
UN
DA
RIE
S M
US
T B
E
RE
SP
EC
TE
D IN
TH
E P
RO
CE
SS
OF
RE
CR
EA
TIN
GM
ES
SA
GE
S. A
CC
IDE
NT
AL R
EA
LIGN
ME
NT
OF
TH
ES
E B
OU
ND
AR
IES
WILL R
EN
DE
R D
AT
A U
SE
LES
S.
PA
CK
ET
, OR
ME
SS
AG
E F
RA
ME
SE
QU
EN
CE
M
US
T B
E R
ES
PE
CT
ED
IN T
HE
PR
OC
ES
S
OF
RE
-AS
SE
MB
LING
FR
AM
ES
. VA
RIA
TIO
NS
IN
NE
TW
OR
K T
RA
NS
IT D
ELA
YS
CA
N
RE
SU
LT IN
OU
T-O
F-S
EQ
UE
NC
E F
RA
ME
A
RR
IVA
LS.
PACKET/FRAME N
UMBER 3
PACKET/FRAME N
UMBER 2
PACKET/FRAME N
UMBER 1
D-064
STXETX
Data C
omm
unications & Fieldbus S
ystems
28
ST
AR
T P
ULS
E(T
he downw
ard-goingleading edge of thispulse is the critical tim
ingreference)
12
34
56
78
ST
OP
PU
LSE
(The unconditional M
AR
K or O
NE
value of this pulse assures that the next ST
AR
T
pulse -- which is alw
ays a MA
RK
-to-SP
AC
E
transition -- will be recognizable)
7 DA
TA
BIT
S(M
ost asynch. systems
operate with 7 data bits)
TH
E 8th. B
IT IS
MO
ST
CO
MM
ON
LY U
SE
DA
S A
PA
RIT
Y B
IT (P
arity checking is a simple
form of error detection)
MA
RK
(Or O
NE
)S
IGN
AL S
TA
TE
SP
AC
E (O
r ZE
RO
)S
IGN
AL S
TA
TE
INC
RE
AS
ING
TIM
E
AS
YN
CH
RO
NO
US
SY
STE
M C
HA
RA
CTE
RIS
TICS
D-001
11
11
00
00
D
ata Com
munications &
Fieldbus System
s29
��������
MO
DE
MM
OD
EM
SY
NC
HR
ON
OU
S S
YS
TE
M C
HA
RA
CT
ER
IST
ICS
MA
ST
ER C
LOC
K IN
SE
ND
ING D
CE
FE
ED
S T
IMIN
G IN
FO
RM
AT
ION T
O
LOC
AL D
TE A
ND R
EM
OT
E D
CE
.
SLA
VE C
LOC
K IN
RE
CE
IVIN
G M
OD
EM
RE
CO
VE
RS IN
CO
MIN
G T
IMIN
GIN
FO
RM
AT
ION A
ND P
AS
SE
S T
O D
TE
DT
E
DC
ED
CE
DT
E
DIR
EC
TIO
N O
F D
AT
A F
LOW
TY
PIC
AL S
YN
CH
RO
NO
US M
ES
SA
GE F
OR
MA
T CO
NS
IST
S O
F:
1. Leading SY
N characters - - usually repeated at least once
2. Message enveloped by S
TX (S
tart of TeX
t) and ET
X (E
nd of TeX
t) characters3. T
railing BC
C (B
lock Check C
haracter) used for error detection.
BC
CE
TXX
XX
STX
SY
NS
YN
D-002
Data C
omm
unications & Fieldbus S
ystems
30
NU
LD
LES
P
SO
HD
C1
DC
2
DC
3
DC
4
STX
ETX
EO
T
EN
Q
AC
K
BE
L
BS
CA
N
ET
B
SY
N
NA
K
6F
Vf
v6
0110
01015 4 3
0@
12345
ABCDE
PQRSTU
abcde
pqrstu
789
GHIJKL
WXYZ
ghijkl
wxyz
DE
L
MNOo n m
SI
SO
CR
FF
VT HT
12 0789ABCDEF
LF1010
1011
1100
1101
1110
1111
1001
1000
0111
0100
0011
0010
0001
0000
01
23
45
67
111110
101100
011010
001000
!"#$&' %()*+,.= < > ; :
\ []} {|~
^-?
/ _
`
Bit P
ositions 7, 6 & 5:
EM
SU
B
ES
C
FS
GS
RS
US
Bit P
ositions4, 3, 2 &
1:
12
34
56
78
Bit P
ositions:
01
00
00
11
: Most S
ignificant Bit (M
SB
)
Least Significant B
it (LSB
) :
46
AS
CII C
HA
RA
CT
ER
SE
T
INT
ER
PR
ET
AT
ION
EX
AM
PL
E:
AS
CII "F
"
HE
X:
DE
CIM
AL 70
D-004
NO
TE
S:
1. The 8th bit, if represented, is a Z
ER
O
2. Data is (alm
ost) always transm
itted LSB
first.
D
ata Com
munications &
Fieldbus System
s31
Circ
uit N
o. 1
Circ
uit N
o. 2
Circ
uit N
o. 3
Circ
uit N
o. 4
Circ
uit N
o. 5
Circ
uit N
o. 6
Circ
uit N
o. 7
Circ
uit N
o. 8
Clo
ck C
ircu
it
With parallel transm
ission of the A
SC
II character pair "OK
", tw
o clock time units are
required to transmit tw
o characters (in this exam
ple w
ith an 8 bit parallel bus configuration).
BIT S
EQ
UE
NC
E R
EP
RE
SE
NT
ING
AS
CII C
HA
RA
CT
ER "O
"B
IT S
EQ
UE
NC
E R
EP
RE
SE
NT
ING
AS
CII C
HA
RA
CT
ER "K
"
ST
AR
T P
ULS
E(T
RA
NS
ITIO
N):
ST
OP
PU
LSE
:
SE
RIA
L vs. PA
RA
LLEL C
OM
MU
NIC
ATIO
N
PA
RA
LLEL D
ATA
SE
QU
EN
CE
S:
D-081
With serial transm
ission of the A
SC
II character pair "OK
" typically 20 clock tim
e units are required to transm
it two
characters (This exam
ple is typical w
ith 7 bits of data per character, one parity bit, one start bit and one stop bit).
SE
RIA
L DA
TA S
EQ
UE
NC
E:
11
10
11
11
00
00
11
01
00
11
01
11
10
01
11
10
"K"
"O"
ST
OP
PU
LSE
:S
TA
RT P
ULS
E(T
RA
NS
ITIO
N)
87
65
43
211
11
10
01
187
65
43
211
10
10
01
0
NO
TE: IN
THE D
ATA
PA
TTER
NS
ILLUS
TRA
TED O
N TH
IS P
AG
E, TH
EFO
LLOW
ING N
OTA
TION A
ND B
ITS
EQ
UE
NC
ES W
ER
E U
SE
D:
"O" C
HA
RA
CTE
R :-
"K" C
HA
RA
CTE
R :-
THE E
IGH
TH B
IT PO
SITIO
N IS
DE
FINE
DA
S A
N E
VE
N P
AR
ITY B
IT IN E
AC
H C
AS
E(P
AR
ITY IS
DIS
CU
SS
ED LA
TER
).
DA
TA IS
TRA
NS
MITTE
D O
N S
ER
IAL
LINE
S LE
AS
T SIG
NIFIC
AN
T BIT
(Bit P
osition No. 1) FIR
ST.
Data C
omm
unications & Fieldbus S
ystems
32
OV
ER
VIE
W O
F T
ELE
CO
MM
UN
ICA
TIO
NS
TE
CH
NO
LOG
IESD-024
AN
ALO
G
DE
DIC
AT
ED
CIR
CU
ITS
WIT
CH
ED
PA
CK
ET
SW
ITC
HE
D
TR
AD
ITIO
NA
L LEA
SE
D,
DE
DIC
AT
ED
, PR
IVA
TE
LIN
E S
ER
VIC
ES
(Require
analog modem
s)
AV
AILA
BLE
AS
DE
DIC
AT
ED
DIG
ITA
L SE
RV
ICE
S(S
uch as DS
0, DS
1, E1, etc.)
GLO
BA
L, UB
IQU
ITO
US
PU
BLIC
SW
ITC
HE
D T
ELE
PH
ON
EN
ET
WO
RK
(PS
TN
) SE
RV
ICE
(Com
monly referred to as "P
OT
S")
BA
SIC
RA
TE
ISD
N S
UP
PO
RT
S
CIR
CU
IT-S
WIT
CH
ED
64 KB
PS
SE
RV
ICE
; TY
PIC
ALLY
CO
NF
IGU
RE
DW
ITH
2 CH
AN
NE
LS C
AP
AB
LE O
FF
UN
CT
ION
ING
AT
128 KB
PS
.
NO
T F
EA
SIB
LE
OB
SO
LES
CE
NT
X.25 P
AC
KE
T S
WIT
CH
ING
SE
RV
ICE
IS S
TILL U
SE
D; F
AS
T P
AC
KE
T
TE
CH
NO
LOG
Y (F
rame R
elay and Cell
Relay/A
TM
) HA
S LA
RG
ELY
RE
PLA
CE
D X
.25
DIG
ITA
L
Data C
omm
unications & Fieldbus S
ystems
33
AN
AL
OG
SIG
NA
L C
HA
RA
CT
ER
IST
ICS
:
DIG
ITA
L S
IGN
AL
CH
AR
AC
TE
RIS
TIC
S
AN
AL
OG
SIG
NA
LS
AR
E IN
FIN
ITE
LY
AN
D C
ON
TIN
UO
US
LY
VA
RIA
BL
E(M
ea
nin
g th
e sig
na
ls are
very co
mp
lex, a
nd
it is very d
ifficult to
e
fficien
tly cha
racte
rize th
eir b
eh
avio
r so th
at e
lectro
nic circu
itsca
n in
terp
ret th
eir m
ea
nin
g)
DIG
ITA
L S
IGN
AL
S A
RE
DIS
CR
ET
E IN
BO
TH
TIM
E A
ND
AM
PL
ITU
DE
(Mea
nin
g th
ey ch
an
ge o
nly a
t pre
dicta
ble
times, a
nd va
ry be
twe
en
a sm
all n
um
be
r of p
redicta
ble
valu
es)
D-005
CO
MP
AR
ISO
N; A
NA
LOG
vs. DIG
ITAL S
IGN
ALSIN
CR
EA
SIN
GT
IME
AX
IS
INC
RE
AS
ING
TIM
E A
XIS
Data C
omm
unications & Fieldbus S
ystems
34
FUN
DAM
ENTAL O
F CO
MM
UN
ICATIO
NS
•Fourier Series Approxim
ation •
Nyquist Theorem
•
Shannon’s Theorem
•M
odulation and Dem
odulation
Data C
omm
unications & Fieldbus S
ystems
35
Data C
omm
unications & Fieldbus S
ystems
36
WE C
AN AD
D SIN
ES TO M
AKE NEW
FUN
CTIO
NS
g2 (t)=1/3sin(2π( 3f )t)
g1 (t)=sin(2πf t)
g3 (t)= g
1 (t) + g2 (t)
Data C
omm
unications & Fieldbus S
ystems
37
FOU
RIER
TRAN
SFOR
M
Jean B. Fourier found that any periodic function can be
expressed as an infinite sum of sine function.
()
()
∑∑
∞=
∞=
++
=1
10
/2
sin/
2cos
2 1)
(n
in
iP
itb
Pit
aa
ts
ππ
Data C
omm
unications & Fieldbus S
ystems
38
FOU
RIER
APPRO
XIMATIO
NS
Data C
omm
unications & Fieldbus S
ystems
39
BAUD
RATE VS. BIT R
ATE
•Transm
ission speed can be measured in bits per
second(bps). •
Technically, transmission is rated in baud, the num
ber of changes in the signal per second that the hardw
are generates.
•U
sing RS
-232 standard to comm
unicate, bit rate rate = baud rate.
•In general, bit rate rate = N
* baud rate, where N
is the num
ber of signals in a string.
Data C
omm
unications & Fieldbus S
ystems
40
BAUD
RATE VS. BIT R
ATE
•Sender sends the bit string, by b
1 b2 …
bn .
•The transm
itter alternately analyzes each string and transm
its a signal component uniquely determ
ined by the bit values. O
nce the component is sent, the
transmitter gets another bit string and repeats this
process. •
The different signal components m
ake up the actual transm
itted signal. The frequency with w
hich the com
ponents change is the baud rate. •
At the receiving end, the process is reversed. The receiver alternately sam
ples the incoming signal and
generates a bit string.
Data C
omm
unications & Fieldbus S
ystems
41
BAUD
RATE VS. BIT R
ATE
•C
onsequently, the bit rate depends on two things: the
frequency with w
hich a component can change (baud
rate) and n, the number of bits in the string. That is
why the form
ula:(signal may have up to 2
n different am
plitudes) bit rate = n * baud rate
Data C
omm
unications & Fieldbus S
ystems
42
NYQ
UIST SAM
PLING
THEO
REM
•D
r. Harry N
yquist (1920) •
Nyquist show
ed that if ƒ is the maxim
um frequency
the medium
can transmit, the receiver can com
pletely reconstruct by sam
pling it 2ƒ times per second on a
perfectly noiseless channel. •
In other words, the receiver can reconstruct the
signal by sampling it at intervals of 1/(2ƒ) second.
•For exam
ple, if the max frequency is 4000 H
z, the receiver needs to sam
ple the signal 8000 times per
second or using 2ƒ as the baud rate. •
Bit rate = 2ƒ * n.
Data C
omm
unications & Fieldbus S
ystems
43
ANY LIM
IT ON
BIT RATE?
•The form
ula Bit rate = 2ƒ * n seems to im
ply that there is no upper bound for the data rate given the m
aximum
frequency. Unfortunately, this is not true for
two reasons.
–Am
plitude limitations
–N
oise
Data C
omm
unications & Fieldbus S
ystems
44
HO
W ABO
UT R
EAL WO
RLD
HAR
DW
ARE?
•First, if w
e used amplitude to represent data bits,
each time w
e separate the amplitude into sm
aller ranges to represent m
ore data bits, the receiver must
be more sophisticated (and m
ore expensive) to be able to detect sm
aller differences. If the differences becom
e too small, w
e eventually exceed the ability of a device to detect them
(or a medium
to react to them
).
Data C
omm
unications & Fieldbus S
ystems
45
LIMITATIO
NS O
N R
EAL HAR
DW
ARE
Data C
omm
unications & Fieldbus S
ystems
46
SIGN
AL TO N
OISE R
ATIO
•S/N
is known as the signal-to-noise ratio in decibels.
•Because S
is usually much larger than N
, the ratio is often scaled dow
n logarithmically and the unit is
measured in bels and 1 dB = 0.1 bel.
•You could expect som
ewhere around 1000-1 S/N
on a typical telephone circuit.
Data C
omm
unications & Fieldbus S
ystems
47
SHAN
NO
N’S TH
EOR
EM
•Bit rate = Bandw
idth * log 2 (1+S/N) bps.
•According to this result, a bit rate around 35,000 bps is an upper lim
it for conventional modem
s. •
Rem
ember telephone specs!
Data C
omm
unications & Fieldbus S
ystems
48
SHAN
NO
N’S TH
EOR
EM AN
D 56K M
OD
EMS
•According to this result, a bit rate around 35,000 bps is an upper lim
it for conventional modem
s. •
How
about 56k modem
s?
Data C
omm
unications & Fieldbus S
ystems
49
TELEPHONEEXCHANGE
LOC
AL LO
OP
INTE
R-E
XC
HA
NG
E N
ETW
OR
KLO
CA
L LOO
P
TE
LEP
HO
NE S
YS
TE
M S
CH
EM
AT
IC V
IEW
LOC
AL LO
OP FE
ATU
RE
S:
INTE
R-E
XC
HA
NG
E N
ETW
OR
K FE
ATU
RE
S:
D-060
TELEPHONEEXCHANGE
Medium
-to- long-distance connections are typical (fiber optic, copper, radio microw
ave and/or satellite)E
ach customer uses only a fractional part of a high-capacity, shared resource
Inter-exchange segment lengths: a few
kilometers (m
iles) to perhaps 25,000 kilometers (15,000 m
iles)T
he long distances involved require signal amplification or regeneration - a unidirectional process
Costs dem
and shared, high capacity facilities making m
ultiplexing essential - a unidirectional processT
hus, the configuration is always based on tw
o unidirectional comm
unication paths
2-wire m
etallic cable connection (copper) where one pair of conductors is dedicated to each subscriber
Typical cable length is 3 to 8 kilom
eters (2 to 5 miles)
Short cable lengths perm
it passive connections -- amplifiers are not typically required
Disbursed term
ination points (one per customer site) m
ake end-to-end multiplexing im
practicalT
hus, bi-directional signal flow on one pair of w
ires is practical
Data C
omm
unications & Fieldbus S
ystems
50
01
11
00
11
D-023
EX
AM
PLE
S O
F DIG
ITAL S
IGN
AL FO
RM
ATS
10
11
01
10
SIM
PLE
UN
IPO
LAR
NO
N-R
ET
UR
NT
O Z
ER
O (N
RZ
) FO
RM
AT
BIP
OLA
R N
ON
-RE
TU
RN
TO
Z
ER
O (B
NR
Z) F
OR
MA
T
UN
IPO
LAR
RE
TU
RN
TO
ZE
RO
F
OR
MA
T (50%
Duty C
ycle)
BIP
OLA
R R
ET
UR
N T
O Z
ER
O,
OR
ALT
ER
NA
TE
MA
RK
IN
VE
RT
ED
(AM
I) FO
RM
AT
MA
NC
HE
ST
ER
EN
CO
DE
D,
OR
DIP
HA
SE
FO
RM
AT
101010101
Data C
omm
unications & Fieldbus S
ystems
51
D-265
GE
NE
RA
L RE
LATIO
NS
HIP
: DIS
TAN
CE
vs. DA
TA R
ATE
With digital com
munication technology, there is a general relationship betw
een the maxim
um
feasible distance of comm
unication and maxim
um feasible data rate -- for any given technology
and/or implem
entation.
In the box illustration, the trade-off between m
aximum
data rate and distance is shown to follow
the classic hyperbola curve.
Different technologies w
ill support different hyperbola curves, but for any given (leve
l of
sophistica
tion o
f) technology, it will alw
ays be possible to double the data rate so long as one is w
illing to half the distance capability.
This characteristic is comm
on to all digital form
at signals (ele
ctrical o
r optica
l), and answers questions like:
How
far can I stretch an EIA
-232-C
interface cable ?
What is the m
aximum
feasible data rate for tw
isted pair (UTP
) cable ?
How
fast can line drivers operate ?
Why can't LA
N system
s be stretched to greater physical dim
ensions ?T
RA
NS
MIS
SIO
N D
IST
AN
CE
ACHIEVABLE DATA RATE
HY
PE
RB
OLIC
CU
RV
ES
HA
VE
THE
CH
AR
AC
TE
RIS
TIC
OF
FOLLO
WIN
G TH
E LIN
E
OF
CO
NS
TAN
T P
RO
DU
CT
OF
THE
TW
O A
XE
S;
TH
US
(in this case)
(DA
TA
RA
TE) x (D
IST
AN
CE
) = CO
NS
TAN
T
●●● ●
Data C
omm
unications & Fieldbus S
ystems
52
CO
MP
AR
ISO
N: A
NA
LOG A
MP
LIFIC
AT
ION vs.
DIG
ITA
L RE
GE
NE
RA
TIO
N
SO
UR
CE
SIG
NA
L
Digital signals are typically view
ed as rectangular pulses, as suggested by the 'source signal' in both sketches.
Signals inevitably w
eaken and become
distorted over the distance of com
munication, as suggested by the
'weakened signal' view
.
Am
plifiers simply increase the signal
strength without reshaping or repairing
the distortion.
All telecom
munications system
s are exposed to the sam
e signal deterioration (w
eakening plus distortion) -- but digital regeneration is able to overcom
e this problem
.
Digital repeaters (regenerators) fully
reconstruct the source signal, delivering a signal that is effectively identical to the source data.
WE
AK
EN
ED
SIG
NA
L
AM
PLIFIE
DS
IGN
AL
SO
UR
CE
SIG
NA
L
WE
AK
EN
ED
SIG
NA
L
REGENERATEDSIGNAL
60
Data C
omm
unications & Fieldbus S
ystems
53
DIG
ITAL TE
LEP
HO
NE N
ETW
OR
K O
VE
RV
IEW
CO
DE
C
CO
DE
C
TIM
E D
IVIS
ION
MU
LTIP
LEX
ER
SY
ST
EM
S
TIM
E D
IVIS
ION
MU
LTIP
LEX
ER
SY
ST
EM
S
CO
DE
C
CO
DE
C
���
MO
DE
M
���
MO
DE
M
MU
LTIP
LEX
SY
ST
EM
SIG
NA
LC
ON
VE
RS
ION
TE
RM
INA
LS
YS
TE
MS
TE
RM
INA
LS
YS
TE
MS
SIG
NA
LC
ON
VE
RS
ION
HIG
H S
PE
ED
DIG
ITA
L LINK
MU
LTIP
LEX
SY
ST
EM
D-246
continued ....
TER
MIN
AL S
YS
TEM
S:
Term
inal system com
ponents are any user-selected terminal devices (voice, video, graphics, etc.).
In most countries, term
inal systems are privately ow
ned, although historically the telephone companies
have jealously guarded their right of ownership.
Data C
omm
unications & Fieldbus S
ystems
54
AAAA
MO
DE
M
DC
E
MO
DE
M
DC
E
PR
OTO
TYP
E LIN
K M
ULTIP
LEX
ER
SY
STE
M C
ON
FIGU
RA
TION
DT
E
DT
E
RE
MO
TE
MU
LTIP
LEX
ER H
OS
T M
ULT
IPLE
XE
R
D-008
Link multiplexers m
ay function as frequency division multiplexers, tim
e division multiplexers or
statistical time division m
ultiplexers. The comm
on concept is that the multiplexer-to-m
ultiplexer segm
ent is adapted to carry the aggregate data volume for the various services being connected.
Multiplexing is often integrally packaged w
ith other functions, such as circuit or packet switching
capability, modem
s, data compression, etc.
Data C
omm
unications & Fieldbus S
ystems
55
EN
CO
DIN
G D
AT
A F
OR T
RA
NS
MIS
SIO
N
-- TH
E R
OLE
OF T
HE D
CE
D-224
���
�����
DT
ED
CE
DC
ED
TE
���
�����
DT
ED
CE
DC
ED
TE
MO
DE
MM
OD
EM
LINE D
RIV
ER
LINE D
RIV
ER
AN
ALO
G T
EC
HN
OLO
GY
DIG
ITA
L TE
CH
NO
LOG
Y
The digital signals from
Data T
erminal E
quipment (at the serial interface) are not capable of w
orking over a significant distance (T
he EIA
-232 interface has a design limitation of 50 feet - 30 m
eters).
Thus, in order to com
municate over greater distances, these signals need to be converted in som
e w
ay -- into signals that are more robust, and conform
to the features of the facilities available.
That is, for analog com
munications facilities, an analog signal is required and for digital
comm
unications facilities, a digital signal is required.
Thus the D
CE perform
s a signal encoding function, and in the case of analog comm
unication facilities, it also perform
s an analog-to-digital conversion -- or a mo
dulation-demodulation function.
Data C
omm
unications & Fieldbus S
ystems
56
D-240
BIT R
AT
E vs. B
AU
D R
AT
E
11
10
01
00
11
10
01
00
1010
INC
RE
AS
ING T
IME
INC
RE
AS
ING T
IME
INC
RE
AS
ING T
IME
DIG
ITAL S
IGN
AL
AN
ALO
G S
IGN
AL
DIG
ITAL S
IGN
AL
AN
ALO
G S
IGN
AL
INC
RE
AS
ING T
IME
10
10
11
00
10
DA
TA S
EQ
UE
NC
E
10
10
11
00
10
DA
TA S
EQ
UE
NC
E
TW
O B
ITS P
ER B
AU
D C
AS
E
ON
E B
IT P
ER B
AU
D C
AS
EW
here one bit of information is
represented by one electrical signal elem
ent -- or symbol, the bit
rate and Baud rate have the sam
e num
erical value.
Thus, if, the data rate is 1200 bps,
then the signal value also changes 1200 tim
es per sec. or 1200 Baud.
If two bits of inform
ation are coded as one sym
bol, the bit rate and Baud
rate are related by a 2 -to- 1 ratio.
Thus, if the data rate is 1200 bps,
then the rate at which the signal
values change is half that rate, or 600 tim
es per second, or 600 Baud.
Thus, bit rate is the m
eaningful param
eter to use in describing transm
ission rate -- it directly specifies the rate of inform
ation transm
ission -- Baud rate does not.
Data C
omm
unications & Fieldbus S
ystems
57
BE
LL (AT&
T) MO
DE
M S
UM
MA
RY
D-179
The AT&
T organization was at one tim
e the leader in developing modem
technology.
Som
e examples of "B
ell-compatible" m
odems are:
TY
PE
103: (1)
TY
PE
201C:
TY
PE
202S:
TY
PE
208A:
TY
PE
209:
TY
PE
212A:
With the advent of a liberalized telephone interconnect policy (a
llow
ing co
nnectio
n o
f custo
mer
ow
ned e
quip
ment) in the early 1980's, com
petitive specialist organizations passed AT&
T by in this m
arket. These are largely based on ITU (e
x CC
ITT
) "V-S
eries" modem
standards.
(1) Type 103 modem
s were acoustically coupled, thus requiring no m
etallic wiring connection.
(2) Types 208 and 209 modem
s operate half duplex on 2-wire connections, full duplex on 4-w
ires.
DA
TA
RA
TE
300 bps
2400 bps
1200 bps
4800 bps
9600 bps
1200 bps
DU
PL
EX
Full
Half
Half
Full/H
alf (2)
Full/H
alf (2)
Full
TIM
ING
Asynch.
Synch.
Synch.
Synch.
Synch.
Synch/A
synch.
LIN
E T
YP
E
Dial (2W
)
Dial (2W
)
Dial (2W
)
Leased (4W)
Leased (4W)
Dial (2W
)
NO
TE
S:
Data C
omm
unications & Fieldbus S
ystems
58
ITU-T S
TAN
DA
RD
MO
DE
M S
UM
MA
RY
D-180
Most significant, current developm
ents in modem
technology are coordinated through the ITU
-T (form
erly C
CIT
T -- se
e N
ote
1 b
elo
w). A
ll ITU-T m
odem standards carry a "V
" prefix, and are com
monly referred to as the "V
-Series" standards.
Som
e examples of ITU
-T modem
s are:
V.22
V.22 bis
V.27
V.29
V.32
V.32 bis
V.33
V.34
(1) The IT
U-T
functioned for many years as the C
CIT
T -- only being renam
ed in 1994. Thus, m
any docum
ents and references still use this earlier name.
(2) Type V
.27 and V.29 m
odems operate half duplex on 2-w
ire connections, full duplex on 4-wires
DA
TA
RA
TE
1200 bps
2400 bps
4800 bps
9600 bps
9600 bps
14,400 bps
14,400 bps
28,800 bps
DU
PL
EX
Full
Full
Full/H
alf (2)
Full/H
alf (2)
Full
Full
Full/H
alf (2)
Full
TIM
ING
Asynch.
Synch/A
synch
Synch.
Synch.
Synch/A
synch.
Synch/A
synch.
Synch.
Synch/A
synch.
LIN
E T
YP
E
Dial (2W
)
Dial (2W
)
Leased (2W/4W
)
Leased (2W/4W
)
Dial (2W
)
Dial (2W
)
Leased (2W/4W
)
Dial (2W
)
NO
TE
:
Data C
omm
unications & Fieldbus S
ystems
59
TELEPHONEEXCHANGE
2-WIR
E LO
CA
L LOO
PIN
TE
R-E
XC
HA
NG
E N
ET
WO
RK
DC
E
���
DC
E
DT
E
2-WIR
E LO
CA
L LOO
P
TELEPHONEEXCHANGE
DT
E
SC
HE
MA
TIC V
IEW - 2-W
IRE A
ND 4-W
IRE D
AT
A
D-281
TELEPHONEEXCHANGE
DC
E
���
DT
ED
TE
DC
E
TELEPHONEEXCHANGE
4-WIR
E LO
CA
L LOO
PIN
TE
R-E
XC
HA
NG
E N
ET
WO
RK
4-WIR
E LO
CA
L LOO
P
4-wire (i.e. dedicated access) configurations have a dedicated local loop cable pair for each
direction of information flow
(thus no conflict or comprom
ise is created by opposing traffic).
Thus, a "divided highw
ay" configuration prevails all the way from
the DC
E to the D
CE
.
2-wire (e.g. dial access) data configurations share a com
mon pair of local loop cable.
This shared access local loop m
ust serve both directions of information flow
.
Data C
omm
unications & Fieldbus S
ystems
60
LINE D
RIV
ER
S
DC
E
����
DC
E
DTE
DT
E
EIA
-232-C IN
TE
RF
AC
E
CO
NN
EC
TIO
N (T
ypical)
EIA
-232-C IN
TE
RF
AC
E
CO
NN
EC
TIO
N (T
ypical)
4-WIR
E T
WIS
TE
D P
AIR
TE
LEP
HO
NE C
AB
LE
ME
DIU
M C
AR
RIE
SP
RO
PR
IET
AR
Y D
IGIT
AL
SIG
NA
LS B
ET
WE
EN
PA
IRS O
F D
CE
's.
LINE D
RIV
ER
S ... D
IGIT
AL D
CE
's
D-210
LINE D
RIV
ER FE
ATU
RE
S:
Maxim
um separation of about
16 km (10 M
iles) and m
aximum
data rate of about 256 kbps.
Line drivers need unloaded cable pairs -- a restriction to use beyond about 5 km
/ 3 mi.
Most use copper cable m
edia; fiber optic line drivers are also used.
Line drivers are data comm
unications equipment (D
CE
) that comm
unicate with digital signals --
exactly comparable to the w
ay modem
s comm
unicate with analog signals.
KE
Y C
ON
CE
PT: T
hey are always equipped in pairs -- like m
odems.
In fact, line drivers are often erroneously called modem
s -- but remem
bering the definition of "m
odem" as a contraction of m
odulate-demodulate (analog processes), the term
is inappropriate.
Correct, alternative term
s for line drivers are local area data sets (LAD
S), lim
ited distance data sets (LD
DS
), and data service units (DS
U) -- although this last term
implies a particular type of line driver
suitable for access to public wide area data netw
orks (such as AT
&T
's DD
S N
etwork).
Data C
omm
unications & Fieldbus S
ystems
61
LOA
DIN
G C
OILS
ON C
OP
PE
R C
AB
LE
D-211
INC
RE
AS
ING F
RE
QU
EN
CY
INC
RE
AS
ING F
RE
QU
EN
CY
AMPLITUDE AMPLITUDE
NO
RM
AL
TE
LE
PH
ON
E
CH
AN
NE
L B
AN
DW
IDT
H
PR
OF
ILE
DIS
TO
RT
ED
TE
LE
PH
ON
E
CH
AN
NE
L B
AN
DW
IDT
H
PR
OF
ILE
TE
LEP
HO
NE C
EN
TR
AL
OF
FIC
E E
XC
HA
NG
E
LOA
DIN
G C
OILS
TO C
US
TO
ME
R P
RE
MIS
ES
LOA
DIN
G C
OILS
AR
E T
YP
ICA
LLY IN
ST
ALLE
D E
VE
RY 6000 ft. (F
OLLO
WIN
G
TH
E F
IRS
T P
AIR A
T 3000 ft.) -- O
N LO
OP LE
NG
TH
S E
XC
EE
DIN
G 3 m
iles.
Copper telephone local loops have a characteristic
known as distributed capacitance -- caused by the long
parallel configuration of the cable conductors.
This distributed capacitance im
pairs the ability of the telephone connection to reproduce the higher frequency sound com
ponents of speech.
The effect accum
ulates with cable length, m
aking long loops the m
ost seriously affected.
The industry solution to the problem
is to insert coils of w
ire in series with the loop conductors at periodic
intervals along the cable -- as illustrated below. T
he resulting coil inductance effectively cancels the distributed capacitance of the cable, yielding a net undistorted bandw
idth profile for the channel.
Loading coils improve the quality of the telephone connection for voice com
munication (and analog
modem
services) but render it unusable for digital signaling such as used by the digital line driver.
Data C
omm
unications & Fieldbus S
ystems
62
FUN
CTIO
NS
OF LIN
K P
RO
TOC
OLS
Link protocols are required to create a structured, reliable environment for data to be exchanged.
Elem
ents of this role are:
Provide an environm
ent where in
tegrity of data is assured (in
tegrity fo
cuse
s on th
e re
quire
ment
that d
ata
should
be d
elive
red to
the a
ppro
pria
te d
estin
atio
n -- in
tact).
Provide for accu
racy in the data delivered (assu
rance
of e
rror d
ete
ction a
nd co
rrectio
n, so
that
oth
er, h
igher la
yer fu
nctio
ns ca
n sa
fely a
ssum
e d
ata
and co
ntro
l info
rmatio
n is co
rrect).
In combination, these integrity and accuracy concerns result in a form
al process of cu
stod
y transfer regarding data.
Additional responsibilities include:
Flo
w co
ntro
l -- responding to congestion in the network, or tem
porary jeopardy of buffer overflow
.
Man
agem
ent o
f con
tentio
n -- w
ithout which an ethernet LA
N or a m
ultipoint line could not function.
D-175
Data C
omm
unications & Fieldbus S
ystems
63
LINK
LAY
ER
PR
OTO
CO
LS
D-103
Link layer protocols provide an organized, structured data delivery capability, validating received data, thus confirm
ing the correct delivery destination and completeness of delivery (in
tegrity
verifica
tion) and correct m
essage content (accu
racy ve
rificatio
n).
TE
LE
TY
PE
PR
OT
OC
OL
(As
yn
ch
ron
ou
s S
ys
tem
s)
Teletype protocol (TT
Y) is unstructured, in that there are no em
bedded guarantees of data integrity or accuracy.
Teletype protocol was developed for the electro-m
echanical teletypewriter system
s used 60 years ago -- and presum
es no higher level of equipment "intelligence" than these early devices provided.
BIN
AR
Y S
YN
CH
RO
NO
US
PR
OT
OC
OL
(Sy
nc
hro
no
us
Sy
ste
ms
)
Binary S
ynchronous (BIS
YN
C) is an inherently half-duplex protocol w
hich was the first w
idely used structured protocol -- offering generally appropriate accuracy and integrity control. In m
ost cases, B
ISY
NC
delivers poor facilities utilization efficiency.
HIG
H L
EV
EL
DA
TA
LIN
K C
ON
TR
OL
PR
OT
OC
OL
(Sy
nc
hro
no
us
Sy
ste
ms
)
HD
LC, patterned after IB
M'S
Synchronous D
ata Link Control (S
DLC
) protocol, is a link service that m
eets most current application needs for accuracy, m
essage integrity and efficiency. HD
LC is
inherently insensitive to code formats and m
essage content, making it a convenient, general
purpose link layer support protocol.
D
ata Com
munications &
Fieldbus System
s64
TE
LET
YP
E (T
TY
) PR
OT
OC
OL
D-104
The first T
eletype protocol systems predated the com
puter era, and were com
prised of electro-m
echanical terminal devices (teletypew
riters) used for human-to-hum
an comm
unication.
Teletype protocol has a m
ajor benefit in being simple and inexpensive to support, an has evolved as
the default link layer protocol for applications where its inherent lim
itations are tolerable -- which
includes personal computer applications connected by the standard serial port.
The absolutely "dum
b" characteristics of the electro-mechanical teletypew
riter (and all systems
which em
ulate them, including P
Cs) create the follow
ing features:
TT
Y system
s lack any kind of data buffering ability.N
o retransmission w
hen errors occur in the basic TT
Y protocol.
Only full-duplex is supported because devices are not able to m
anage half-duplex sequences.Lack of address recognition ability results in only point-to-point operation being feasible.
Teletype protocol system
s tend to be satisfactory in applications where a hum
an is involved in the process (i.e. either hum
an-to-human m
essaging, or human interactive term
inal service).
When m
achine-to-machine com
munication is required (w
ith inherent "zero tolerance" of errors), basic unaugm
ented TT
Y protocol is unacceptable.
A num
ber of auxiliary Application-layer protocols (such as X
MO
DE
M, K
ER
MIT
, BLA
ST
, MN
P and
V.42) have been used to com
pensate for some of these lim
itations.
Data C
omm
unications & Fieldbus S
ystems
65
HA
LF-D
UP
LEX vs. F
ULL D
UP
LEX
PR
OT
OC
OL O
PE
RA
TIO
N
D-068
Half D
uplex Protocol:
Half duplex is sim
ple to implem
ent, simple to operate and very effective in sim
ple applications.
Half duplex protocols (such as B
ISY
NC
) were the first structured link protocols developed specifically
for computer applications. T
hey were a rem
arkable improvem
ent over the unstructured Teletype
protocols previously used.
Inefficiency occurs through only transmitting in one direction at a tim
e -- and waiting tim
e delays are required to reverse the direction of transm
ission twice for each block of data sent.
Full Duplex P
rotocol:
Full duplex operation supports sim
ultaneous bi-directional service. Since w
aiting to acknowledge
each transmission (as is done w
ith half-duplex protocols) would prevent effective full duplex
operation, full duplex implem
entations always use a m
essage sequence numbering schem
e.
Message references, such as requests for retransm
ission, are made using these num
bers. The
number of different num
bers available for numbering the m
essages is called the window
size.
Full duplex protocols support efficient utilization of expensive telecom
munications facilities.
Com
plexity of implem
entation slowed early adoption, but this is a non-issue in current practice.
D
ata Com
munications &
Fieldbus System
s66
BIN
AR
Y S
YN
CH
RO
NO
US
ME
SS
AG
E F
EA
TU
RE
S
SYN
STX
ETX
SYN
SYN
BCC
BCC
VA
RIA
BLE
LEN
GT
H D
AT
A F
IELD
. TY
PIC
AL B
LOC
K LE
NG
TH
S M
AY B
E 200 T
O 2000 C
HA
RA
CT
ER
S.
D-065
NO
RM
AL M
OD
E B
ISY
NC requires that this data field contain no control characters (ie. binary bit
patterns that happen to replicate any of the control codes). This requirem
ent effectively limits data
content to alpha-numeric text characters.
TRA
NS
PA
RE
NT M
OD
E B
ISY
NC perm
its any conceivable bit pattern to be transmitted. T
his is accom
plished through a data content scanning and code substitution process.
RE
PE
AT
ED 'S
YN
CH
RO
NO
US ID
LE'
CH
AR
AC
TE
RS A
RE S
EN
T FIR
ST
TO P
ER
MIT B
IT AN
D C
HA
RA
CT
ER
SY
NC
HR
ON
IZA
TIO
N T
O B
EE
ST
AB
LISH
ED
.
ON
E O
R M
OR
E B
LOC
K C
HE
CK C
HA
RA
CT
ER
S
AR
E C
OM
PU
TE
D D
AT
A. T
HE B
CC IS
NO
T A
CH
AR
AC
TE
R -- IT IS
DE
RIV
ED F
RO
M D
AT
A
FIE
LD C
ON
TE
NT
S. IT
S P
UR
PO
SE IS
TO
EN
AB
LE T
HE R
EC
EIV
ING D
EV
ICE T
O V
ALID
AT
E
INC
OM
ING D
AT
A.
TH
E N
ET M
ES
SA
GE
IS B
RA
CK
ET
ED B
Y T
HE
CO
NT
RO
L CH
AR
AC
TE
R
PA
IR; S
TX and E
TX
.
Data C
omm
unications & Fieldbus S
ystems
67
BIN
AR
Y S
YN
CH
RO
NO
US
TR
AN
SP
AR
EN
T MO
DE F
EA
TU
RE
S
SYN
STX
ETX
SYN
DLE
BCC
BCC
D-066
DLE
SYN
DLE
DLE
DLE
DLE
Transparent B
ISY
NC retains the variable length data field -- and data is scanned by the sending
DT
E, searching for "D
LE" bit patterns em
bedded in the data field. For every occurrence discovered,
a second DLE
is inserted, and the data is transmitted m
odified.
At the receiving D
TE
, the data field is unambiguously m
arked by the control character pairs D
LE/S
TX and D
LE/E
TX
. Fraudulent D
LEs, w
hich would otherw
ise confuse the logic, are marked as
frauds by the a second DLE
(no legitimate control code sequences ever use a pair of D
LEs).
The receiving D
TE has the responsibility of rem
oving all inserted DLE
s in the data field, thus restoring it to its original unm
odified format.
INS
ER
TE
D D
LE C
HA
RA
CT
ER
S
FO
LLOW E
VE
RY O
CC
UR
RE
NC
E O
F H
AP
PE
NS
TA
NC
E D
LE
BIT
-PA
TT
ER
NS W
HIC
H M
AY O
CC
UR
IN LE
GIT
IMA
TE B
INA
RY D
AT
A.
TH
E N
ET M
ES
SA
GE C
ON
TE
NT IS
N
OW B
RA
CK
ET
ED B
Y T
WO
CO
NT
RO
L CH
AR
AC
TE
RS A
T BO
TH
EN
DS
; DLE
/ST
X and D
LE/E
TX
.
D
ata Com
munications &
Fieldbus System
s68
HA
LF-DU
PLE
X LIN
K P
RO
TOC
OL
SE
QU
EN
CE
OF E
VE
NTS
D-069
RE
QU
ES
T P
ER
MIS
SIO
N T
O T
RA
NS
MIT
(EN
Q)
PE
RM
ISS
ION
GR
AN
TE
D (A
CK
)
TR
AN
SM
IT F
IRS
T B
LOC
K O
F D
AT
A (D
ELIM
ITED
BY
ST
X/E
TX
)
AS
SU
ME
RE
CE
IVE
D C
OR
RE
CT
LY, V
ER
IFY
RE
CE
I PT (A
CK
)
TR
AN
SM
IT N
EX
T B
LOC
K O
F D
AT
A (D
ELIM
ITE
D B
Y S
TX/E
TX
)
AS
SU
ME
RE
CE
IVE
D C
OR
RE
CT
LY, V
ER
IFY
RE
CE
IPT (A
CK
)
TR
AN
SM
IT N
EX
T B
LOC
K O
F D
AT
A (D
ELIM
ITE
D B
Y S
TX/E
TX
)
AS
SU
ME
ER
RO
R(S
) IN T
RA
NS
MIS
SIO
N, R
EJE
CT
ME
SS
AG
E (N
AK
)
RE
TR
AN
SM
IT LA
ST
BLO
CK
OF
DA
TA
AS
SU
ME
RE
CE
IVE
D C
OR
RE
CT
L Y, V
ER
IF Y R
EC
EIP
T (A
CK
)
AS
SU
ME
NO
MO
RE
DA
TA
(TA
SK
CO
MP
LETE
); SE
ND
EO
T
NO
RE
SP
ON
SE
EX
PE
CT
ED
INCREASING TIMES
TA
TIO
N "A
"S
TA
TIO
N "B
"
SESSIONINITIATIONSEQUENCE
DATATRANSFERSEQUENCE
SESSIONTERMINATIONSEQUENCE
Data C
omm
unications & Fieldbus S
ystems
69
VA
RIA
BLE L
EN
GT
H D
AT
A F
IEL
D
FLAG
ADDRESSCONTROL
FLAG
CR
C E
RR
OR
CH
EC
K FIE
LD
HIG
H LE
VE
L DA
TA LIN
K C
ON
TR
OL
FR
AM
E S
TR
UC
TU
RE
D-067
FLAG FIE
LD: T
he FLA
G field is a distinct bit pattern sequence of "01111110" m
arking the ends of the variable length H
DLC
frame. T
his makes it necessary to verify that no flag patterns happen to
occur in the message, using a scan-insert-scan-delete sequence sim
ilar to that used for BIS
YN
C.
AD
DR
ES
S FIE
LD: T
his feature is of little value in point-to-point applications -- but it is essential for m
ultipoint operation. The address field is typically either one or tw
o octets in length.
CO
NTR
OL FIE
LD: B
its in this field support the logical mechanism
that controls the operation of the com
municating devices. S
equence numbers allow
transmission of new
data without w
aiting for acknow
ledgement of previous data, and control field logic also provides for retransm
ission requests and system
initialization functions.
DA
TA FIE
LD: T
he DA
TA field is variable length. It is typically required that this field contain an
integer number of octets -- w
ith 2,000 octets being a typical length.
CR
C E
RR
OR C
HE
CK FIE
LD: T
he CR
C E
RR
OR C
HE
CK field is a tw
o-octet error check sequence. T
he CR
C calculation is typically perform
ed in interface hardware (thus no process overhead).
Data C
omm
unications & Fieldbus S
ystems
70
D-221
01
10
01
11
01
11
10
01
11
11
00
11
11
11
00
01
10
01
11
01
11
10
01
11
11
00
11
11
11
00
00
01
11
11
10
01
11
11
10
01
10
01
11
01
11
10
01
11
11
00
11
11
11
00
----- F
LA
G -
----
----- F
LA
G -
----
❉❉
❉
OR
IGIN
AL D
AT
A
(Incl. control fields)
2 EX
AM
PLE
S O
F FIVE
"1" BIT S
EQ
UE
NC
ES
AN
D T
WO S
TU
FF
ED
BIT
S (M
AR
KE
D )
DE
LIVE
RE
D D
ATA
IS ID
EN
TIC
AL
TO T
HE S
OU
RC
E
HD
LC B
IT ST
UF
FIN
G F
OR T
RA
NS
PA
RE
NC
Y
The control logic of any link protocol is critically dependent on being able to unam
biguously differentiate betw
een control signals and user data. With H
DLC
, this is handled by the structure of the fram
es being bracketed by the flag pattern at each end of every frame -- all fields are located in
relation to the flag positions.
It follows that the flag bit pattern m
ust be uniquely reserved for this role, and happenstance occurrences of the flag bit sequence in data (and the internal address, control and/or C
RC
) must be
somehow
neutralized.
Imm
ediately prior to appending the flags and sending a frame of data, H
DLC
systems invoke a bit
stuffing procedure such that for every occurrence of a string of five consecutive "1" bits, a "0" bit is added -- a stuffed bit .
The receiving station m
ust subsequently locate and remove any stuffed bits -- a sim
ple task, since every "0" bit w
hich follows five "1" bits is unconditionally a stuffed bit, and m
ust be discarded.
Data C
omm
unications & Fieldbus S
ystems
71
S-FR
AM
E(S
upervisory Form
at)
I-FRA
ME
(Information F
ormat)
U-FR
AM
E(U
nnumbered F
ormat)
15
67
82
34
-------- BIT P
OS
ITION
S IN
CO
NTR
OL FIE
LD --------
010
11
P/F
P/F
P/F
SE
ND S
EQ
.N
UM
BE
RR
CV
. SE
Q.
NU
MB
ER
RC
V. S
EQ
.N
UM
BE
R
MO
DIF
IER
FC
N. B
ITS
SU
PV
.B
ITS
MO
DIF
IER
FC
N. B
ITS
03
74
65
12
3-BIT (M
odulo 8)C
IRC
ULA
R C
OU
NTE
R
continued ....D
-222
The 8-bit C
ontrol Field takes one
of three forms -- or form
ats:
Information Form
at; which is
always used w
hen data is actually being sent,
Supervisory Form
at, which is
used to assert normal control over
the link, and
Unnum
bered Format, w
hich perform
s extraordinary control tasks, such as resetting the link.
HD
LC C
ON
TR
OL F
IELD
CO
MP
ON
EN
TS
The three-bit send and receive sequence num
ber fields are used to provide a m
essage counting capability; they operate as a modulo-8
circular counter as suggested in the sketch to the right.
These counter fields establish a m
essage sequence numbering
capability, where the lim
ited repertoire (8 possible values) means that
there can never be more than 8 uniquely identified m
essages in transit (i.e. unacknow
ledged) in any particular one-way link).
Data C
omm
unications & Fieldbus S
ystems
72
continued ....D
-223
HD
LC C
ON
TRO
L FUN
CTIO
NS
, cont.
Supervisory Form
at Control Field C
ontents:
The tw
o-bit field of supervisory data is encoded as follows:
00 - RR (or R
eceive Ready C
ode) -- indicates that the sending station is ready to receive data, and is used either to: (a) A
cknowledge receipt of a m
essage when there is no data to send (a sim
ple positive acknow
ledgement) or
(b) To reset a flow
control comm
and (see code 01 next).
Information Form
at Control Field C
ontents:
Tw
o sequence numbers are carried -- S
end and Receive num
bers
The S
end Sequence N
umber is one increm
ent greater than the number of the last fram
e sent to this particular station.T
he Receive S
equence Num
ber is a piggybacked confirmation of correct m
essage receipt for data m
oving in the opposite direction, and is the number of the last correctly received data fram
e from
that station.
The "P
/F" bit is interpreted as a poll bit for the m
aster station transmissions (its value is used to
indicate whether a m
essage is a Polling m
essage or not) and for slave stations it is a final bit (its value used to indicate w
hether a given message is the final, or last fram
e in a sequence). Note that
the P/F bit appears in all three m
essage formats.
Data C
omm
unications & Fieldbus S
ystems
73
HD
LC C
ON
TR
OL F
UN
CT
ION
S, cont.
D-282
Supervisory Form
at Control Field C
ontents, cont.
01 - RN
R (or R
eceive Not R
eady code) -- is used to force flow control on the sending station.
10 - Go-B
ack-N R
eject -- which requires the sending station to back up to the m
essage number
indicated, and resend everything that follows that point in the sequence (i.e. not including the
indicated frame) .
11 - Selective R
eject - which requires the sending station to resend only the single fram
e following
the indicated sequence number -- and then continue as before the interruption.
Unnum
bered Format C
ontrol Field Contents:
There are 5 bits encoded as m
odifier bits -- used to invoke system initialization, system
reset and other extraordinary tasks that are required to adm
inister the link, but which do not contribute to
normal, m
essage transfer functions.
HD
LC w
as derived from the S
ynchronous Data Link C
ontrol, or SD
LC link protocol developed by
IBM
.
Although it is capable of supporting m
ultipoint lines (as per the P/F bit definition), m
ost HD
LC
applications and variants operate in point-to-point mode. A
good example is P
oint-to-Point P
rotocol (P
PP
) widely used on point-to-point links either to, or joining segm
ents of, the Internet.
There are countless other H
DLC
implem
entations, both proprietary and public domain.
Data C
omm
unications & Fieldbus S
ystems
74
STA
TION "A
"S
TATIO
N "B
"
ME
SS
AG
E IN
TR
AN
SIT W
ES
TB
OU
ND
ME
SS
AG
E IN
TR
AN
SIT E
AS
TB
OU
ND
DA
TA
SO
UR
CE
: O
UT
PU
T M
ES
SA
GE
BU
FF
ER
DA
TA S
INK
: IN
PU
T M
ES
SA
GE
BU
FF
ER
D-177
ME
SS
AG
E S
EQ
UE
NC
E N
UM
BE
R
CIR
CU
LAT
ION E
XA
MP
LE
54
41
43
32
63
52
❉
❉
❉
(1) Num
bers in the left side of the above boxes (solid black digits) are sequence numbers for S
tation "A
"; numbers in the right side (gray shaded digits) are sequence num
bers for Station "B
".(2) E
ntries marked w
ith the asterisk ( ) are arbitrary starting assumptions.
NO
TE:
INC
OM
ING (R
eceive) ME
SS
AG
E
SE
QU
EN
CE N
UM
BE
RS A
RE C
OP
IED
OV
ER T
O T
HE O
UT
GO
ING (S
end) S
EQ
UE
NC
E N
UM
BE
R F
IELD
FO
R
TR
AN
SM
ISS
ION W
ITH T
HE N
EX
T M
ES
SA
GE
.
DA
TA
SO
UR
CE
: O
UT
PU
T M
ES
SA
GE
BU
FF
ER
DA
TA S
INK
: IN
PU
T M
ES
SA
GE
BU
FF
ER
Data C
omm
unications & Fieldbus S
ystems
75
THROUGHPUT EFFICIENCY
LOW E
RR
OR
RA
TE C
AS
E
ME
DIU
M E
RR
OR
RA
TE C
AS
E
HIG
H E
RR
OR
RA
TE C
AS
E
Unfortunately, error rates are not all that stable (i.e. they vary from
minute-to-m
inute) and are different for tw
o implem
entations of the same netw
ork technology -- thus attempting to pick an optim
um packet
size for a given network technology is at best an educated guess.
Generally w
ide area networks (W
AN
s) exhibit a much higher error rate than local area netw
orks (LAN
s) -- being the reason w
hy packet sizes are typically much sm
aller on WA
Ns than on LA
Ns.
RE
LAT
ION
SH
IP: O
PT
IMU
M P
AC
KE
T SIZ
ED
EP
EN
DS O
N E
RR
OR R
AT
E E
XP
EC
TA
TIO
N
T-196
For every netw
ork environment, there
will alw
ays be an optimum
message
unit, or packet size, largely determined
by the error rate encountered in that environm
ent.
For very sm
all packet sizes, operating efficiency w
ill increase with increasing
packet size.
By increasing packet size indefinitely
(i.e. to very large packet sizes) , the now
large packets will encounter so
many errors (and require so m
any resulting retransm
issions) that efficiency falls off quickly.
INC
RE
AS
ING (N
ET
) PA
CK
ET S
IZE
Data C
omm
unications & Fieldbus S
ystems
76
D-171
EIA
-232-C F
EA
TU
RE
S/F
UN
CT
ION
S/D
EF
ICIE
NC
IES
The T
elecomm
unications Industries Association (form
erly, the Electronic Industries A
ssociation) has defined effectively all equipm
ent interface standards in North A
merica.
The E
IA-232-C
interface started out life as the RS
-232, and went through 3 revisions and nam
e changes to becom
e the EIA
-232-C. It w
as further redefined/refined as the EIA
-232-D -- and
remains the m
ost widely used equipm
ent interface standard in the world.
Earlier versions w
ere more data rate-restricted; E
IA-232-C
and D revisions are lim
ited to 20 kbps. -- in less sophisticated tim
es, this was not a serious lim
itation but it currently is a key constraint.
The interface is designed to connect adjacent pieces of equipm
ent, with a nom
inal maxim
um
separation of 50 feet between D
TE and D
CE
. Current practice often w
orks with m
uch greater separations.
Depending on data rate, electrom
agnetic environment and cable type, this 50-foot lim
itation may
be reasonably stretched to a few hundred feet -- perhaps as far as 500.
The interface w
as specified without a unique connector designation (although the A
mphenol
DB
-25 shell connector has become the de facto standard, and the E
IA-232-D
version actually specifies the D
B-25), and lacked detail as to gender expectations and m
echanical restraint.
The interface provides for asym
metric (P
rimary/S
econdary) modem
support, which, since it is
rarely used, results in many unused pin connections that could either be better used to serve
different functions, or eliminated to allow
a smaller connector.
Data C
omm
unications & Fieldbus S
ystems
77
AA
MODEM
DC
ED
TE
TE
LEP
HO
NE
LINE
CO
NN
EC
TO
R
WILL B
E E
ITH
ER
:
RJ-11 M
OD
ULA
R JA
CK
FO
R
2-WIR
E D
IAL D
EV
ICE
AC
CE
SS
, or
RJ-45 M
OD
ULA
R JA
CK
FO
R
4-WIR
E D
ED
ICA
TE
D LIN
E
CO
NN
EC
TIO
N
TH
E C
AB
LE E
ND
TE
RM
INA
TIN
G A
T T
HE
DC
E IS
A M
ALE
GE
ND
ER
CO
NN
EC
TO
R,
... TH
ER
EF
OR
E, T
HE
CH
AS
SIS
CO
NN
EC
TO
RO
N T
HE
DC
E IS
FE
MA
LE G
EN
DE
R
TH
E F
OR
MA
L EIA
-232-C IN
TE
RF
AC
E
AC
TU
ALLY
OC
CU
RS
AT
TH
IS P
OIN
T(at the D
CE
connection rather than DT
E)
EIA
-232-C C
ON
NE
CTIO
N FE
ATU
RE
S
TH
E S
TA
ND
AR
D D
OE
S N
OT
AN
TIC
IPA
TE
A C
ON
NE
CT
OR
ON
TH
IS E
ND
OF
TH
E C
AB
LE, A
ND
TH
ER
EF
OR
E N
O G
EN
DE
R
SP
EC
IFIC
AT
ION
IS P
RO
VID
ED
. IN P
RA
CT
ICE
TH
E D
TE
WILL
ALW
AY
S H
AV
E A
DE
TAC
HA
BLE
CA
BLE
AN
D C
ON
SE
QU
EN
TLYA
CO
NN
EC
TO
R G
EN
DE
R C
ON
VE
NT
ION
IS N
EE
DE
D.
D-026
TELE
PH
ON
E C
AB
LE
●●
Data C
omm
unications & Fieldbus S
ystems
78
D-029
EIA
-232-C S
IGN
AL A
ND C
ON
TR
OL
VO
LTA
GE C
ON
VE
NT
ION
S
10
11
01
10
BIP
OLA
R N
ON
-R
ET
UR
N T
O Z
ER
O
(BN
RZ
) FO
RM
AT
+3-3 0
+25
-25
VA
LID E
IA-232-C
INT
ER
FA
CE E
LEC
TR
ICA
L SIG
NA
LS M
AY B
E A
NY
WH
ER
EIN T
HE R
AN
GE B
ET
WE
EN 3 A
ND 25 V
OLT
S. N
OR
MA
L OP
ER
AT
ING R
AN
GE
IS T
YP
ICA
LLY 12 T
O 15 V
OLT
S. S
HA
DE
D A
RE
AS IN
DIC
AT
E T
HE R
AN
GE
OF IN
VA
LID O
R IN
DE
TE
RM
INA
TE S
IGN
AL V
ALU
ES
.
OF
FO
NO
FF
ON
OF
FO
FF
ON
OF
F
DA
TA S
IGN
AL LO
GIC
AL
INT
ER
PR
ET
AT
ION O
FIN
DIC
AT
ED V
OLT
AG
E
PA
TT
ER
N
CO
NT
RO
L SIG
NA
L LOG
ICA
LIN
TE
RP
RE
TA
TIO
N O
FIN
DIC
AT
ED V
OLT
AG
E
PA
TT
ER
N
VOLTAGE SCALE VERTICALLY
TIM
E S
CA
LE H
OR
IZO
NT
AL
LY
Data C
omm
unications & Fieldbus S
ystems
79
1. PR
OTE
CT
IVE
GR
OU
ND
2. TR
AN
SM
ITTE
D D
ATA
3. RE
CE
IVE
D D
ATA
4. RE
QU
ES
T TO
SE
ND
5. CLE
AR
TO S
EN
D
6. DA
TA S
ET
RE
AD
Y8. R
EC
EIV
ED
LINE
SIG
NA
L DE
TE
CTO
R
9. RE
SE
RV
ED
FOR
TES
TIN
G
10. RE
SE
RV
ED
FOR
TES
TIN
G
11. UN
AS
SIG
NE
D
12. SE
CO
ND
AR
Y R
CD
'D LIN
E S
IGN
AL D
ET
.
13. SE
CO
ND
AR
Y C
LEA
R T
O S
EN
D
7. GR
OU
ND
& C
OM
MO
N
RE
TUR
N
14. SE
CO
ND
AR
Y T
RA
NS
MITT
ED
DA
TA
15. TRA
NS
MITTE
R S
IGN
AL E
LEM
EN
T TIM
ING
16. SE
CO
ND
AR
Y R
EC
EIV
ED
DA
TA
17. RE
CE
IVE
R S
IGN
AL E
LEM
EN
T TIMIN
G
18. UN
AS
SIG
NE
D
19. SE
CO
ND
AR
Y R
EQ
UE
ST TO
SE
ND
20. DA
TA TE
RM
INA
L RE
AD
Y
21. SIG
NA
L QU
ALITY
DE
TE
CTO
R
22. RIN
G IN
DIC
ATO
R
23. DA
TA S
IGN
AL R
ATE
SE
LEC
TOR
24. TRA
NS
MITTE
R S
IGN
AL E
LEM
EN
T TIMIN
G (D
TE)
25. UN
AS
SIG
NE
D
D-022
EIA
-232-C IN
TER
FAC
E P
IN A
SS
IGN
ME
NTS
ILLUS
TR
AT
ION
SH
OW
S "M
ALE
" GE
ND
ER
DB
-25 SH
ELL C
ON
NE
CT
OR
Data C
omm
unications & Fieldbus S
ystems
80
EIA
-232-C FU
NC
TION
AL D
ETA
IL
1 2 3 4 5 6 7 8 910111213141516171819202122232425
FG
TX
DR
XD
RT
SC
TS
DS
RG
ND
DC
D
SD
CD
SC
TS
ST
XD
TX
CLK
SR
XD
RX
CLK
SR
TS
DT
RS
QD
RI
RA
TE
XTC
LK
Fram
e Ground
Transm
itted Data
Received D
ataR
equest to Send
Clear to S
endD
ata Set R
eadyS
ignal Ground
Data C
arrier Detect
Positive T
est Voltage
Negative T
est Voltage
Unassigned
Secondary C
arrier Detect
Secondary C
lear to Send
Secondary T
ransmitted D
ataT
ransmitter C
lockS
econdary Received D
ataR
eceiver Clock
Unassigned
Secondary R
equest to Send
Data T
erminal R
eadyS
ignal Quality D
etectR
ing IndicatorD
ata Rate S
electorE
xternal Clock
Unassigned
✔✔✔✔✔✔✔✔✔✔✔✔✔✔
✔✔✔✔✔✔✔
✘✘✘✘
✘✘✘✘✘✘✘✘✘
✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘
✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘✘
PIN
NO
.A
BB
RE
V-
IATIO
NFU
NC
TION
DE
SC
RIP
TION
SU
BS
ET E
QU
IPP
ED
: 4 9 15 25
D-074
SIGN
AL TO
:D
TE DC
E
Data C
omm
unications & Fieldbus S
ystems
81
1. RE
CE
IVE
D LIN
E S
IGN
AL D
ETE
CT
2. RE
CE
IVE
D D
ATA
4. DA
TA TE
RM
INA
L RE
AD
Y
5. GR
OU
ND
AN
D C
OM
MO
N R
ETU
RN
3. TRA
NS
MITT
ED
DA
TA
6. DA
TA S
ET
RE
AD
Y
7. RE
QU
ES
T TO S
EN
D8. C
LEA
R TO
SE
ND
9. RIN
G IN
DIC
ATO
R
D-053
EIA
-232-C IN
TER
FAC
E - 9-P
IN S
UB
SE
T P
IN A
SS
IGN
ME
NTS
ILLUS
TR
AT
ION
SH
OW
S "M
ALE
" GE
ND
ER
DB
-9 SH
ELL C
ON
NE
CT
OR
Data C
omm
unications & Fieldbus S
ystems
82
1. SH
IELD
GR
OU
ND
2. TR
AN
SM
ITTED
DA
TA
3. RE
CE
IVE
D D
ATA
4. RE
QU
ES
T TO S
EN
D
5. CLE
AR
TO
SE
ND
6. DC
E R
EA
DY
8. RE
CE
IVE
D LIN
E S
IGN
AL D
ET
EC
TOR
9. RE
SE
RV
ED
FO
R TE
STIN
G
10. RE
SE
RV
ED
FOR
TE
STIN
G
11. UN
AS
SIG
NE
D
12. SE
CO
ND
AR
Y R
CD
'D LIN
E S
IGN
AL D
ET
.
13. SE
CO
ND
AR
Y C
LEA
R T
O S
EN
D
7. GR
OU
ND
& C
OM
MO
N
RE
TU
RN
14. SE
CO
ND
AR
Y T
RA
NS
MITT
ED
DA
TA
15. TRA
NS
MITTE
R S
IGN
AL E
LEM
EN
T TIM
ING
16. SE
CO
ND
AR
Y R
EC
EIV
ED
DA
TA
17. RE
CE
IVE
R S
IGN
AL E
LEM
EN
T TIMIN
G
18. LOC
AL LO
OP
BA
CK
19. SE
CO
ND
AR
Y R
EQ
UE
ST TO
SE
ND
20. DTE
RE
AD
Y
21. RE
MO
TE LO
OP
BA
CK
/SIG
NA
L QU
ALITY
DE
TE
CTO
R
22. RIN
G IN
DIC
ATO
R
23. DA
TA S
IGN
AL R
ATE
SE
LEC
TOR
24. TRA
NS
MITTE
R S
IGN
AL E
LEM
EN
T TIMIN
G (D
TE)
25. TES
T MO
DE
D-038
EIA
-232-D IN
TER
FAC
E P
IN A
SS
IGN
ME
NTS
ILLUS
TR
AT
ION
SH
OW
S "M
ALE
" GE
ND
ER
DB
-25 SH
ELL C
ON
NE
CT
OR
NO
TE
: DIF
FER
EN
CE
S C
OM
PA
RE
D W
ITH E
IA-232-C
US
AG
E
AR
E M
AR
KE
D W
ITH
AS
TER
ISK
S
✽ ✽ ✽ ✽
✽ ✽
Data C
omm
unications & Fieldbus S
ystems
83
UN
BA
LAN
CE
D E
LEC
TRIC
AL C
IRC
UIT --
SU
CH
AS
EIA
-232-C. TH
E G
RO
UN
D
RE
TUR
N P
ATH
IS S
HA
RE
D B
Y A
LL E
LEC
TRIC
AL S
IGN
ALS
.
UN
BA
LAN
CE
D E
LEC
TRIC
AL C
IRC
UIT --
SU
CH
AS
EIA
-423. GR
OU
ND
RE
TU
RN
P
AT
H IS
ISO
LAT
ED
FR
OM
OT
HE
R
FUN
CTIO
NS
, BU
T ELE
CTR
ICA
L BA
LAN
CE
IS
LOS
T TH
RO
UG
H TH
E G
RO
UN
D
CO
NN
EC
TION
.
BA
LAN
CE
D E
LEC
TRIC
AL C
IRC
UIT
-- S
UC
H A
S E
IA-422. B
OTH
CO
ND
UC
TO
RS
A
RE
ELE
CTR
ICA
LLY E
QU
AL (N
EITH
ER
IS
DE
SIG
NA
TED
AS
"GR
OU
ND
" C
ON
NE
CTIO
N). T
HE
RE
SU
LTIN
G
SY
MM
ET
RY
CR
EA
TES
SU
PE
RIO
R
NO
ISE
/INTE
RFE
RE
NC
E IM
MU
NIT
Y.
TR
AN
SM
IT, O
R
DR
IVE
R C
IRC
UIT
RE
CE
IVE
R
CIR
CU
ITC
AB
LE C
ON
NE
CTIO
N C
IRC
UIT
D-079
BA
LAN
CE
D &
UN
BA
LAN
CE
D IN
TER
FAC
E C
IRC
UITS
INP
UT
OU
TP
UT
INP
UT
OU
TP
UT
INP
UT
OU
TP
UT
Data C
omm
unications & Fieldbus S
ystems
84
1. SH
IELD
GR
OU
ND
2. SIG
NA
LING
RA
TE
IND
ICA
TOR
3. UN
US
ED
4. SE
ND
DA
TA
5. SE
ND
TIM
ING
6. RE
CE
IVE
DA
TA
10. LOC
AL
LOO
PB
AC
K
20. RE
CE
IVE
CO
MM
ON
21. UN
US
ED
22. SE
ND
DA
TA
23. SE
ND
TIM
ING
24. RE
CE
IVE
DA
TA
25. RE
QU
ES
T TO
SE
ND
D-025
EIA
-449 INTE
RFA
CE
PIN
AS
SIG
NM
EN
TS
ILLUS
TR
AT
ION
SH
OW
S "M
ALE
" GE
ND
ER
DB
-37 SH
ELL C
ON
NE
CT
OR
14. RE
MO
TE LO
OP
BA
CK
15. INC
OM
ING
CA
LL
16. SE
LEC
T F
RE
QU
EN
CY
17. TE
RM
INA
L TIMIN
G
18. TE
ST
MO
DE
19. SIG
NA
L GR
OU
ND
32. SE
LEC
T S
TA
ND
BY
33. SIG
NA
L QU
ALIT
Y
34. NE
W S
IGN
AL
35. TE
RM
INA
L TIM
ING
36. STA
ND
BY
IND
ICA
TO
R
37. SE
ND
CO
MM
ON
11. DA
TA
MO
DE
12. TE
RM
INA
L RE
AD
Y
13. RE
CE
IVE
R R
EA
DY
7. RE
QU
ES
T T
O S
EN
D
8. RE
CE
IVE
TIM
ING
9. CLE
AR
TO
SE
ND
26. RE
CE
IVE
TIM
ING
27. CLE
AR
TO
SE
ND
28. TE
RM
INA
L IN S
ER
VIC
E29. D
AT
A M
OD
E
30. TE
RM
INA
L RE
AD
Y
31. RE
CE
IVE
R R
EA
DY
Data C
omm
unications & Fieldbus S
ystems
85
1. SH
IELD
GR
OU
ND
2. TR
AN
SM
ITTE
D D
ATA
3. RE
CE
IVE
D D
ATA
4. RE
QU
ES
T TO
SE
ND
5. CLE
AR
TO
SE
ND
6. DC
E R
EA
DY
8. RE
CE
IVE
D LIN
E S
IGN
AL D
ET
EC
T
9. RE
CE
IVE
R S
IGN
AL E
LEM
EN
T TIMIN
G
10. RE
CE
IVE
D LIN
E S
IGN
AL D
ET
EC
T
11. TR
AN
SM
IT SIG
NA
L ELE
ME
NT T
IMIN
G (D
TE)
12. TRA
NS
MIT S
IGN
AL E
LEM
EN
T T
IMIN
G (D
CE
)
13. CLE
AR
TO
SE
ND
7. GR
OU
ND
& C
OM
MO
N
RE
TU
RN
14. TRA
NS
MITT
ED
DA
TA
15. TRA
NS
MIT S
IGN
AL E
LEM
EN
T TIMIN
G (D
CE
)
16. RE
CE
IVE
D D
AT
A
17. RE
CE
IVE
R S
IGN
AL E
LEM
EN
T TIMIN
G
18. LOC
AL LO
OP
BA
CK
19. RE
QU
ES
T T
O S
EN
D20. D
TE
RE
AD
Y
21. RE
MO
TE
LOO
PB
AC
K
22. DC
E R
EA
DY
23. DT
E R
EA
DY
24. TRA
NS
MIT S
IGN
AL E
LEM
EN
T TIM
ING
(DTE
)
25. TES
T MO
DE
D-039
EIA
-530 INTE
RFA
CE
PIN
AS
SIG
NM
EN
TS
ILLUS
TR
AT
ION
SH
OW
S "M
ALE
" GE
ND
ER
DB
-25 SH
ELL C
ON
NE
CT
OR
NO
TE
: DU
PLIC
AT
ED
TR
AN
SM
IT S
IGN
AL E
LE
ME
NT
TIM
ING
AS
SIG
NM
EN
TS
PR
OV
IDE
F
OR
CLO
CK
SO
UR
CE
TIM
ING
TO
BE
PR
OV
IDE
D B
Y E
ITH
ER
DT
E O
R D
CE
Data C
omm
unications & Fieldbus S
ystems
86
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAA
AAA
AAA
AAA
AAA
AAA
AAA
100 bps1kbps
10 kbps100 kbps
1 Mbps
10 Mbps
24
68
24
68
24
68
24
68
24
68
10 100 1000 10,0002 4 6 82 4 6 8 2 4 6 8
DISTANCE IN FEET
EIA
-422 DIS
TA
NC
E/D
AT
A R
AT
E E
NV
ELO
PE
EIA
-423 D
IST
AN
CE
/DA
TA
RA
TE
E
NV
ELO
PE
EIA
-232D
IST
AN
CE
/DA
TA
RA
TE
E
NV
ELO
PE
D-078
DIS
TAN
CE
vs. DA
TA R
ATE
RE
LATIO
NS
HIP
S O
F CO
MM
ON
INTE
RFA
CE
STA
ND
AR
DS
Data C
omm
unications & Fieldbus S
ystems
87
K-030
RS
-485 INT
ER
FA
CE
ST
AN
DA
RD
The R
S-485 interface standard has som
e strong similarities to the R
S-422 interface:
It uses the same balanced electrical configuration
It uses essentially the same signaling voltage conventions
The key concept difference is the R
S-422 is a point-to-point service (o
pera
ting o
n a
two-p
air, o
r 4-w
ire co
nnectio
n), w
hile the RS
-485 supports multi-drop, or bus configurations (o
pera
ting o
ver a
tw
o-w
ire, o
r single
pair co
nnectio
n) -- see sketch.
RS
-485 operates up to 56 kbps (com
pare
d w
ith R
S-4
22 o
pera
tion to
10 m
bps u
nder so
me
circum
stance
s).
This m
ulti-drop bus format configuration lends itself to netw
ork connection of field instruments --
suitable to applications where the sim
plicity of the configuration is desirable, and high perform
ance is not required.
BU
S-C
ON
NE
CT
ED
DE
VIC
EB
US
-CO
NN
EC
TE
DD
EV
ICE
BU
S-C
ON
NE
CT
ED
DE
VIC
E
TW
IST
ED
PA
IR C
AB
LE
Data C
omm
unications & Fieldbus S
ystems
88
A
DIR
EC
T CO
NN
EC
TIVITY
: DTE
-DTE
D-040
?A
DIR
EC
T, S
IMP
LE C
ON
NE
CT
ION
IS IN
OP
ER
AB
LE. T
RA
NS
MIT
AN
D R
EC
EIV
E D
AT
A S
IGN
ALS
(O
N P
INS
2 and 3 RE
SP
EC
TIV
ELY
) AR
E IN
DIR
EC
T C
ON
FLIC
T. O
TH
ER
INT
ER
FA
CE
F
UN
CT
ION
S (LIK
E R
EA
DY
ST
AT
US
AN
D R
TS
/CT
S) W
OU
LD B
E S
IMILA
RLY
MIS
HA
ND
LED
.
PIN
NO
.
237
PIN
NO
.
237
DT
ED
TE
-- TE
RM
INA
LS T
RA
NS
MIT
ON
PIN
2
-- TE
RM
INA
LS R
EC
EIV
E O
N P
IN 3
-- (A C
OM
MO
N, O
R G
RO
UN
D C
ON
NE
CT
ION
A
T P
IN 7 IS
ALW
AY
S R
EQ
UIR
ED
)
Data C
omm
unications & Fieldbus S
ystems
89
1 2 3 4 5 6 7 815172024
1 2 3 4 5 6 7 815172024
1 2 3 4 5 6 7 820
1 2 3 4 5 6 7 820
D-075
RE
PR
ES
EN
TATIV
E M
OD
EM
ELIM
INA
TOR
, OR
NU
LL MO
DE
M C
ON
FIGU
RA
TION
S
CA
SE
"A"
AS
YN
CH
RO
NO
US
SY
STE
M E
XA
MP
LE
CA
SE
"B"
SY
NC
HR
ON
OU
SS
YS
TEM
EX
AM
PLE
NO
TE
: (1) DE
TA
ILS O
F S
PE
CIF
IC S
YS
TE
MS
, AN
D IN
TE
RC
ON
NE
CT
ION
SU
BT
LET
IES
PR
EV
EN
T D
EF
INIT
ION
OF
A
UN
IVE
RS
ALLY
"CO
RR
EC
T" M
OD
EM
ELIM
INA
TO
R W
IRIN
G C
ON
FIG
UR
AT
ION
. TH
ES
E E
XA
MP
LES
AR
E
INT
EN
DE
D A
S ILLU
ST
RA
TIO
NS
OF
HO
W S
UC
H C
ON
FIG
UR
AT
ION
S M
IGH
T B
E E
ST
AB
LISH
ED
.
(2) TH
IS E
QU
IPM
EN
T C
ON
NE
CT
ION
PR
OB
LEM
IS G
EN
ER
ALLY
AS
SO
CIA
TE
D W
ITH
TH
E N
EE
D T
O
DIR
EC
TLY
CO
NN
EC
T D
TE
-to-DT
E; IT
IS A
N ID
EN
TIC
AL P
RO
BLE
M IN
SIT
UA
TIO
NS
WH
ER
E
DIR
EC
T D
CE
-to-DC
E C
ON
NE
CT
ION
S A
RE
RE
QU
IRE
D.
Data C
omm
unications & Fieldbus S
ystems
90
continued ....D
-735
PIN
NO
.
237
PIN
NO
.
237
?T
ER
MIN
ALS
TR
AN
SM
IT O
N P
IN 2
TE
RM
INA
LS R
EC
EIV
EO
N P
IN 3
A C
OM
MO
N, O
R G
RO
UN
DC
ON
NE
CT
ION O
N P
IN 7 IS
A
LSO R
EQ
UIR
ED
CR
OS
SO
VE
R C
AB
LES
It is always a source of confusion to have to deal w
ith the inevitable asymm
etry of electrically cabled connections.
The problem
always arises w
hen data cables are used to connect two pieces of
equipment; E
IA/T
IA R
S-232 serial data cables, 10/100 B
AS
E-T E
thernet cables, and US
B
cables are three examples of cable system
s where this detail m
ust be dealt with.
Transm
itting, or sending devices are always required at one end of a w
ired link, and a receiving circuit is required at the other end -- a fundam
entally unavoidably asymm
etric feature of w
ired connections (see the illustration in the accompanying box).
The tw
o types of devices connected w
ith the RS
-232 are modem
s and term
inal equipment (D
ata C
omm
unications Equipm
ent, or D
CE
s and Data T
erminal
Equipm
ent, or DT
Es) -- the
terminal, or D
TE view
is suggested in the illustration here.
A 'straight through' cable w
ill clearly not w
ork in this case.
Data C
omm
unications & Fieldbus S
ystems
91
CR
OS
SO
VE
R C
AB
LES
, cont.
D-736
PIN#
1 TD
+
PIN#
2 TD
-
PIN#
3 RD
+
PIN#
6 RD
-
PIN#
1 TD
+
PIN#
2 TD
-
PIN#
3 RD
+
PIN#
6 RD
-
continued ....
'Com
puter' -- and the netw
ork-side device is the 'Hub'.
In the Ethernet case, the
problem is a little m
ore com
plicated, because the com
puter is connected with a
short access cable to a wall
plate that is wired as a 'hub' --
which is then connected via
house cabling to a connector that plugs into a real 'hub'
100/10 BASE-T CROSSOVER
The norm
al approach to dealing with this problem
in the case of serial, RS
-232 connections (e.g. w
here two D
TE
s need to be directly connected) is to either modify a
cable so that Pin #2 on one end connects to pin #3 on the other (the result is called a "null
modem
cable") or construct an adapter unit in which the cross over w
iring is located (called a "m
odem elim
inator")
In the case of 10/100 BA
SE
-T Ethernet connectivity, the problem
is identical -- except the term
inology is different.
Here the 'term
ination' device is more com
monly called the 'E
thernet Device' or sim
ply the
Data C
omm
unications & Fieldbus S
ystems
92
CR
OS
SO
VE
R C
AB
LES
, cont.
D-737
PIN N
UM
BE
R1 2 3 4 5 6 7 8
RJ-45 D
ATA
CO
NN
EC
TOR
(RE
CE
PTA
CLE
V
IEW )
CO
MP
UT
ER
-EN
D C
ON
NE
CT
OR
PE
RIP
HE
RA
L-EN
D C
ON
NE
CTO
R
When using U
SB devices to connect com
puters to peripherals (the primary role w
ith US
B),
the computer end alw
ays uses the rectangular connector (below left), and the device
connects to the squarish connector (below right)
The standard E
thernet 100/10 Base-T
connector configuration is illustrated in the box to the right.
The designers of the U
niversal Serial
Bus (U
SB
) solved this problem quite
elegantly by specifying two different
connectors, completely avoiding
conflict between device roles.
Data C
omm
unications & Fieldbus S
ystems
93
K-077
WIR
ELE
SS S
EN
SO
R N
ET
WO
RK
S
Environm
ental controls B
linds, drapery and shade controls M
edical sensing and monitoring
Universal rem
ote control to a set-top box Industrial and building autom
ation
Achieving an effective, econom
ical connection to sensors and/or controllers is an on-going problem
for industrial network designers.
An em
erging wireless technology defined by the IE
EE 802.15.4 standard -- augm
ented by a set of application-oriented protocols going by the trade nam
e of ZigB
ee -- shows prom
ise of delivering effective solutions in this application. F
irst product target Q4-2004.
ZigB
ee is actually a set of networking, security and application softw
are standards that sits atop the 802.15.4 low
-data wireless standard approved by the IE
EE
.
Unlike other w
ireless standards such as 802.11 or 802.16, ZigB
ee and 802.15.4 are designed to carry lim
ited amounts of data at a m
aximum
rate of 250K bps (bits per second) -- w
ith a major
priority on robust, secure comm
unication in a strongly power conservative form
at.
The w
ww
.zigbee.org website carries considerable background inform
ation on this promising
technology, and suggests the following m
ainly intelligent-building-oriented applications as potential w
ay of exploiting this new w
ireless innovation:
Lighting controls A
utomatic m
eter reading W
ireless smoke and C
O detectors
HV
AC control
Hom
e security
Data C
omm
unications & Fieldbus S
ystems
94
K-023
AN
ALO
G IN
ST
RU
ME
NT
INT
ER
FA
CE
S
The industrial com
puting industry has used several analog instrument interfaces -- or analog
inputs (AI) -- for the past num
ber of years.
A partial list of signal types/interfaces that are used is:
4 - 20 mA
current loop signal (ove
rwhelm
ingly th
e m
ost co
mm
only u
sed a
nalo
g in
terfa
ce
signal typ
e)
0 - 5 Volt signal (a
n in
terfa
ce typ
e th
at is g
enera
lly difficu
lt to b
e su
ccessfu
l with
, due to
the
need to
main
tain
a h
igh im
pedance
inte
rface
-- to m
inim
ize vo
ltage lo
ss in a
ssocia
ted ca
blin
g
-- which
in tu
rn in
crease
s vuln
era
bility to
noise
inductio
n)
frequency and pulse rate signal (ofte
n re
ferre
d to
as a
ccum
ula
tor sig
nals, co
mm
only
asso
ciate
d w
ith flo
w ra
te m
easu
rem
ent)
resistance value (com
monly a
ssocia
ted w
ith th
erm
oco
uple
-base
d te
mpera
ture
measu
rem
ent
-- which
is typica
lly limite
d b
y pow
er d
issipatio
n lim
its in th
e th
erm
oco
uple
to m
illivolt-le
vel
signals, w
hich
are
very vu
lnera
ble
to n
oise
pro
ble
ms)
It is a significant engineering challenge to design data acquisition systems so that am
bient electrical noise is controlled, so as to preserve the usefulness of the signals being m
easured (typ
ically a
ccom
plish
ed th
rough u
se o
f shie
lded w
iring a
nd lo
w im
pedance
circuits), w
hile respecting the need for intrinsic safety in m
any applications.
Data C
omm
unications & Fieldbus S
ystems
95
K-014
Analog instrum
ents have historically supported a standard 4-20 mA
current loop interface to associated control equipm
ent -- over a dedicated 2-wire shielded cable pair.
This current loop prevailed in the industry for m
any years, in spite of several distinct disadvantages:
The low
levels of current required are subject to interference distortion problems that lim
it the accuracy and confidence of m
easured results.
The dedicated (typ
ically sh
ield
ed) cable pair required to support each instrum
ent interface results in a large, bulky cable plant, w
hich is expensive to install and difficult to maintain
--especially in hazardous environments.
Cable term
inations are also bulky, resulting in the cable termination cabinets and racking
occupying typically more control room
space than the functional systems they serve.
The unsophisticated characteristics of the current loop interface (e
.g. n
o o
pportu
nity fo
r ca
rrying te
st or ca
libra
tion sig
nals o
ver th
e d
edica
ted ca
ble
pair w
ithout d
isturb
ing th
e
measu
red re
sults), m
aking maintenance and calibration procedures very disruptive.
The traditional current loop interface, and the associated hub-and-spoke cabling
configuration is not capable of supporting sophisticated microprocessor-based
instruments, w
ithout adding greatly to the bulk and expense of the cable plant.
4-20 mA
CU
RR
EN
T LO
OP
INS
TR
UM
EN
T IN
TE
RF
AC
ES
Data C
omm
unications & Fieldbus S
ystems
96
K-015
Intelligent, or "smart" instrum
ents can be defined in several ways, but their key differentiating
features are:
They contain em
bedded microprocessor and other digital electronic com
ponents.
They support a sophisticated interface to associated control system
s which enable a
bus-cabled wiring system
.
They are capable of perform
ing self-calibration procedures and test functions non-intrusively (w
ithout d
isruptin
g th
e p
roce
ss bein
g m
onito
red).
The concept of intelligent instrum
entation led to the industry initiatives to develop a fieldbus standard, enabling the bus-connection of num
erous devices in an industrial network, in m
any w
ays the equivalent of an office local area network.
The fieldbus initiative has been plagued by the proliferation of num
erous fieldbusses, all essentially incom
patible -- leading to considerable cynicism as to w
hether a real inter-operable, industry consensus standard w
ill ever emerge.
The so called F
ou
nd
ation
Field
bu
s shows prom
ise as the possible solution to this difficulty, but it w
ill take time to determ
ine if it will yield the desired result.
INT
ELLIG
EN
T IN
ST
RU
ME
NT
S
Data C
omm
unications & Fieldbus S
ystems
97
INS
TR
UM
EN
TS
K-055
Instrumentation is a subject quite distinct from
SC
AD
A, and includes its ow
n unique body of technical know
ledge and conventional practices.
It is not uncomm
on that SC
AD
A technicians are required to share responsibility w
ith their colleagues in the instrum
ent group, and vice versa -- but there is always a clear distinction in
the areas of responsibility and the roles of the two groups.
Som
e important features of electronic instrum
entation include:
The reason for the instrum
ent's existence is measurem
ent of some im
portant process variable -- and precision, stability over tim
e, calibration accuracy and results validation are essential priorities.
Instrumentation, by definition, exists in very close proxim
ity to the primary processes and
systems being m
easured. Flow
meters, pressure and tem
perature sensors, for example
are imm
ersed in the oil, gas or water fluids that are basic to the business.
Instruments alw
ays measure som
e secondary electrical effect that is configured in some
way to be dependent on the prim
ary phenomenon being m
easured.
There is a trem
endous amount of creativity and innovation in the instrum
ents industry. D
esigners have been successful in creating instruments that deliver effective results using
an incredible array of different devices and effects.
Data C
omm
unications & Fieldbus S
ystems
98
K-056
FLO
WD
IRE
CT
ION
PR
ES
SU
RE
SE
NS
OR
S
FLO
WD
IRE
CT
ION
Tw
o of many fluid flow
measurem
ent schemes are illustrated here:
FLO
W M
EA
SU
RE
ME
NT C
ON
CE
PT
S
OR
IFIC
E
Orifice (P
ressure Differential M
eter)
An orifice is a deliberate constriction in
a flow (such as in a pipeline) that w
ill result in a pressure build-up up-stream
and a pressure reduction dow
nstream.
This pressure difference (or differential)
is proportional to the fluid flow rate and
with the help of a suitable calculation,
may be used to m
easure flow.
Impeller (D
isplacement M
eter)
An im
peller is a wheel of som
e sort that m
oves in the flow of a fluid (like a w
ater w
heel).
Wheel rotation rate is proportional to
flow rate.
DIS
PLA
CE
ME
NT
ME
TER
PR
ES
SU
RE
DIFFE
RE
NTIA
LM
ETE
RIMP
ELLE
R
Data C
omm
unications & Fieldbus S
ystems
99
Day Tw
o
►The O
SI M
odel
►C
omm
unications Media
►E
rror Detection
►LA
N S
tandardsData C
omm
unications & Fieldbus S
ystems
100
AP
PLIC
AT
ION LA
YE
R
PR
ES
EN
TA
TIO
N LA
YE
R
SE
SS
ION LA
YE
R
TR
AN
SP
OR
T LAY
ER
NE
TW
OR
K LA
YE
R
LINK LA
YE
R
PH
YS
ICA
L LAY
ER
AP
PLIC
AT
ION LA
YE
R
PR
ES
EN
TA
TIO
N LA
YE
R
SE
SS
ION LA
YE
R
TR
AN
SP
OR
T LAY
ER
NE
TW
OR
K LA
YE
R
LINK LA
YE
R
PH
YS
ICA
L LAY
ER
D-247
LAY
ER
ED A
RC
HIT
EC
TU
RE S
YS
TE
M C
ON
CE
PT
EN
DU
SE
RE
ND
US
ER
MO
DE
MM
OD
EM
TER
MIN
OLO
GY
/CO
NC
EP
T DIS
CU
SS
ION
:
OB
JEC
TIV
E: T
o facilitate useful com
munication betw
een end users (which are
either people or programs).
They are not able to com
municate directly, so
they delegate the task to underlying system(s).
Each layer has a specific task to perform
in support of this com
munication, but only the
bottom layer function can actually pass data to
the other system -- all others delegate
downw
ards.
Thus, the actual inform
ation flow follow
s a "U"
shaped path as shown.
TE
LEC
OM
MU
NIC
AT
ION LIN
K
FU
NC
TIO
NA
L CO
MM
UN
ICA
TIO
N
RE
QU
IRE
DS
YS
TE
M
"A"
SY
ST
EM
"B"
The International O
rganization for Standardization (IS
O) has rigorously defined a seven-layer
architecture structure, specifying protocols and responsibilities for each layer, with a view
to creating an open architecture for com
puter networking (ie. one in w
hich all vendors can participate).
This largely theoretical standard is know
n as the OP
EN S
YS
TEM
S IN
TER
CO
NN
EC
TION (O
SI)
AR
CH
ITEC
TUR
E.
This concept w
as originally inspired by IBM
'S S
ystem N
etwork A
rchitecture (SN
A), w
hich is a proprietary, or closed architecture (ie. an environm
ent in which only IB
M system
s can participate) .
Data C
omm
unications & Fieldbus S
ystems
101
SE
SS
ION
LAY
ER
TRA
NS
PO
RT LA
YE
R
AP
PLIC
ATIO
N LA
YE
R
PR
ES
EN
TATIO
N LA
YE
R
D-303
FUN
CTIO
NS
OF N
ETW
OR
K-R
ELA
TED
LAY
ER
S O
F THE
OS
I MO
DE
L
OS
I LAY
ER
ED
ST
RU
CT
UR
E
PH
YS
ICA
L L
AY
ER
NE
TW
OR
K L
AY
ER
DA
TA
LIN
K L
AY
ER
Physical layer functions apply to electrical bit
level characteristics of the physical interface.E
xamples of representative standards are
EIA
-449, EIA
-232-C and E
IA-232-D
.
Link layer functions apply to the task of organizing data bits into a structured form
at (a
nd in
the p
roce
ss atte
ndin
g to
erro
r contro
l, flo
w co
ntro
l, link m
anagem
ent a
nd d
ata
in
tegrity).
Exam
ples of representative link layer protocols are B
ISY
NC
, HD
LC, C
SM
A/C
D, etc.
Netw
ork layer functions apply to the task of m
anaging resources by controlling switching
and routing functions involving potentially m
any different links, so as to create an end-to-end coherent netw
ork.A
comm
on addressing scheme is essential at
this layer, enabling network level flow
control, congestion control, and com
mon routing
procedures.E
xamples are X
.25 packet layer protocol and the IP
portion of the TCP
/IP protocols.
Data C
omm
unications & Fieldbus S
ystems
102
D-304
FUN
CTIO
NS
OF H
OS
T SY
STE
M-R
ELA
TED
LAY
ER
S O
F THE
OS
I MO
DE
L
OS
I LAY
ER
ED
ST
RU
CT
UR
E
PH
YS
ICA
L LAY
ER
NE
TW
OR
K LA
YE
R
DA
TA
LINK
LAY
ER
TR
AN
SP
OR
T L
AY
ER
SE
SS
ION
LA
YE
R
PR
ES
EN
TA
TIO
N L
AY
ER
AP
PL
ICA
TIO
N L
AY
ER
The transport layer sends data to the network
layer in segments sized to suit the netw
ork's m
aximum
transmission unit, or M
TU lim
it.The transport layer is principally responsible for end-to-end flow
control, and message
packetization/segmentation and reassem
bly.
Session layer functions apply to the task of
initiating, managing and term
inating sessions, and support different m
essage sizes and flow
mechanism
s (FD
X, H
DX
), and provide for recovery in event of low
er layer failure.A
n example of session (a
long w
ith tra
nsp
ort
and n
etw
ork) functions is N
ET
BIO
S.
The presentation layer defines the structure of data -- effectively ensuring that data syntax requirem
ents are taken care of.P
resentation layer functions provide for code conversion (e
.g. A
SC
II to E
BC
DIC
) and graphical user interface (G
UI) support.
The application layer manages
comm
unication between application
processes.E
xamples of application layer functions are
SM
TP, FTP
and Telnet functions in TCP
/IP.
Data C
omm
unications & Fieldbus S
ystems
103
D-335
THR
EE
CLO
SE
LY R
ELA
TED
CO
NC
EP
TS:
DO
WN
SIZIN
G, C
LIEN
T/SE
RV
ER
and OP
EN
SY
STE
MS
OP
EN
SY
ST
EM
S
The issue of O
PE
N S
YS
TE
MS
speaks to the desire to w
ork with inform
ation system
s that are:
capable of interoperationsupportive of application and data portabilityscalable across a range of platform
sizes, vendors and configurations
●●●
DO
WN
SIZ
ING
The D
OW
NS
IZIN
G issue involves
carefully specifying and selecting the optim
um
(RIG
HT
SIZ
ED
) hardware
platform to host a given
application suite.
Dow
nsizing is not a new
concept, having been the essential focus of departm
ental computing
strategies in the 1970's.
TH
E C
OM
MO
N G
RO
UN
D
The prim
ary thrust of new inform
ation system
s development effort focuses on
the area in this diagram that is com
mon
to all three concepts.
That is, appropriate Inform
ation S
ystems design strategies typically
embrace A
LL
TH
RE
E concepts.
CLIE
NT
/SE
RV
ER
SY
ST
EM
S
DO
WN
-S
IZIN
GO
PE
NS
YS
TEM
S
CL
IEN
T/S
ER
VE
R P
AR
AD
IGM
Key concept: C
LIEN
T/S
ER
VE
R
empow
ers the desktop systems and
along with that, superior:
performance, to support the G
UI
economy, using the available P
Cs
data managem
ent features of RD
BM
S
●●●
Data C
omm
unications & Fieldbus S
ystems
104
K-084
GO
OD N
EW
SF
EA
TU
RE
S
BA
D N
EW
SF
EA
TU
RE
S
OP
EN
CO
MP
AR
ISO
N O
F ISS
UE
S:
PR
OP
RIE
TA
RY vs. O
PE
N S
CA
DA T
EC
HN
OLO
GIE
S
PR
OP
RIE
TA
RY
Increased security risks
End user organization tends to be
responsible for systems integration
and modification (w
ith training im
plications).
System
s tend to be simpler
System
s tend to be efficient
"Security by obscurity"
SC
AD
A operations is m
ore autonom
ous
Implem
entations are very inflexible
Staff has lim
ited scope in modifying
details/features
Uncom
petitive -- single source prices (especially after-m
arket).
Multi-vendor environm
ent results in com
petitive prices
Com
modity m
anufacturing results in better com
ponent costs
Facilitates data access and
exchange with IT
, managem
ent, etc.
Data C
omm
unications & Fieldbus S
ystems
105
D-132
TW
IST
ED
PA
IR C
AB
LE C
HA
RA
CT
ER
IST
ICS
UN
SH
IELD
ED
TW
IST
ED
PA
IR T
ELE
PH
ON
E C
AB
LE
Unshielded T
wisted P
air (UT
P) cable is universally used
in telephone and data systems. T
here are three technical features that prim
arily determine its
performance characteristics:
1. Wire G
auge; most w
ire is specified in terms of the
Am
erican W
ire Gau
ge (A
WG
) number (T
ypica
l house
cablin
g is A
WG
No. 2
6; h
eavie
r cable
so
metim
es u
sed is A
WG
No. 2
4).
2. The num
ber of twists per foot w
ill be a factor in determining a cable's suitability for LA
N service
(Conve
ntio
nal U
TP
has 1
twist p
er 1
8 in
ches, h
igh p
erfo
rmance
cable
s have
1 tw
ist eve
ry 6
inch
es o
r less) .
3. The type and dim
ensions of insulation will determ
ine cable capacitance; comm
only low
capacitance cable is specified in demanding applications, such as high speed LA
N installations.
The purpose of m
anufacturing cable with a helical tw
ist to the individual cable pairs is to minim
ize electrom
agnetic energy transfer to and from the cable pairs (th
is energ
y transfe
r is the p
rincip
al
cause
of cro
sstalk, a
nd o
ther p
erfo
rmance
-limitin
g in
terfe
rence
).
Sh
ield
ed
Tw
iste
d P
air (S
TP
) cable is a related technology, and although technically superior to U
TP
, there is minim
al interest in using ST
P due to cost, physical bulk and installation difficulty.
Data C
omm
unications & Fieldbus S
ystems
106
SU
MM
AR
Y -- T
WIS
TE
D P
AIR C
AB
LELIM
ITA
TIO
NS W
ITH F
AS
T ET
HE
RN
ET
There are a num
ber of standard UT
P/S
TP (unshielded and shielded tw
isted pair, respectively) cable types that are suitable to fast E
thernet service.
Each of these are designated on the basis of a 'C
ategory' type, as in Category 5 -- or often
simply C
at-5.
Applicable features, distance and data rate lim
itations are:
CA
TEG
OR
Y
Category 5
Category 5+
AT
&T P
roprietaryC
ategory 5+
Category 6 (S
TP
)
Category 7 (S
TP
)(A
/V A
pplications)
DA
TA R
ATE
100 Mbps
1 Gbps
1 Gbps
1 Gbps
1 Gbps
DISTA
NC
E
100 m
75 m
100 m
100 m
100 m
BA
ND
WID
TH
100 MH
z
300 MH
z
300 MH
z
350 MH
z
750 MH
z
D-732
Data C
omm
unications & Fieldbus S
ystems
107
D-733
It is comm
only thought that many cable installations have been im
plemented w
ith com
ponents that meet C
ategory 5 specs but where installation practices have resulted in
sub-standard system perform
ance.
This issue w
ill become m
ore acute as organizations move aw
ay from traditional 10M
bps E
thernet, to 100Mbps and higher rate E
thernets and/or AT
M.
Som
e installation practices that are capable of causing below-standard perform
ance are:
Excessively tight cable bending radius
Modifying the tw
ist pitch (over- or under-twisting of the pairs, typically caused by bad
termination practice)
Rem
oval of too much jacket m
aterial at the termination point
Not respecting pathw
ay separation requirements (e.g. creating an interference problem
by allow
ing power cables to lie too close to data cables)
Overstressing (stretching) cable during installation
Overtightening cable ties and/or inadequate cable support
INS
TA
LLAT
ION P
RA
CT
ICE
S T
HA
T MA
Y IM
PA
IRP
ER
FO
RM
AN
CE O
F CA
TE
GO
RY 5 U
TP
Data C
omm
unications & Fieldbus S
ystems
108
CO
AX
IAL C
AB
LE C
HA
RA
CT
ER
IST
ICS
CE
NT
ER
CO
RE
C
ON
DU
CT
ORCY
LIND
RIC
AL
INS
ULA
TO
RO
R D
IELE
CT
RIC
OU
TE
R
CO
ND
UC
TO
R,
OR
SH
IELD
OU
TE
RP
RO
TE
CT
IVE
JAC
KE
T
D-131
Coaxial cable derives its nam
e from
the fact that its two
conductors share a comm
on axis; they are thus co
-axia
l ...
Coaxial cable features w
hich are im
portant are:
The outer shield conductor
completely envelopes the inner
conductor. Thus, coaxial
cable:
(a) Contains the desired
electromagnetic energy, or signals (i.e
. limits ra
dia
tion o
f signals -- w
hich
can ca
use
in
terfe
rence
and se
curity th
reats) .
(b) Excludes undesired electrom
agnetic energy, or signals from entering the cable and
conflicting with desired signals (i.e
. limits in
com
ing in
terfe
rence
signals).
Coaxial cable is specified in term
s of impedance; a characteristic determ
ined by the relative diam
eters of the conductors, and measured in O
hms (e
.g. R
G-5
8 ca
ble
is 50 O
hm
s). It is always
critical to respect the specified cable impedance in coaxial cable applications.
Data C
omm
unications & Fieldbus S
ystems
109
FIB
ER O
PT
IC C
AB
LE C
HA
RA
CT
ER
IST
ICS
������
������
������
������
THE
RE A
RE M
AN
Y D
IFFER
EN
TP
AC
KA
GIN
G TE
CH
NIQ
UE
S U
SE
DW
ITH F
IBE
R O
PTIC
CA
BLE
-- SO
ME A
RE M
UC
H M
OR
EC
OM
PLE
X T
HA
N T
HE O
NE
ILLUS
TR
AT
ED
SILIC
ON
E
CO
AT
ING
BU
FF
ER
JAC
KE
TS
TR
EN
GT
HM
EM
BE
RS
(TY
PIC
ALLY
FIB
ER M
AT
ER
IAL)
OU
TE
R
JAC
KE
T(F
OR P
HY
SIC
AL
PR
OT
EC
TIO
N)D-164
PA
CK
ING
ME
DIU
M
CO
RE
FIBE
RS
(SILIC
A)
There are principally tw
o types of fiber optic cable (and a number of variations w
ithin each category):
Multim
ode Fiber:
The active core fiber diam
eter is typically 50.5 or 62.5 microm
eters (compare to hum
an hair which is
about 100 microm
eters) , -- sufficiently large that multiple light rays (or paths) coexist. E
ach path will have
a subtly different transit time delay, resulting in a capacity lim
iting problem know
n as pulse dispersion.
Single M
ode Fiber (Mono-m
ode Fiber)
The active core diam
eter is typically 8 -to- 10 microm
eters, which is sm
all enough to restrict light transm
ission to a single ray of light (more correctly, a single m
ode of propagation), effectively eliminating
pulse dispersion as a performance constraint.
Data C
omm
unications & Fieldbus S
ystems
110
D-768
FIBE
R O
PTIC
CA
BLE
CR
OS
S-S
EC
TION
S
By far, the m
ajor share of the bulk in fiber optic cables is associated w
ith the features of the cable that provide strength and toughness.
The sketches at the right show
three different cable fabrication concepts -- each w
ith exactly the sam
e optical fiber capacity.
In each case, there are just four optical fibers (at the 12, 3, 6 and 9 o'clock positions) adjacent to the very center core w
ire.
The largest, arm
ored cable is 64.7 mm (2.55
inches) in diameter; the m
id-sized cable is 36.8 m
m (1.45 inches) -- and the sm
allest, minim
ally sheathed cable is 21 m
m (0.83 inches)
diameter.
Electrical pow
er is fed down the cable from
shore stations, using a copper foil sheath w
rapped around the steel wires (black outer
ring, lower diagram
) - and sea water as the
return path.
Data C
omm
unications & Fieldbus S
ystems
111
PU
LSE
DIS
PE
RS
ION
CO
NC
EP
T
D-243
To understand the pulse dispersion effect, consider tw
o light rays traveling along an optical fiber -- Indicated here as the gray dashed arrow
representing a direct, axial light ray, and the black, solid arrow
s which reflect off the fiber sidew
alls -- representing a less optimally aligned light path:
These tw
o light pulses start off at the same instant, but follow
paths of different lengths, and thus arrive at different tim
es.
When they recom
bine at the optical detector, the resulting sum of all input pulse com
ponents has becom
e distorted by being broadened as a result of pu
lse disp
ersion; in reality there are a vast
number of light ray com
ponents, resulting in a more uniform
ly dispersed light pulse than this simple
example w
ould suggest (as in
dica
ted b
y the sm
ooth
dotte
d lin
e cu
rve).
Note that both the distance traveled
(cable
length
), and pulse duration (data
rate
) affect the amount of
pulse dispersion encountered. Pulse dispersion thus lim
its the product of data rate and cable length -- and fiber cables are specified in term
s of megabit-per-second-m
eters.
AS
SU
ME
D O
RIG
INA
L LIG
HT
PU
LSE
RE
SU
LTIN
G D
ISP
ER
SE
D
LIGH
T P
UL
SE
RE
FLE
CT
ED
LIGH
T R
AY A
RR
IVE
S LA
TER
DIR
EC
T LIGH
T R
AY
AR
RIV
ES
SO
ON
ER
Data C
omm
unications & Fieldbus S
ystems
112
OP
TICA
L FIBE
R FA
BR
ICA
TION
Fiber pre-form at collapse
Com
pleted Pre-Form
s ready to be drawn
Data C
omm
unications & Fieldbus S
ystems
113
FIBE
R S
PE
CTR
UM
Data C
omm
unications & Fieldbus S
ystems
114
Cop
yrigh
t CA
NO
VA
SYS
TE
MS
CO
RP.
W-163
FREE SPACE OPTICS
TYPICAL TE
RMINAL D
EVICE
continued ....
VIS
ION S
AFE
TY FA
CTO
RS
FS
O system
s operate in the infrared spectrum
and generate no energy in the visible light spectrum
where hum
an vision is most vulnerable.
Pow
er levels are very low -- typical F
SO lasers
operate at about 10 mW (com
pare with laser
pointers that typically operate at about 5 mW
.)
Free Space O
ptics (otherwise know
n as optical wireless) is an open-air laser technology capable
of supporting effective high-capacity short-range communications applications.
Several configurations are feasible, including point-to-point, point-to-m
ultipoint, and meshed
topologies.
Operating data rates of up to about 10 G
bps are feasible -- although more com
monly system
s operate at m
egabit rates.
Operating distances of up to about 4 km
. are practical, depending on atmospheric conditions.
(40508)
14
7D
ata Com
munications &
Fieldbus System
s115
Cop
yrigh
t CA
NO
VA
SYS
TE
MS
CO
RP.
FREE SPACE OPTICS, cont.
W-164
Advantages of FS
O Technology
Uses unlicensed electrom
agnetic spectrum; no regulatory delays or fees are a concern.
Equipm
ent/infrastructure cost is very low com
pared with m
icrowave or fiber and elapsed tim
e to operate is very brief -- typically installation and testing tim
e is a few hours.
The inherently point-to-point, full duplex characteristic of F
SO is easily adapted to suit m
ore com
plex multiport and m
esh topologies -- supporting complex applications w
ith redundant connectivity.
Disadvantages of FS
O Technology
Weather conditions that obscure a visual sight path also obscure F
SO signals; thus feasible path
lengths are very much affected by local atm
ospheric conditions -- particularly fog and precipitation (rain and particularly snow
).
Building m
ovement, how
ever slight, can affect the laser beam alignm
ent -- although many products
claim to support an auto-tracking capability that autom
atically maintains alignm
ent through a small
range of movem
ent.
FSO is vulnerable to being obstructed by birds and really any object that m
ight find its way into the
laser beam's path -- an argum
ent in favor of a more sophisticated m
esh network configuration,
where alternate paths are available.
(30330)
Data C
omm
unications & Fieldbus S
ystems
116
Microw
ave radio signals are really like any radio comm
unication signals; two key features
differentiate microw
ave radio from low
er frequency radio applications:
Since m
icrowave system
s operate at a very high frequency, the distance the signal can travel during one cycle of the signal frequency (th
e w
aveleng
th) is very short -- literally a "m
icro-wave".
This very high frequency of operation m
akes it possible to carry more end-user signal inform
ation (su
ch a
s voice
channels o
r data
signals) than low
er frequency systems.
The propagation characteristics of m
icrowave radio are such that only line-of-sight com
munication
is feasible (contra
st with
shortw
ave
radio
, which
can p
ropagate
beyo
nd th
e h
orizo
n).
Microw
ave systems typically com
municate w
ith carrier frequencies in the range from about
2,000,000,000 Hz (2
GH
z) to around 15,000,000,000 Hz (1
5 G
Hz). F
requencies above about 10 G
Hz are adversely affected by rain and fog in the path.
Physical separation distances betw
een microw
ave radio stations are typically limited by the
curvature of the earth, and the need to maintain sufficient vertical clearance at the m
iddle of the path
(typica
l maxim
um
feasib
le p
ath
length
s are
about 4
0 m
iles, o
r 65 km
.)
D-244
MIC
RO
WA
VE
RA
DIO
SY
ST
EM
CH
AR
AC
TE
RIS
TIC
S
UN
DE
SIR
ED
, IND
IRE
CT
S
IGN
AL P
AT
H
DE
SIR
ED
, DIR
EC
T S
IGN
AL P
ATH
WH
ER
E S
MO
OT
H E
AR
TH
OR
W
AT
ER
SU
RFA
CE
S A
RE
LO
CA
TE
D A
T M
ID-P
AT
H,
TR
OU
BLE
SO
ME
R
EF
LEC
TIO
NS
OC
CU
R,
CA
US
ING
INTE
RF
ER
EN
CE
Data C
omm
unications & Fieldbus S
ystems
117
D-245
CO
MM
UN
ICA
TIO
N S
AT
ELLIT
E
PO
SIT
ION
ING
CO
NS
IDE
RA
TIO
NS
Geosynchronous satellite orbits (w
hich
perm
it a sa
tellite
's positio
n in
space
to
ap
pe
ar sta
tion
ary fro
m e
arth
) require that satellites orbit eastw
ard over the equator (in
the
dire
ction
of e
arth
's ro
tatio
n) at an altitude exactly
consistent with a 24 hour period.
There are a lim
ited number of practical
"parking spots" in space; satellites cannot be positioned any closer than about 2 degrees of longitude separation -- assum
ing moderate cost
earth-based antennas. Actually, this 2
degree separation is a time separation
-- all of these satellites share an identical orbit -- separated by about 8 m
inutes and 780 miles.
The area of coverage of a satellite is called its footprint, the shape of w
hich is custom designed for
each nation's geographic shape (see illu
stratio
n).
International satellites have been typically operated by the Intelsat organization (by in
tern
atio
nal
agre
em
ent) -- although com
petitive international services now exist. C
omm
unication within one
country (or re
gio
nal se
rvice) is m
anaged by a domestic authority.
●●●●
●●●●
D
ata Com
munications &
Fieldbus System
s118
D-183
Earth-to-satellite distance = 35,888 km
, or 22,300 miles, w
hich results in an earth-to-satellite propagation tim
e delay of 125 milliseconds. R
adio signals propagate at the speed of light (3
00
,00
0 km
, or 1
86
,00
0 m
iles p
er se
con
d).
Minim
um round-trip tim
e delay (the
time
it take
s for a
sign
al to
trave
l from
a
po
int o
n e
arth
up
to th
e sa
tellite
, do
wn
to a
no
the
r po
int o
n e
arth
, an
d th
en
ba
ck again
via th
e sa
tellite
to th
e p
oin
t of o
rigin
) is 4 x 125 = 500 milliseconds.
TIM
E D
ELA
Y IS
SU
ES
IN C
OM
MU
NIC
AT
ION
SA
TE
LLITE
SY
ST
EM
S
Actual round-trip tim
e delays are greater than the above estimate for three reasons:
1. Earth stations are rarely located on the equator. Latitudes other than zero increase the path length
(note
that th
e d
otte
d lin
e to
a n
orth
ern
poin
t is longer th
an th
e d
ash
ed lin
e to
the e
quato
r).2. A
similar longitude offset increases path length
(The tw
o e
arth
statio
ns a
nd th
e sa
tellite
will n
eve
r be a
ll thre
e o
n e
xactly th
e sa
me lo
ngitu
de).
3. Signal processing delays in electronic system
s (the tw
o e
arth
statio
ns a
nd th
e sp
ace
pla
tform
).
Thus a typical round-trip tim
e delay budget is 650 to 750 msec.
GE
OS
YN
CH
RO
NO
US
CO
MM
UN
ICA
TIO
N
SA
TE
LLITE
S A
RE
ALW
AY
S P
OS
ITIO
NE
DO
VE
R T
HE
EA
RT
H'S
EQ
UA
TO
R
NO
TE: E
arth diameter
and satellite separation distance are scaled approxim
ately correctly.
Data C
omm
unications & Fieldbus S
ystems
119
D-262
WIR
ELE
SS
SY
ST
EM
S: M
INIM
IZIN
G C
OV
ER
AG
E
Traditionally, radio com
munication system
s have been engineered to maxim
ize the range of effective coverage from
any transmitter site -- resulting in poor use of radio frequency spectrum
.
Cellular radio technology (re
ferre
d to
as "m
obile
" service
in m
any p
arts o
f the w
orld
) was the first
break with this tradition, w
here the radius of cell coverage is designed to be deliberately limited.
Cellular system
s require a greater number of transm
itter sites, and some kind of interconnection --
but have the key advantage of a remarkably superior efficiency of spectrum
use, due to the fact that frequencies can be reused w
ithout interference in cells that are separated from one another
(thus g
reatly in
creasin
g to
tal syste
m ca
pacity).
Capacity can be further increased by subdividing cells (to
a lim
it of d
ow
n to
a fe
w h
undre
d m
ete
rs ra
diu
s).
A m
aximum
power, m
aximum
height transm
itter will
comm
and a coverage radius of perhaps 50 km
(30 miles) --
but there will be
no other service able to use the sam
e frequency in the sam
e area.
Multiple low
power
transmitters w
ill cover the sam
e service area, but it w
ill now be
possible to re-use frequencies several tim
es in a given coverage area.
D
ata Com
munications &
Fieldbus System
s120
WIR
ELE
SS D
AT
A N
ET
WO
RK
ING
TE
CH
NO
LOG
Y - O
VE
RV
IEW
D-606
continued ....
Wireless data netw
orking applications fit into two broad categories:
Low S
peed, Wide A
rea Technologies
A num
ber of products are on the market that enable integrated access to w
ide area data networks for
mobile/cellular or portable and trunked radio users.
This class of system
typically accesses an earth-based (terrestrial) infrastructure -- such as cellular telephone netw
orks, or similar dedicated data netw
orks.
Operating speed is typically m
easured in tens of kilobits per second (practically limited to about
50 kbps), and freedom of physical m
ovement is a m
ajor business priority.
Applications include courier services, law
enforcement and vehicle dispatch/fleet m
anagement.
High S
peed Local Technologies
The originally-popular IE
EE 802.11b w
ireless LAN standard operates at 11 M
bps, which com
pares favorably w
ith the first generation, 10 Mbps shared cable E
thernet LAN technology. T
he newer
versions of the IEE
E 802.11 W
LAN standard (e.g. -a and -g) operate at up to 54 M
bps.
The range of operation of this class of equipm
ent is typically limited to a distances in the order of one
hundred meters -- and is very dependent on interior building structures.
Data C
omm
unications & Fieldbus S
ystems
121
WIR
ELE
SS D
AT
A N
ET
WO
RK
ING
TE
CH
NO
LOG
Y - O
VE
RV
IEW
, cont.
D-607
The high cost of connectivity has historically been an obstacle to grow
th in wireless data applications,
where costs per w
orkstation have typically been in the order of 5 to 10 times greater than equivalent
wired services.
The availability of standards (as w
ith the IEE
E 802.11-series of W
LAN standards) and com
mercial
deployment of reasonable w
ide-area, public packet data services (as GP
RS
/ED
GE and 1xR
TT
/EV
-DO
offered on the 2 and 2.5 G m
obile/cellular networks) have created a high level of public interest.
Trem
endous progress in cost reduction, compact packaging and im
proved battery life has created many
opportunities that were previously not feasible.
A sam
pler of the many applications w
hich make good use of the technology include:
Industrial applications - Com
plexity in managing a w
ired infrastructure and challenges in calibrating instrum
entation argue strongly for wireless netw
orking solutions.
Health care - a roam
ing pen-based or palm-top com
puter can be used to enter patient data and/or access existing data records from
the bedside.
Extending office netw
ork functions - Thin-client technology enables full netw
orked Window
s functionality w
ith the 10's of kilobit data rates supported by 2G public w
ireless packet networks.
Data C
omm
unications & Fieldbus S
ystems
122
W-332
INC
RE
AS
ING D
AT
A R
AT
E (M
bps)
INCREASING RANGE
WW
AN
(Cellular)
WM
AN
WLA
N
WP
AN
IEE
E802.15.4Z
igBee
0.010.1
110
1001000
IEE
E802.15
Bluetooth
IEE
E802.11a/b/g
WiF
i
IEE
E802.16W
iMax
IEE
E802.15.3a
UW
B
TH
E A
PP
RO
XIM
AT
E R
ELA
TIO
NS
HIP B
ET
WE
EN
SO
ME W
IRE
LES
S T
EC
HN
OLO
GIE
S
2G -
GS
M/G
PR
S &
CD
MA 1xR
TT
3G -
UM
TS - W
CD
MA &
EV
-DO C
DM
A
Data C
omm
unications & Fieldbus S
ystems
123
D-295
MO
BILE
DA
TA C
OM
MU
NIC
AT
ION S
YS
TE
MS
SA
FE
TY
TAX
I
TR
AN
SIT
IND
US
TR
IAL
BA
SE S
TA
TIO
N S
UB
-SY
ST
EM C
ON
SIS
TS O
FR
AD
IO T
RA
NS
MIT
TE
R/R
EC
EIV
ER
, LAN
D-LIN
EIN
TE
RF
AC
E A
ND C
ON
TR
OL LO
GIC
TX
/RX
COMM'NCONTROL
DC
ED
CE
MA
INS
ER
VE
R
CO
MM
UN
ICA
TION
S C
ON
TRO
LLER M
AN
AG
ES
THE LIN
K LA
YE
R P
RO
TOC
OLS
US
ED
OV
ER TH
E R
AD
IO C
HA
NN
EL
TYP
ICA
LLY TR
AN
SM
ITTER
SA
RE D
ES
IGN
ED
/LOC
ATE
D FO
RM
AX
IMU
M A
RE
A C
OV
ER
AG
E
DIG
ITAL D
ATA
LINK O
RM
OD
EM
-CO
NN
EC
TED TE
LEP
HO
NE
LINE TO
BA
SE S
TATIO
N
Up until the recent availability of packet-based cellular/m
obile services (such as G
PR
S and 1X
RT
T), m
obile data services were served by analog radio
technology, and wire-line access w
as provided by either leased line (as illustrated below
) or over circuit switched telephone lines.
Data C
omm
unications & Fieldbus S
ystems
124
D-296
MO
BILE
DA
TA C
OM
MU
NIC
AT
ION S
YS
TE
MS
, cont.
Data transm
ission rates over wireless infrastructure are constrained by generally lim
ited RF
bandwidth -- and the typically high error rate over this type of channel.
Where data applications are delivered over a voice-oriented fram
ework, the m
aximum
feasible data rates are typically less than 20 kbps -- com
monly in the 10 to 12 kilobit range.
The new
er packet radio technologies (GP
RS and 1X
RT
T) have been developed to extend this
limit -- potentially to rates in the order of 100 kbps -- and so-called 2.5 and 3G
technologies (like E
DG
E and U
MT
S) prom
ise to support rates up to several hundred kbps.
Vehicle m
otion causes severe variation in signal strength - and thus the most dem
anding applications and services are generally lim
ited to stationary (or slowly m
oving) user terminals.
Limited data rate (and other accom
panying features of mobile service, such as sm
all screen displays and battery pow
er limitations) generally require a unique approach to system
s and hum
an interface design.
There are m
any interesting applications for mobile data technology: public safety
(police/fire/ambulance), taxi, transit and other fleet dispatch applications, train control system
s, courier package tracking, auto rental check-in service -- and fixed applications w
here landline infrastructure is not an option, such as rem
ote weather station and pipeline control station access.
Data C
omm
unications & Fieldbus S
ystems
125
Cop
yrigh
t CA
NO
VA
SYS
TE
MS
CO
RP. C
ELL S
IZE IS
HIG
HLY
VA
RIA
BLE
The com
mon denom
inator of all modern w
ireless comm
unication is the deliberately limited coverage
range of cellular systems.
These cells range over a w
ide scale of dimensions:
Picocells: V
ery small coverage zones that are effective w
ithin buildings (office workgroups or
residences) -- which m
ay or may not be associated w
ith device mobility. E
xamples include
Bluetooth-enabled system
s and wireless LA
Ns.
Microcells: C
overage areas typical of mobile/cellular telephone system
s in very dense urban environm
ents.
Macrocells: C
overage areas typical of less dense, suburban mobile/cellular telephone system
s.
Satellite cells: C
overage areas for Low and M
edium E
arth Orbit (LE
O and M
EO
) satellite system
s are 'cellular' footprints of hundreds of kilometers/m
iles in diameter. G
eosynchronous E
arth Orbit (G
EO
) satellite systems like the T
huraya project support continent-sized footprints.
The degree to w
hich mobility is supported by each of these cell types is highly variable, although in
general, the larger the cell structure the more likely that a high degree of intra-cell m
obility is expected and supported.
Rather than being determ
ined by cell size, inter-cell mobility is determ
ined by the features of the system
architecture and features of the applications being served.
1)D
ata Com
munications &
Fieldbus System
s126
W-073
Whenever m
ore than one comparable strength propagation path exists betw
een a transmitter
and receiver (which
is typica
lly the ca
se) the tw
o (o
r typica
lly more
) signals will arrive at the
receiver with differing phase values.
The destructive interference effect of these m
ultiple signals arriving over different paths is m
ultipath propagation, and results in multipath fading varying over tim
e.
Radio paths betw
een stationary devices will dem
onstrate multipath fading effects that are
described by the Rayleigh statistical m
odel -- so called Rayleigh fading.
Radio paths betw
een devices in motion (o
ne e
nd o
f the p
ath
or b
oth
) will sim
ilarly exhibit R
ayleigh fading statistics -- but in this case variable in both space and time.
Multipath propagation is exacerbated by factors com
mon in m
obile comm
unications:
It is not possible to engineer a particular path -- highly variable conditions prevail as a mobile
unit moves around its service area.
Om
nidirectional antennas are unable to discriminate betw
een the preferred and secondary path signals -- and thus offer no relief for the m
ultipath problem.
Signal reflections off m
ajor buildings and other structures in urban environments typically
create serious multipath fading problem
s.
MU
LTIP
AT
H P
RO
PA
GA
TIO
N
(00105)
Data C
omm
unications & Fieldbus S
ystems
127
Multipath fading phenom
ena are a greater problem w
ith narrow bandw
idth signals, since the sharp nulls w
hich reduce signal strength below usable levels are very phase specific, and the
sharpest, most troublesom
e nulls will only occur in a very narrow
bandwidth.
Planning radio signal strengths and propagation effectiveness in an urban environm
ent is challenged by the unpredictability of m
ultipath-induced signal fades, variability of terrain which
distorts median signal strength, physical obstructions (m
an-m
ade a
nd n
atu
ral), and frequently,
exposure to high noise levels caused by automobiles, pow
er infrastructure, signs, etc.
MU
LTIP
AT
H P
RO
PA
GA
TIO
N, cont.
SIGNAL LEVEL (db)
0
-10
-20
-30
-40
-501 λ
Observable nulls in
received signal strength are spaced in the order of 1 w
avelength apart.
Occurrence of
multipath fading
events is highly variable; greater regularity is observable w
hen a sm
all number of
signals are present.
✽ ✽
05)
Data C
omm
unications & Fieldbus S
ystems
128
The depth of m
ultipath fades (and a
ssocia
ted e
rror co
nse
quence
s) is affected by a number of
factors, including propagation effects and carrier frequency of operation.
Perhaps the m
ost critical issue is the occupied bandwidth of the signal being view
ed, since extrem
ely deep multipath fades are only observed w
ith very narrow bandw
idth carrier signals.
Accordingly, the follow
ing time-dom
ain view of m
ultipath effects is simplistic.
The accom
panying time-dom
ain view of m
ultipath signal strength variations is overlaid with a
sequence of four packets, where the horizontal scale is im
plicitly assumed to be tim
e-calibrated.
It is evident from this sketch that som
e packets will be
hopelessly corrupted by the noise burst that accom
panies deep fading events (pro
bably p
acke
ts A
and D
in th
is exa
mple
).
Since effective perform
ance in marginal coverage
areas will alw
ays be a key operating priority, a signal processing and error m
itigation strategy that is effective through these fading events is key to the success of data transm
itted over mobile w
ireless channels.
TIM
E-D
OM
AIN
VIE
W O
FM
ULT
IPA
TH
EF
FE
CT
SSIGNAL LEVEL (db)
0
-10
-20
-30
-40
-501 λ
AAA
AAA
AAA
AAAA
AAAA
AAAA
AAAAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAAAAA
AAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAAAAA
AAA
AAAA
AAAA
AAAA
AAAA
AAAA
AAAAAA
AB
CD
Data C
omm
unications & Fieldbus S
ystems
129
WIR
ELE
SS
SU
BS
CR
IBE
R
TE
CH
NO
LOG
Y
The m
ost capital-intensive portion of a traditional telephone system is the local loop infrastructure
-- the subscriber lines.
The num
erous configurations and equipment concepts that can m
ake telephone and enhanced services available w
ithout traditional copper (or fib
er o
ptic) cable connections are referred to as
subscriber radio or wireless subscriber system
s.
Three m
ajor application areas are developing for this class of equipment:
In underdeveloped countries, the cost of conventional cabled technology is cost-prohibitive, and im
pedes other development efforts -- w
ireless offers an instant telephone solution, and is very cost-effective even in sparse com
munities.
Many developed countries are m
oving toward deregulation of local access telephone service.
Com
petitors to the traditional telephone companies are anxious to offer state-of-the-art
services without the incredible cost and delay in getting to m
arket with a cabled product.
These em
erging technologies support enhanced service possibilities parallel to conventional w
ired technology -- enabling traditional, established telephone companies to position
themselves for a m
ore competitive m
arket.
Data C
omm
unications & Fieldbus S
ystems
130
WIR
ELE
SS S
UB
SC
RIB
ER
TE
CH
NO
LOG
Y, cont.
Alternative technology distribution system
s -- such as w
ireless subscriber (and including cable television) have no choice but to replicate the trunk distribution netw
ork of a telephone system -- N
etwork segm
ents "1" and "2".
The actual subscriber wireless system
supports only the last portion of this distribution service -- segm
ent "3" above.
The need to conserve radio frequency
spectrum dictates that the concept of
cellular frequency reuse must prevail
(minim
um radius of coverage per
transmitter, m
any low pow
er cell sites able to reuse radio spectrum
throughout an urban area ... ).
The trunk distribution system
in an alternate, com
petitive carrier model of
service could be served by microw
ave radio links or fiber optic cable system
s
In urban applications, the subscriber radio links are very short -- m
easured in a few
hundreds of meters.
In underdeveloped countries, the subscriber links m
ay be much longer,
sometim
es up to tens of kilometers.
12
3
D-648
(10831)
Data C
omm
unications & Fieldbus S
ystems
131
INC
RE
AS
ING D
AT
A R
AT
E (M
bps)
INCREASING RANGE
WW
AN
(Cellular)
WM
AN
WLA
N
WP
AN
IEE
E802.15.4Z
igBee
0.010.1
110
1001000
IEE
E802.15
Bluetooth
IEE
E802.11a/b/g
WiF
i
IEE
E802.16W
iMax
IEE
E802.15.3a
UW
B
TH
E A
PP
RO
XIM
AT
E R
ELA
TIO
NS
HIP B
ET
WE
EN
SO
ME W
IRE
LES
S T
EC
HN
OLO
GIE
S
2G -
GS
M/G
PR
S &
CD
MA 1xR
TT
Data C
omm
unications & Fieldbus S
ystems
132
1
ULT
RA W
IDE
BA
ND (U
WB
) RA
DIO
An em
erging class of short-range, very high data rate wireless service has com
e to be known
as ultra wideband, or U
WB radio -- consistent w
ith the IEE
E 802.15.3a specifications.
The architecture for U
WB that has been adopted is based on orthogonal frequency division
multiplex (O
FD
M) technology, w
herein 122 sub-carriers are modulated (using Q
PS
K) in each
of potentially 13 bands of 528 MH
z of microw
ave spectrum.
Data rates ranging from
55 through 480 Mbps per R
F band are achievable. V
ery low pow
er consum
ption is a key design goal, with rem
ote devices capable of supporting the direct sequence spread-spectrum
service for a year on a single AA
A battery.
Initial applications targeted by this technology are Personal A
rea Netw
ork (PA
N) service,
including a super-Bluetooth w
ireless desktop model, hom
e entertainment netw
orks and autom
otive applications.
The incredibly high bandw
idth sustained by this technology is attracting interest in other areas, w
here the multipath resistance and frequency/phase azim
uth resolution permits accurate
position determination.
For exam
ple, RF
ID devices m
ay be located with resolutions of 1 cm
over distances of 100 m
eters or more -- and backscatter from
reflecting media m
ay permit detection of people or other
targets behind walls in a portable, sim
plified radar kind of application (an application of interest to firefighters and law
enforcement agencies).
3)
D
ata Com
munications &
Fieldbus System
s133
W-266
IEE
E 802.16 -- W
IMA
X F
EA
TU
RE S
UM
MA
RY
The IE
EE 802.16 standard defines a w
ireless metropolitan-area (W
ireless MA
N) netw
orking technology -- a w
ireless "last mile" solution.
It is essentially a standardized approach to the applications intended for Local Multipoint
Distribution S
ervice (LMD
S) and M
ulti-channel Multipoint D
istribution Service (M
MD
S) -- both
being fixed microw
ave local distribution system concepts.
Specifically, the IE
EE 802.16 and 802.16a standards provide this sum
mary feature set:
Range: up to 50 km
(30 miles)
Frequencies of operation:
802.16 - 11 -to- 40 GH
z 802.11a - 2 -to- 11 G
Hz
Data rates of up to 280 M
bps
Suitable for fixed link service and "nom
adic" users (i.e. not a dynamically m
obile service)
The technology is Intended as a com
munity-w
ide service, suited to delivering telephony, broadband Internet and cable-television style entertainm
ent programm
ing.
One application that is attracting a lot of interest is the use of this new
technology to provide connectivity to 802.11 hot spots, creating a com
munity-w
ide wireless LA
N netw
ork.
(31211)
Data C
omm
unications & Fieldbus S
ystems
134
Multiplexing and m
ultiple access are closely related topics.
Without intending to lim
it the generality of the discussion, consider only the tim
e-domain dim
ension to this subject; T
ime D
ivision Multiplex
(T
DM
) and Tim
e Division M
ultiple A
ccess (TD
MA
) systems.
The key sim
ilarity is the data sequences received at the system
input (R
X) are identical, being
time-interleaved data elem
ents representing num
erous unrelated data sources.
The key difference is m
ultiple access technologies elim
inate the source m
ultiplexer (TX
); instead distributing the tim
e sequencing responsibility am
ongst the comm
unity of source devices.
MU
LTIP
LE A
CC
ES
S vs. M
ULT
IPLE
XIN
G
TIME
DIV
ISIO
N M
ULTIP
LEX
ING
RX
TX1
23
12
3
123
123
TIME
DIV
ISIO
N M
ULTIP
LE A
CC
ES
S
RX
22
123
123
2
11
1
33
3
TIM
E S
LOT
S A
RE
MA
NA
GE
DB
Y S
EN
DIN
G M
ULT
IPLE
XE
R
SO
UR
CE
DE
VIC
ES
SH
AR
E R
ES
PO
NS
IBILIT
Y
FO
R T
IME
SLO
T C
OO
RD
INA
TIO
N (V
IA A
M
ULT
IPLE
AC
CE
SS
CO
NT
RO
L SY
ST
EM
)
Data C
omm
unications & Fieldbus S
ystems
135
Multiplexing is a fundam
ental feature of many com
munications system
, involving the ability to share a com
munications channel of som
e type by a number of sim
ultaneous users.
Generally, the term
'multiplexing' is used to describe a channel sharing environm
ent where all
parties share a small num
ber of comm
on physical locations -- as in a point-to-point service.
A very closely related term
is 'multiple access' w
hich applies to a multiplexed channel
environment w
here the user population is disbursed over a broader service area, as in the case of population of m
obile radio users.
Thus, the various channel sharing techniques used to equitably and efficiently share radio
spectrum am
ongst a population of mobile users are referred to as m
ultiple access techniques.
There are four such sharing m
echanisms that are im
portant to wireless services, based on
sharing in the space dim
ension, in the time dim
ension, in the frequ
ency dim
ension and in a cod
e dim
ension:
Sp
ace Divisio
n M
ultip
le Access
The structuring of cellular w
ireless systems w
ith deliberately defined and managed physical
dimensions to each cell is essentially w
hat SD
MA
is all about.
SD
MA
makes it possible for m
ultiple users to occupy the same frequencies at the sam
e time in a
comm
on service area by deliberately restricting each to a different subset of that service area.
MU
LTIP
LEX
ING
AN
D M
ULT
IPLE
AC
CE
SS
T
EC
HN
IQU
ES
Data C
omm
unications & Fieldbus S
ystems
136
Freq
uen
cy Divisio
n M
ultip
le Access
Multiplexing in the frequency dom
ain is perhaps the oldest, most fam
iliar channel sharing m
echanism.
With F
DM
A, individual users are assigned different, non-interfering frequencies of operation.
The fam
iliar spectrum m
anagement schem
e used in broadcast services is FD
MA
, where each
broadcaster operates in a coordinated, protected frequency segment of the available spectrum
.
Tim
e Divisio
n M
ultip
le Access
Tim
e division multiplexing techniques yield efficient, effective channel sharing -- strictly in context
of digital comm
unication.
The essential concept of T
DM
/TD
MA
systems is that the entire channel is assigned to each user
sequentially, so that operation is characterized by intermittent, high rate bursts of data being sent.
TD
M/T
DM
A system
s are highly flexible, making it possible to dynam
ically assign channel resources on an 'as required' basis; busy users receiving a larger share of shared capacity than less dem
anding users.
MU
LTIP
LEX
ING
AN
D M
ULT
IPLE
AC
CE
SS
T
EC
HN
IQU
ES
, cont.
Data C
omm
unications & Fieldbus S
ystems
137
Co
de D
ivision
Mu
ltiple A
ccess
The new
est, and therefore least familiar channel sharing technology is C
DM
/CD
MA
.
With C
DM
A, each individual user's data stream
is modulated w
ith a unique code -- the uniqueness of w
hich is the basis of channel sharing.
The ability of spread spectrum
systems to support m
ultiple simultaneous, co-channel users is
keyed to the concept of each user using a unique chipping code that is orthogonal (see
acco
mpanyin
g b
ox) to that used by all other sim
ultaneous users.
MU
LTIP
LEX
ING
AN
D M
ULT
IPLE
AC
CE
SS
T
EC
HN
IQU
ES
, cont.
OR
THO
GO
NA
LITY
Orthogonality is a m
athematical
concept, in which tw
o sets of data values are independent of each other.
The three axes of three-dim
ensional space (x, y a
nd z) are orthogonal; the
projection of each axis on the other tw
o is unconditionally zero.
Expressed in m
atrix algebra term
inology, two sets of data are
orthogonal to each other if their dot product is zero.
Thus, tw
o examples of orthogonal
data are:
(1, 0, 0) * ( 0, 1, 0) = 0 and(3,-2, 4) * (-2, 3, 3) = 0
Data C
omm
unications & Fieldbus S
ystems
138
Each of the m
edium-sharing, m
ultiple access mechanism
s are imperfectly m
anaged, such that guard space is required betw
een each of the concurrent users.
This guard space becom
es, for each multiple access m
echanism:
SD
MA
-- a physical guard space
With S
DM
A, overlapping radio coverage forces a physical separation betw
een user groups.
FD
MA
-- a frequency guard space
Unique frequencies are the basis of F
DM
A operation -- and in this case individual users m
ust be assigned frequencies sufficiently separated so as to m
inimize adjacent channel interference.
TD
MA
-- a time guard space
With T
DM
A, absolute control over tim
ing of individual devices' data bursts is impossible, and a
guard-time (g
ap) betw
een individual transmissions is necessary to m
anage interference.
CD
MA
-- a code guard space
The selection of the code space used for C
DM
A is a highly m
athematical, com
plex process -- but the m
athematical concept of code distance is the analog here.
GA
PS
AN
D G
UA
RD
BA
ND
S
Data C
omm
unications & Fieldbus S
ystems
139
Frequency D
ivision Multiple A
ccess (FD
MA
) involves a number of channel m
anagement
schemes -- all of w
hich separate users in the frequency domain (i.e
. assig
n in
divid
ual u
sers
uniq
ue, n
on-in
terfe
ring fre
quencie
s)
Most F
DM
A system
s and frequency allocations are designed for full duplex, telephony-style (tw
o
open, co
ntin
uously a
vaila
ble
channels) operation w
here the transmit and receive signals are
assigned different frequencies -- a configuration known as freq
uen
cy divisio
n d
up
lex, or FDD
.
As in the exam
ple below (b
ase
d o
n th
e 9
00 M
Hz G
SM
frequency p
lan) each pair of frequencies is
typically offset by a uniform value -- here 45 M
Hz.
FR
EQ
UE
NC
Y D
IVIS
ION
MU
LTIP
LE
AC
CE
SS
FR
EQ
UE
NC
IES
AN
D C
HA
NN
EL A
LLOC
AT
ION
S S
HO
WN
AR
E C
ON
SIS
TE
NT
WIT
H 900 M
Hz G
SM
124 FD
D C
HA
NN
ELS
AR
E A
SS
IGN
ED
IN T
WO
25 MH
z. FR
EQ
UE
NC
Y B
AN
DS
-- WH
ICH
AR
E
SE
PA
RA
TE
D B
Y A
20 MH
z GA
P.
EA
CH
SID
E O
F T
HE
FD
D C
HA
NN
EL N
OM
INA
LLY O
CC
UP
IES
200 kHz,
TH
E F
DD
CH
AN
NE
L-PA
IRS
AR
E S
EP
AR
AT
ED
BY
45 MH
z.
12
34
123124
TRA
NS
MIT-FR
OM
-MO
BILE
12
34
123124
TRA
NS
MIT-TO
-MO
BILE
890.2M
Hz
915M
Hz
935.2M
Hz
960M
Hz
20 MH
zS
EP
AR
AT
ION
GS
M E
XA
MP
LE
OF
FR
EQ
UE
NC
Y A
LL
OC
AT
ION
FO
R F
DM
A - F
DD
SE
RV
ICE
S
Data C
omm
unications & Fieldbus S
ystems
140
Tim
e Division M
ultiple Access (T
DM
A) provide for tim
e-coordinating a given digital com
munications m
edium so that m
ultiple users are able to share this comm
on resource.
TD
MA
offers a much higher level of flexibility than F
DM
A, in that tim
e slots allocated to individual users are not necessarily all of the sam
e duration, or repetition rate, nor are they necessarily m
anaged in a highly structured way (e
.g A
loha is th
e u
ltimate
ly unstru
cture
d a
ccess m
eth
od) .
Tim
e Divisio
n D
up
lex (TD
D) is the term
used for use of the same R
F channel for both transm
it and receive functions (se
e illu
stratio
n b
elo
w) -- the essential feature is a m
anageable, predictable tim
e separation between the transm
it and receive functions.
TIM
E D
IVIS
ION
MU
LTIP
LE A
CC
ES
S
TIM
E-S
LOT
ALLO
CA
TIO
NS
SH
OW
N A
RE
ON
LY T
YP
ICA
L OF
TD
MA
OP
ER
AT
ION
(TH
E D
IFF
ER
EN
T T
DM
A
ST
AN
DA
RD
S [e
.g G
SM
, IS-5
4, IS
-136, e
tc.] GE
NE
RA
LLY
US
E D
IFF
ER
EN
T N
UM
BE
RS
OF
TIM
E S
LO
TS
)IN
TH
IS E
XA
MP
LE, W
HIC
H IS
CO
NS
IST
EN
T W
ITH
TH
E D
EC
T C
OR
DLE
SS
PH
ON
E S
YS
TE
M, 12
TIM
E S
LOT
S A
RE
ALLO
CA
TE
D F
OR
ON
E D
IRE
CT
ION
OF
CO
MM
UN
ICA
TIO
N, F
OLLO
WE
D B
Y 12
IN T
HE
RE
VE
RS
E D
IRE
CT
ION
-- WIT
H A
FR
AM
ING
PA
TT
ER
N T
HA
T R
EP
EA
TS
EV
ER
Y 10 m
s, resulting in 417 µ
s IND
IVID
UA
L TIM
E S
LOT
S.
BY
SE
PA
RA
TIN
G T
HE
TR
AN
SM
IT A
ND
RE
CE
IVE
FU
NC
TIO
NS
IN T
IME
, WIT
H T
HIS
TD
D
FE
AT
UR
E, IT
IS P
OS
SIB
LE T
O S
HA
RE
RF
CIR
CU
ITR
Y B
ET
WE
EN
A D
EV
ICE
'S R
EC
EIV
ER
AN
D
TR
AN
SM
ITT
ER
.
TR
AN
SM
IT-F
RO
M-M
OB
ILET
RA
NS
MIT-T
O-M
OB
ILE
DE
CT
EX
AM
PL
E O
F T
IME
AL
LO
CA
TIO
N F
OR
TD
MA
SE
RV
ICE
S
32
11
11
23
21
11
12
Data C
omm
unications & Fieldbus S
ystems
141
Spread spectrum
technology was developed as a U
.S. m
ilitary initiative, the key elements of
which w
ere declassified in the 1980's.
A num
ber of features of the technology that were attractive to m
ilitary comm
unications are sim
ilarly attractive in civilian applications -- see accompanying box.
Spread spectrum
technology deliberately modulates a transm
itter's spectrum so as to occupy
considerably more bandw
idth than would be required using conventional m
odulation techniques.
On the surface of it, this tactic w
ould seem to w
asteful of spectrum -- but spread spectrum
is arguably m
ore spectrum-efficient than m
ore conventional channel sharing technologies, like F
DM
A and T
DM
A.
A key elem
ent of spread spectrum technology, as adapted to civilian applications is the ability to
efficiently share spectrum w
ith m
ultiple simultaneous users --
each of which uses a unique
frequency spreading code sequence that ensures each user's signals are recoverable.
The resulting channel sharing
scheme is C
ode Division
Multiple A
ccess, or CD
MA
. SP
RE
AD
SP
EC
TR
UM
SP
RE
AD
SP
EC
TRU
M FE
ATU
RE
S
INIT
IAL M
ILITA
RY
OB
JEC
TIV
E
Jam-proof com
munication
Stealthy operation
Superb security
NE
T C
IVILIA
N B
EN
EF
IT
Interference imm
unity
Minim
al frequency coordination
Superb security
Data C
omm
unications & Fieldbus S
ystems
142
There are tw
o spread spectrum m
ethods or processes:
Direct S
equ
ence S
pread
Sp
ectrum
(DS
SS
)
With D
SS
S, a high rate pseudo-random
binary code (calle
d th
e ch
ippin
g co
de) is E
xclusively O
R'd against the data stream
-- resulting in a similarly high rate binary sequence that is directly
modulated as a w
ide band carrier signal.
Chipping code sequences are in fact not random
-- they are carefully chosen sequences that dem
onstrate a high degree of orthogonality with all other sim
ultaneously used chipping codes.
In civilian applications, chipping code rates are typically between 10 and 100 tim
es the data rate (in
milita
ry applica
tions th
e n
um
ber is m
ore
typica
lly 10,0
00), resulting in a channel spreading
factor (bandw
idth
incre
ase
) of a similar num
ber.
Freq
uen
cy Ho
pp
ing
Sp
read S
pectru
m (F
HS
S)
With F
HS
S, the available spectrum
is divided into a number of discrete channels -- m
uch as is done w
ith FD
MA
. Transm
itters and receivers operate in a carefully orchestrated sequence, pseudo-random
ly selecting first one and then another channel on which to com
municate a burst
of data.
With slow
hopping FH
SS
, the carrier stays on frequency long enough to transmit several bits of
user data (slow
FH
SS
is a sta
ndard
optio
n w
ith G
SM
system
s) -- and with fast hopping F
HS
S
systems, the carrier frequency changes several tim
es per user data bit interval.
SP
RE
AD
SP
EC
TR
UM
, cont.
Data C
omm
unications & Fieldbus S
ystems
143
Direct S
equence Spread S
pectrum (D
SS
S) system
s perform an exclusive-O
R betw
een the user's data sequence and a spreading code chipping sequence, as illustrated in the accom
panying box.
The degree of frequency spreading is the spreading factor -- w
hich is the ratio of the chipping sequence rate to the user data rate (in
this sim
ple
exa
mple
the ch
ippin
g se
quence
is the 7
-bit
data
patte
rn 1
001010, so
the sp
readin
g fa
ctor is 7
:1) .
The m
odulation spectrum is spread, or w
idened by the spreading factor; that is, with all other
system features unchanged (e
.g. m
odula
tion a
nd filte
ring stra
tegy) the spread signal w
ill be wider
than the single channel spectrum by this factor.
SP
RE
AD
ING
WIT
H D
SS
S
US
ER
DA
TA
10
CH
IPP
ING
SE
QU
EN
CE
10
11
00
01
01
10
00
X-O
R'd R
ES
ULT
ING
SIG
NA
L
EX
CLU
SIV
E-O
RT
RU
TH
TA
BLE
AB
AB
00
00
11
10
11
10
10
11
00
00
10
01
11
Data C
omm
unications & Fieldbus S
ystems
144
CO
DE
DIV
ISIO
N M
ULT
IPLE
AC
CE
SS
This and follow
ing pages illustrate the concepts of a DS
SS
CD
MA
signal processing sequence:
Data E
nco
din
g S
equ
ence: (S
ee 'D
ata
Enco
din
g S
equence
' illustra
tion n
ext p
age)
Prior to com
mencing this transm
it sequence, the CD
MA
system w
ill have assigned the two
distinct and orthogonal 6-bit spreading codes.
These tw
o codes (101100 and 110110) can be shown to be orthogonal as follow
s: Consider
the logical 1 value to have an arithmetic +1 value and the logical 0 to have an arithm
etic -1 value -- then bit-by-bit, the product of these tw
o codes is +1 -1 -1 +1 -1 +1 = 0
The tw
o transmitted data sequences are obtained by applying the E
xclusive OR
function to the the source data and the spreading code -- and are sent at the rate of the spreading code sequence.
Data R
ecovery S
equ
ence (S
ee 'D
ata
Reco
very' illu
stratio
n fo
llow
ing)
The tw
o transmitted data sequences com
bine additively in the receiver, as illustrated -- resulting in a three-state com
bined signal.
Note how
critically dependent this additive signal recovery will be on
precise transmit tim
e coordination and precise signal strength control at the signal source transm
itters.
EX
CL
US
IVE
"OR
" F
UN
CT
ION
DE
FIN
ITIO
N
INP
UT
1 IN
PU
T 2
OU
TP
UT
0011
0101
0110
12
D
ata Com
munications &
Fieldbus System
s145
W-119
CO
DE
DIV
ISIO
N M
ULT
IPLE
AC
CE
SS
, cont.
Data E
ncoding Sequence:
US
ER
#1
US
ER
#2
SO
UR
CE
DA
TA
SP
RE
AD
ING
CO
DE
(101100)
TR
AN
SM
ITT
ED
DA
TA
SO
UR
CE
DA
TA
SP
RE
AD
ING
CO
DE
(110110)
TR
AN
SM
ITT
ED
DA
TA
10
1
10
11
00
10
11
00
10
11
00
01
00
11
10
11
00
01
00
11
00
1
11
01
10
11
01
10
11
01
10
11
01
10
11
01
10
00
10
01
(1) 6-bit spreading codes used here are shorter than typically used -- but are orthogonal.(2) T
ransmitted data is E
xclusive-OR
of Source D
ata and Spreading C
ode✽
Notes:
(01028)
Data C
omm
unications & Fieldbus S
ystems
146
0
CO
DE
DIV
ISIO
N M
ULT
IPLE
AC
CE
SS
, cont.
Data R
ecovery Sequence:
US
ER
#1 DA
TA
CO
MB
INE
DR
EC
EIV
ED
SIG
NA
L
01
00
11
10
11
00
01
00
11
11
01
10
11
01
10
00
10
01
US
ER
#2 DA
TA
0 +1-1(1) CD
MA
systems are very dependent on careful, accurate control of the signal strength
recovered from all sim
ultaneously received signals.(2) T
his is because the "Com
bined Received S
ignal" shown here is an additive value and it is
vital that all contributing signals contribute a like amount of signal from
each received sym
bol.
✽ N
otes:
8)
Data C
omm
unications & Fieldbus S
ystems
147
W-127
CO
DE
DIV
ISIO
N M
ULT
IPLE
AC
CE
SS
, cont.
Data D
ecod
ing
Seq
uen
ce: (See 'D
ata
Deco
din
g S
equence
' illustra
tion n
ext p
age)
The data recovery sequence w
ith CD
MA
is critically dependent on precise amplitude and
phase coordination of incoming m
ultiple access users, so that their signals can be simply
combined (o
r added) in the receiver.
The resulting signal is a m
ultistate sequence -- shown here as a three-state because of
additively combining tw
o 2-state signals.
The data decoding sequence involves calculating the bit-by-bit product of the incom
ing signal and each of the in-service spreading codes -- here just tw
o.
It is convenient to assign an algebraic value to the binary spreading codes (i.e. vie
w th
em
as
havin
g va
lues +
1 a
nd -1
com
pare
d w
ith th
e m
ore
com
mon 1
and 0
valu
atio
n).
The bit-by-bit products from
this process are accumulated, or integrated to yield a soft decision
process.
Note that the decision is not necessarily assisted by all bit intervals in the spreading code (i.e
. so
me b
it inte
rvals d
o n
ot co
ntrib
ute
an in
crease
d sig
nal a
ccum
ula
tion, o
r inte
gra
l), but over the source data bit interval (a
nd th
ere
fore
num
ero
us sp
readin
g co
de sym
bol in
terva
ls) a usable binary detection process results.
3
(01028)
Data C
omm
unications & Fieldbus S
ystems
148
W-121
CO
DE
DIV
ISIO
N M
ULT
IPLE
AC
CE
SS
, cont.
Data D
ecoding Sequence:
CO
MB
INE
DR
EC
EIV
ED
SIG
NA
L0 +1-1
Each user signal is integrated (a
ccum
ula
ted) over the source data bit interval
using the following calculation and the num
eric calibration shown on the left
margin above: -1 x (com
bined receiver signal) x (user spreading code)
US
ER
#1S
PR
EA
DIN
GC
OD
E
10
11
00
10
11
00
10
11
00
11
01
10
11
01
10
11
01
10
US
ER
#2S
PR
EA
DIN
GC
OD
E
US
ER
#1S
IGN
AL
INT
EG
RA
L
US
ER
#2S
IGN
AL
INT
EG
RA
L
+1-1+1-1
✽ N
ote:
(01028)
Data C
omm
unications & Fieldbus S
ystems
149
W-311
WIR
ELE
SS M
OD
EM
S
A w
ireless modem
is a device that performs the function of all m
odems, in that it takes serial data
from a com
puter and modulates it onto the carrier signal associated w
ith some kind of
telecomm
unications channel -- in this case a wireless channel.
Historically, w
ireless modem
s operated over VH
F or UH
F analog voice radio channels (and some
still do), but the major interest in current tim
e relates to devices that provide an interface to cellular/m
obile packet data networks like G
PR
S (associated w
ith GS
M m
obile networks) and 1xR
TT
(associated with C
DM
A m
obile networks).
Conceptually, w
hat these devices provide is the functionality of a digital mobile phone w
ithout the keypad and screen -- but w
ith an EIA 232 serial data port suitable for interfacing to external
hardware.
Wireless m
odems com
e with a variety of features and options:
Various packet radio technologies (e.g. G
PR
S and 1xR
TT
) and frequency bands (e.g. 850/1900 or 900/1800 M
Hz.).
External pow
er requirements -- they typically use a D
C pow
er feed (such as is readily available for vehicle-m
ounted and SC
AD
A applications).
Many offer an integrated G
PS receiver option, as is often required for m
obile applications.
Directly connected telem
etry ports for simple S
CA
DA or data collection applications.
Data C
omm
unications & Fieldbus S
ystems
150
W-312
WIR
ELE
SS M
OD
EM
S - S
AM
PLE
R
Multitech S
ystems
MultiM
odem G
SM
/GP
RS and C
DM
A(w
ww
.multitech.com
)
WebT
ech Wireless S
ystems Inc.
WebTech 4000 W
ireless Modem
(ww
w.w
ebtechwireless.com
)
Sierra W
ireless M
P 775 G
PS - G
PR
S/E
DG
E W
ireless Modem
(ww
w.sierraw
ireless.com)
Cypress Industrial W
ireless Solutions
CT
M-100 C
DM
A W
ireless MO
dem(w
ww
.cypress.bc.ca)
Data C
omm
unications & Fieldbus S
ystems
151
continued ....
K-035
GR
OU
ND
ING G
RID S
YS
TE
M,
US
UA
LLY S
UB
-SU
RF
AC
E
TE
LEH
ON
EE
XC
HA
NG
EB
UILD
ING
X
RE
SID
UA
L EF
FE
CT
IVE
GR
OU
ND IM
PE
DA
NC
E
Ground P
otential Rise (G
PR
) -- and often called Earth P
otential Rise (E
PR
) -- is a hazard phenom
enon caused by overloading grounding/earthing systems of generally industrial
installations through lightning strikes or major electrical faults. S
ee sketch below:
GR
OU
ND P
OT
EN
TIA
L RIS
E
TH
US
, A M
AS
SIV
E C
UR
RE
NT W
ILL S
UR
GE T
HR
OU
GH T
HE S
ITE
'S
GR
OU
ND
ING S
TR
UC
TU
RE
S A
ND
INT
O T
HE E
AR
TH C
ON
NE
CT
ION
(FO
LLOW
ING T
HE B
IG A
RR
OW
)
ELE
CTR
ICA
L SU
BS
TATIO
N E
XA
MP
LE
AS
SU
ME S
OM
E F
AU
LT ON T
HE S
TA
TIO
N (IN
TH
E
EQ
UIP
ME
NT M
AR
KE
D "X
") CR
EA
TE
S A P
AT
H F
RO
M T
HE
HIG
H V
OLT
AG
E C
IRC
UIT T
O G
RO
UN
D
Data C
omm
unications & Fieldbus S
ystems
152
GR
OU
ND P
OT
EN
TIA
L RIS
E, cont.
continued ....K
-036
When a m
ajor electrical fault, or lightning strike occurs, massive, uncontrolled currents surge
into the grounding system of the installation -- very briefly (for typically a few
milliseconds).
Lightning currents are naturally limited in duration; in the case of an electrical failure, circuit
breakers will blow
and eliminate the current flow
.
How
ever, briefly, these currents can easily peak at 10,000 Am
peres or more, all of w
hich will
flow into the grounding structures provided for this purpose and thus into the earth.
There is alw
ays a small, unavoidable residual effective im
pedance between the station
ground and so-called 'remote earth' -- w
hich is the ultimate connection point. T
his value is often in the order of 1 or 2 O
hms.
A sim
ple Ohm
's Law calculation tells us that w
hen a 10,000 Am
pere surge current flows
through a 1 Ohm
impedance, it w
ill generate a 10,000 Volt surge -- w
hich appears as a transient that everything -- and everyone -- on the station w
ill exhibit when referenced to any
remotely connected facilities, such as a w
ired telephone connection.
With norm
al, minim
al currents flowing through the grounding system
at the telephone exchange, its building ground is at norm
al "earth" potential -- and all metallic cables leaving
the building (including the cable to the telephone in the sketch) are at neutral earth potential.
Data C
omm
unications & Fieldbus S
ystems
153
GR
OU
ND P
OT
EN
TIA
L RIS
E, cont.
K-037
Som
eone talking on this telephone would experience the above-described 10,000 V
olt potential difference betw
een their feet on the ground and hands on the telephone!
There are several w
ays of ensuring that this is not an unavoidable hazard -- among the best is
use of non-metallic fiber optic com
munication cables (w
hich cannot carry any so-called longitudinal currents), or through a m
icrowave radio link that avoids the issue com
pletely.
In addition to "off-site" issues, there are other hazards to personnel and equipment that are
caused by non-uniformity in the station grounding system
:
Personnel "S
tep Hazards" -- T
he sub-surface grounding grid system is a m
esh of large gauge copper conductors perhaps one m
eter apart, bonded together at the cross-points. S
paces between these w
ires (where one m
ight step) will be at a low
er potential than directly over them
. This difference can present a hazard voltage betw
een one's feet.
Personnel "Touch H
azards" -- Sim
ilarly, the ground under one's foot and a piece of station steel in one's hand can have enough potential difference to be lethal.
Equipm
ent Hazards -- S
imilarly, m
etallic cables and/or equipment assem
blies on the station m
ay be imperfectly grounded and dam
aging voltages can develop between com
ponents that are only nom
inally grounded as checked by low-voltage test equipm
ent.
Data C
omm
unications & Fieldbus S
ystems
154
ER
RO
RS A
ND T
HE
IR E
FF
EC
T ON O
PE
RA
TIO
NS
D-279
9
9
Errors occur in the process of transm
itting data due to any of a number of external effects (com
ponent m
alfunctions, electrical disturbances like lightning, operation of electro-mechanical equipm
ent like m
otors and generators, operation of neon or fluorescent lighting systems, etc.).
With m
ost applications, it is highly desirable to minim
ize the frequency with w
hich undetected/uncorrected errors are allow
ed into the application process -- where they result in
corruption of data bases, broken protocol sequences or worse.
Detected
errors must be dealt w
ith reliably and effectively so as to avoid these undesirable consequences and resulting loss of inform
ation integrity.
Undetected errors m
ust be effectively eliminated -- although there w
ill always be a m
athematical
probability of their residual presence in received data.
Errors occur unevenly distributed (i.e. in clum
ps or bursts) in data streams due to the kinds of events
that cause them. T
hat is, if one bit is errored due to some external event, there is an increased
likelihood that other nearby bits are damaged also.
Error experience is typically expressed as a bit error rate (B
ER
) -- with a statem
ent like "we expect an
error rate of 1 in 10 on this circuit" -- or as a probability, as in "the bit error probability on this circuit is 1 in 10 " -- m
eaning the chances of any single bit being flawed is one in a m
illion. These suggested
BE
R/probability figures are typical of low
speed data links before error detection/correction measures
are applied (the so-called "raw" error rate) .
(41102)
Data C
omm
unications & Fieldbus S
ystems
155
TH
QE
UC
IK
BR
ON
WF
OX
..
.
1 01 01 0
00011
11
11
11
11
11
11000
01
001 000
100 01
11010
00 100 10000 11
0 1100 1
0001 000
0001 00
0001 0000
111 1001 111 100
11
000 100
000 10 1
11100 1
011100
1 0000 1
1 00010
1 011010
1 011010
1 011010
10
11
10
11
00
01
11
11
11
11
00
0
10111 000
00
00
00
D-034
PA
RITY
ER
RO
R D
ETE
CTIO
N C
ON
CE
PTS
A C
HA
RA
CT
ER
-SE
QU
EN
TIA
L VIE
W O
FD
AT
A IS
DE
SC
RIB
ED
AS
HO
RIZ
ON
TA
L,O
R LO
NG
ITU
DIN
AL, O
R B
LOC
K
SE
QU
EN
CE
.
A BIT-SEQUENTIAL VIEW OF DATA IS DESCRIBED AS VERTICAL OR BIT-SEQUENCE.
VE
RT
ICA
L PA
RIT
Y B
ITS
AR
E A
PP
EN
DE
D T
O E
AC
H C
HA
RA
CT
ER
, T
YP
ICA
LLY B
Y A
DD
ING
AN
8th. BIT
TO
A 7-B
IT A
SC
II CH
AR
AC
TE
R.
HO
RIZ
ON
TA
L PA
RIT
Y IS
IMP
LEM
EN
TE
D
AS
A B
CC
(BLO
CK
CH
EC
K C
HA
RA
CT
ER
)A
PP
EN
DE
D T
O T
HE
EN
D O
F A
BLO
CK
OF
D
AT
A. D
AT
A B
LOC
KS
AR
E T
YP
ICA
LLYH
UN
DR
ED
S O
F C
HA
RA
CT
ER
S IN
LEN
GT
H.
Parity calculations involve defining the value of a "parity bit" such that the original data segm
ent plus parity bit w
ill always have an even (o
r odd w
hen u
sing o
dd p
arity) num
ber of 'one' bits in the now
larger data segment. E
ven parity is more com
mon than odd parity, and is illustrated here:
(60113)
Data C
omm
unications & Fieldbus S
ystems
156
CY
CLIC
RE
DU
ND
AN
CY
CH
EC
K (C
RC
) E
RR
OR
DE
TEC
TION
D-106
Cyclic R
edundancy error detection calculations involve doing a long division calculation on the data.
The remainder w
hich results from this calculation becom
es the block check data which -- is
transmitted along w
ith the desired information.
The receiving system repeats the identical long division calculation, and checks for consistency
between the received C
RC
check data and the results of its own calculation.
The mathem
atical basis of CR
C error detection is derived using polynom
ial algebra, where all binary
sequences are regarded as polynomials for purposes of analysis.
Exam
ple: 11010001
10011 110001100000
10011
-----
10111
10011
-----
01001
00000
-----
10010
10011
-----
00010
00000
-----
00100
00000
-----
01000
00000
-----
10000
10011
-----
0011
TH
E R
EM
AIN
DE
R, O
R F
RA
CT
ION
PA
RT
OF
T
HE
RE
SU
LT, B
EC
OM
ES
TH
E B
LOC
K
CH
EC
K S
EQ
UE
NC
E (E
QU
IVA
LEN
T T
O
PA
RIT
Y B
ITS
) US
ED
AT
TH
E R
EC
EIV
ING
S
ITE
TO
VA
LIDA
TE
TH
E D
AT
A.
TH
E Q
UO
TIE
NT
IS T
HE
INT
EG
ER
PO
RT
ION
O
F T
HE
RE
SU
LT -- T
HE
VA
LUE
OF
WH
ICH
ISO
F N
O P
AR
TIC
ULA
R S
IGN
IFIC
AN
CE
HE
RE
.
TH
E D
EF
INIT
ION
OF
AN
AP
PR
OP
RIA
TE
DIV
ISO
R P
OLY
NO
MIA
L IS A
BLA
CK
A
RT
; IT W
ILL ALW
AY
S H
AV
E O
NE
MO
RE
BIT
TH
AN
TH
E R
EM
AIN
DE
R.
US
ING
PO
LYN
OM
IAL A
LGE
BR
A N
OT
AT
ION
TH
IS 5-bit V
ALU
E W
OU
LD B
E E
XP
RE
SS
ED
A
S:
X + X
+ 1
TH
E IN
FO
RM
AT
ION
FIE
LD T
O B
E T
RA
NS
MIT
TE
DW
ILL BE
MU
CH
LON
GE
R T
HA
N T
HIS
12 BIT
E
XA
MP
LE (R
EA
L-WO
RLD
DA
TA
FIE
LDS
IN H
DLC
F
RA
ME
S A
RE
CO
MM
ON
LY 10,000 bits LO
NG
)
4
(61209)
Data C
omm
unications & Fieldbus S
ystems
157
CY
CLIC
RE
DU
ND
AN
CY C
HE
CK (C
RC
) E
RR
OR D
ET
EC
TIO
N, cont.
D-107
4
X + X
+ 1
X + X
+ X + 1
X + X
+ X + 1
X + X
+ X + X
+ X + X
+ X + X
+ X + X
+ X + X
+ X + X
+ 1
16 15 2
16 12 5
Som
e comparative C
RC polynom
ial values (used in different applications) are:
Preceding (hypothetical) exam
ple:
IBM
's EB
CD
IC B
ISY
NC
:
HD
LC, S
DLC
and ITU
-T:
Ethernet:
32 26 23 22 16 12 11 10 8 7 5 4 2
Cyclic redundancy error detection techniques have several key advantages:
CR
C system
s are simply im
plemented (typically in hardw
are on the I/O card).
CR
C error checking is the m
ost effective technique for any given error check field size (16 bit CR
C's
typically yield a billion-fold increase in undetetected error rate).
CR
C error detection has becom
e the industry de-facto standard error detection technique.
(30421)
Data C
omm
unications & Fieldbus S
ystems
158
ER
RO
R C
OR
RE
CTIO
N TE
CH
NIQ
UE
S
D-112
There are two basic error correction techniques (a
nd se
vera
l varia
tions o
f each
). These are Autom
atic R
epeat Request (A
RQ
) and Forward E
rror Correction (F
EC
).
Au
tom
atic
Re
pe
at R
eq
ue
st requires that the receiving device validate incom
ing data to determine if
there are any errors in the data:
If the received data appears to be correct, the receiving station will respond w
ith a positive acknow
ledgement m
essage -- sometim
es just an AC
K character.
If the data are found to be incorrect, the response will be a negative acknow
ledgement
(possib
ly just a
NA
K ch
ara
cter) rejecting the data as received.
The sending station thus becomes responsible for retransm
itting the flawed data -- potentially m
ore than once -- until it is received correctly.
Fo
rwa
rd E
rror C
orre
ctio
n systems operate w
ithout any feedback, or referral to the sending station.
Data are sent w
ith additional redundancy, such that the receiving station has sufficient information
about the message content to enable it to not only locate errors in data, but to m
ake corrections.
Since FE
C system
s are limited as to the num
ber of bits that can be reliably corrected, seriously degraded data channels w
ill result in too many errors for dependable handling, delivering an
unacceptable residual error rate.
(61209)
Data C
omm
unications & Fieldbus S
ystems
159
CO
MP
AR
ISO
N; F
EC vs. A
RQ
D-113
Autom
atic Repeat R
equest:
Autom
atic repeat request systems pass all data through a uniform
test of accuracy; thus no questionable data is ever accepted.
Since seriously degraded channel conditions w
ill result in an unusual frequency of error occurrences, system
performance w
ill be seen to deteriorate, due to excessive retransmission attem
pts. AR
Q
systems thus deliver assured quality, but offer uncontrolled data throughput and delay.
This is a favorable com
promise w
ith business systems, w
here we w
ould rather suffer a throughput and/or delay penalty than try to deal w
ith errored data.
Forward E
rror Correction:
Forw
ard error correction systems m
aintain a constant pace of data transmission that is insensitive to
prevailing channel conditions.
Since seriously degraded channel conditions w
ill result in an unusual frequency of error occurrences, system
performance w
ill deteriorate, due to excessive uncorrectable errors in the data.
FE
C system
s thus deliver a constant data throughput rate, but offer uncontrolled quality of delivered data -- an undesirable characteristic for business system
s.
For this reason, F
EC is typically delivered in conjunction w
ith AR
Q -- a hybrid approach.
(30421)
Data C
omm
unications & Fieldbus S
ystems
160
AU
TOM
ATIC
RE
PE
AT R
EQ
UE
ST
(AR
Q) E
RR
OR
RE
CO
VE
RY
D-108
Autom
atic Repeat R
equest (AR
Q) error recovery techniques are basic to business data
comm
unication systems.
Ha
lf-Du
ple
x P
roto
co
ls like BIS
YN
C invoke a sim
ple process, whereby A
CK
(ackn
ow
ledge) or N
AK
(n
egative
ackn
ow
ledge, o
r reje
ct) messages are anticipated in response to a transm
itted message.
This process is known as a S
top
an
d W
ait A
RQ
.
There is no ambiguity to this A
CK
/NA
K reference, because there is never m
ore than one message
being handled at any given time.
Fu
ll-Du
ple
x P
roto
co
ls invoke one of tw
o alternate AR
Q control techniques:
Go
Ba
ck
-N A
RQ
provides for the receiving station to pass advice back to the sending station identifying the m
essage number of the last correctly received m
essage.
The sending station is then responsible for retransmitting all m
essage segments that w
ere sent subsequent to that specific m
essage segment.
Se
lec
tive
AR
Q provides for the receiving station to save in buffer all m
essage segments --
specifically including those received subsequent to a flawed data segm
ent.
Thus, the sending station only needs to selectively re-send message segm
ents identified by the receiving station as flaw
ed.
(61209)
Data C
omm
unications & Fieldbus S
ystems
161
AU
TOM
ATIC
RE
PE
AT R
EQ
UE
ST
(AR
Q) E
RR
OR
RE
CO
VE
RY
, cont.
D-109
1
1
2
2
WA
ITW
AIT
DATA
DATA
ACK
WA
ITW
AIT
3
3W
AIT
WA
IT
DATA
NAK
3
3
DATA
ACK
4
4
DATA
ACK
ER
RO
R✽
WA
IT
WA
IT
WA
IT
WA
IT
WA
IT
ST
OP
AN
D W
AIT
AR
Q (H
AL
F D
UP
LE
X)
GO
BA
CK
"N" A
RQ
(FU
LL
DU
PL
EX
)
SE
LE
CT
IVE
AR
Q (F
UL
L D
UP
LE
X)
DATA
SE
ND
ING
S
TA
TIO
N
RE
CE
IVIN
G
ST
AT
ION
ACKACK
1
1
DATA
2
2
DATA
3
3
SE
ND
ING
S
TA
TIO
N
RE
CE
IVIN
G
ST
AT
ION
ACKNAK
45
63
3
7
7
8
8
1
1
2
2
34
52
2
6
6
7
7
8
8
1
1W
AIT
ER
RO
R✽
WA
IT
ACK
ER
RO
R✽
ACK
ACK
ACKACKNAK
ACKACKACKACK
45
6
ACK
1
1
DATA
2
2
DATA
3
3
SE
ND
ING
S
TA
TIO
N
RE
CE
IVIN
G
ST
AT
ION
ACKNAK
45
63
3
4
4
5
5
6
6
7
7
81
27
7
8
8
1
1
2
2
3
3W
AIT
ER
RO
R✽
WA
IT
ACK
ER
RO
R✽
WA
ITACK
ACK
ACKACKNAK
ACKACKACKACK
WA
IT
34
5ACK
ACK
ACK
ACK
ACK
ACK
(30311)
Data C
omm
unications & Fieldbus S
ystems
162
FOR
WA
RD
ER
RO
R C
OR
RE
CTIO
N
D-110
12
34
56
87
910
1112
1314
15
12
34
56
87
11 AC
TU
AL D
AT
A B
ITS
AR
E M
AP
PE
D IN
TO
16 B
EF
OR
E B
EIN
G T
RA
NS
MIT
TE
D, A
DD
ING
4 CH
EC
K
BIT
S (S
HO
WN
BE
LOW
IN G
RA
Y N
UM
ER
ALS
)
910
11
10
00
01
11
11
1
10
11
00
01
11
10
10
1
The four parity check bits (Bits 1
, 2, 4
and 8
) are defined using even parity calculations, involving a subset of the full sequence:
Bit N
o. 1 is calculated using Bits 3, 5, 7, 9, 11, 13 and 15 (i.e
. all o
dd
nu
mb
ere
d b
its -- or a
ll me
mb
ers o
f eve
ry se
con
d g
rou
p o
f on
e b
it).
Bit N
o. 2 is calculated using Bits 3, 6, 7, 10, 11, 14 and 15 (i.e
. all m
em
be
rs of e
very se
con
d g
rou
p o
f two
bits).
Bit N
o. 4 is calculated using Bits 5, 6, 7, 12, 13, 14 and 15 (i.e
. all m
em
be
rs of e
very se
con
d g
rou
p o
f fou
r bits).
Bit N
o. 8 is calculated using Bits 9, 10, 11, 12, 13, 14 and 15 (i.e
. all m
em
be
rs of e
very se
con
d g
rou
p o
f eig
ht b
its).
Following is an illustration of a H
amm
ing Code im
plementation. The H
amm
ing Code is useful for
illustration -- in practice, more pow
erful codes (e.g
. the R
eed-S
olo
mon co
des) w
ould be used:
DA
TA
AS
TR
AN
SM
ITT
ED
NO
W H
AS
4 E
XT
RA
BIT
S -- B
EIN
G R
ED
UN
DA
NT
IN
FO
RM
AT
ION
TO
BE
US
ED
TO
FIN
D
AN
D C
OR
RE
CT
ER
RO
RS
IN
TR
AN
SM
ISS
ION
.
(61208)
Data C
omm
unications & Fieldbus S
ystems
163
Assum
e there is an error in transmission, such that one bit is incorrect. A
ssume this is bit N
o. 9. To perform
the Ham
ming error correction calculation, w
e start by checking to see if the parity calculations (in
volvin
g b
it positio
ns 1
, 2, 4
and 8
) are still apparently correct.
There are four tests to perform -- designed to verify the correctness of each of these parity bits:
Bit N
o. 1 was defined w
ith bits 3, 5, 7, 9, 11, 13 and 15; This set no longer has even parity and this test fails.
Bit N
o. 2 was defined w
ith bits 3, 6, 7, 10, 11, 14 and 15; This set still has even parity and this test p
asses.B
it No. 4 w
as defined with bits 5, 6, 7 , 12, 13, 14 and 15; T
his set still has even parity and this test passes.
Bit N
o. 8 was defined w
ith bits 9, 10, 11, 12, 13, 14 and 15; This set no longer has even parity and the test fails.
We enter these test results into a 4-bit register, using a "0" value for each test that passes, and a "1"
for each failure. This resulting 1-0-0-1 bit pattern is the binary value of 9, which identifies the bit
position we arbitrarily flaw
ed.
Thus, we have the necessary inform
ation to correct this single bit error condition.
FOR
WA
RD
ER
RO
R C
OR
RE
CTIO
N
D-111
12
34
56
87
910
1112
1314
15
10
11
00
01
11
10
10
1
12
34
56
87
910
1112
1314
15
10
11
10
01
11
10
10
1
DA
TA
IS T
RA
NS
MIT
TE
D -- F
RO
M T
HE
S
EN
DIN
G S
TA
TIO
N
-- TO
--
TH
E R
EC
EIV
ING
ST
AT
ION
-- WIT
H A
N
ER
RO
R A
SS
UM
ED
IN B
IT P
OS
ITIO
N N
O. 9.
ER
RO
R✽
12
48
00
11
9(61208)
D
ata Com
munications &
Fieldbus System
s164
FO
RW
AR
D E
RR
OR C
OR
RE
CT
ION
AP
PLIC
AT
ION E
XA
MP
LER
OU
TE
R C
ON
NE
CT
ED LA
N S
YS
TE
MS V
IA S
AT
ELLIT
E LIN
K:
GE
OS
YN
CH
RO
NO
US S
AT
ELLIT
E
64 kbps DA
TA LIN
K T
O T
HE
AP
PLIC
AT
ION P
RO
CE
SS
FEC
TX
RX
RO
UT
ER
RO
UT
ER
D-035
TX
RX
FEC
TH
E E
LEC
TR
ON
IC P
AC
KA
GE IN
T
HE E
AR
TH S
TA
TIO
N T
ER
MIN
AL
CO
NT
AIN
S A T
RA
NS
MIT
TE
R (T
X),
RE
CE
IVE
R (R
X), A
ND F
OR
WA
RD
ER
RO
R C
OR
RE
CT
ING (F
EC
) C
OD
EC
, OR C
OD
ER
-DE
CO
DE
R.
128 kbps DA
TA
LINK
S O
N
SA
TE
LLITE R
AD
IO
CH
AN
NE
L
SE
CO
ND
AR
Y, O
R B
AC
KS
TO
P E
RR
OR
CO
NT
RO
L IS P
RO
VID
ED B
Y A
RQ LIN
K
PR
OT
OC
OLS
SU
PP
OR
TE
D B
Y T
HE
DA
TA T
ER
MIN
AL E
QU
IPM
EN
T
TH
E P
RIM
AR
Y E
RR
OR C
ON
TR
OL
ME
CH
AN
ISM IS
FE
C B
ET
WE
EN C
OD
EC
'S
WH
ICH A
RE P
AR
T OF T
HE
SA
TE
LLITE E
AR
TH S
TA
TIO
NS
.
(41102)
Data C
omm
unications & Fieldbus S
ystems
165
RIN
G A
RC
HITE
CTU
RE
LAN S
YS
TEM
BU
S A
RC
HITE
CTU
RE
LAN S
YS
TEM LA
N C
ON
FIG
UR
AT
ION C
ON
CE
PT
S
HU
B A
RC
HITE
CTU
RE
LAN S
YS
TEM
D-009
Both of the early leading
LAN architectures originally
promoted their ow
n unique cabling and connectivity m
odel
Both evolved early on to
operate in a hub-centric, U
TP architecture.
(99999)
Data C
omm
unications & Fieldbus S
ystems
166
RE
LATIO
NS
HIP -- IE
EE LA
N S
TAN
DA
RD
S
TO O
SI LA
YE
RE
D A
RC
HITE
CTU
RE D
EFIN
ITION
S
D-115
PHYSIC
AL LAYER
DATA LIN
K LAYER
NETW
OR
K LAYER
HIG
HE
R LA
YE
RS
IEEE 802.1 -- INTER
NETW
OR
KING
ALTHO
UG
H O
FFICIALLY C
OVER
ED BY TH
E SCO
PE O
F THE IEEE 802.1 M
AND
ATE, THE N
ETWO
RK AN
D
HIG
HER
LAYER FU
NC
TION
S OF LAN
SYSTEMS AR
E SU
PPOR
TED BY PR
OPR
IETARY N
ETWO
RK
OPER
ATING SYSTEM
S (NO
S's) SUC
H AS
NETW
ARE AN
D VIN
ES.
IEEE 802.2 -- LOG
ICAL LIN
K CO
NTR
OL
802.3 MAC
80
2.3
PH
YS
ICA
LC
SM
A/C
D
IEEE 802.1 -- SYSTEM MANAGEMENT
IEEE 802.1 -- OVERVIEW
NO
TE
: 'MA
C' R
EF
ER
S T
O T
HE M
ED
IUM A
CC
ES
S C
ON
TR
OL F
UN
CT
ION
, WH
ICH IS
R
EG
AR
DE
D A
S A S
UB
-LAY
ER IN
CO
NT
EX
T O
F IE
EE LA
N S
TA
ND
AR
DS
.
IEE
E LA
N S
TA
ND
AR
DS
OS
I ST
AN
DA
RD
S
80
2.1
1
PH
YS
ICA
LW
IRE
LE
SS
LA
N
80
2.1
5
PH
YS
ICA
LB
LU
ET
OO
TH
PA
N
80
2.1
6
PH
YS
ICA
LW
IRE
LE
SS
MA
N
802.11 MAC
802.15 MAC
802.16 MAC
(99999)
Data C
omm
unications & Fieldbus S
ystems
167
SIX T
EC
HN
ICA
L FE
AT
UR
ES
OF LO
CA
L AR
EA N
ET
WO
RK
S
2. AC
CE
SS C
ON
TRO
L
3. TOP
OLO
GY
4. HU
B TY
PE
5. SW
ITCH
ING TY
PE
6. SIG
NA
L TYP
E
D-010
1. ME
DIA TY
PE
Characteristics of com
munications m
edium affect system
capacity, quality, cost and ease of use.
Options include U
TP
, ST
P, coaxial cable, fiber optic cable and w
ireless.
Access control m
ethods (protocols) affect system perform
ance, dimension
constraints, cost and complexity.
Options include contention access m
ethods, such as ethernet's CS
MA
/CD
and controlled access methods, such as token passing m
echanisms.
Topology (or netw
ork shape) options can be viewed as either a physical
topology (such as the ethernet physical hub topology) or an electrical topology (such as the ethernet logical bus topology).
There is a scale of sophistication in hub options, ranging from
effectively passive first-generation hubs, through highly intelligent sw
itching hubs.
There w
as a time in the 1980's w
hen circuit switching technology w
as thought to be an option of interest but packet sw
itching is the only option at present.
Sim
ilarly, analog, broadband technology (using cable television practices) w
as once of interest but digital baseband is our only priority at this time.
(99999)
Data C
omm
unications & Fieldbus S
ystems
168
CO
NTE
NTIO
N B
AS
ED
MU
LTIPLE
AC
CE
SS
M
ETH
OD
S/P
RO
TOC
OLS
D-086
AL
OH
A
Each user transm
its whenever it has data to send, w
ithout any attempt at first determ
ining system
status (busy/id
le), or developing any physical layer view
of transmission success or failure.
Relies on upper layer functions to validate m
essage transmission success.
Works w
ell with lightly loaded system
s; theoretical maxim
um capacity about 18%
of channel capacity.V
ariant: SLO
TT
ED
ALO
HA
operation provides system-w
ide clocking to mark tim
e slots, ensuring either 100%
collision or 100% m
issed collisions (i.e. n
o p
artia
l messa
ge o
verla
ps).
CO
NC
EP
T: c
ollis
ion
s result from tw
o stations transmitting at the sam
e (or o
verla
ppin
g) tim
es, resulting in garbled transm
issions. C
AR
RIE
R S
EN
SE
MU
LT
IPL
E A
CC
ES
S
CS
MA
operation provides for a system status check prior to initiating transm
ission.T
hus, collisions will only occur as a result of effectively sim
ultaneous decisions to transmit.
Relies on higher layer functions to validate m
essage transmission success, as w
ith ALO
HA
.O
nce transmission is initiated, all transm
issions run to completion (i.e
. no d
ete
ction o
f collisio
ns)
means no ability to abort failed transm
issions.
(60506)
Data C
omm
unications & Fieldbus S
ystems
169
CO
NTE
NTIO
N B
AS
ED
MU
LTIPLE
AC
CE
SS
M
ETH
OD
S/P
RO
TOC
OLS
, cont.
D-087
CA
RR
IER
SE
NS
E M
UL
TIP
LE
AC
CE
SS
WIT
H C
OL
LIS
ION
DE
TE
CT
ION
Concept is sim
ilar to CS
MA
, except CS
MA
/CD
adds ability to monitor the progress and continuing
success of data transmission, and take rem
edial action when collisions occur.
Whenever collisions are detected (b
y each
of th
e tw
o co
nte
ndin
g sta
tions), they respond by first
sending a jamm
ing signal to ensure no attempt is m
ade to decode the garbled message content;
Secondly, each station starts a random
timer, w
hich defines the waiting tim
e before repeating the "listen-before-you-transm
it" CS
MA
logical process.A
ssuming contending stations select different random
timer intervals, tw
o stations should not encounter subsequent conflict.In the event of m
ultiple unsuccessful transmission attem
pts (due to
collisio
ns), the interval w
ithin w
hich the random tim
er is set is forced to grow w
ith each failure. It actually doubles w
ith every repeated collision event -- for up to 10 occurrences, and tolerates a further 6 collisions prior to giving up.
CA
RR
IER
SE
NS
E M
UL
TIP
LE
AC
CE
SS
WIT
H C
OL
LIS
ION
AV
OID
AN
CE
CS
MA
/CA
is similar to C
SM
A/C
D, except a pre-specified scheduling sequence is invoked every
time the channel goes idle, preventing sim
ultaneous attempts at transm
ission.C
SM
A/C
A is really a hybrid contention-controlled m
ode of operation due to this scheduling m
echanism.
(60506)
Data C
omm
unications & Fieldbus S
ystems
170
LAN
INTE
RC
ON
NE
CTIO
N A
LTER
NA
TIVE
S
D-145
LAN
ALA
N B
?
LAN
interconnection is motivated by a desire to:
Extend the physical dim
ensions, or range beyond the limits of a single LA
N, or
Expand the traffic carrying capacity beyond that of a single LA
N, or
Increase the manageability and/or apparent reliability by segm
enting the system into m
ore m
anageable pieces.
Equipm
ent options include:
Repeaters
Bridges
Routers
Gatew
ays
(50813)
Data C
omm
unications & Fieldbus S
ystems
171
LAN
INT
ER
CO
NN
EC
TIO
N
ALT
ER
NA
TIV
ES
Connectivity betw
een LAN
's is achieved with four different types of devices (a
nd so
me h
ybrid
so
lutio
ns w
hich
share
featu
res o
f these
four):
RE
PE
AT
ER
S
Repeaters provide a m
eans of extending a single, contiguous LAN
to distances that cannot be supported w
ithout such aid.
Repeaters sim
ply regenerate the electrical (or o
ptica
l, as th
e ca
se m
ay b
e) signals on the LA
N; they
do not interpret any portion of the message, and assert no discretion over forw
arding data (ie. a
ll packe
ts are
unco
nditio
nally fo
rward
ed).
Repeater connected LA
N's rem
ain a single logical LAN
(e.g
. limita
tions o
n th
e n
um
ber o
f addre
sses
supporte
d o
n a
LA
N a
re n
ot o
verco
me b
y a re
peate
r).
BR
IDG
ES
Bridges provide a w
ay of transparently internetworking tw
o logically distinct networks so as to yield
the perception of having a single, contiguous wider area, higher capacity LA
N than can be achieved
with repeaters.
D-134
(60113)
Data C
omm
unications & Fieldbus S
ystems
172
LAN
INT
ER
CO
NN
EC
TIO
N
ALT
ER
NA
TIV
ES
, cont.
D-135
BR
IDG
ES
, co
nt.
Bridges are ideally suited as a m
eans of interconnecting a small num
ber of departmental LA
N's
(perh
aps u
p to
3 o
r 4), particularly w
here the comm
unity of users are uniformly trustw
orthy (Brid
ges
pro
vide n
o a
dm
inistra
tive o
r secu
rity partitio
nin
g ca
pability).
Most bridges are self learning, in that they construct an address table w
hich associates the address of each device observed on each LA
N w
ith that LAN
. They then use this table to make forw
arding decisions -- such that only traffic addressed to a device on the alternate LA
N passes through the
bridge.
RO
UT
ER
S
Routers are suited to interconnecting a larger num
ber of departmental LA
N's than bridges. R
outers are m
ore complex devices than bridges, and m
ore easily integrated into an automated netw
ork m
anagement system
-- and can be configured to yield effective security partitions in a network
(refe
rred to
as "fire
walls" in
this m
ode o
f use
).
Routers perform
a more dem
anding role than bridges, and the resulting processing burden makes
routing a more com
putationally intensive task than bridging (acco
rdin
gly, ro
ute
rs are
typica
lly cu
stom
, multip
roce
ssor h
ard
ware
solu
tions w
here
as b
ridge fu
nctio
ns ca
n b
e su
pporte
d b
y a
conve
ntio
nal P
C).
(50813)
Data C
omm
unications & Fieldbus S
ystems
173
LAN IN
TE
RC
ON
NE
CT
ION
ALT
ER
NA
TIV
ES
, cont.
D-384
RO
UTE
RS
, cont.
Routers require a vision of the netw
ork topology -- a vision that can be developed in several ways
(most com
monly, routers exchange reachability inform
ation with each other using special route discovery
protocols, making all routers aw
are of all networks in a given system
).
Routers are protocol sensitive, in that routing decisions are m
ade on the basis of information contained in
the network layer protocol headers (such as IP
in TC
P/IP
and IPX in N
ovell's Netw
are). Thus, the ability
of a router to make appropriate decisions about forw
arding packets is dependent on routers being know
ledgeable about all network layer protocols operating in a given system
.
GA
TEW
AY
S
Gatew
ays are used to interconnect dissimilar (and otherw
ise incompatible) netw
orks, providing:
access to external proprietary systems, like IB
M's S
ystems N
etwork A
rchitecture (SN
A)
access to the global Internetaccess to general W
AN applications accessible via public X
.25 or frame relay netw
orks.
Gatew
ays are often application-specific (such as e-mail gatew
ays that manage e-m
ail services, or e-com
merce gatew
ays that arbitrate e-comm
erce traffic for a server farm).
(21106)
D
ata Com
munications &
Fieldbus System
s174
DE
LIVE
RIN
G D
ATA
US
ING
FLAT A
ND
HIE
RA
RC
HIC
AL A
DD
RE
SS
ES
D-442
Hierarchical addressing m
akes it possible to route a letter to an arbitrary destination by analyzing the address from
the bottom up:
The first sort decision determines the destination country
The second step determines the destination city etc.
Only the last postal sorting step requires know
ledge of the final destination.
Routers use this m
odel; intermediate routers interpret
network-coded addresses hierarchically, w
ith only the last router required to identify individual end-devices.
Susan Henderson
12345 East Berryhill C
ourtPhiladelphia, PA 34567U
.S.A.
WA
JAX
Forks Pty.
6784 Alberhill B
oulevardT
iddlybank, New
South W
alesA
ustralia
The first-step postal sorting
process has to be able to identify w
hich of a possible 200 (a
pp
roxim
ate
ly) destination countries has been specified.
With a flat address system
, every sorting clerk w
ould have to know
the location of every possible recipient -- som
e 6,000,000,000 possibilities!
Susan Henderson
WA
JAX
Forks P
ty.Flat addresses are superficially sim
pler -- but they require unreasonable detailed know
ledge of all possible destinations w
hen there is a large population of users.
Bridges w
ork with this address m
odel, requiring each bridge to m
aintain exhaustive address tables containing location inform
ation for all possible destinations.
This w
orks well for sm
all networks, but is not a
reasonable approach for complex configurations.
(60607)
Data C
omm
unications & Fieldbus S
ystems
175
D-582
EV
OLU
TION
IN LA
N-to-LA
NC
ON
NE
CTIV
ITY S
TRA
TEG
IES
When an organization initially deploys LA
N services, the obvious first configuration is a free-standing
(non-co
nnecte
d) system
.
As traffic volum
es and/or the physical dimensions of the service area increase, a strategy for defining a
connectivity-based infrastructure becomes necessary.
Directly C
on
nected
LA
N's
Where a sm
all number of LA
N's (p
robably
departm
enta
l in sca
le a
t this p
oin
t) are in use, direct connectivity is a reasonable strategy -- typically using bridges as the preferred entry-level connectivity devices.
Once m
ore than two D
LAN
s (Departm
enta
l LA
Ns)
are involved, it becomes inevitable that there w
ill be through-traffic adding potential congestion problem
s to intermediate netw
orks -- much like
vehicle traffic adds congestion to an urban district w
here there is no through highway available.
The maxim
um num
ber of LAN
s suitable to this configuration is sm
all (perh
aps 4
or 5
) and will
depend on traffic, types of applications and physical dim
ensions of the site.
DLA
N #1
DLA
N #2
DLA
N #3
DLA
N #4
RE
PR
ES
EN
TA
TIV
E D
IRE
CT
CO
NN
EC
T S
ER
VIC
E C
ON
FIG
UR
AT
ION
(61207)
Data C
omm
unications & Fieldbus S
ystems
176
D-583
EV
OLU
TION
IN LA
N-to-LA
NC
ON
NE
CTIV
ITY S
TRA
TEG
IES
, cont.B
ackbo
ne C
on
nected
LA
Ns
With a backbone architecture, a purpose built netw
ork segment is added, w
ith the dedicated mission of
providing connectivity services to the other networks.
Backbone capacity typically exceeds that of
the individual networks it serves (p
erh
aps th
e
most co
mm
on co
nfig
ura
tion is a
100 M
bps
FD
DI b
ackb
one syste
m se
rving a
cluste
r of
10 M
bps e
thern
et d
epartm
enta
l LA
Ns).
The physical dimensions of the backbone
system are highly variable, ranging from
a few
feet (colla
pse
d b
ackb
one lim
ited to
an
equip
ment ra
ck) to tens of miles/kilom
eters (M
AN
-structu
red b
ackb
one).
Backbone netw
orks should have no directly connected w
orkstations or servers (all su
ch
applica
tion-o
riente
d e
quip
ment sh
ould
be o
n
one o
f the D
LA
N se
gm
ents).
Possible backbone-connected resources
would include routers and/or firew
alls to external netw
orks and network m
anagement
automation resources.
DLA
N #1
DLA
N #2
DLA
N #3
DLA
N #4
BA
CK
BO
NE
LAN
R
EA
CH
ES
TO
ALL
DLA
N N
ET
WO
RK
S(C
onnections are by local bridges/routers)
RE
PR
ES
EN
TA
TIV
E B
AC
KB
ON
ES
ER
VIC
E C
ON
FIG
UR
AT
ION
(61207)
Data C
omm
unications & Fieldbus S
ystems
177
D-213
FIBE
R D
ISTR
IBU
TED
DA
TA IN
TER
FAC
E
The Fiber Distributed D
ata Interface (FD
DI) w
as the first recognized high speed LAN
(HS
LA
N) and
metropolitan area netw
ork (MA
N) technology -- developed in 1990 and accepted by A
NS
I as a standard.
Key features of FD
DI are:
100 Mbps dual ring, token passing fiber optic technology
Fault-tolerant, capable of sustaining full operational throughput through any arbitrary single-point failure (d
evice
failu
re o
f cable
bre
ak).
Com
patible with IE
EE
802-Series LA
N standards at 802.2 LLC
sub-layerO
perates on multi-m
ode and single-mode fiber -- or copper tw
isted pair (UT
P) cable.
Maxim
um ring circum
ference allowed = 200 km
.M
aximum
number of attachm
ents = 1000S
upports two classes of stations: dual-attachm
ent and single-attachm
ent (Dual-a
ttach
ment co
nfig
ura
tions h
ave
fu
ll fault-to
lera
nt fe
atu
res).
Applications: cam
pus backbones, data center bus systems,
metropolitan area netw
orks and high-speed desktop applications.
FFOL
(FD
DI F
ollo
w O
n L
AN
) is a planned upward com
patible extension projected to operate at 2.4 G
bps.P
HY
SIC
AL M
ED
IUM
DE
PE
ND
EN
T (P
MD
)
PH
YS
ICA
L L
AY
ER
PR
OT
OC
OL (P
HY
)
ME
DIA
AC
CE
SS
CO
NT
RO
L (M
AC
)
FD
DI L
OW
ER
LA
YE
R S
TR
UC
TU
RE
OS
I L
INK
LA
YE
R
OS
I P
HY
SIC
AL
LA
YE
R
IEE
E 8
02
.2 L
OG
ICA
L
LIN
K C
ON
TR
OL (L
LC
)
(61024)
Data C
omm
unications & Fieldbus S
ystems
178
FD
DI M
AC
LAY
ER
FR
AM
E F
OR
MA
T
D-667
PR
EA
MB
LES
TAR
TD
ELIM
ITER
FR
AM
EC
ON
TRO
LD
ESTINATIO
NAD
DR
ESSS
OU
RC
EA
DD
RE
SS
INFO
RM
ATIO
N P
AY
LOA
DF
RA
ME
CH
EC
KE
ND
DE
LIMITE
RF
RA
ME
ST
AT
US
FRA
ME
FEA
TUR
ES
:
Pream
ble:C
onsists of alternating 1-0 bits for clock synchronization characters -- a m
aximum
of 16 octets
Marks the start of the fram
e, much
like the flag field with H
DLC
Designates the fram
e type being carried (U
LP
identifica
tion) and
designates token/message status
Either a 48 bit U
niversally A
dministered address field, or a 16
bit Locally Adm
inistered address (n
ot typ
ically u
sed
)
Like Destination A
ddress, either a U
niversally or Locally Adm
inistered address
Start D
elimiter:
Fram
e Control:
Destination
Address:
Payload contents (th
ree
com
mo
n
form
ats a
re u
sed
):
IEE
E/IS
O com
patible LLC fram
esS
NA
P-encoded general U
LP data
(such
as IP
da
tag
ram
)IE
EE
/ISO
MA
C layer fram
e encapsulated
CR
C error check
Com
panion to Start D
elimiter
Delivery confirm
ation (simila
r to
IEE
E 8
02
.5)
Information
Payload
Source
Address:
Fram
e Check:
Fram
e Status:
End D
elimiter:
(11206)
Data C
omm
unications & Fieldbus S
ystems
179
D-374
DA
S
DA
S
DA
SD
AS
PR
IMA
RY
RIN
G
SE
CO
ND
AR
Y R
ING
Devices w
hich populate the FD
DI P
rimary R
ing will alw
ays be Dual A
ttachment S
tations (or DA
S) --
supporting full access to both fiber rings. As w
ill be seen later, a non-redundant class of termination
known as a S
ingle Attachm
ent Station (S
AS
) has a useful role in making non-critical connections to
the core ring shown here.
The P
RIM
AR
Y R
ING
operates at 100 Mbps, and alw
ays carries active message traffic
The S
EC
ON
DA
RY
RIN
G operates at 100 M
bps also, and functions in one of two m
odes:
it may be designated as a hot-standby back-up to the prim
ary ring, idling while the system
operates norm
ally -- probably for about 99.999 percent of the time (typ
ical)
FDD
I CO
NF
IGU
RA
TIO
N C
ON
CE
PT
it may carry preem
ptable message traffic, w
hich gets dumped w
hen called upon to back-up the prim
ary ring (incre
asin
gly w
idely u
sed)
DA
SD
AS
(11206)
Data C
omm
unications & Fieldbus S
ystems
180
D-375
FA
ILU
RE
SC
EN
AR
IO 1:
LOS
S O
F N
ET
WO
RK
-CO
NN
EC
TE
D
DE
VIC
E (D
AS
)
✄
FA
ILU
RE
SC
EN
AR
IO 2:
LOS
S O
F C
AB
LE C
ON
TIN
UIT
Y
The F
DD
I dual ring system w
rap
s primary ring traffic onto the secondary ring -- at the nearest
surviving dual attachment station (D
AS
) connection point, thus maintaining operation through any
single failure event.
WR
AP
PO
INT
; Prim
ary ringconnects to secondary ring
FD
DI FA
ILUR
E S
CE
NA
RIO
S
DA
S
DA
S
DA
SD
AS
DA
SD
AS
DA
S
DA
S
DA
SD
AS
DA
SD
AS
(11206)
Data C
omm
unications & Fieldbus S
ystems
181
Illustrated below are tw
o virtual LAN system
s configured on a three-hub physical LAN
.
The key concept is that there no longer needs to be an association betw
een the physical network
configuration and the functional configuration.
VIR
TU
AL LO
CA
L AR
EA N
ET
WO
RK
S -
CO
NC
EP
T ILLUS
TR
AT
ION
K-094
VIR
TU
AL N
ET
WO
RK N
O. 1
SC
AD
A N
ET
WO
RK
(IP A
DD
RE
SS 155.77.10.0)
VIR
TU
AL N
ET
WO
RK N
O. 2
DA
TA W
AR
EH
OU
SIN
G S
YS
TE
M(IP
AD
DR
ES
S 155.77.12.0)
RO
UT
ER
TO P
LAN
T
(41103)
D
ata Com
munications &
Fieldbus System
s182
LAN
PE
RFO
RM
AN
CE
ISS
UE
S
D-232
LAN
systems perform
ance can be limited -- or bottlenecked -- by any of a num
ber of components:
The first tem
ptation is to suspect the physical cable system is out of capacity -- w
hich is typically not the problem
. How
ever, a related problem m
ay be a factor:
Violation of cable length and repeater placem
ent rules (with
eith
er E
thern
et o
r Toke
n R
ing) can
result in excessive numbers of failed transm
ission attempts, w
ith too much tim
e unproductively consum
ed with retransm
issions.
Server perform
ance is the most com
mon system
bottleneck: hard disk access time, R
AM
capacity and configuration, N
IC throughput lim
itations, server cpu type and clock speed are all perform
ance-related features.
To a lesser extent, workstation perform
ance will be a factor, w
ith the two m
ost significant being w
orkstation cpu type, RA
M capacity and clock speed and N
IC characteristics.
System
diagnostic software and/or test equipm
ent can be used to identify these choke points -- or bottleneck conditions.
Conversely -- people responsible for m
aking LAN
systems upgrade decisions should be cautious
about spending sums of m
oney to upgrade components of the system
that may not be the
performance lim
iting assets.
(10420)
Data C
omm
unications & Fieldbus S
ystems
183
Day Three
►S
CA
DA
and PLC
´s
►M
OD
BU
S
►H
AR
T
►A
SI
Data C
omm
unications & Fieldbus S
ystems
184
TELE
PH
ON
EM
OD
EM
LINE
DR
IVE
RLIN
ED
RIV
ER
WIR
ELE
SS
LINK
WIR
ED
LINK
S
TELE
PH
ON
EM
OD
EM
RE
MO
TE
TE
RM
INA
LU
NIT
�������
MTU
CO
MP
UT
ER
SY
ST
EM
INS
TR
UM
EN
TIN
TE
RF
AC
ES
PR
OT
OT
YP
E S
CA
DA S
YS
TE
M
CO
MM
UN
ICA
TIO
NS
EQ
UIP
ME
NT
INT
ER
FA
CE
S
RE
MO
TE
TE
RM
INA
LU
NIT
RE
MO
TE
TE
RM
INA
LU
NIT
The netw
ork infrastructure, com
prising LAN
, W
AN and other
components.
The M
aster Term
inal U
nit (MT
U) consists
of at least three elem
ents:
MT
U com
puterC
omm
unications interfacesH
uman M
achine Interface (H
MI)
The R
emote
Term
inal Units
(RT
Us) and
instrument
interfaces.
HM
I
K-047
(30818)
D
ata Com
munications &
Fieldbus System
s185
RE
MO
TE
TE
RM
INA
LU
NIT
RE
MO
TE
TE
RM
INA
LU
NIT
RE
MO
TE
TE
RM
INA
LU
NIT
INS
TR
UM
EN
TIN
TE
RF
AC
ES
PR
OT
OT
YP
E S
CA
DA S
YS
TE
M
The netw
ork infrastructure, com
prising LAN
, W
AN and other
components.
The M
aster Term
inal U
nit (MT
U) consists
of at least three elem
ents:
MT
U com
puterC
omm
unications interfacesH
uman M
achine Interface (H
MI)
The R
emote
Term
inal Units
(RT
Us) and
instrument
interfaces.
K-049
MTU
CO
MP
UT
ER
SY
ST
EM
CO
MM
UN
ICA
TIO
NS
EQ
UIP
ME
NT
INT
ER
FA
CE
S
HM
I
TELE
PH
ON
EM
OD
EM
TELE
PH
ON
EM
OD
EM
LINE
DR
IVE
RLIN
ED
RIV
ER
WIR
ELE
SS
LINK
WIR
ED
LINK
S
(30818)
Data C
omm
unications & Fieldbus S
ystems
186
K-050
continued ....
ELE
ME
NT
S O
F BA
SIC S
CA
DA S
YS
TE
MS
The three basic elem
ents of SC
AD
A technology are:
Master Term
inal Unit (M
TU)
The M
TU has tw
o key functions:
It controls all remote data collection functions and data flow
s through the system.
It supports the Hum
an-Machine Interface (H
MI) -- also com
monly referred to as the
Man-M
achine Interface (MM
I).
These tasks require the intelligence and inform
ation managem
ent capabilities of a com
puter -- thus MT
Us are alw
ays based on one or more com
puter system(s) w
ith most or
all of these features:
embedded hard disk system
sextensive com
munications I/O
capabilityone or m
ore major color screen displays or projectors
real-time, interrupt-driven operating system
selected redundant elements -- (e.g. uninterruptible, or U
PS pow
er)
(40308)
D
ata Com
munications &
Fieldbus System
s187
K-051
ELE
ME
NT
S O
F BA
SIC S
CA
DA S
YS
TE
MS
, cont.
continued ....
Rem
ote Terminal U
nits (RTU
s)
RT
Us act as agents for the M
TU in field locations w
here the SC
AD
A system
has a need to collect data and/or distribute control signals.
There are typically m
any RT
Us per S
CA
DA system
-- sometim
es hundreds.
The tw
o key functions of the RT
U are:
To interface w
ith the network and support the necessary com
munication protocols and
functions -- enabling the transfer of data and comm
ands to and from the M
TU
To interface w
ith the local instrumentation, collecting or inputting digital status and
values of analog variables -- and performing the inverse function of actuating discrete
(binary) and proportional (analog) controllers.
Historically, R
TU
s were built w
ith discrete logic electronic (i.e. non-programm
able) com
ponents, but the incredible cost and functionality advantage of programm
able devices has resulted in R
TU
s being universally based on microprocessor-driven com
ponents.
The stored-program
is typically kept in some type of read-only m
emory (R
OM
) -- thus the "softw
are" for these systems m
ay be more appropriately term
ed "firmw
are".
(40308)
Data C
omm
unications & Fieldbus S
ystems
188
ELE
ME
NT
S O
F BA
SIC S
CA
DA S
YS
TE
MS
, cont.
K-052
A C
onnecting Netw
ork Infrastructure
The netw
ork infrastructure that connects the MT
U w
ith the associated group of RT
Us is the
most variable and difficult-to-sum
marize part of S
CA
DA system
s:
The scale of distance involved can vary from
a few feet/m
eters to half way around the
world.
The capacity requirem
ents (i.e. bits per second data rate) vary from sim
ple systems that
operate at a few hundred bits per second to high-capacity, tim
e-critical applications that require m
illions of bits per second of network capacity.
Reliability requirem
ents vary from applications w
here the information is "nice to have"
(but with no m
ajor penalty associated with failure) to life- and/or m
ission-critical applications that sim
ply cannot be allowed to fail.
Som
e SC
AD
A system
s operate entirely within an urban setting (e.g. typical m
unicipal w
ater systems), w
ith the result that it is feasible to build a reasonable network
infrastructure based on rented telephone company facilities.
Other system
s (e.g. some pipelines) operate in w
ilderness areas where there is no
telecomm
unications infrastructure, and where the capital cost of creating that
infrastructure will have to be borne by the S
CA
DA project.
(30818)
D
ata Com
munications &
Fieldbus System
s189
K-087
SE
LEC
T BE
FO
RE O
PE
RA
TE vs.
OP
ER
AT
E W
ITH A
CK
NO
WLE
DG
E
The operation of S
CA
DA control procedures m
ay be set up in about three different ways:
Select B
efore Operate (S
ometim
es called "Confirm
Before O
perate")
High value, critical operations (typically those w
hose incorrect operation could result in facilities dam
age or financial loss) are typically handled with this tw
o-step procedure.
The S
CA
DA operator sets up a control com
mand, and essentially arm
s the RT
U, ready to activate
the designated comm
and.
The R
TU then sends a confirm
ing message back to the operator, effectively asking for confirm
ation that this action is w
hat is really intended.
Direct O
perate With A
cknowledgem
ent
This procedure provides a sim
pler framew
ork, with a designated control procedure being
implem
ented at the RT
U, no questions asked. A
n acknowledgm
ent of the action follows.
Direct O
perate Without A
cknowledgem
ent
Very uncom
mon -- this procedure sim
ply performs the designated action w
ith no confirmation.
(41031)
Data C
omm
unications & Fieldbus S
ystems
190
OB
JEC
T-O
RIE
NT
ED P
RO
GR
AM
MIN
G C
ON
CE
PT
S
The central difference betw
een Object-O
riented Program
ming (O
OP
) and more traditional
programm
ing methods is the abstract notion of an object -- a class of data representation that
enables a broad range of entities to be considered in a comm
on class.
A good exam
ple of objects and the range of entities that can be included in the description is visible in the fam
iliar Window
s screen, where files, printers and executable program
s are all represented by sim
ilar on-screen icons.
Probably the m
ost important uniqueness of object-oriented data representation and program
ming is
the property of inheritance -- where features and detailed data structures of prim
itive objects (called the base class) are accum
ulated or aggregated into higher-level structures called derived classes.
This enables sets of prim
itive objects to be defined (somew
hat similar to the w
ay low-level
subroutines can be defined) and then higher level, derived classes of objects can inherit the cum
ulative features of the included base class objects (again somew
hat similar to the w
ay higher level program
ming procedures can sim
ply access the features of lower level subroutines).
This inheritance m
odel is open-ended; A inherits from
B, B
inherits from C
, C inherits from
D ...
Object-oriented program
s are significantly less procedural than conventional programm
ing structures, although ultim
ately there is a sequential or procedural component to their execution.
K-064
(40306)
D
ata Com
munications &
Fieldbus System
s191
K-065
AN O
BJE
CT
-OR
IEN
TE
D V
IEW O
FIN
DU
ST
RIA
L DA
TA S
OU
RC
ES A
ND P
RO
CE
SS
ES
Traditionally, representing the different elem
ents of data that are accessible from, or are delivered
to remote equipm
ent is massive, tedious and fraught w
ith human error.
Even m
ore troublesome, it is typically a very com
plex challenge to add a new class of data (such
as is provided by a new field instrum
ent), increase the significant digits of data resolution or change the update or refresh rate on selected data elem
ents.
These kinds of configuration changes have historically required specialist-level skills to im
plement,
tend to be highly platform/vendor dependent and poorly scalable.
The generality of object-oriented data representation enables:
The convenient database design and subsequent editing of detail-level data elem
ents (such as m
ay be required when replacing a field instrum
ent).
Accom
modation of new
requirements that em
erge after the systems are com
missioned, such as
new H
MI features and display screens or new
process control models.
Com
patibility with m
ajor trends in the IT comm
unity that embrace object-oriented m
ethodologies, and associated data m
anagement, graphical hum
an interfaces and web-based inform
ation access.
(40307)
Data C
omm
unications & Fieldbus S
ystems
192
K-066
MT
U P
LAT
FO
RM R
ED
UN
DA
NC
Y
continued ....
When planning for high levels of availability in S
CA
DA and related industrial com
puting im
plementations, one alw
ays encounters the question of master station redundancy -- "D
o we w
ant it and how
do we achieve it? "
Cold S
tandby/Warm
Standby
These are redundancy, or fault-tolerance m
odes that typically require operator intervention to enable transfer of control by sw
itching or swapping out com
ponents or complete units, w
ith significant gaps in availability. T
hat is, the functionality of systems w
ill be unavailable until a manual
restoration is completed.
This w
ill typically involve transferring power and/or com
munications circuits and initializing system
softw
are (which m
eans data associated with w
ork in progress is lost).
Hot S
tandby
This is a class of fault-tolerance that provides a pow
ered, operational replacement system
ready and w
aiting for service.
Transfer of operational responsibility to the standby system
may be autom
atic or manual. A
s was
the case in the previous example, w
ork in progress will be disrupted -- but w
ith a much sm
aller w
indow of dow
n time here.
(40307)
D
ata Com
munications &
Fieldbus System
s193
K-067
MT
U P
LAT
FO
RM R
ED
UN
DA
NC
Y, cont.
Full Dynam
ic Redundancy
There are a num
ber of techniques for implem
enting a hands-free dynamic transfer of operational
control from one M
TU to a back-up m
achine -- with varying degrees of confidence, cost, com
plexity, and duration of "dow
n time" during the transfer.
Hew
lett Packard's N
on-Stop E
nterprise Division produces the "S
-Series" line of fully redundant
fault-tolerant computer system
s that are the most robust, fail-safe system
s in the market (a
certainty because they have no competition!).
These system
s (originally developed by Tandem
Com
puters in the late 1970's and acquired by HP
with their purchase of C
OM
PA
Q) are designed to be fully fault-tolerant in every respect -- no single
hardware or softw
are failure will interrupt service.
The technology supports standard "m
iddleware" developm
ent technologies like Tuxedo and C
orba -- and is strongly oriented tow
ard web service applications.
In most cases, it is too costly as a solution for S
CA
DA applications, but it w
ould deliver extraordinary availability if used as an M
TU host m
achine.
There are a num
ber of less ambitious fault-tolerant equipm
ent configurations based on Unix and
Microsoft W
indows 2000 technologies -- but none w
ith a "no single point of failure architecture".
(40308)
Data C
omm
unications & Fieldbus S
ystems
194
D-333
SE
RV
ER D
ISK A
RR
AY S
YS
TE
MS
Fault-tolerant disk arrays enable on-line replacem
ent of failed disk hardware, and seam
less reconstruction of data -- transparent to the user
Data is broken into pieces and w
ritten in duplicate to at least two of several disks, using a
process called 'striping'
The m
ost successful of these schemes is the five-level R
edundant Arrays of Inexpensive D
isk (R
AID
) techmnology, w
hich has gained acceptance through MIcrosoft's support:
RA
ID-1: U
ses mirrored disks to provide one-on-one data redundancy; recovery requires
disrupting service.
RA
ID-2: U
ses a more efficient data representation, requiring less than 100%
duplication of data volum
e; not comm
only used.
RA
ID-3 and R
AID
-4: Use disk guarding, w
hich provides data striping on two or m
ore data drives, plus a dedicated error recovery (guard) drive w
hich carries comprehensive error
correcting code information for the data drives. R
AID
-3 writes stripes in bytes; R
AID
-4 writes
in blocks and supports concurrent read and write activities.
RA
ID-5: U
ses distributed data guarding, eliminating the dedicated guard drive -- resulting in all
drives having equal status, and supporting multiple sim
ultaneous reads and writes.
(99999)
D
ata Com
munications &
Fieldbus System
s195
SE
RIA
LC
OM
MU
NIC
AT
ION
PO
RT CP
U
ME
MO
RY
TE
RM
INA
L ST
RIP F
OR
CO
NN
EC
TIN
G O
UT
PU
TS
TE
RM
INA
L ST
RIP F
OR
CO
NN
EC
TIN
G IN
PU
TS
INP
UT S
IGN
AL
INT
ER
FA
CE
OU
TU
T SIG
NA
LIN
TE
RF
AC
E
RT
U B
LOC
K D
IAG
RA
M
MO
DE
M O
R
OT
HE
RC
OM
MU
NIC
AT
ION
SD
EV
ICE
NO
TE T
HIS D
IAG
RA
M IS
R
EM
AR
KA
BLY
SIM
ILAR
TO A P
LC B
LOC
K
DIA
GR
AM
-- TH
US T
HE
CO
MM
ON
INT
ER
CH
AN
GE
AB
LE
RO
LES
.
K-060
TW
O-W
AY E
XT
ER
NA
LC
OM
MU
NIC
AT
ION
(31011)
Data C
omm
unications & Fieldbus S
ystems
196
K-041
CO
NT
RO
LS A
RE IM
PLE
ME
NT
ED A
S E
ITH
ER
CLO
SE
D LO
OP O
R O
PE
N LO
OP S
YS
TE
MS
CO
NT
RO
LLER
AC
TU
AT
OR
PLA
NT O
RP
RO
CE
SS
CO
NT
RO
LS
IGN
AL
AC
TU
AT
ING
SIG
NA
L
PLA
NT
/P
RO
CE
SS
OU
TP
UT
+
-
OP
EN
-LOO
P C
ON
TRO
L
HU
MA
N
OP
ER
AT
OR
SE
NS
OR
INP
UT
SO
PE
RA
TO
RC
ON
TR
OLS
INIT
IAL
INP
UT
continued ....
Open Loop S
ystems
An open loop process is a one-w
ay process -- input control signals pass through a logical sequence, or process and result in m
odifying the behavior of the plant or process.
Where S
CA
DA system
s are used, information is collected from
the output and presented to a hum
an operator who w
ill assess output information and possibly im
plement changes to input
conditions. There is no autom
atic feedback from the output back to input.
Data C
omm
unications & Fieldbus S
ystems
197
continued ....
CO
NT
RO
LS A
RE IM
PLE
ME
NT
ED A
S E
ITH
ER
CLO
SE
D LO
OP O
R O
PE
N LO
OP S
YS
TE
MS
, cont.
K-042
Open Loop S
ystems, cont.
A good exam
ple of an open loop control system is the fuel control m
echanism used in an
automobile.
The desired input is defined by the position of the accelerator pedal; the actuator is the throttle
valve and the process output is a modified engine operating speed.
Hum
an observations will typically result in an assessm
ent of the suitability of a given engine speed and the operator w
ill change the accelerator pedal position accordingly.
SC
AD
A system
s almost alw
ays operate as open loop control processes -- with the process
control operator performing a function analogous to the autom
obile driver above.
The lack of any feedback m
echanism for allow
ing output conditions to automatically influence
input signals puts some constraints on the process:
The system
response/operating dynamics m
ust be compatible w
ith human reaction tim
es.
Hum
an vigilance is required to ensure safe, continued operation.
Data C
omm
unications & Fieldbus S
ystems
198
continued ....
CO
NT
RO
LLER
AC
TU
AT
OR
PLA
NT O
RP
RO
CE
SS
CO
NT
RO
LS
IGN
AL
AC
TU
AT
ING
SIG
NA
LD
ES
IRE
DIN
PU
T
K-043
CLO
SE
D-LO
OP C
ON
TRO
L
SE
NS
OR
PLA
NT
/P
RO
CE
SS
OU
TP
UT
+
-
Closed Loop S
ystems
Closed loop control system
s take advantage of process output information, and derive a signal
(using some kind of sensor) that is passed back to the system
input -- resulting in the autom
atic adjustment of the controlled process.
Closed loop control system
s have the potential for being unstable in some or all-operating
conditions -- and must be carefully designed to operate reliably under all conditions.
CO
NT
RO
LS A
RE IM
PLE
ME
NT
ED A
S E
ITH
ER
CLO
SE
D LO
OP O
R O
PE
N LO
OP S
YS
TE
MS
, cont.
(30615)
Data C
omm
unications & Fieldbus S
ystems
199
K-044
JAM
ES W
ATT'S
FLY-B
ALL G
OV
ER
NO
R
CO
NT
RO
LS A
RE IM
PLE
ME
NT
ED A
S E
ITH
ER
CLO
SE
D LO
OP O
R O
PE
N LO
OP S
YS
TE
MS
, cont.
Closed Loop S
ystems, cont.
Exam
ples of closed loop control systems include:
An autom
obile "cruise-control" system for
regulating throttle position based on the speed of the vehicle.
A household furnace or air conditioner that uses
a simple on-off sw
itching thermostat to activate
the required temperature control system
.
James W
att's fly-ball governor, (see accom
panying illustration) which w
as one of the first closed loop control system
s.
In each of these examples, a desired input value
(set point) is modified by inform
ation returned via a feedback link, enabling the input instructions to the control process to be m
odified or adjusted so as to m
ore exactly conform to the desired input value, or
set point.
Data C
omm
unications & Fieldbus S
ystems
200
K-028
RE
AL-T
IME
OP
ER
AT
ION
S
Industrial computer system
s are typically required to service their "clients" -- robots, instruments,
device actuators, controllers, etc. -- in real-time, m
eaning that results/outputs must be assured of
being available while the input conditions are still valid.
There are num
erous alternative definitions of real-time com
puting, but all interpretations of the concept em
phasize a guarantee of timeliness of results, w
here the meaning of "tim
eliness" depends on application-specific details.
Exam
ples of real-time com
puting applications include:
a servo control sub-system in an aircraft fly-by-w
ire system that m
ust respond quickly enough to m
odify flight attitude to assure timely reaction to changing flight circum
stances
a factory production system that involves a robot arm
picking something off a conveyor belt --
in which delayed response from
the arm w
ould mean the object had m
oved beyond recovery reach
In both cases, the computational result m
ust be complete in a tim
e-frame that w
ill assure timely
accomplishm
ent of the overall system's operating objectives.
The achievem
ent of effective real-time com
puting is more than a com
puter hardware/softw
are problem
, since it includes data recovery delays, network transit delays, output delays and
actuator operation delays.
Data C
omm
unications & Fieldbus S
ystems
201
K-029
FE
AT
UR
ES
OF
RE
AL-T
IME
OP
ER
AT
ING
SY
ST
EM
SR
eal-Tim
e Operating S
ystems (R
TO
S) have distinctly different features and requirem
ents, com
pared with typical business applications.
Sum
mary features that are necessary to support tim
e-critical, event-driven industrial computing
applications include:
multi-thread operation - alternatively know
n as reentrancy, is the ability to support multiple
concurrent processes with a single m
emory-resident version of key softw
are modules.
preemptible tasking - capability of designating tim
e-critical processes as top priority, and therefore eligible to be scheduled for im
mediate execution.
interrupt driven (asyn
chro
nous sch
edulin
g) - capable of responding to external events that
trigger hardware-level interrupts (to
norm
al, o
therw
ise se
quentia
l pro
cessin
g)
controlled, short intervals with interrupts disabled - processes w
hich normally operate w
ith interrupts enabled m
ust disable interrupts while context sw
itching activities are under way
(and w
hile
certa
in d
iscrete
pro
cesse
s opera
te) -- but m
ust assure a rigorous bound on the m
aximum
time interval that w
ill be encountered with interrupts disabled.
The unique data handling processes associated w
ith real time applications are frequently w
ell served by a tw
o-stage architecture, where the front-end process is supported by a R
TO
S (a
nd
desig
ned to
be le
an a
nd sim
ple
), feeding a conventional scheduled environment operated on a
backend process.
Data C
omm
unications & Fieldbus S
ystems
202
K-039
RE
LAY B
AS
ICS & C
ON
CE
PT
S
BA
SIC R
ELA
Y FE
ATU
RE
S
BA
TT
ER
YN
/ON
/C
SW
ITC
H
CO
NT
RO
LC
IRC
UIT
There w
as a time w
hen industrial control systems w
ere entirely based on the use of electrom
agnetic relays to sense electrical operating conditions and activate control functions.
Those days are gone -- but the design processes used to im
plement relay-based logic are still
important in the developm
ent, implem
entation and use of programm
able logic controllers (PLC
s).
The essential feature of electrom
agnetic relays is the way in w
hich the magnetic field that results
from a controlled current flow
actuating a switched electrical contact for a different circuit.
In the diagram here, w
ith no current flowing in the
control circuit, the resting position of the relay contacts is dow
n, making contact betw
een the bottom
pair of contact points.
The resting position of a relay w
ith no current flow
ing in its magnetic coil is the "N
ormal" position,
so contacts that are closed in this condition are "N
ormally C
losed" or N/C -- and contacts that are
open in this condition are "Norm
ally Open", or N
/O.
When the sw
itch is closed, and current flows in the
relay coil, these contact conditions reverse -- but the "N
ormal" definition rem
ains as before.
(30616)
Data C
omm
unications & Fieldbus S
ystems
203
K-040
IN T
HE B
EG
INN
ING P
RO
CE
SS
ES W
ER
EC
ON
TR
OLLE
D W
ITH R
ELA
YS
The first autom
ation solutions for process control and related applications were based on
electro-magnetic relays -- m
any, many of them
.
In order to manage the m
yriad of details required to operate complex processes w
ith this class of device, a disciplined, system
atic way of representing relay control inputs and outputs w
as developed -- the result of w
hich is the "ladder diagram"
The first P
LC im
plementations w
ere designed as functional replacements for relay-based
control systems -- and w
ere supported by technicians already familiar w
ith the logical processes of designing, im
plementing, testing and m
aintaining relay-based systems.
For this reason, P
LCs inherited the sam
e logical system for representing their operation as the
technology they replaced.
Thus, the "ladder diagram
" lives on as a design concept that is imbedded in the program
ming
tools that are used to develop modern P
LC instructions, or program
s.
Of course, num
erous extensions to the original relay-equivalent ladder diagram program
ming
features have emerged, including com
munications support, hum
an interface functions and tim
er-based process sequencing to name a few
.
Data C
omm
unications & Fieldbus S
ystems
204
K-053
LAD
DE
R D
IAG
RA
MS
The first process and industrial control autom
ation solutions were based on
electro-magnetic relays -- typically m
any of them.
In order to manage the am
ount of detail involved in managing com
plex control processes w
ith this class of device, it was useful for the industry to develop a system
atic, disciplined w
ay of diagramm
ing and working w
ith relay controls -- particularly with regard to their
various inputs and outputs -- the result of which is the "ladder diagram
"
The first P
LC im
plementations w
ere designed as functional replacements for relay-based
control systems, and w
ere supported by technicians already familiar w
ith the logical processes of designing, im
plementing, testing and m
aintaining relay-based systems.
For this reason, P
LCs inherited the sam
e logical system for representing their operation as
the relay-based technology they replaced.
Thus, the "ladder diagram
" lives on as a design concept that is imbedded in the
programm
ing tools that are used to develop modern P
LC instructions, or program
s.
Of course, num
erous extensions to the original relay-equivalent ladder diagram
programm
ing features have emerged, including com
munications support, hum
an interface functions and tim
er-based process sequencing to name a few
.
(30906)
Data C
omm
unications & Fieldbus S
ystems
205
BA
TT
ER
YC
OIL
SW
2S
W1
K-059
LAD
DE
R D
IAG
RA
M C
ON
CE
PT
S
INP
UT
S
SW
1S
W2
OU
TP
UT
CO
IL1
EN
D
VE
RY S
IMP
LE R
ELA
Y C
IRC
UIT A
ND
CO
RR
ES
PO
ND
ING LA
DD
ER D
IAG
RA
M
Elem
ents of a simple Ladder D
iagram are illustrated
in the box on the right.
SW
1 and SW
2 are switches; S
W1 is norm
ally open (N
O) and S
W2 is norm
ally closed (NC
).
The battery positive rail is the left-side vertical line;
the battery negative rail is the right-side vertical line.
Sequential logic (w
hich is remarkably sim
ple here) flow
s top-to-bottom -- either looping, or term
inating on the "E
ND
" comm
and.
Each logical input and output is assigned a m
emory
address value so that operations can reference the T
RU
E or F
ALS
E condition of each elem
ent.
Elem
ents are considered to be FA
LSE w
hen in their norm
al, or resting state, and TR
UE w
hen activated.
Thus, N
O inputs are F
ALS
E w
hen open; NC inputs
are TR
UE w
hen open.
Outputs are energized only w
hen all input conditions are T
RU
E.
(31011)
Data C
omm
unications & Fieldbus S
ystems
206
FU
NC
TIO
N B
LOC
K IN
ST
RU
CT
ION
S
K-057
The basic P
LC ladder diagram
programm
ing approach is satisfactory for doing simple
things, but becomes unw
ieldy with m
ore complex tasks.
Exam
ples include:
Program
ming a shift register or a push-pop stack
Configuring a com
munication m
odule (serial or network card)
continued ....
Tasks of this sort could be program
med w
ith combinations of standard, sim
ple instructions - but the function block w
as introduced in order to make these m
ore complex tasks easier.
Just as with an arithm
etic rung in a ladder diagram, a function block rung has three parts:
Input conditions consisting of combinations of exam
ine instructions. There w
ill typically be several input conditions that are "exam
ined", or used to drive the function block.
The actual function block, w
hich will often depend on a num
ber of locations - or variables -- that w
ill affect its action.
Outputs, w
hich behave like relay contacts, are the means by w
hich the function block interacts w
ith other ladder rungs in the system.
(31011)
Data C
omm
unications & Fieldbus S
ystems
207
FU
NC
TIO
N B
LOC
K IN
ST
RU
CT
ION
S, cont.
K-058
Like inputs, there are comm
only several outputs:
Num
erical outputs are comm
only used by other rungs (function blocks) as inputs to their arithm
etic instructions.
Relay coil (bit-level, or binary) outputs
can be "examined", or read by exam
ine instructions in other functions.
There has been a clear trend tow
ard P
LC m
anufacturers converting timing,
counting, and arithmetic instructions to
the more user-friendly function block
format.
Program
ming docum
entation available for any particular P
LC w
ill explain the program
ming of the various available
function block instructions in detail.
INP
UT
S
SW
1S
W2
FU
NC
TIO
NB
LOC
K
SW
3
SW
4
OU
TP
UT
CO
IL1
OU
TP
UT
CO
IL2
EN
D
FUN
CTIO
N B
LOC
K U
SE
D IN
A
SIM
PLE
LAD
DE
R D
IAG
RA
M
(31011)
Data C
omm
unications & Fieldbus S
ystems
208
K-024
PR
OG
RA
MM
AB
LE LO
GIC
CO
NT
RO
LLER
S
The P
rogramm
able Logic Controller originated in the late 1960's as a discrete electronic logic
system intended to replace hardw
ired relay control systems.
Relay system
s have been designed using a so-called ladder diagram, or ladder logic -- effectively
a Boolean logic schem
e where relay features (su
ch a
s make
-befo
re-b
reak a
nd la
tchin
g
opera
tion) are central to the logical operation of the system
.
With P
LCs, these relay features are m
odeled and embedded in the program
mable
characteristics of the controller.
Although the program
ming interface to P
LCs typically focuses on the logical relay-based
equivalent, the PLC
is really a computer w
ith computer-like com
ponents, including:
Central P
rocessing Unit (C
PU
)M
emory U
nitInput InterfacesO
utput Interfaces
The P
LC has been adapted to countless applications, m
any well beyond the original scope
intended by the original concept -- but the core application remains local control of com
plex m
achines based on Boolean logic conditions (su
ch a
s tank le
vels, m
oto
r or p
um
p sta
tus,
pre
ssure
conditio
ns, p
ositio
n o
f mova
ble
parts, e
tc.)
(11206)
Data C
omm
unications & Fieldbus S
ystems
209
K-054
PLC
BLO
CK D
IAG
RA
M���
SE
RIA
LC
OM
MU
NIC
AT
ION
PO
RT CP
U
ME
MO
RY
TE
RM
INA
L ST
RIP F
OR
CO
NN
EC
TIN
G O
UT
PU
TS
TE
RM
INA
L ST
RIP F
OR
CO
NN
EC
TIN
G IN
PU
TS
PC S
ER
VE
S A
S H
UM
AN
INT
ER
FA
CE F
OR B
OT
H
PR
OG
RA
MM
ING
AC
CE
SS A
ND A
S A
N
OP
ER
AT
ION
AL H
MI/M
MI
INP
UT S
IGN
AL
INT
ER
FA
CE
OU
TU
T SIG
NA
LIN
TE
RF
AC
E
Data C
omm
unications & Fieldbus S
ystems
210
K-026
OP
PO
RT
UN
ITIE
S A
ND
AD
VA
NT
AG
ES
OF
PLC
s
The P
LC has proven to be a rem
arkable efficiency tool to industry, offering the following key
advantages over precursor electro-mechanical hardw
are solutions:
Superior flexibility to accom
modate various applications
Superior reliability (m
ain
tenance
is radica
lly simplifie
d)
Superior econom
y (cost to
insta
ll and o
pera
te/m
ain
tain
)Investm
ent is recyclable (a so
ftware
change ch
anges th
e e
ntire
system
)D
evelopment effort is portable (o
ne so
ftware
deve
lopm
ent e
ffort ca
n se
rve m
any in
stalla
tions)
Other features that have no direct com
parison to hardware relay logic solutions includes:
PLC
s can be applied to applications where critical response tim
es must be guaranteed
PLC
s can support networked com
munication w
ith peers and/or master control system
sP
LCs can support graphical, user-friendly hum
an interface sub-systems
PLC
s can support a wide range of sophisticated instrum
ents, creating a systems control
capability that is incomparable to precursor technology.
The range of P
LC product size, sophistication, capacity and cost is rem
arkable; PLC
s can be cost-effectively used in applications w
here only a handful of relays might otherw
ise be used, all the w
ay to big, complex, netw
orked multi-m
illion dollar process control applications.
With this range of applications and scale of capacity available, the understanding the technology
and capabilities of PLC
systems is a technical specialty in its ow
n right.
(11206)
Data C
omm
unications & Fieldbus S
ystems
211
K-025
CR
OS
S-S
EC
TIO
N O
F P
LC A
PP
LICA
TIO
NS
The P
rogramm
able Logic Controller has been adapted to m
any industrial automation
applications -- but most are applied to factory autom
ation (FA
) systems, and m
ost are stand-alone configurations operating independently of other control system
s.
A sam
pler of PLC
applications includes:
Injection molding
Warehouse autom
ationP
ump (w
ate
r, chem
icals, fo
od p
roducts) control
Rolling m
illsIndustrial m
ixing and blending applicationsP
roduction line testingB
ottling and filling applicationsM
otor controlsP
aper machine controllers
Speed control (su
ch a
s ele
vato
rs, esca
lato
rs and co
nve
yor syste
ms)
Cutting, drilling, w
elding and stamping applications
Rem
ote Term
inal Unit (R
TU
) interfaces to SC
AD
A system
sP
etrochemical distillation/cracking/blending/m
ixing applicationsF
low control system
s (aggre
gate
s, liquid
s, discre
te ite
ms)
Data C
omm
unications & Fieldbus S
ystems
212
K-027
PR
OG
RA
MM
AB
LE LO
GIC C
ON
TR
OLLE
RS
-- PA
RT
IAL V
EN
DO
R LIS
T
A sam
pler of the better known P
LC vendors includes:
(ww
w.abb.com
)(w
ww
.rockwell.com
)(w
ww
.aromat.com
/pgh.htm)
(ww
w.controlm
icrosystems.com
)(w
ww
.cutler-hamm
er.eaton.com)
(ww
w.gefanuceur.com
)(w
ww
.hitachi-ds.com)
(ww
w.idec.com
)(w
ww
.keyence.com)
(ww
w.global.m
itsubishielectric.com)
(ww
w.om
ron.com)
(ww
w.schneider-electric.com
)(w
ww
.seprol.com)
(ww
w.sea.siem
ens.com/controls/)
(ww
w.tic.toshiba.com
)
AB
BA
llen Bradley (R
ockwell A
utomation)
Arom
at C
ontrol Microsystem
s E
aton / Cutler-H
amm
er G
E Fanuc (G
eneral Electric)
Hitachi
IDE
CK
eyenceM
itsubishiO
mron
Schneider E
lectric (Modicon)
Seprol
Siem
ensToshiba
(30615)
Data C
omm
unications & Fieldbus S
ystems
213
K-017
The technical feasibility of using m
icroprocessor-based intelligence as an integral part of field instrum
ents opens many doors of opportunity for enhancing the operational effectiveness of
industrial networked control system
s.
With this opportunity com
es the inevitable requirement that a different m
eans of com
municating w
ith this class of instrumentation is necessary -- m
ore consistent with the
functions of business local area networks.
From
this need and opportunity the fieldbus concept was born.
Unfortunately, like m
any good ideas, the fieldbus concept has not lived up to industry expectations because of a lack of standards and industry consensus about the m
yriad of technical details that m
ust be coordinated in actually making it all happen.
This problem
is especially acute in the industrial context because:
The m
any vendors involved have a vested interest in promoting their proprietary solutions,
which lock custom
ers into their complete system
s solution.
Reliability and safety priorities discourage experim
entation and end-user innovation
The industry does not have a history of creating system
s from plug-and-play m
odular com
ponents -- systems integration is a com
plex, specialists domain.
Industrial practice and preferred vendors vary from country-to-country, creating regional
centers of influence.
FIE
LDB
US
-- SY
ST
EM
CO
NC
EP
TS
Data C
omm
unications & Fieldbus S
ystems
214
K-018
There are at least 10 alternative fieldbus technologies m
aking claim to a "standard" industrial
network solution. O
f these, the following are m
ost credible:
Pro
fibu
s DP
/PA
■ Netw
ork technology similar to IE
EE
802.4 Token B
us■ S
tandardized as a Germ
an National S
tandard (D
IN 1
9245, P
art 3
/4), and is a E
uropean "interim
fieldbus" standard CE
NE
LEC
50170.■ S
iemens and other G
erman industry suppliers have been principal advocates.
■ Prim
ary/initial application is networking P
LC equipm
ent■ P
rofibus DP
is a 12 Mbps technology, P
rofibus PA
operates at 31.35 kbps.
LO
Nw
orks
■ Features a non-determ
inistic network technology (u
ses C
SM
A/C
D a
ccess te
chniq
ue like
E
thern
et)
■ LON
works is not a form
al standard technology, although it is an openly published technical specification and coordinated by A
SH
RA
E of B
AC
net.■ A
major application area is w
ith control applications in intelligent buildings; (HV
AC
, fire-safety, security, elevator controls, etc.), although m
any applications in general fieldbus use (p
etro
chem
icals, w
ate
r contro
l, food p
roce
ssing a
nd e
nerg
y managem
ent)
have been successfully developed.■ LO
Nw
orks networks operate at 1.25 M
bps full duplex over various media.
FIE
LDB
US
- ALT
ER
NA
TIV
E "S
TA
ND
AR
DS
"
(11206)
Data C
omm
unications & Fieldbus S
ystems
215
K-019
Wo
rldF
IP
■ Works w
ith a twisted pair or fiber optic bus architecture netw
ork■ W
orldFIP
is a European standard technology (o
rigin
ally d
eve
loped a
nd sp
onso
red in
F
rance
) ■ W
orldFIP
technology has been very influential in the creation of Foundation F
ieldbus standards -- being the source technology m
ost closely adopted.■ F
IP is a F
rench National S
tandard, and is one of four European C
EN
ELE
C 50170
"interim fieldbus" standards.
■ FIP
is based on a 31.25 kbps, 1 Mbps, 2.5 M
bps or 6 Mbps peer-to-peer netw
ork infrastructure.
IEC
/ISA
SP
50 Field
bu
s
■ The IE
C/IS
A S
P50 F
ieldbus standardization initiative is an attempt to pull together m
uch of the divergent architecture of the various "fieldbusses".
■ Part of w
hat makes this initiative challenging is the vested interest groups; several
national interest groups (nota
bly P
rofib
us a
nd W
orld
FIP
) and groups of equipment
manufacturers, none of w
hich have much m
otivation to simplify the user's situation.
■ The design features of the S
P50 initiative are intended to be flexible, w
ith a view to
accomm
odating as many of the other technology features as possible.
■ The S
P50 F
ieldbus supports a 31.25 kbps service, as well as 1, 2.6 and 5 M
bps mode.
of operation.
FIE
LDB
US
- ALT
ER
NA
TIV
E "S
TA
ND
AR
DS
", cont.
Data C
omm
unications & Fieldbus S
ystems
216
K-020
Fo
un
datio
n F
ieldb
us
■ Foundation F
ieldbus is the result of an industry-sponsored initiative to unify the diverse segm
ents of the fieldbus industry. ■ T
he Foundation F
ieldbus technology development and standards are coordinated w
ith the IE
C/IS
A S
P50 initiative, although there is no com
mitm
ent to synchronizing the two
standards tracks.■ T
here are two categories of com
munication services defined (a
s with
seve
ral o
f the
oth
er fie
ldbusse
s) :● T
he H1 Instrum
ent Bus - oriented tow
ard servicing field instruments
■ operates at 31.25 kbps.■ 1.9 km
maxim
um cable length
● The H
2 System
s Netw
ork Bus - oriented tow
ard supporting networked connectivity
between P
LCs, D
CS
systems, hum
an interface subsystems, etc.
■ operates at 1.0 or 2.5 Mbps.
■ 750 m m
aximum
cable length
■ Intrinsically safe operation is supported in both bus configurations.■ B
oth configurations operate over (nom
inally sh
ield
ed ) tw
isted pair cable media.
■ Both configurations support bus-pow
ered instruments, although the bus pow
ered feature lim
its the number of devices that can be connected per fieldbus segm
ent (and re
stricts th
e H
2 se
rvice to
1.0
Mbps).
■ The standard is m
uch more than a netw
ork technology; it includes a so-called User
Layer set of features that facilitate coordination of measurem
ent units and point descriptions (b
ase
d o
n a
pro
cedure
and d
ata
repre
senta
tion m
eth
od re
ferre
d to
as
functio
n b
locks).
FIE
LDB
US
- ALT
ER
NA
TIV
E "S
TA
ND
AR
DS
", cont.
(11206)
Data C
omm
unications & Fieldbus S
ystems
217
K-021
FO
UN
DA
TIO
N F
IELD
BU
S --
CO
NC
EP
TU
AL C
ON
FIG
UR
AT
ION
BR
IDG
E
H2 (M
egab
it rate) Bus
1.0 or 2.5 Mbps operation
Tw
isted pair or fiber optic cableS
upports redundant mode
Bus pow
ered devices supported750 m
. maxim
um length @
1 Mbps
Intrinsically safe at 1.0 Mbps
H1 (K
ilob
it rate) Bus
31.25 kbps operationT
wisted pair or fiber optic
cableIntrinsically safe operationB
us powered devices supported
1,900 m. m
aximum
length
CO
NT
RO
LLER
Data C
omm
unications & Fieldbus S
ystems
218
K-045
CO
MP
AR
ISO
N A
ND U
SA
GE O
F CO
MM
ON
IND
US
TR
IAL N
ET
WO
RK
S
NE
TWO
RK
AR
CN
et
AS
I
CA
NO
pen
ControlN
et
DeviceN
et
Ethernet
FoundationF
ieldbus
Interbus
LonWorks
Modbus
Profibus D
P/P
A
LEV
EL
Enterprise
Device
Device
Control
Device
Enterprise
Control
Device
Device
Control/D
evice
Control/D
evice
ME
DIA
Flat or U
TP
UT
P
Coax, F
iber
UT
P
UT
P, F
iber
UT
P
UT
P, F
iber
UT
P, F
iber,P
ower line
UT
P
UT
P, F
iber
UT
P, C
oax,F
iber
MA
X R
ATE
10 Mbps
167 Kbps
1 Mbps
5 Mbps
500 Kbps
10 Gbps
100 Mbps
500 Kbps
1.25 Mbps
38.4 Kbps
12 Mbps
MA
X D
IST.
100 m
10 Km
1 Km
30 Km
500 m
2 Km
1.9 Km
400 m
2 Km
500 m
24 Km
MA
X N
OD
ES
255
31
127
9964
1024
240
256
32000
250
127
US
A U
SA
GE
9%2%3%
15%
22%
73%
3%7%4%
29%
8%
INT'L U
SA
GE
12%
9%
12%
14%
17%
50%
7%7%6%
22%
26%
NO
TE
: Usage data taken from
a 1999 survey by Control E
ngineering apparently indicating the percentage of organizations actually using each technology (i.e. installed base at that tim
e)
(30616)
Data C
omm
unications & Fieldbus S
ystems
219
K-031
TH
E M
OD
BU
S /A
RC
HIT
EC
TU
RE
PR
OT
OC
OL
Modbus is a nom
inally proprietary comm
unications architecture/protocol initially developed as part of the M
odicon brand of PLC
s and attached equipment.
Modbus is a w
idely used industrial comm
unications environment that specifies both:
an equipment configuration (e
ither a
bus o
r star to
polo
gy)
a comm
unications protocol ( a m
aste
r-slave
pollin
g p
roto
col)
The m
aster-slave architecture of Modbus supports only one m
aster device per bus segment,
consistent with a single (n
on-re
dundant) P
LC interacting w
ith a number of slave devices.
Applications w
hich require interaction with higher-level system
s (such
as H
um
an M
ach
ine
Inte
rface
s [HM
Is] or p
ossib
ly SC
AD
A syste
ms) are often configured redundantly -- requiring
duplicate Modbus netw
orks to support redundant operations.
A new
er version of Modbus (M
odbus P
lus) is a m
ore sophisticated architecture/protocol, but is not nearly as w
idely accepted as an industry standard.
Modbus P
lus is a token-passing, distributed control network system
-- not remarkably different
from the com
mon token passing LA
N system
s used in business comm
unications applications.
A key feature of this approach is it offers peer-to-peer capability, in distinct contrast to the
master/slave com
munication in basic M
odbus.
Data C
omm
unications & Fieldbus S
ystems
220
K-032
TH
E M
OD
BU
S N
ET
WO
RK
TO
PO
LOG
IES
The tw
o comm
on Modbus netw
ork configurations are bus and star topology -- and several variations w
ithin each. Typical im
plementations are suggested in the accom
panying sketches.
MA
ST
ER
DE
VIC
EB
US
TO
PO
LO
GY
MO
DE
M
MO
DE
M
SLA
VE
DE
VIC
E
MO
DE
M
SLA
VE
DE
VIC
E
CO
NF
IGU
RA
TIO
N S
HO
WN
SU
GG
ES
TS
T
ELE
PH
ON
E C
ON
NE
CT
IVIT
Y. 4-W
AY
4-W
IRE
BR
IDG
ES
WO
ULD
SU
PP
OR
T A
LO
CA
L HA
RD
-WIR
ED
CO
NF
IGU
RA
TIO
N
MA
ST
ER
DE
VIC
E
ST
AR
TO
PO
LO
GY
DIG
ITA
LB
RID
GE
SLA
VE
DE
VIC
E
SLA
VE
DE
VIC
E
SLA
VE
DE
VIC
E
DIG
ITA
L BR
IDG
E IS
AN
AC
TIV
E
ELE
CT
RO
NIC
RS
-232 (typica
lly) B
RO
AD
CA
ST
OR
RE
PE
AT
ER
UN
IT
(11206)
Data C
omm
unications & Fieldbus S
ystems
221
K-033
TH
E M
OD
BU
S P
RO
TO
CO
L
The M
odbus protocol is a master-slave polling regim
e, where sim
ple polling instructions are issued to addressed destination devices, and a corresponding response is received.
Typical M
odbus master station polling m
essage format is illustrated in the accom
panying box.
The key to flexibility w
ith the Modbus protocol is the approxim
ately 21 standard function codes (p
lus 2
00-o
dd re
serve
d/sp
ecia
l use
codes).
The polling m
essage example suggested in the accom
panying box consists of five fields:
Slave A
ddress (05) -- allows up to 247 slave addresses per bus segm
ent.
Function C
ode (03) -- designates the functional purpose of this m
essage (se
e co
de su
mm
ary n
ext
page)
Read at S
tart Register
(00F1) designates the
register number at w
hich data recovery is to begin
SLA
VE
AD
DR
ES
SF
UN
CT
ION
CO
DE
RE
AD
A
T S
TA
RT
R
EG
IST
ER
NU
MB
ER
OF
R
EG
IST
ER
ST
O R
EA
D
ER
RO
RC
HE
CK
CO
DE
TY
PIC
AL M
OD
BU
S P
OLLIN
G M
ES
SA
GE
FIE
LD
DE
SC
RIP
TIO
NS
EX
AM
PLE
FIE
LD
CO
NT
EN
TS
(HE
X F
OR
MA
T)
0503
00F1
0002E
3
Data C
omm
unications & Fieldbus S
ystems
222
K-034
TH
E M
OD
BU
S P
RO
TO
CO
L, cont.
Modbus protocol fields, cont.
Num
ber of registers to Read (0002) -- specifies the num
ber of data register contents to be recovered in this transaction
Error C
heck Code (E
3) -- data integrity calculation used to validate received data in preceding m
essage.
A sam
pler of Modbus function codes is given in the accom
panying box.
Data is alw
ays sent as an asynchronous character, and will be encoded either as:
AS
CII representations of H
EX
characters, orB
inary codes (8-b
it byte
s, or o
ctets)
010203040506070809
RE
AD
CO
IL ST
AT
US
RE
AD
INP
UT
ST
AT
US
RE
AD
HO
LDIN
G R
EG
IST
ER
SR
EA
D IN
PU
T R
EG
IST
ER
SF
OR
CE
SIN
GLE
CO
ILR
ES
ET
SIN
GLE
RE
GIS
TE
RR
EA
D E
XC
EP
TIO
N S
TA
TU
SLO
OP
BA
CK
DIA
GN
OS
TIC
TE
ST
PR
OG
RA
M
101114151617192021
PO
LL PR
OG
RA
M C
OM
PLE
TE
FE
TC
H E
VE
NT
CO
UN
TE
RP
OLL P
RO
GR
AM
CO
MP
LET
EF
OR
CE
MU
LTIP
LE C
OILS
PR
ES
ET
MU
LTIP
LE R
EG
IST
ER
SR
EP
OR
T S
LAV
E ID
RE
SE
T C
OM
MU
NIC
AT
ION
S LIN
KR
EA
D G
EN
ER
AL R
EF
ER
EN
CE
WR
ITE
GE
NE
RA
L RE
FE
RE
NC
E
SA
MP
LE
R O
F M
OD
BU
S F
UN
CT
ION
CO
DE
S
(11206)
Data C
omm
unications & Fieldbus S
ystems
223
K-061
ET
HE
RN
ET
/IP
Ethernet/IP
is an emerging standard specification for open architecture industrial
networking that is confusing in its first encounter:
The "IP
" part of the specification does not denote Internet Protocol as one m
ight anticipate, but rather Industrial P
rotocol -- Thus, it is E
thernet Industrial Protocol.
Having said that, E
thernet/IP is based on a T
CP over Internet P
rotocol framew
ork.
It is a whole m
ulti-layer protocol specification with m
uch more broad im
plications than sim
ply adapting Ethernet to the industrial environm
ent -- although that is an included feature of the technology.
The m
ost important feature of E
thernet/IP is not the adaptation of IT
-oriented lower layer
technologies like Ethernet and T
CP
/IP but the unique A
pplication Layer, which is the
industrially oriented Control and Inform
ation Protocol (C
IP).
CIP provides a set of real-tim
e I/O - and peer-to-peer m
essaging services that supports the M
TU in its com
munication w
ith both the RT
U and w
ith the HM
I.
CIP is an application-layer protocol that w
as in place prior to Ethernet/IP
developments. It
was created initially to provide a sim
ilar set of application-layer messaging services for
ControlN
et and DeviceN
et.
(40308)
D
ata Com
munications &
Fieldbus System
s224
K-062
CIP LA
YE
RE
D A
RC
HIT
EC
TU
RE V
IEW
INTE
RN
ET P
RO
TOC
OL
TCP
UD
P
IEE
E 802.3 E
THE
RN
ET - LLC
& M
AC LA
YE
RS
IEE
E 802.3 E
THE
RN
ET - P
HY
SIC
AL LA
YE
R
CO
NTR
OL-
NE
TTR
AN
S-
PO
RT
DE
VIC
E-
NE
TTR
AN
S-
PO
RT
CTD
MA
CA
NC
SM
A/N
BA
PH
YS
ICA
LP
HY
SIC
AL
CIP M
ES
SA
GE R
OU
TING & C
ON
NE
CTIO
N M
AN
AG
EM
EN
T
CIP D
ATA
MA
NA
GE
ME
NT S
ER
VIC
ES (E
xplicit Messages &
I/O M
essages)
CIP A
PP
LICA
TION LA
YE
R & A
PP
LICA
TION O
BJE
CT LIB
RA
RY
VA
RIO
US D
EV
ICE P
RO
FILES (S
EN
SO
RS
, CO
NTR
OLS
, etc.)
CONTROL AND INFORMATIONPROTOCOL
EN
CA
PS
ULA
TION
(40306)
Data C
omm
unications & Fieldbus S
ystems
225
K-063
DE
VIC
EN
ET
'S C
SM
A/N
BA P
RO
TO
CO
L
DeviceN
et is an industry-standard, open specification device-level network that has been designed
with a high priority on real-tim
e control applications.
A key feature of this class of application is the need for a highly predictable, or determ
inistic network
delivery subsystem.
Ethernet shares the C
arrier Sense M
ultiple Access (C
SM
A) feature of this protocol, w
hich really m
eans that devices respect active traffic on the network, and w
ait for a gap in the traffic before starting to transm
it, much like the w
ay vehicular traffic yields the right-of-way.
Ethernet tolerates the inevitable collisions that occur in this process by detecting them
, accepting the destruction of data, and queuing the traffic for later retransm
ission -- which results in
troublesome delays for real-tim
e, or delay-sensitive traffic.
The C
ontrol Area N
etwork (C
AN
) over which D
eviceNet operates is based on a unique variation on
this idea -- instead of detecting and accepting collisions, it uses what they call C
arrier Sense
Multiple A
ccess with N
on-destructive Bitw
ise Arbitration (C
SM
A/N
BA
).
CS
MA
/NB
A em
ploys a dynamic arbitration m
echanism, such that w
hen a collision is detected, one of the colliding sending devices is allow
ed to continue to completion, rather than w
aiting for a later clear opportunity.
This difference m
akes the CS
MA
/NB
A access protocol m
uch more determ
inistic and thus suitable to tim
e-critical applications.
(40306)
Data C
omm
unications & Fieldbus S
ystems
226
MO
DB
US
PR
OTO
CO
L
►D
efines a message structure that
controllers will recognize and use
►Independent to the type of netw
orks (R
S-232, RS-422/485, Ethernet, etc)
►D
escribes the process a controller uses to :o
request access to another device, o
respond to requests from the other
devices, o
detects and reports errors.►
Master-slave arrangem
ent
►Establishes a com
mon form
at for the layout and contents of m
essage fields
Data C
omm
unications & Fieldbus S
ystems
227
MO
DB
US
TRA
NS
AC
TION
►D
ata exchange between devices is called
“transaction”►
Only the m
aster can initiate transactions (“query”)
►The slave respond by supplying the requested data to the m
aster, or by taking the action requested in the query.
►The m
aster can address individual slaves, or can initiate a broadcast m
essage to all slaves. ►
Maxim
um 246 slaves can be addressed (1-247)
►Slaves return a m
essage (called a ‘response’) to queries that are addressed to them
individually. ►
Responses are not returned to broadcast
queries from the m
aster.
Data C
omm
unications & Fieldbus S
ystems
228
ME
SS
AG
E FO
RM
AT
►STA
RT
ostarting delim
iter of a message
►A
DD
RESS
oaddress of destination devices
oaddress 0 is used for broadcast
►FU
NC
TION
ocode of m
aster’s comm
and function
►D
ATA
oadditional inform
ation which the slave m
ust use to take the action defined by the function code
►LR
C/C
RC
CH
ECK
om
essage frame check
►EN
Do
ending delimiter of a m
essage
ASC
IIForm
at
RTU
Format
Data C
omm
unications & Fieldbus S
ystems
229
FUN
CTIO
N C
OD
ES
►Each code represents the m
aster’s com
mand to the slaves
►C
ategories :o
Data query (coil/register)
oS
tatus/register change comm
ando
Program
ming com
mand
oD
iagnosticso
Device inform
ation query►
Some function codes are not
supported by all devices
Data C
omm
unications & Fieldbus S
ystems
230
ER
RO
R C
HE
CK
ING
►Error checking m
ethod :o
Parity check (even/odd) for each character (optional)
oFram
e check for entire message (C
RC
/LRC
)►
Both the character check and m
essage frame check (C
RC
/LRC
) are generated in the master
device and applied to the message contents before transm
ission►
The slave device checks each character and the entire message fram
e during receipt and generates C
RC
’/LRC
’►
If the LRC
/CR
C generated by m
aster device is not the same as LR
C’/C
RC
’ generated by slave device, the m
essage contains some errors
Data C
omm
unications & Fieldbus S
ystems
231
AN
EX
AM
PLE
OF M
OD
BU
S TR
AN
SA
CTIO
N
►R
ead inputs 10197–10218 from slave
device 17►
Message form
at : RTU
►A
ddress = 17 = 11 hex►
Function codes = 02 (Read Input
Status)►
Datao
Starting input address = 10197
relative to 10000 = 00C4 hex (P
LC
address)o
Num
ber of input points = 20 points = 14 hex.
Data C
omm
unications & Fieldbus S
ystems
232
MO
DB
US
PLU
S
►Local area netw
ork system designed
for industrial control applications. ►
Each network supports up to 32
addressable node devices (64 if repeater is used)
►1 M
bps data transfer rate ►
Token passing derivative that uses the M
OD
BU
S messaging structure
►Provides host level peer-to-peer com
munication for the netw
ork devices.
►A
pplications include transferring of process control and supervisory m
essages.
Data C
omm
unications & Fieldbus S
ystems
233
MO
DB
US
PLU
S FR
AM
E (1)
►M
odbusm
essages sent on Modbus
Plus networks are em
bedded into the Logical Link C
ontrol (LLC) level fram
e.
►M
odbusm
essage fields consist of 8–bit bytes, similar to those used w
ith RTU
framing.
►The Slave A
ddress field is converted to a Modbus
Plus routing path by the sending device.
►The C
RC
field is not sent in the Modbus
message, because it w
ould be redundant to the CR
C
check performed at the H
igh–level Data Link C
ontrol (HD
LC) level.
►The rest of the m
essage remains as in the standard serial form
at.
►The application softw
are (e.g., MSTR
blocks in controllers, or Modcom
III in hosts) handles the fram
ing of the message into a netw
ork packet.
Data C
omm
unications & Fieldbus S
ystems
234
MO
DB
US
PLU
S FR
AM
E (2)
Data C
omm
unications & Fieldbus S
ystems
235
MO
DB
US
TCP
►A
variant of the MO
DB
US fam
ily of simple, vendor-neutral com
munication protocols intended
for supervision and control of automation equipm
ent.
►C
overs the use of MO
DB
US m
essaging in an ‘Intranet’ or ‘Internet’ environment using the
TCP/IP protocols.
►U
se mostly for Ethernet attachm
ent of PLC’s, I/O
modules, and ‘gatew
ays’ to other simple
field buses or I/O netw
orks.
►B
eing promoted as ‘de-facto standard’
Data C
omm
unications & Fieldbus S
ystems
236
MO
DB
US
TCP
FRA
ME
►Em
beds a MO
DB
US fram
e into a TCP fram
e
►The connection-oriented TC
P protocol is used rather than the datagram-oriented
UD
P to keep control of an individual ‘transaction’
►The M
OD
BU
S checksum is not used as the Ethernet TC
P/IP link layer checksum
mechanism
s are relied upon to guarantee data integrity
Data C
omm
unications & Fieldbus S
ystems
237
DA
TA H
IGH
WA
Y P
LUS
(DH
+)
►A
local area network designed to support rem
ote programm
ing for factory-floor applications.o
Connects a m
aximum
of 64 devices per link (although 15 or fewer nodes are recom
mended per
link)oP
LC-5 and S
LC 5/04 program
mable controllers
oC
olor graphics systems
oP
ersonal computers
oH
ost computers
oN
umerical controls
oP
rogramm
able RS
-232-C/R
S-422 devices
oC
abling(1770-CD
shielded twin-axial):
otrunk-line -10,000 ft m
ax o
drop-lines -100 ft max
o“floating m
aster” token passing arrangement
o57.6k bit/s data rate
Data C
omm
unications & Fieldbus S
ystems
238
DH
PLU
S N
ETW
OR
K
Data C
omm
unications & Fieldbus S
ystems
239
DH
-485
►D
H-485 is a local area netw
ork (LAN
) designed for factory-floor applications.
oC
onnects up to 32 devices, including SLC
500 and MicroLogix
1000 programm
able controllers, color graphics system
s, and personal computers (w
ith RS
Linxsoftw
are)
►The D
H-485 link together w
ith auxiliary RS-232-C
(DF1 protocol) links m
ake up the D
H-485 netw
ork.
►A
single PC connected to netw
ork can be used to program all the SLC
500 controllers (using SLC
500 programm
ing software)
oTrunkline
length 4000 ft max
o19.2k bit/s m
ax data transfer rate
Data C
omm
unications & Fieldbus S
ystems
240
DH
-485 NE
TWO
RK
Data C
omm
unications & Fieldbus S
ystems
241
HA
RT
(Highw
ay Addressable R
emote Transducer)
►Interconnects sm
art transmitter in a tw
o-wire netw
orko
1200 bps data rateo
Shielded Tw
isted Pair (2 w
ire loop)o
Support m
ultivariable transmitter w
ith the use of HA
RT splitter
oC
an be used as either conventional transmitter or sm
art transmitter
►N
ormally not used for control caused by low
data rate
►Sm
art instrument benefits :
oIncreased reliability
oS
elf-diagnosticso
Greater accuracy and precision
Data C
omm
unications & Fieldbus S
ystems
242
HA
RT LA
YE
RS
Data C
omm
unications & Fieldbus S
ystems
243
HA
RT B
AS
ICS
HA
RT (H
ighway A
ddressable Rem
ote Transducer) was developed by Fisher-R
osemount
to retrofit 4-to-20mA current loop transducers w
ith digital data comm
unication.
►N
ormally not used for control caused by low
data rate
►H
AR
T modulates the 4-20m
A current with a low
-level frequency-shift-keyed (FSK
) sine-w
ave signal, without affecting the average analogue signal.
►H
AR
T uses low frequencies (1200H
z and 2200 Hz) to deal w
ith poor cabling, its rate is 1200 B
aud -but sufficient.
►H
AR
T uses Bell 202 m
odem technology, A
DS
L technology was not available in 1989, at
the time H
AR
T was designed
Data C
omm
unications & Fieldbus S
ystems
244
HA
RT FR
AM
E FO
RM
AT
preamble
startaddress
comm
andbytecount
[status]data
datachecksum
11..5
5..20(xFF)
11
[2](slave response)
0..25(recom
mended)
1
Hart fram
e format (character-oriented):
Data C
omm
unications & Fieldbus S
ystems
247
HA
RT M
ES
SA
GE
STR
UC
TUR
E
Data C
omm
unications & Fieldbus S
ystems
248
HA
RT C
OM
MA
ND
S
►The H
AR
T comm
and set provides uniform and consistent com
munication for all field devices
Host applications m
ay implem
ent any of the necessary comm
ands for a particular application.
►U
NIVER
SAL :
oU
nderstood by all devices o
Provide access to inform
ation useful in normal operations (e.g., read prim
ary variable and units).
►C
OM
MO
N PR
AC
TICE
oP
rovide functions implem
ented by many, but not necessarily all, H
AR
T comm
unication devices.
►D
EVICE SPEC
IFIC
oR
epresent functions that are unique to each field device.o
Access setup, calibration, and construction inform
ationo
Available from
device manufacturers.
Data C
omm
unications & Fieldbus S
ystems
249
HA
RT C
OM
MA
ND
S
Data C
omm
unications & Fieldbus S
ystems
250
HA
RT -C
omm
ands►
Universal com
mands (m
andatory):•
identification,•
primary m
easured variable and unit (floating point format)
•loop current value (%
) = same info as current loop
•read current and up to four predefined process variables
•w
rite short polling address•
sensor serial number
•Instrum
ent manufacturer, m
odel, tag, serial number, descriptor,
range limits, …
►C
omm
on practice (optional)•
time constants, range,
•E
EP
RO
M control, diagnostics,…
►Total: 44 standard com
mands, plus user-defined com
mands
►Transducer-specific (user-defined)•
calibration data,•
trimm
ing,…
Data C
omm
unications & Fieldbus S
ystems
251
HA
RT -IM
PO
RTA
NC
E
Practically all 4-20m
A devices come equipped w
ith HA
RT today
About 40 M
illion devices are sold per year.
more info:
http://ww
w.thehartbook.com
/default.asp
http://ww
w.hartcom
m.org/
Data C
omm
unications & Fieldbus S
ystems
252
AS
I A
ctuator Sensor Interface
►Very sim
ple sensor bus for building automation, com
bining power and data on the sam
e w
ires, transmitting m
ostly binary signals
D0 = sensor 1
D1 = sensor 2
D2 = actuator 1
D3 = actuator 2
P0
up to 4 sensorsor/and
4 actuatorsenergy
AS
-InterfaceS
lave IC
1 module
enclosure
oneconnection
Watchdog
Data C
omm
unications & Fieldbus S
ystems
253
AS
I
►M
echanically coded flat cable -two w
ires for data and power
►insulation piercing connectorso
simple &
safeo
protection class up to IP67,
oeven after disconnecting
►directly connected slaveso
sensors, actuatorso
valve terminals
oelectrical m
odules etc.
vampire-connector
Data C
omm
unications & Fieldbus S
ystems
254
AS
I►
Master-slave principleo
up to 31 slaves on one lineo
cycle time < 5 m
s►
Each slave can have up to 4 digital inputs + 4 digital outputso
additional 4 parameter bits / slave
oM
ax. 248 digital Inputs and Outputs
►A
utomatic address num
bering via bus connection
master
controller
ToSlave1
Slave1
ToSlave2
Slave2
ToSlave1
Slave1
ToSlave31
Slave31
master calls
slave response
Data C
omm
unications & Fieldbus S
ystems
255
AS
I TOP
OG
RA
PH
Yo
no terminating resistor necessary
ofree tree structure of netw
orko
protection class up to IP67
ounshielded 2-w
ire cable
odata and pow
er on one cableo
extension: 100 m (300 m
with
extender)
starline
branch linestree
controller
Master
controller
Master
Master
Master
Slave
Slave
Slave
Slave
Slave
Slave
Slave
Slave
Slave
Slave S
laveS
lave
Slave
Slave
Slave
Slave
SlaveS
lave Slave
Slave
controllercontroller
Data C
omm
unications & Fieldbus S
ystems
256
INTE
RB
US
-S►
Discrete M
anufacturing bus
+Standard in C
ENELEC
+1700 products, 270 m
anufacturers,375.000 applications
+G
ood experience in field wiring
(intelligent wiring bar)
+Easy to engineer
+Easy to program
(IEC 61131)
+Far extension (400m
-13 km)
+G
ood response time
+C
onformance test
-M
arket centered on manufacturing
-Lim
ited number of variables (4096 bits)
-R
ing structure sensitive to disruptions
-Sensitive to m
isplacement
-C
lumsy and slow
message service
-M
edium user com
munity
-Few
and costly tools
-Strong ties to Phoenix C
ontact
Data C
omm
unications & Fieldbus S
ystems
257
INTE
RB
US
-S TO
PO
LOG
Y
Master
400 m betw
eendevices
bus coupler
localbus (flat cable)
IO
loop (2 wire, includes pow
er)
IOIO
BC
remote "bus"(ring)
BA
BA
5-wire
optical fibresalso available
Data C
omm
unications & Fieldbus S
ystems
258
Day Four
►C
AN
BU
S
►P
rofiBU
S
►Foundation Fieldbus
►S
afety & R
eliability
Data C
omm
unications & Fieldbus S
ystems
259
►C
AN
is an important em
bedded protocolo
Prim
arily automotive, but used in m
any other places
►C
AN
specifies:o
Physical layer
oP
rotocol layero
Message filtering layer (w
ith add-on protocols)
►N
ote o
How
message prioritization achieved
oH
ow “sm
all” nodes can be kept from overloading w
ith received messages
CA
N O
VE
RV
IEW
Data C
omm
unications & Fieldbus S
ystems
260
THE
DE
VE
LOP
ME
NT O
F CA
N
►The developm
ent of CA
N began w
hen more and m
ore electronic devices were
implem
ented into modern m
otor vehicles. Exam
ples of such devices include engine m
anagement system
s, active suspension, AB
S, gear control, lighting control, air
conditioning, airbags and central locking. All this m
eans more safety and m
ore comfort for
the driver and of course a reduction of fuel consumption and exhaust em
issions.
►To im
prove the behavior of the vehicle even further, it was necessary for the different
control systems (and their sensors) to exchange inform
ation. This was usually done by
discrete interconnection of the different systems (i.e. point to point w
iring). The requirem
ent for information exchange has then grow
n to such an extent that a cable netw
ork with a length of up to several m
iles and many connectors w
as required. This produced grow
ing problems concerning m
aterial cost, production time and reliability.
Data C
omm
unications & Fieldbus S
ystems
261
BE
FOR
E C
AN
Data C
omm
unications & Fieldbus S
ystems
262
WITH
CA
N
►The solution to this problem
was the
connection of the control systems via a
serial bus system. This bus had to fulfill
some special requirem
ents due to its usage in a vehicle. W
ith the use of CA
N,
point-to-point wiring is replaced by one
serial bus connecting all control systems.
This is accomplished by adding som
e C
AN
-specific hardware to each control unit
that provides the "rules" or the protocol for transm
itting and receiving information via
the bus.
Data C
omm
unications & Fieldbus S
ystems
263
THE
CA
N B
US
►C
AN
is a broadcast type of bus. o
This means that all nodes can "hear" all transm
issions. There is no way to send a m
essage to just a specific node; all nodes w
ill invariably pick up all traffic. The CA
N hardw
are, how
ever, provides local filtering so that each node may react only on the “interesting”
messages.
Data C
omm
unications & Fieldbus S
ystems
264
BA
SIC
CO
NFIG
UR
ATIO
N
Data C
omm
unications & Fieldbus S
ystems
265
CA
N B
US
OV
ER
VIE
W
►The physical layer uses differential transm
ission on a twisted pair w
ire. The bus uses N
on-Return To Zero (N
RZ) w
ith bit-stuffing.
►The nodes are connected to the bus in a w
ired-andfashion: if just one node
is driving the bus to a logical 0, then the whole bus is in that state regardless
of the number of nodes transm
itting a logical 1.
►M
ax. transfer rate of 1000 kilobits per second at a maxim
um bus length of 40
meters or 130 feet w
hen using a twisted w
ire pair which is the m
ost comm
on bus m
edium used for C
AN
.
►M
essage length is short with a m
aximum
of 8 data bytes per message and
there is a low latency betw
een transmission request and start of transm
ission. The m
essages are protected by a CR
C type checksum
Data C
omm
unications & Fieldbus S
ystems
266
CA
N B
US
OV
ER
VIE
W
►The bus access is handled via the advanced serial com
munications protocol
Carrier S
ense Multiple A
ccess/Collision D
etection with N
on-Destructive
Arbitration. This m
eans that collision of messages is avoided by bitw
ise arbitration w
ithout loss of time.
►There is no explicit address in the m
essages, instead, each message carries
a numeric value w
hich controls its priority on the bus, and may also serve as
an identification of the contentsof the m
essage.
►A
n elaborate error handling scheme that results in retransm
itted messages
when they are not properly received.
►There are effective m
eans for isolating faults and removing faulty nodes from
the bus.
Data C
omm
unications & Fieldbus S
ystems
267
BA
SIC
BIT E
NC
OD
ING
Data C
omm
unications & Fieldbus S
ystems
268
CA
N B
US
CH
AR
AC
TER
STIC
S
Data C
omm
unications & Fieldbus S
ystems
269
BU
S C
HA
RA
CTE
RIS
TICS
–W
IRE
D A
ND
Only if all nodes transm
it recessive bits (ones), the B
us is in the recessive state.If any one node transm
its a dominant bit (zero), the
bus is in the dominant state.
oT is Transmitter, R
is receiver. Note nodes can therefore check the line w
hile transmitting. This is
important particularly during arbitration.D
ata Com
munications &
Fieldbus System
s270
BU
S A
CC
ES
S A
ND
AR
BITR
ATIO
N –
CS
MA
/CD
ND
A
CSM
A/C
D N
DA –
Carrier Sense M
ultiple Access/C
ollision avoidance by Non D
estructive arbitration
Data C
omm
unications & Fieldbus S
ystems
271
BU
S TR
AN
SM
ISS
ION
SP
EE
DA
rbitration limits bus speed. M
aximum
speed = 2 x tpdtpd
= propagation delay of electrical medium
Data C
omm
unications & Fieldbus S
ystems
272
THE
CA
N P
RO
TOC
OL
►Specifies how
small packets of data m
ay be transported from point A
to point B using a
shared comm
unications medium
.
►It (quite naturally) contains nothing on topics such aso
flow control
otransportation of data larger than can fit in a 8-byte m
essageo
node addresseso
establishment of com
munication, etc.
Data C
omm
unications & Fieldbus S
ystems
273
HIG
HE
R LA
YE
R P
RO
TOC
OLS
►H
igher layer protocols are used in order to
ostandardize startup procedures including bit rate setting
odistribute addresses am
ong participating nodes or kinds of messages
odeterm
ine the layout of the messages
oprovide routines for error handling at the system
level
►Som
e high layer protocols
oD
evice neto
CA
NK
ingdomo
CA
Nopen
Data C
omm
unications & Fieldbus S
ystems
274
THE
CA
N S
TAN
DA
RD
►The C
AN
standard defines four message types
oD
ata Frame –
the predominantly used m
essage typeo
Rem
ote Frame
oE
rror Frame
oO
verload Frame
►The m
essages uses a clever scheme of bit-w
ise arbitration to control access to the bus, and each m
essage is tagged with a priority.
►The C
AN
standard also defines an elaborate scheme for error handling and confinem
ent.
►C
AN
may im
plemented using different physical layers, and there are also a num
ber of different connector types in use.D
ata Com
munications &
Fieldbus System
s275
THE
DA
TA FR
AM
E
►Sum
mary: "H
ello everyone, here's some data labeled X, hope you like it!"
►The D
ata Frame is the m
ost comm
on message type. It com
prises the following m
ajor parts (a few
details are omitted for the sake of brevity):
oThe A
rbitration Field, which determ
ines the priority of the message w
hen two or m
ore nodes are contending for the bus. The A
rbitration Field contains:o
For CA
N 2.0A
, an 11-bit Identifier and one bit, the RTR
bit, which is dom
inant for data fram
es. o
For CA
N 2.0B
, a 29-bit Identifier (which also contains tw
o recessive bits: SR
R and ID
E)
and the RTR
bit.
othe D
ata Field, which contains zero to eight bytes of data.
othe C
RC
Field, which contains a 15-bit checksum
calculated on most parts of the m
essage. This checksum
is used for error detection.
oan A
cknowledgem
ent Slot; any
CA
N controller that has been able to correctly receive the
message sends an A
cknowledgem
ent bit at the end of each message. The transm
itter checks for the presence of the A
cknowledge bit and retransm
its the message if no acknow
ledge was
detected.
Data C
omm
unications & Fieldbus S
ystems
276
CA
N D
ATA
FRA
ME
SN
ote 1: It is worth noting that the presence of an A
cknowledgem
ent Bit on the bus does not m
ean that any of the intended
addressees has received the message. The only thing w
e know is that one or m
orenodes on
the bus has received it correctlyN
ote 2: The Identifier in the Arbitration Field is not, despite of its nam
e, necessarily identifying the contents of the m
essage.
►C
AN
2.0A (“standard C
AN
” 11-bit ID) D
ata Frame.
►C
AN
2.0B (“extended C
AN
” 29-bit ID) D
ata Frame.
Data C
omm
unications & Fieldbus S
ystems
277
THE
RE
MO
TE FR
AM
E
►Sum
mary: "H
ello everyone, can somebody please produce the data labeled X?"
►The R
emote Fram
e is just like the Data Fram
e, with tw
o important differences:
oIt is explicitly m
arked as a Rem
ote Frame (the R
TR bit in the A
rbitration Field is recessive), and
othere is no D
ata Field.
►The intended purpose of the R
emote Fram
e is to solicit the transmission of the
corresponding Data Fram
e. If, say, node A transm
its a Rem
ote Frame w
ith the Arbitration
Field set to 234, then node B, if properly initialized, m
ight respond with a D
ata Frame w
ith the A
rbitration Field also set to 234.
►R
emote Fram
es can be used to implem
ent a type of request-response type of bus traffic m
anagement. In practice, how
ever, the Rem
ote Frame is little used. It is also w
orth noting that the C
AN
standard does not prescribethe behaviouroutlined here. M
ost CA
N controllers
can be programm
ed either to automatically respond to a R
emote Fram
e, or to notify the local C
PU instead.
Data C
omm
unications & Fieldbus S
ystems
278
RE
MO
TE FR
AM
E (C
ON
TD.)
►There's one catch w
ith the Rem
ote Frame: the D
ata Length Code m
ust be set to the length of the expected response m
essage. Otherw
ise the arbitration will not w
ork.
►Som
etimes it is claim
ed that the node responding to the Rem
ote Frame is starting its
transmission as soon as the identifier is recognized, thereby "filling up" the em
pty Rem
ote Fram
e. This is not the case.
►A
Rem
ote Frame (2.0A
type):
Data C
omm
unications & Fieldbus S
ystems
279
THE
ER
RO
R FR
AM
E
Summ
ary: (everyone, aloud) "OH
DEA
R, LET'S TR
Y A
GA
IN"
Sim
ply put, the Error Fram
e is a special message that
violates the framing rules of a C
AN
message. It is
transmitted w
hen a node detects a fault and will cause all
other nodes to detect a fault -so they will send E
rror Fram
es, too. The transmitter w
ill then automatically try to
retransmit the m
essage. There is an elaborate scheme of
error counters that ensures that a node can't destroy the bus traffic by repeatedly transm
itting Error Fram
es.
The Error Frame
The Error Fram
e consists of an Error Flag, w
hich is 6 bits of the sam
e value (thus violating the bit-stuffing rule) and an E
rror Delim
iter, which is 8 recessive bits. The E
rror D
elimiter provides som
e space in which the other nodes on
the bus can send their Error Flags w
hen they detect the first E
rror Flag.
Data C
omm
unications & Fieldbus S
ystems
280
THE
OV
ER
LOA
D FR
AM
E
Summ
ary: "I'm
a very busy little 82526 device, could you please wait for a m
oment?"
►The O
verload Frame is m
entioned here just for completeness. It is
very similar to the Error Fram
e with regard to the form
at and it is transm
itted by a node that becomes too busy. The O
verload Frame is
not used very often, as today's CA
N controllers are clever enough not
to use it. In fact, the only controller that will generate O
verload Frames
is the now obsolete 82526
Data C
omm
unications & Fieldbus S
ystems
281
ISO
PH
YS
ICA
L LAY
ER
One of the m
ost comm
on and cheapest im
plementations is to use
a twisted w
ire pair. The bus lines are then called "C
AN
_H" and "C
AN
_L".
The two bus lines C
AN
_H
and CA
N_L are driven by
the nodes with a
differential signal.
The twisted w
ire pair is term
inated by terminating
resistors at each end of bus line, typically 120 ohm
s.
Data C
omm
unications & Fieldbus S
ystems
282
CA
N A
ND
EM
I
Due to the differential
nature of transmission
CA
N is insensitive to
electromagnetic
interference, because both bus lines are affected in the sam
e way w
hich leaves the differential signal unaffected.
To reduce the sensitivity against electrom
agnetic interference even m
ore, the bus lines can additionally be shielded. This also reduces the electrom
agnetic emission
of the bus itself, especially at high baud rates.
Data C
omm
unications & Fieldbus S
ystems
283
STA
ND
AR
DIZA
TION
•Vehicle bus system applications can be separated in three different categories according to their
real-time capabilities.
•Class A for a low
speed bus with bit rates up to 10 kbps, e.g
for body control applications,•C
lass B for a low
speed bus with bit rates from
10 kbps to 125 kbps, e.g. for dashboard and diagnostics,•C
lass C for a high speed bus w
ith bit rates from 125 kbps to 1 M
bps for real time
applications like engine managem
ent, Gearbox, A
BS
etc.
For the use of CA
N in vehicles tw
o standards have been defined for the bus interface:
CA
N H
igh Speed according to IS
O-IS
11898 for bit rates betw
een 125 kbps and 1 M
bps C
AN
Low S
peed according to ISO
-IS
11519-2 for bit rates up to 125 kbps
Data C
omm
unications & Fieldbus S
ystems
284
BU
S LE
VE
LS A
CC
OR
DIN
G TO
ISO
-IS 11898
•These are the bus levels according to ISO
-IS 11898. A recessive bit is represented by both C
AN
bus lines driven to a level of about 2.5 V
so that the differential voltage between C
AN
_H and C
AN
_L is around 0 V.•A dom
inant bit is represented by CA
N_H
going to about 3.5 V and C
AN
_L going to about 1.5 V. This results in a differential voltage for a dom
inant bit of about 2V.
Data C
omm
unications & Fieldbus S
ystems
285
A B
AS
IC C
AN
CO
NTR
OLLE
R
►C
heap CA
N controller –
CPU
could get overrun with m
essages even if it didn’t need them
.
Data C
omm
unications & Fieldbus S
ystems
286
FULL C
AN
CO
NTR
OLLE
R
►H
ardware m
essage filters sort & filter m
essages without interrupting C
PU
Data C
omm
unications & Fieldbus S
ystems
287
CA
N (S
AE
J1939) EX
AM
PLE
: CA
TER
PILLA
R 797
Data C
omm
unications & Fieldbus S
ystems
288
CA
TER
PILLA
R E
XA
MP
LE
Data C
omm
unications & Fieldbus S
ystems
289
CA
TER
PILLA
R E
XA
MP
LE
Data C
omm
unications & Fieldbus S
ystems
290
Data C
omm
unications & Fieldbus S
ystems
291
►Fieldbus is the generic nam
e of a family of industrial netw
ork systems for real-tim
e distributed control. H
uman M
achine Interface (HM
I)at the top, Program
mable Logic C
ontrollers (PLCs)in
the middle, and the Fieldbus
at the bottom.
FactoryLevel
Cell
Level
FieldLevel
Bus C
ycleTim
e< 1000 m
s
Bus C
ycleTim
e< 100 m
s
Bus C
ycleTim
e< 10 m
s
MM
S, TCP/IP B
ackbone
Profibus-FMS
Profibus-DP
Profibus-PA
I/OD
riveValves
FieldD
eviceTrans-m
itterField
Device
AreaC
ontroller
PR
OFIB
US
Data C
omm
unications & Fieldbus S
ystems
292
PR
OFIB
US
►P
RO
cessFieldB
US
oD
IN 19245
oFM
S (Fieldbus
Message S
pecification)o
DP
(Distributed P
eripheral)o
PA
(Process A
utomation)
►32 stations per segm
ent
►9.6, 19.2, 93.75 kbps for segm
ent length less than 1200 m
Data C
omm
unications & Fieldbus S
ystems
293
PR
OFIB
US
(CO
NS
OR
TIUM
FIELD
BU
S)
►V
endor-independent ensured by EN
50170 and EN
50254 International Standard
►O
pen field bus standard for a wide range of applications in m
anufacturing and process autom
ation
►C
onforms to O
SI reference m
odel
►A
llows com
munication betw
een devices of different manufacturers w
ithout any special interface adjustm
ents
►C
an be used for both high-speed time critical applications and com
plex comm
unication tasks
►C
ontinuing further technical developments prepared for the future
►W
idely accepted in European industrial com
munity
Data C
omm
unications & Fieldbus S
ystems
294
ME
DIU
M A
CC
ES
S P
RO
TOC
OL
►Token passing procedure
oE
nsures that the bus access right (token) is assigned to each master w
ithin a precisely defined tim
eframe
oThe token is passed around the logical token ring once to all m
asters within a
maxim
um token rotation tim
eo
Used only for com
munication betw
een complex stations (m
asters)
►M
aster-slave procedureo
Perm
its the master (active station) w
hich currently owns the token to access the
assigned slaves (passive stations)o
Enables the m
aster to send messages or retrieve them
from the slaves inithe
following configurations :
•P
ure master-slave system
•P
ure master-m
aster system (token-passing)
•A
combination of the tw
o
Data C
omm
unications & Fieldbus S
ystems
295
ME
DIU
M A
CC
ES
S C
ON
TRO
L►
Token passing among m
aster stations on list of active station (LAS
)
►Transm
issionfrom
slavestations
onlyby
masterstation’s
polling
Data C
omm
unications & Fieldbus S
ystems
296
PR
IOR
ITY M
EC
HA
NIS
M
►Tw
o priority classes: high and lowo
TTR: target rotation tim
eo
TRR
: real rotation time
oTTH
: token holding time, TTH
=TTR –
TRR
►A
t least one high priority message per token reception regardless of TTH
►A
dditional high priority messages only if TTH
is still positive
►Low
priority messages after high priority m
essage transmission if TTH
is still positive
Data C
omm
unications & Fieldbus S
ystems
297
PR
IOR
ITY M
EC
HA
NIS
M
Data C
omm
unications & Fieldbus S
ystems
298
PR
OFIB
US
PR
OTO
CO
L AR
CH
ITEC
TUR
E
Data C
omm
unications & Fieldbus S
ystems
299
CO
MM
UN
ICA
TION
PR
OFILE
S
►D
efines how users transm
it their data serially via the comm
on transmission m
edium
►D
ecentralized Periphery (D
P)
oThe m
ost frequently usedo
Optim
ized for speed, efficiency, and low connection cost
oD
esigned especially for comm
unication between autom
ation systems and distributed
peripherals (PLC
and field devices)o
Suitable as a replacem
ent for conventional, parallel signal transmission
►Fieldbus
Message S
pecification (FMS
)o
Universal com
munication profile for dem
anding comm
unication taskso
Offers m
any sophisticated application functions for comm
unication between intelligent
devices (PLC
s and PC
s)o
Will be less frequently used in the future w
ith the usage of TCP
/IP in cell level netw
ork
Data C
omm
unications & Fieldbus S
ystems
300
H1 M
AC
RO
CY
CLE
Data C
omm
unications & Fieldbus S
ystems
301
FIELD
BU
S M
ES
SA
GE
SP
EC
IFICA
TION
(FMS
)
►A
pplication layero
program invocation m
anagement
ovariable access
oevent m
anagement
ocontext m
anagement (connection establishm
ent)o
virtual fieldbusdevice object
oobject dictionary m
anagement
Data C
omm
unications & Fieldbus S
ystems
302
AV
AILA
BLE
BLO
CK
S IN
DFI302
RE
SO
UR
CE
DE
SC
RIP
TION
RS
RE
SO
UR
CE
TRAN
SDU
CER
BLOC
KSD
ES
CR
IPTIO
ND
IAG
Diagnostics Transducer
HC
Hardw
are Configuration Transducer
MB
CF
Modbus C
onfigurationTR
DTransducer
INPU
T TRAN
SDU
CER
BLOC
KS
DE
SC
RIP
TION
TEM
PTem
perature Transducer
INP
UT FU
NC
TION
BLO
CK
SD
ES
CR
IPTIO
NA
IAnalog Input
DI
Discrete Input
MA
IM
ultiple Analog InputM
DI
Multiple D
iscrete InputP
UL
Pulse Input
Data C
omm
unications & Fieldbus S
ystems
303
AV
AILA
BLE
BLO
CK
S IN
DFI302
CO
NTR
OL A
ND
CA
LCU
LATION
FUN
CTIO
N B
LOC
KS
DE
SC
RIP
TION
PID
PID
Control
EP
IDE
NH
AN
CE
D P
ID –
It has all the standard features plus : bum
pless or hard transfer from a “m
anual” m
ode to an “automatic” m
ode and bias.
AP
ID
AD
VANC
ED
PID
–It has all the standard features
plus: bumpless or hard transfer from
a“m
anual” mode to an “autom
atic” mode, bias.
Adaptive gain, P
I sampling, deadband for error,
special treatment for error, IS
A or parallel algorithm,..
AR
THA
rithmetic -9 predefined equation types
SP
LTS
plitter -split ranging and sequencing.C
HA
RS
ignal Characterizer -20 points curve
INTG
IntegratorA
ALM
Analog A
larmIS
EL
Input Selector -4 inputs
SP
GS
etpoint Ram
p Generator
TIME
Timer
Data C
omm
unications & Fieldbus S
ystems
304
AV
AILA
BLE
BLO
CK
S IN
DFI302
CO
NTR
OL A
ND
CA
LCU
LATION
FUN
CTIO
N B
LOC
KS
DE
SC
RIP
TION
LLAG
Lead LagD
ENS
Density
CT
Constant
FFET
FLIP-FLO
P and ED
GE
TRIG
GE
RM
BC
SM
odbus Control S
laveM
BS
SM
odbus Supervision S
laveM
BC
MM
odbus Control M
asterM
BC
SM
odbus Control S
laveA
EQ
UA
dvanced Equations
OU
TPU
T FUN
CTIO
N B
LOC
KS
DE
SC
RIP
TION
AO
Analog O
utputD
OD
iscrete Output
MA
OM
ultiple Analog O
utputM
DO
Multiple D
iscrete Output
STE
PS
tep Output P
ID
Data C
omm
unications & Fieldbus S
ystems
305
PRO
FIBUS uses a single, open com
munication protocol
(PRO
FIBUS D
P, Decentralized Periphery) for all applications
The protocol uses the
“Master-S
lave“ model:
One device (m
aster) controls one or m
ore other devices (slaves).
The protocol uses the “Token P
assing“ model:
The “token“ is transmitted
across the network; the
station in possession of the token controls the access to the netw
ork.
CO
MM
UN
ICA
TION
PR
OTO
CO
L DP
DP Slave
1
PRO
FIBUS D
PM
aster Class
1PR
OFIBU
S DP
Master C
lass2
DP Slave2
DP Slave
3
Token
Slave1
Slave3
Slave2
Slave3
Cyclic
Access
( Master 1)
Acyclic
Access
(Master 2)
.....
Cycle
DP Slave
1
PRO
FIBUS D
PM
aster Class
1PR
OFIBU
S DP
Master C
lass2
DP Slave2
DP Slave
3
Token
Slave1
Slave3
Slave2
Slave3
Cyclic
Access
( Master 1)
Acyclic
Access
(Master 2)
.....
Cycle
Data C
omm
unications & Fieldbus S
ystems
306
PRO
FIBUS D
P exists in
three versions:
DP-V0: O
verall comm
and structure, cyclic data exchange
D
P-V1: Extension by
acyclic data exchange et al.
D
P-V2: Further extension by tim
e stamp, clock
synchronization et al.
Time
FunctionalLevels
DP-V2
Data Exchange Broadcast(Publisher /Subscriber)
IsochronousM
ode(E
quidistance
plus extensions:
Clock Synchronization& Tim
eStam
ps
HART on PR
OFIBUS
Up/Dow
nload (Segmentation)
Redundancy
DP-V1
AcyclicData Exchange
between
PCorPLC
and Slave Devices
plus extensions:
Integration within
Engineering: EDD
and FDT
Portable P
LC S
oftware Function B
locks (IEC 61131-3)
Fail-Safe Comm
unication (PRO
FIsafe)
Alarms
DP-V0
C
yclic Data Exchangebetw
eenPLC
and Slave Devices
plusextensions:
G
SDC
onfiguration
Diagnosis
Device Features
Time
FunctionalLevels
DP-V2
Data Exchange Broadcast(Publisher /Subscriber)
IsochronousM
ode(E
quidistance
plus extensions:
Clock Synchronization& Tim
eStam
ps
HART on PR
OFIBUS
Up/Dow
nload (Segmentation)
Redundancy
DP-V1
AcyclicData Exchange
between
PCorPLC
and Slave Devices
plus extensions:
Integration within
Engineering: EDD
and FDT
Portable P
LC S
oftware Function B
locks (IEC 61131-3)
Fail-Safe Comm
unication (PRO
FIsafe)
Alarms
DP-V0
C
yclic Data Exchangebetw
eenPLC
and Slave Devices
plusextensions:
G
SDC
onfiguration
Diagnosis
Device FeaturesDevice Features
CO
MM
UN
ICA
TION
PR
OTO
CO
L DP
Data C
omm
unications & Fieldbus S
ystems
307
PRO
FIBU
S D
P (RS-485)
32 devices max. (incl. controller) on one segm
ent. D
evices must be daisy chained; no spur lines.
Segm
ent must be term
inated (T). B
aud rate depends on segment length.
Repeater are possible, 9 m
ax. per segment.
Use of “recom
mended grounding m
ethods”.
Controller
T
Devices
Segment
Data C
omm
unications & Fieldbus S
ystems
308
PRO
FIBU
S PA - Trunk topology
One m
ain cable (trunk) and spur lines M
aximum
length of spurs depends on number of spurs
T-connectors with or w
ithout short-circuit protection W
ith optional overvoltage protection
T L
ink
/ C
ou
ple
r T
PRO
FIBU
S DP
PRO
FIBU
S PA
Trunk
Spur lines
Devices
Data C
omm
unications & Fieldbus S
ystems
309
PRO
FIBU
S PA - Star topology
Junction Box w
ith or without short-circuit protection
All spur lines com
e from the junction box:
Lin
k/
Co
up
ler
T PR
OFIB
US D
P PR
OFIB
US PA
Junction B
ox T
Devices
Data C
omm
unications & Fieldbus S
ystems
310
PRO
FIBU
S PA - Trunk-and-spur topology
Short-circuit protection at the spur lines
Clearly arranged and easy to docum
ent
Lin
k/
Co
up
ler
T PR
OFIB
US D
P PR
OFIB
US PA
T Junction
Box
Junction B
ox
Spur lines
Trunk
Devices
Data C
omm
unications & Fieldbus S
ystems
311
PRO
FIBU
S PA - Ring topology
Two redundant links/couplers
High availability of the trunk
Short-circuit protection at the spur
Lin
k/
Co
up
ler
T
T L
ink
/ C
ou
ple
r
Junction B
ox Junction
Box
Junction B
ox
PRO
FIBU
S PA Intern
Spur lines
Trunk
Devices
Data C
omm
unications & Fieldbus S
ystems
312
TRA
NS
MIS
SIO
N TE
CH
Com
munication
Technology
Transmission
Technologies
PRO
FIBUS D
P (DP-V0, -V1, -V2)
Wired
RS485 / R
S485-IS
MBP
/ MBP
-IS
Engineering TechnologiesGSD, EDD, FDT / DTM, TCI
Optical
Glass, PC
F, PlasticW
ireless
PROFIdrive
PA Devices
Encoder
Ident Systems
Weighing & Dosage
HART on PROFIBUS
LabAutomation
SpecificApplication Profiles
Com
mon
Application Profiles
XY
. . . .
PRO
FIsafe, I&M, iPar-Server,
Time S
tamp, R
edundancy, …
Com
munication
Technology
Transmission
Technologies
PRO
FIBUS D
P (DP-V0, -V1, -V2)
Wired
RS485 / R
S485-IS
MBP
/ MBP
-IS
Engineering TechnologiesGSD, EDD, FDT / DTM, TCI
Optical
Glass, PC
F, PlasticW
ireless
PROFIdrive
PA Devices
Encoder
Ident Systems
Weighing & Dosage
HART on PROFIBUS
LabAutomation
SpecificApplication Profiles
Com
mon
Application Profiles
XY
. . . .
PRO
FIsafe, I&M, iPar-Server,
Time S
tamp, R
edundancy, …
Wired, O
ptical, and Wireless
Data C
omm
unications & Fieldbus S
ystems
313
RS - 485
PRO
FIBU
S DP
MB
P PR
OFIB
US PA
MB
P- IS PR
OFIB
US PA
Baud rate
9.6 ... 12.000 kBit/s31.25 kBit/sec
31.25 kBit/sec
Devices/segm
ent (max.)
3232
32
Devices/segm
ent (typic.)14 ... 20
4 ... 6
Cable length m
ax.1200
1900 m1000 m
Spur line length max.
120 m60 m
Wired transm
ission
RS
485; MB
P
R
S 485-IS
; MB
P-IS
(Intrinsically Safe)
TRA
NS
MIS
SIO
N TE
CH
Data C
omm
unications & Fieldbus S
ystems
314
Wired transm
ission
MB
P transmission technology
Fieldbus standard IEC 61158-2 for
MB
P transmission technology
Up to 32 nodes in one segm
ent
Data transm
ission rate 31.25 Kbit/s
Per field device: M
in. working voltage 9 V D
CM
in. current consumption 10 m
A
Transmission of digital com
munication signal in zero-m
ean M
anchester II coding (MBP
) through ± 9 mA
amplitude
Signal transm
ission and remote pow
er supply using twisted-pair
cable
Fieldbus cable type A
Connection of field devices via stubs (spur) to a m
ain cable (trunk) for trouble-free disconnection of devices w
ithout affecting other nodes
Max. total length of m
ain cable, including all stubs, is 1900 m
TRA
NS
MIS
SIO
N TE
CH
Data C
omm
unications & Fieldbus S
ystems
315
Optical transm
ission
Various types of fiberoptic cables are supported.
Typical topology structures are star and ring, linear structures are also possible.
The im
plementation of a fiberoptic cable netw
ork involves the use of electrooptical converters.
Fiber type C
ore diameter [µm
] Transm
ission range M
ulti-mode glass fiber
62,5 / 125 2 - 3 km
Single-m
ode glass fiber 9 / 125
> 15 km P
lastic fiber 980 / 1000
Up to 100 m
HC
S®
fiber 200 / 230
Approx. 500 m
TRA
NS
MIS
SIO
N TE
CH
Data C
omm
unications & Fieldbus S
ystems
316
Wireless transm
ission
PR
OFIB
US
& P
RO
FINE
T International do not specify its ow
n wireless solution.
Num
erous wireless solutions are available for P
RO
FIBU
S
from different vendors using gatew
ays to translate the electric signals into a w
ireless signal. S
ee the product guide on the PR
OFIB
US
website and search
for wireless. Various types of fiberoptic cables are supported.
TRA
NS
MIS
SIO
N TE
CH
Data C
omm
unications & Fieldbus S
ystems
317
AS
SE
T MA
NA
GE
ME
NT
Controller
One variable,
One direction
Conventional system
Very limited system
view, device details are „invisible“.
PRO
FIBU
S
Expanded system
view, device details are „visible“
Devices
Unlike conventional com
munication system
s, PR
OFIBU
S allows a detailed “view
into field devices”.
M
ultiple variables, Tw
o directions
Controller
Devices
T T
Data C
omm
unications & Fieldbus S
ystems
318
Example from
a chemical plant:
“Level in reactor 2B gets out of spec”
Conventional system
(left) reports just undefined “Failure”.
PR
OFIB
US
(right) reports exact diagnosis information.
“Failure” U
ndefined inform
ation
Controller
One variable,
one direction
Same failure
„Level in reactor B2 out of spec“
happens in both systems.
“Level in reactor B2
out of spec” Exact inform
ation
Multiple variables,
two directions
Controller
T T
AS
SE
T MA
NA
GE
ME
NT
Data C
omm
unications & Fieldbus S
ystems
319
DIAG
NO
STICS
Process control, m
aintenance, condition monitoring, …
PRO
FIBU
S PA
Device A
Device C
D
evice B
Before profile 3.02 w
as introduced
all diagnosis messages have been provided to all users.
>> Difficult to m
anage by the operators
Data C
omm
unications & Fieldbus S
ystems
320
With Profile 3.02
Diagnosis m
essages are mapped to categories already by the
manufacturer, categories com
ply with N
AM
UR
NE
107. P
lant operator gets categorized information.
Maintenance departm
ent gets full information.
PRO
FIBU
S PA
Device A
Diag 1
Diag2
Diag 3
Diag 4
------
Diag n
Maintenance R
equired Failure
Functional C
heck O
ut of S
pecification D
iagnosis messages are
mapped to 4 categories.
Plant operator M
aintenance departm
ent
DIAG
NO
STICS
Data C
omm
unications & Fieldbus S
ystems
321
In hazardous environments, fieldbus system
s must
comply w
ith two IEC
standards: IE
C 60079: E
xplosive atmospheres
IEC
61158-2: Fieldbus/Physical layer specification
H
azardous zones and PRO
FIBUS solution
Zone 0, 1 and 2 define areas of a plant, where explosive
substances may exist in the air and an electrical spark
could trigger an explosion
The respective PR
OFIB
US
solution limits the energy going to
the bus and the devices to eliminate the danger of generating
a sparc
The „Intrinsically Safe (IS)“ version of the MBP physical layer
(MBP-IS) com
plies with this approach.
INTR
INS
ICA
LLY S
AFE
ISS
UE
S
Data C
omm
unications & Fieldbus S
ystems
322
Data of M
BP and M
BP-IS physical layers
N
ote: R
S485 is also available in an IS
-version, which runs at low
er power
levels with a special coupler and a special w
iring. .
MB
P PR
OFIB
US PA
MB
P- IS PR
OFIB
US PA
Baud rate
31.25 kBit/sec31.25 kBit/sec
Voltage24 ... 30 V
13,2 V
Current
1000 mA
110 mA
Devices/segm
ent (max.)
32
Devices/segm
ent (typic.)14 ... 20
4 ... 6
Cable length m
ax.1900 m
1000 m
Spur line length max.
120 m60 m
INTR
INS
ICA
LLY S
AFE
ISS
UE
S
Data C
omm
unications & Fieldbus S
ystems
323
Fieldbus Intrinsically Safe Concept (FISC
O)
The FIS
CO
(Fieldbus Intrinsically Safe C
oncept) provides easy and fast design of P
RO
FIBU
S PA installations in
hazardous areas. FIS
CO
enables to get IS approval w
ithout individual calculations. FIS
CO
requirements:
Only one pow
er source permitted.
All other com
ponents are drains. M
aximum
overall cable length 1000 m
Maxim
um spur line length 60 m
P
ower supply, coupler and field devices m
ust be FISC
O certified.
INTR
INS
ICA
LLY S
AFE
ISS
UE
S
Data C
omm
unications & Fieldbus S
ystems
324
From Intrinsic Safety to the H
igh-Power-Trunk
Intrinsic safety (I.S
.) is the method of choice for
instrument connections in hazardous areas.
I.S. does not satisfy com
pletely the needs regarding to cable length and num
ber of devices compared to applications
outside of hazardous areas. The H
igh-Pow
er Trunk Concept solves this lim
itation for use in hazardous areas.
INTR
INS
ICA
LLY S
AFE
ISS
UE
S
Data C
omm
unications & Fieldbus S
ystems
325
PRO
FIBUS P
A intern
T
FieldDevices
Terminator
Segment coupling
and power supply
PROFIB
US DP
PROFIB
US PA
Spur: Ex i
Trunk: Ex e
Zone 2C
lassI, D
iv. 2
Zone 1C
lassI, D
iv. 1/2
General P
urpose/ Safe A
rea
FieldBarriers
Zone 0C
lassI, D
iv. 1
T
FieldDevices
Terminator
Segment coupling
and power supply
PROFIB
US DP
PROFIB
US PA
Spur: Ex i
Trunk: Ex e
Zone 2C
lassI, D
iv. 2
Zone 1C
lassI, D
iv. 1/2
General P
urpose/ Safe A
rea
FieldBarriers
Zone 0C
lassI, D
iv. 1
High-Pow
er-Trunk to supply Zone 1 The trunk is installed w
ith increased protection in zone 1 to allow
increased supply current for more field devices.
The field devices are connected using Ex i ignition protection.
Data C
omm
unications & Fieldbus S
ystems
326
PRO
FIBUS P
A
intern
FieldDevices
T
Terminator
Segment coupling
and power supply
High pow
er, withoutignition
protection
PROFIB
US DP
PROFIB
US PA
Spur: Ex ic/nLEnergy lim
ited
Trunk: Ex nAN
on sparking
Zone 2C
lassI, D
iv. 2
Coupler
With
Short-circuitProtection
General P
urpose/ Safe A
rea
FieldDevices
T
Terminator
Segment coupling
and power supply
High pow
er, withoutignition
protection
PROFIB
US DP
PROFIB
US PA
Spur: Ex ic/nLEnergy lim
ited
Trunk: Ex nAN
on sparking
Zone 2C
lassI, D
iv. 2
Coupler
With
Short-circuitProtection
General P
urpose/ Safe A
rea
High-Pow
er-Trunk to supply zone 2
Data C
omm
unications & Fieldbus S
ystems
327
Objective: R
educe the risk to an acceptable level.
PRO
FIsafe e. g. modified
process design
Risk of a technical setup
Risk
Zero risk
Acceptable risk
Risk
reduction m
easures
Unfeasible zero risk
Other m
easures
Safety Instrumented
Systems (SIS)
Data C
omm
unications & Fieldbus S
ystems
328
SIL: A perform
ance criteria of a Safety Instrum
ented System
(SIS
)
which describes, am
ong other things, the Probability of
Failure on Dem
and (PFD
). S
IL covers four levels SIL 1 to S
IL 4.
PFD
: A value that indicates the probability of a system
failing to respond to an actual demand.
PFD
is also referred to as “safety unavailability”.
Safety Integrity Level (S
IL)P
robability of failure on dem
and (PFD
) per year
Risk R
eduction Factor (1 / P
FD)
SIL 1>= 10 -2 to <10 -1
100 to 10
SIL 2>= 10 -3 to <10
-21000 to 100
SIL 3>= 10 -4 to <10 -3
10 000 to 1000
SIL 4>= 10 -5 to <10 -4
100 000 to 10 000
PR
OFIsafe is standardized in IE
C 61784-3-3
and complies w
ith SIL 3 according to IE
C 61508.
PRO
FIsafe
Data C
omm
unications & Fieldbus S
ystems
329
Standard and safety comm
unication on the same bus
B
lack Channel:
Not safety-related com
ponents such as AS
ICs, links, cables etc.
PRO
FIsafe (Safety Function, Safety Layer) P
art of the safety-related comm
unication system, located above layer 7
Safety Layers checks addressing, signature, fault tolerance tim
e etc.
Safety-related components (I/O
s, controller, control systems)
These are not part of PR
OFIsafe!
Not safety-related functions, e.g. diagnosis
SF
1 7 2
1 7 2
Standard I/O
Standard C
ontroller
Safety Input
Safety C
ontroller
1 2 7
1 2 7
1 2 7
1 7 2
1 7 2
SF SF
SF SF
Safety O
utput
PRO
FIsafe
Data C
omm
unications & Fieldbus S
ystems
330
WO
RLD
FIP
►Field Instrum
entation Protocol
oFrench standard
►N
etwork configuration and transm
ission speed
o32 stations per segm
ento
up to 256 stationso
31.25kbps,1M
bps,2.5M
bps
Data C
omm
unications & Fieldbus S
ystems
331
ME
DIU
M A
CC
ES
S C
ON
TRO
L
►C
entralized pollingo
Bus arbitrator
oP
roducer-consumer m
odelo
Bus arbitrator transm
itting an ID fram
e for a certain pair of producer and consum
erso
Elem
entary cycle with a period equal to that of m
ost frequent message
oA
periodicvariable and m
essage exchange after periodic data transm
issiono
Filling for excess timeD
ata Com
munications &
Fieldbus System
s332
ME
DIU
M A
CC
ES
S C
ON
TRO
L
Data C
omm
unications & Fieldbus S
ystems
333
ME
DIU
M A
CC
ES
S C
ON
TRO
L
Data C
omm
unications & Fieldbus S
ystems
334
►IE
C 61158 Fieldbus
standard for use in industrial control systems (IE
C/S
C/65C
)
oType 1: IE
C Fieldbus
Technical Specifications
oType 2: C
ontrol Net
oType 3: P
rofibuso
Type 4: Pnet
oType 5: H
SE
of the FieldbusFoundation
oType 6: S
wiftN
eto
Type 7: WorldFip
oType 8: Interbus
IEC
61158
Data C
omm
unications & Fieldbus S
ystems
335
►IS
O 15745 Industrial autom
ation systems and integration -O
pen systems
application integration framew
orks (TC 184 S
C5)
oP
art 2: ISO
11898 based control systems
•D
eviceNet, C
AN
Kingdom
, CA
NO
pen
oP
art 3: IEC
61158-based control systems
•P
-Net, P
rofibus, WorldFIP
, ControlN
et, Interbus
oP
art 4: Ethernet-based control system
s •
Ethernet/IP
, AD
S-net, FL-net
ISO
15745
Data C
omm
unications & Fieldbus S
ystems
336
FOU
ND
ATIO
N FIE
LDB
US
►The Fieldbus m
arket is dominated by Foundation Fieldbus and P
RO
FIBU
S. B
oth technologies use the sam
e physical layer but are not interchangeable.
►A
s a general guide, applications which are controlled and m
onitored by PLC
s tend tow
ards PR
OFIB
US
, and applications which are controlled and m
onitored by a DC
S
(digital/distributed control system) tend tow
ards FOU
ND
ATIO
N Fieldbus.
►Foundation Fieldbus
is an all-digital, serial, two-w
ay comm
unications system that
serves as the base-level network in a plant or factory autom
ation environment.
Developed and adm
inistered by the FieldbusFoundation.
Data C
omm
unications & Fieldbus S
ystems
337
FIELD
BU
S S
YS
TEM
NIC
HE
S
Data C
omm
unications & Fieldbus S
ystems
338
HO
W IS
FIELD
BU
S D
IFFER
EN
T FRO
M 4-20M
A?
oIn 4-20m
A circuits, the pow
er supply, field device and input card are connected in series.
oIn a Fieldbus
system, the pow
er supply, field devices and input card are connected in parallel.
4-20mA
Power
Supply
fieldbus
Power
Supply
Input card
Data C
omm
unications & Fieldbus S
ystems
339
TYP
ICA
L FIELD
BU
S IN
STA
LLATIO
N
Safe area
Redundant,
isolated Fieldbus
power
conditioner
Field wiring hub
with spur short-
circuit protection
FO
UN
DATIO
NFieldbus
Devices
FIELD
CO
NTR
OL R
OO
M
Fieldbuscontrol
system (D
CS)
Data C
omm
unications & Fieldbus S
ystems
340
►Tw
o related implem
entations of FOU
ND
ATION
fieldbus have been introduced to m
eet different needs within the process autom
ation environment.
oH
1is a bi-directional com
munications protocol used for com
munications am
ong field devices and to the control system
. It works at 31.25 kbit/s and generally
connects to field devices. It provides comm
unication and power over standard
twisted-pair w
iring. H1 is currently the m
ost comm
on implem
entation.
oH
SE
(High-speed E
thernet) works at 100 M
bit/s and generally connects input/output subsystem
s, host systems, linking devices, gatew
ays, and field devices using standard E
thernet cabling. It doesn't currently provide power over
the cable.
CU
RR
EN
TS
TATE
Data C
omm
unications & Fieldbus S
ystems
341
CU
RR
EN
TS
TATE
Data C
omm
unications & Fieldbus S
ystems
342
FOU
ND
ATIO
N FIE
LDB
US
IN IN
DU
STR
Y
►Installed base w
orldwide (published by Fieldbus
Foundation, 2008):o
>10,000 systems
o>700,000 field instrum
ents
►D
iverse adoption:o
9 of the top 10 oil & gas com
panieso
24 of the top 25 pharmaceutical m
anufacturerso
23 of the top 25 chemical producers
o15 of the top 20 pulp &
paper companies
o10 of the top 20 food &
beverage companies
►A
RC
Advisory G
roup:“…
powerful value proposition for fieldbus
in the hybrid industries-namely, food &
beverage and pharm
aceutical, particularly when it com
es to easing the process of validation and regulatory com
pliance and the integration of batch procedural operations w
ith continuous processes. The function block structure of Foundation technology lends itself to these processes very w
ell.”
Data C
omm
unications & Fieldbus S
ystems
343
MA
RK
ET D
ISTR
IBU
TION
Global
LA12 %
NA43 %
AP
13 %MEA
7%
EU R25%
Other8%
Mining and M
etals3%
Education3%
Pulp and Paper
4%
Food &
Beverage
9%
Pharm-
aceutical9%
Power
13%
Oil &
Gas
29%
Chem
ical22%
Industry
Data C
omm
unications & Fieldbus S
ystems
344
►Fieldbus C
omponents:
oB
us Terminal
oE
therCA
To
Fieldbus Box
oLightbus
oP
C Fieldbus C
ardso
Sw
itcheso
And m
ore …
SU
PP
OR
TING
TEC
HN
OLO
GY
Data C
omm
unications & Fieldbus S
ystems
345
WH
EN
SH
OU
LD FIE
LDB
US
BE
CO
NS
IDE
RE
D?
►‘G
reen Field’ Facility
►Facility U
pgradeo
New
unit operation
►P
rocess Autom
ation Focuso
≥ 60% of sensors are analogue
►E
xisting cabling infrastructure is overloaded
Data C
omm
unications & Fieldbus S
ystems
346
WH
Y U
SE
FOU
ND
ATIO
N FIE
LDB
US
?
►Integrated digital architecture
►“Future proof”o
Sensors can be field upgraded w
ith new features
►E
conomic savings
oD
ifferent incentives at each stage of a facility life cycle:•
Capital/C
onstruction•
Com
missioning
•O
perations, maintenance
oR
educed infrastructure
Data C
omm
unications & Fieldbus S
ystems
347
RE
-US
ING
INS
TALLE
D C
AB
LES
Three existing4-20 m
APairs
J-Box
TT
Fieldbusdevices
Three existingpairs, one usedfor fieldbus
J-Box
Data C
omm
unications & Fieldbus S
ystems
348
Replace
Operations &
Maintenance
Engineering, C
onstruction,&
Com
missioning/S
tart-up
EC
ON
OM
IC LIFE
CY
CLE
Cash Flow
Analog
fieldbus
Time
Data C
omm
unications & Fieldbus S
ystems
349
EN
GIN
EE
RIN
G, C
ON
STR
UC
TION
& S
TAR
T-UP
Time
startup
Cash Flow
Reduce im
plementation
risk and cost
Engage in FE
ED
stageS
ooner and faster start-up
Data C
omm
unications & Fieldbus S
ystems
350
TER
MIN
ATIO
N C
OU
NT
FT-100
+-
FCV-100+-
TT-100
+-
Field
PT-101
+-
123456789101112
Junction Box
123456789101112
Marsh C
ab.
1234
+-+-+-+-
+-+-+-+-
I.S.
Barrier
8
2420
5
164
82
15
+-+-+-+-+-+- I/OA
ssembly
Conventional analog term
ination
Fieldbusterm
ination (also includedin conventional term
ination count)M
arshalling done in Junction Box w
ith Fieldbusso
Marsh C
ab is optional
Totals:75 Analog34 Fieldbus
4 pr4 pr
4 pr
1 pr1 pr
1 pr
Data C
omm
unications & Fieldbus S
ystems
351
EN
GIN
EE
RIN
G -N
ETW
OR
K LO
AD
ING
►N
umber of devices per netw
ork segment
o15-20 m
Aper transm
ittero
20-25 mA
per valve
►S
egment Length
oD
efined by Fieldbusstandard
•function of num
ber of devices•
Segm
ent topology
►V
oltage Calculations
o9 volt m
inimum
at field deviceo
design for 12 volts
►B
us Bandw
idtho
Balance cycle tim
e against num
ber of deviceso
Function of host control system
►V
irtual Com
munication R
esourceso
Required for com
munication
between devices and host
Data C
omm
unications & Fieldbus S
ystems
352
OP
ER
ATIO
NS
& M
AIN
TEN
AN
CE
Cash Flow
Higher sustained
production with
minim
ize costs
Faster time-to
profitS
afer operation
Fewer unscheduled
outages
Shorter scheduled outage
Longer duration to scheduled outage
Data C
omm
unications & Fieldbus S
ystems
353
ME
AS
UR
EM
EN
T AC
CU
RA
CY
IS InterfaceI/O
card
PV = 392.8mb
12.83mA
PV = 393.1mb
12.87mA
IS InterfaceH
1 I/O card
PV = 392.8mb
PV = 392.8mb
PV = 392.8mb Field JB
+ m
arshalling
Field JB +
marshalling 12.86m
A
PV = 392.8mb
PV = 392.8mb
Leakage + noiseC
onversion error
Conversion
error
4-20mA
Fieldbus
Data C
omm
unications & Fieldbus S
ystems
354
DIA
GN
OS
TICS
SA
VE
WA
STE
D M
AIN
TEN
AN
CE
0% 5%
10%
15%
20%
25%
30%
35%
Routin
ech
eck
No
Pro
ble
mC
alibratio
nsh
iftZe
ro O
ffPlu
gged
lines
Faile
d
63% of instrum
ent maintenance labor results in no action taken: w
aste of resources
35%
28%
20%
6%6%
4%
Shell G
lobal Solutions
Data C
omm
unications & Fieldbus S
ystems
355
FIELD
BU
S P
RO
JEC
T CO
STS
►E
stimated P
roject Savings
oM
aterials and Field Devices
•S
ome increase
oInstallation Labor•
Savings up to 50%
o
Com
missioning:
•S
ignificant savings up to 60%
►O
verall Capital S
avings of 25–30%H
owever savings w
ill only be realized if the project is planned as a fieldbus
project from the start.
Data C
omm
unications & Fieldbus S
ystems
356
►C
osts below are a per device cost
oTraditional field instrum
ents $890o
Ff devices w/ single-pair hom
erun $672o
Ff devices w/ m
ulti-pair homerun $652
oFf devices w
/ connectors $567
FF DE
VIC
E C
OS
TS
Data C
omm
unications & Fieldbus S
ystems
357
►A
dvantages
oR
educes installation costs, wiring,
less termination and few
er screw
driver turns.o
Reduces hardw
are requirements
and Capital E
xpenditures. o
Reduces O
perating expenses through plant efficiencies, better asset m
anagement, and less
maintenance.
►Lim
itations
oFieldbus system
s are more com
plex, so users need to be m
ore extensively trained or m
ore highly qualified
oThe price of Fieldbus com
ponents is higher
oFieldbus test devices are m
ore com
plex compared to a (high-spec)
multim
eter that can be used to read and sim
ulate analog 4-20 mA
signals
oO
ne or more Fieldbus standards
may predom
inate in future and others m
ay become obsolete. This
increases the investment risk w
hen im
plementing Fieldbus.
AD
VA
NTA
GE
S A
ND
LIMITA
TION
S
Data C
omm
unications & Fieldbus S
ystems
358
VE
ND
OR
S
Data C
omm
unications & Fieldbus S
ystems
359
PR
OTO
CO
L AR
CH
ITEC
TUR
E
Data C
omm
unications & Fieldbus S
ystems
360
ME
SS
AG
E FO
RM
AT
►P
DU
= Protocol D
ata Unit
►P
CI = P
rotocol Control Inform
ation
Data C
omm
unications & Fieldbus S
ystems
361
PH
YS
ICA
L PR
OFILE
S : M
AN
CH
ES
TER
CO
DE
►S
elf-clocking
►S
ynchronous
►H
alf-duplex
Data C
omm
unications & Fieldbus S
ystems
362
DA
TA LIN
K LA
YE
R
►C
ontrols transmission of m
essages onto the fieldbus
►M
anages access to the fieldbusthrough a determ
inistic centralized bus scheduler (Link Active
Scheduler -LA
S)
►D
ata transfer :o
Scheduled (C
yclic)o
Unscheduled (Token P
assing)D
evice Hierarchy :
•Link Master
•Basic D
evice
Data C
omm
unications & Fieldbus S
ystems
363
SC
HE
DU
LED
DA
TA TR
AN
SFE
R
►The m
essage in the data buffer is broadcastto all devices on the fieldbusw
hen the LA
S issues the com
pel data (CD
) to the publisher►
The subscribers listen to the message broadcast
Data C
omm
unications & Fieldbus S
ystems
364
UN
SC
HE
DU
LED
DA
TA TR
AN
SFE
R
►The m
essage in the queue buffer is transmitted on the fieldbus
when the LA
S issues
the pass token message to device x
►The m
essage can be sent to a single destination or to multiple destinations
(multicast)
Data C
omm
unications & Fieldbus S
ystems
365
DE
VIC
E D
ES
RIP
TION
S A
ND
ED
DL
►W
hat is a Device D
escription?
►A
clear, and unambiguous, structured text description that precisely
describes field device data to host systems.
Data C
omm
unications & Fieldbus S
ystems
366
DE
VIC
E D
ES
CR
IPTIO
NS
►A
DD
contains the following inform
ation about the parameters of a device:
oA
ttributes like coding, name, engineering unit, w
rite protection, how to display etc.
oThe arrangem
ent of the parameters in a m
enu structure, names of m
enus and submenus.
oInform
ation about the relation of parameters to others.
oInform
ation about help texts and help procedures.
oInform
ation about necessary operating interactions (e.g. calibration), also called methods.
oInform
ation about visualization tools (i.e.: charts and graphs)
Data C
omm
unications & Fieldbus S
ystems
367
ED
DL TE
CH
NO
LOG
Y E
NH
AN
CE
ME
NTS
►W
hat is the Electronic
Device D
escription Language?
►The international standard for developing E
lectronic Device D
escriptions (E
DD
s) according to the IEC
61804-2.
►The Fieldbus
Foundation FF-900 “Device D
escription Language S
pecification” is fully conformant to the IE
C 61804-2 E
DD
L Standard.
Data C
omm
unications & Fieldbus S
ystems
368
IEC
61804-2ED
DL
2004
19921993
19941995
19961997
19981999
20002001
20022003
20042005
20062007
FF DD
L1996
HA
RT
DD
L1992
ISPD
DL
1993PR
OFIB
US
DD
L2000
ED
DL H
ISTO
RY
Data C
omm
unications & Fieldbus S
ystems
369
ED
DL TE
CH
NO
LOG
Y E
NH
AN
CE
ME
NTS
►E
lectronic Device D
escription Language Specifications
oIE
C 61804
oFunction B
locks (FB) for Process C
ontrol•
Part 1: Overview
of system aspects
•Part 2: Specification of FB
concept and Electronic Device D
escription Language (ED
DL)
oC
overs DD
L used in the FOU
ND
ATIO
N fieldbus™
, HA
RT®
and Profibuscom
munication protocol
oB
ecame an International Standard in M
arch 2004
Data C
omm
unications & Fieldbus S
ystems
370
DE
VIC
E D
ES
RIP
TION
S
►W
hat is ED
DL?
oE
DD
is an extended description of the data, user interface and comm
unication in a device w
hich is used by PC
applications and handhelds for engineering, com
missioning, m
onitoring, operation and diagnostics.
oE
DD
s are written in a language called E
lectronic Device D
escription Language (E
DD
L).
oE
DD
s are operating system (O
S) and com
munication protocol independent:
•E
DD
s are not executable code, do not affect OS
configuration•
Adding a new
ED
D w
ill not disrupt existing applications•
ED
Ds support H
AR
T, PR
OFIB
US
, FOU
ND
ATIO
N fieldbus™
& other
protocols
Data C
omm
unications & Fieldbus S
ystems
371
ED
DL B
EN
EFITS
•E
stablished –m
illions of devices worldw
ide•
System
Independent (including OS
)•
A single E
DD
for all hosts•
Robust R
evision Control
•Testing and R
egistration•
International Standard
•U
niformity
•S
upported by major suppliers
•B
ackward com
patible•
Preserves investm
ent•
Easy to im
plementD
ata Com
munications &
Fieldbus System
s372
ED
DL C
OO
PE
RA
TION
PR
OJE
CT
►Joint Fieldbus
Foundation, PN
O and H
CF project to specify visualization and data storage
managem
ent extensions
Data C
omm
unications & Fieldbus S
ystems
373
WH
Y E
XTE
ND
THE
LAN
GU
AG
E
►C
omplex devices use advanced
visualization and data storage m
anagement
•valve signatures
•sensor calibration curves
•persistent data storage
•trends
►H
uge worldw
ide ED
D investm
ent •
development tools
•testing tools
•P
C, handheld hosts
Proven technology: E
DD
has been in use for over 10 years
Easy to use:
Developers and end users like it
Open and interoperable
Data C
omm
unications & Fieldbus S
ystems
374
•Attributes for D
isplay Coding
•Engineering U
nits•M
enu Structure
•Menus, S
ubmenu N
ames
•Param
eter Relationships
•Help Texts
•Help P
rocedures•M
ethods for operating interactions like calibrationS
ystem Independent (including O
S)
IEC 61804-2 Standard
Protocol documentation
Product SpecificationH
ost Mapping Spec.
ED
DL C
OM
BIN
ES
Data C
omm
unications & Fieldbus S
ystems
375
ED
DL C
OO
PE
RA
TION
TEA
M C
HA
RTE
R
oE
xtension to the IEC
61804-2 International Standard
oO
perating System
& C
omm
unication Protocol Independent
oN
o Changes to E
xisting Device D
escriptions
oP
rotects Investment in D
D-based D
evices, Tools & Training
oS
ingle Technology for Suppliers and E
nd Users
oR
educes Device D
evelopment Tim
e & D
evelopment C
ost
oIm
proves Asset M
anagement C
apability
Data C
omm
unications & Fieldbus S
ystems
376
ED
DL E
XTE
NS
ION
S
►C
hartingo
Enables graphical data display real-tim
e (continuous)•
New
CH
AR
T construct to define display characteristics•
New
SO
UR
CE
construct enables multiples curves on a C
HA
RT
•N
ew A
XIS
construct
►G
raphingo
Enables graphical display of static Y
-t and XY
data•
New
GR
AP
H construct to define display characteristics
•N
ew W
AV
EFO
RM
construct enables multiple curves on a G
RA
PH
.•
New
AX
IS construct
►Im
proved Data Storage
oE
nables DD
Developer to securely store data on host
•N
ew FILE
construct describes parameters that w
ill be stored•
New
LIST construct is used w
ith FILE to access specific param
eters
Data C
omm
unications & Fieldbus S
ystems
377
GR
AP
HS
A GR
AP
His used to present
one or more w
aveforms.
Waveform
s may be device
data, calculated data or persisted data.
Radar C
alibration
AXIS
FILE/LIST
GR
APH
WAVEFO
RM
Data C
omm
unications & Fieldbus S
ystems
378
CH
AR
TS
Blue line is a reference retrieved
via FILE and A
RR
AY
A CH
AR
Tis used to present
the real-time (continuous)
device values
AXIS
CH
AR
T
SOU
RC
E
Valve Step Response
Data C
omm
unications & Fieldbus S
ystems
379
VIS
UA
LIZATIO
NS
AN
D P
ER
SIS
TEN
T DA
TA
►V
isualization Built-ins
oM
en
uD
is
pl
ay
for enabling “Wizard-like” interface using enhanced
Menus
►List (persistent storage)o
Li
st
In
se
rt
inserts an element into a list
oL
is
tD
el
et
eE
le
me
nt
At
deletes an element from
a list
►Im
proved User Interface (U
I) o
DD
Developer can describe screen layout
•E
nhanced ME
NU
construct with screen layout attributes (e.g. dialog
boxes)
Data C
omm
unications & Fieldbus S
ystems
380
IMA
GE
S A
ND
EN
HA
NC
ED
VIS
UA
LIZATIO
N
IM
AG
E
Language codes permit different
images for each language
supportedin E
DD
L
Data C
omm
unications & Fieldbus S
ystems
381
SC
AD
AS
EC
UR
ITY
►S
CA
DA
insecuritym
ayhave
contributedto
theend
ofthe►
SC
AD
Ainsecurity m
ay have contributed to the end of the C
old War
►S
CA
DA
may be of substantial interest to m
ajor terrorists
►S
CA
DA
systems m
ay suffer sabotage by disgruntled insiders,acting
individuallyinsiders, acting individually
►S
CA
DA
may
have“big”technicalfailures
►S
CA
DA
may have big
technical failures
Data C
omm
unications & Fieldbus S
ystems
382
“The Most M
onumental N
on-Nuclear E
xplosion and Fire Ever
Seen From
Space."
►Thom
as C. R
eed, Ronald R
egan’s Secretary of the A
ir Force, described in his book A
t The Abyss
(Ballantine, 2004, IS
BN
0-89141-821-0) how the U
nited States
arranged for the Soviets to receive intentionally flaw
ed process control software
for use in conjunction w
ith the US
SR
's natural gas pipelines, pipelines which w
ere to t
itill
dd
hd
fth
US
SR
generate critically needed hard currency for the US
SR
.
Reed stated that "The pipeline softw
are that was to run the pum
ps, turbines, and values
was
programm
edto
gohayw
ireaftera
decentintervalto
resetpump
values was program
med to go hayw
ire, after a decent interval, to reset pump
speeds and valve settings to produce pressures far beyond those acceptable to pipeline joints and w
elds."
The result? A three-kiloton blast in a rem
ote area of Siberia in 1982, w
hich, only by som
e miracle, apparently didn't result in any deaths. (For context, the H
alifax Fire M
useumlists
them
assive1917
MontB
lancship
explosionin
theH
alifaxH
arboratM
useum lists the m
assive 1917 Mont B
lanc ship explosion in the Halifax H
arbor at a force of 2.9 kilotons.)
Data C
omm
unications & Fieldbus S
ystems
383
Nation-S
tates Aren’t the O
nly Ones Interested in S
CA
DA
Security
►‘A
forensic summ
ary of the investigation, prepared in the Defense
Departm
ent, said the bureau found "multiple casings of sites" nationw
ide. R
outed through telecomm
unications switches in the G
ulf, Indonesia and P
akistan, the visitors studied emergency telephone system
s, electrical ,
gy
py
,generation and transm
ission, water storage and distribution, nuclear
power plants and gas facilities.
‘ Som
eofthe
probessuggested
planningfora
conventionalattackU
SS
ome of the probes suggested planning for a conventional attack, U
.S.
officials said. But others hom
ed in on a class of digital devices that allow
remote control of services such as fire dispatch and of equipm
ent such as pipelines
More
information
aboutthosedevices
andhow
toprogram
pipelines. More inform
ation about those devices --and how to program
them
--turned up on al Qaeda com
puters seized this year, according to law
enforcement and national security officials.’
Data C
omm
unications & Fieldbus S
ystems
384
SA
BO
TAG
EB
YIN
SID
ER
SM
AY
ALS
OS
AB
OTA
GE
BY
INS
IDE
RS
MA
Y A
LSO
PO
SE
A R
ISK
TO S
CA
DA
SY
STE
MS
►In 2000, in M
aroochyS
hire, Queensland, V
itekB
odenreleased
millions of liters of untreated sew
age using a wireless laptop,
apparently taking revenge against former em
ployers. He w
as arrested
convictedand
jailed*
arrested, convicted and jailed.*
*Tht
ftht
*The rest of the story…
Data C
omm
unications & Fieldbus S
ystems
385
WIR
ELE
SS
NE
TWO
RK
PO
RO
SITY
IS C
OM
MO
N
►‘P
aul Blom
gren[…
] measures control system
vulnerabilities. Last year, his com
pany assessed a large southwestern utility that serves about four m
illion custom
ers.“ Our people drove to a rem
ote substation," he recalled. "Without
leavin g their vehicle, they noticed a wireless netw
ork antenna. They plugged g
,y
yp
ggin their w
ireless LAN
cards, fired up their notebook computers, and
connected to the system w
ithin five minutes because it w
asn't using passw
ords[
]Within
15m
inutesthey
mapped
everypiece
ofequipmentin
passwords. […
] Within 15 m
inutes, they mapped every piece of equipm
ent in the operational control netw
ork. Within 20 m
inutes, they were talking to the
business network and had pulled off several business reports.
Data C
omm
unications & Fieldbus S
ystems
386
THE
($50B)9/14/2003
US
BLA
CK
OU
TTH
E ($50B
) 9/14/2003 U.S
. BLA
CK
OU
T
►“S
tarting around 14:14, FE [FirstE
nergy] control room operators lost
the alarm function that provided audible and visual indications w
hen a significant piece of equipm
ent changed from an acceptable to problem
atic status. Analysis of
the alarm problem
performed by FE
after the blackout suggests that the alarm
till
“t
lld”
hili
lt
With
thft
processor essentially “stalled” while processing an alarm
event. With the softw
are unable to com
plete that alarm event and m
ove to the next one, the alarm
processor buffer filled and eventually overflowed. A
fter 14:14, the FE control
computerdisplays
didnotreceive
anyfurtheralarm
snorw
ereany
alarms
beingcom
puter displays did not receive any further alarms, nor w
ere any alarms being
printed or posted on the EM
S’s alarm
logging facilities.“FE
operators relied heavily on the alarm processor for situational aw
areness, since
theydid
nothaveany
otherlargescale
visualizationtoolsuch
asa
dynamic
since they did not have any other large-scale visualization tool such as a dynamic
map board. The operators w
ould have been only partially handicapped without the
alarm processor, had they know
n it had failed. How
ever, by not knowing that they
were
operatingw
ithoutanalarm
processorthe
operatorsdid
notrecognizesystem
were operating w
ithout an alarm processor, the operators did not recognize system
conditions w
ere changing and were not receptive to inform
ation received later from
MIS
Oand neighboring system
s. The operators were unaw
are that in this situation they
neededto
manually,and
more
closely,monitorand
interprettheS
CA
DA
they needed to manually, and m
ore closely, monitor and interpret the S
CA
DA
information they w
ere receiving.”
Data C
omm
unications & Fieldbus S
ystems
387
BLA
CK
OU
TB
LAC
KO
UT
Data C
omm
unications & Fieldbus S
ystems
388
MU
ND
AN
EA
TTAC
KS
CO
ULD
TAR
GE
TS
CA
DA
NE
TWO
RK
MU
ND
AN
E A
TTAC
KS
CO
ULD
TAR
GE
T SC
AD
AN
ETW
OR
K
FIBE
R A
S E
AS
ILY A
S P
OW
ER
LINE
S►
SC
AD
Asystem
sare
oftenphysically
distributedoverlarge
areas,making
physicalsecurity►
SC
AD
Asystem
s are often physically distributed over large areas, making physical security
a challenge. Sim
ple vandalism is a real/w
ell known risk:
--“[…] vandals shot out approxim
ately 80 individual insulators on the BP
A C
ougar-Thurston 115
000ltt
ii
lii
ittt
fi
tthtti
Thd
li115,000 volt transm
ission line causing it to go out of service at that time. The vandalism
occurred near C
ougar Dam
, which is approxim
ately 25 miles east of E
ugene. BP
A crew
s replaced the dam
aged insulators at an estimated cost of $6,000. E
ven though no electrical service to E
WE
Band Lane E
lectric Cooperative custom
ers was disrupted by the vandalism
, p
py
,E
ugene Water and E
lectric had to purchase additional power to serve its custom
ers during the 13 hours that it took to repair the dam
aged line.”
‘AW
ashingtonm
anw
hoadm
ittedto
tampering
with
more
than20
highvoltage
--A
Washington m
an who adm
itted to tampering w
ith more than 20 high-voltage
transmission tow
ers in four Western states said yesterday he w
as trying to point out the pow
er system's vulnerabilities. "I intended to loosen the bolts and by doing so illustrate the
vulnerabilities of these towers," P
oulintold the judge. P
oulinsaid in a telephone interview
before his arrest that he considered his actions necessary to point out that he w
as able to dam
age the towers despite being "62 years old, overw
eight, arthritic, diabetic, half-blind and a cancer patient living on a m
inimum
of 12 medication pills a day.“’
Data C
omm
unications & Fieldbus S
ystems
389
SC
AD
AS
EC
UR
ITYTO
DA
YS
CA
DA
SE
CU
RITY
TOD
AY
: W
HE
RE
EN
TER
PR
ISE
SE
CU
RITY
WA
S 10 Y
EA
RS
AG
O
►“The present state of security for S
CA
DA
is not comm
ensurate with
thethreatorpotentialconsequences
Theindustry
hasgenerated
athe threat or potential consequences. The industry has generated a large base of relatively insecure system
s, with chronic and pervasive
vulnerabilities that have been observed during security assessments.
Arbitrary applications of technology, inform
al security, and the fluid vulnerability environm
ent lead to unacceptable risk. […] S
ecurity for S
CA
DA
istypically
fiveto
tenyears
behindtypicalinform
ationS
CA
DA
is typically five to ten years behind typical information
technology (IT) systems because of its historically isolated stovepipe
organization.”
Data C
omm
unications & Fieldbus S
ystems
390
THE
“HID
DE
NH
ALF”O
FTH
EN
ETW
OR
KTH
E H
IDD
EN
HA
LF O
F THE
NE
TWO
RK
►Traditionally
network
andsecurity
peoplehave
focusedvirtually
allour►
Traditionally network and security people have focused virtually all our
attention on the “enterprise” side of the network, ignoring the parallel
“hidden” half of the network associated w
ith process control systems and
embedded
systems
embedded system
s.
►P
rocess control systems and em
bedded systems use different protocols,
yy
pdifferent jargon, and no one ever really m
entioned them. They w
ere out of sight and out of m
ind, and “handled” by hardware guys.
Data C
omm
unications & Fieldbus S
ystems
391
UN
FOR
TUN
ATE
LY“H
IDD
EN
”DO
ES
NO
TA
LWA
YS
EQ
UA
LU
NFO
RTU
NA
TELY
, “HID
DE
N” D
OE
S N
OT A
LWA
YS
EQ
UA
L P
HY
SIC
ALLY
SE
PA
RA
TED
►In the old days, process control system
s used proprietary protocols and ran w
ith serial comm
unications (e.g., modem
s) or on physically separated (“air gapped”) private dedicated netw
orks, but that’s no longer always the
case.
►These days, process control system
s often run using MO
DB
US
/TCP
on the enterprise LA
N and over the Internet; process control traffic m
ay be i
ld
ithb
ilP
2Pt
ffiV
IPt
ffit
comm
ingled with w
eb pages, email, P
2P traffic, V
oIP traffic, etc.
Data C
omm
unications & Fieldbus S
ystems
392
BU
TD
ON
’TTA
KE
MY
WO
RD
FOR
ITB
UT D
ON
T TAK
E M
Y W
OR
D FO
R IT…
►‘M
ISC
ON
CE
PTIO
N#1
“ThS
CA
DA
tid
hi
ll►
‘MIS
CO
NC
EP
TION
#1 –“The S
CA
DA
system resides on a physically
separate, standalone network.”
‘Most S
CA
DA
systems w
ere originally built before and often separate from
other corporate networks. A
s a result, IT managers typically operate on the
assumption that these system
s cannot be accessed through corporate p
yg
pnetw
orks or from rem
ote access points. Unfortunately, this belief is usually
fallacious.’
“Understanding S
CA
DA
System
Security V
ulnerabilities”
Data C
omm
unications & Fieldbus S
ystems
393
SE
RIO
US
CO
NS
EQ
UE
NC
ES
OF
SE
RIO
US
CO
NS
EQ
UE
NC
ES
OF
SC
AD
A-R
ELA
TED
CO
MP
RO
MIS
ES
►W
hile enterprise network security is undeniably im
portant, unlike enterprise netw
orksecurity
SC
AD
Acom
promises
canhave
realworld
lifesafety
impacts
network security, S
CA
DA
comprom
ises can have real world life safety im
pacts.
►E
nterprise network security breach: financial consequences, custom
er privacy p
yq
,p
yis com
promised, system
s need to be rebuilt, spam gets sent, etc., but life goes
on.
►S
CA
DA
security breach? Property can be destroyed and people can be hurt
or killed.
Data C
omm
unications & Fieldbus S
ystems
394
SIM
PLE
PR
OTO
CO
LSS
IMP
LE P
RO
TOC
OLS
►B
ecauseS
CA
DA
devicesw
ithem
beddedcontrollers
tendto
havelim
ited►
Because S
CA
DA
devices with em
bedded controllers tend to have limited
computational pow
er, and were historically connected via low
speed serial lines, S
CA
DA
protocols tend to be quite simple, w
ith little or no protection i
tfi
ltt
ki
tfd
il
fi
ttk
against spoofing, replay attacks, or a variety of denial of service attacks.
►‘In
adem
onstrationata
recentsecurityconference
[JeffDagle
aP
NN
L►
In a demonstration at a recent security conference, [Jeff D
agle, a PN
NL
(Pacific N
orthwest N
ational Laboratory) EE
] hacked into his testbedsystem
and tripped an electrical breaker. The breaker then signaled the S
CA
DA
software
thatithadopened
Butthe
SC
AD
Acontrollerdid
notS
CA
DA
software that it had opened. B
ut the SC
AD
Acontroller did not
respond because it had not instructed the breaker to open. It was a classic
denial-of-service attack. "We w
ere demonstrating a w
eakness at the t
lllit
lf"id
Dl
’protocol level itself," said D
agle.’
Data C
omm
unications & Fieldbus S
ystems
395
LON
G LIFE
CY
CLE
DE
VIC
ES
►Industrial plants, and the instrum
entation they include, tend to be long life cycle projects –
ten, fifteen or twenty year project lives are by no m
eans uncom
mon. A
s a result, the devices that may be deployed as part of that
construction may be virtual antiques by the tim
e the facility is finally decom
missioned, and there’s no provision for refreshing those devices the
pg
way you m
ight upgrade out of date PC
s in some office.
►‘"A
tii
ftd
'tk
thS
CA
DA
t"
idR
bt
►‘"A
nti-virus software doesn't w
ork on these SC
AD
Asystem
s," said Robert
Childs, inform
ation security analyst at the Public S
ervice Com
pany of New
M
exico, who spoke at N
etSec
about the challenges in working w
ith SC
AD
Avendors to get them
to comply w
ith the new rules. "M
any of these systems
are based on old Intel 8088 processors, and security options are limited to
us.“
Data C
omm
unications & Fieldbus S
ystems
396
WIN
DO
WS
-BA
SE
D C
ON
TRO
L STA
TION
S
►S
CA
DA
devices are often controlled from central m
onitoring i
(MTU
“i
li
”)H
ii
llh
stations (MTU
s, or “master term
inal units”). Historically those
were U
nix-based systems, but m
any contemporary M
TUs
are now
MicrosoftW
indows
based.now
Microsoft W
indows based.
Data C
omm
unications & Fieldbus S
ystems
397
HA
RD
-TO-U
PG
RA
DE
RE
MO
TE D
EV
ICE
S
►R
emote devices (R
TUs
and PLC
s) also tend to be hard to upgrade :
--the device may use an O
S and application that w
as burned to RO
M, and
which is not rew
ritable (“upgrade” = replacing RO
Ms)
--the device may be physically sealed and not upgradeable, or be located in a
difficult location, or have no removable m
edia
---the vendor may no longer be in business, or m
ay not be producing upgrades, or the vendor m
ay not be allowing
upgrades
Data C
omm
unications & Fieldbus S
ystems
398
CE
RTIFY
ING
PA
TCH
ES
►A
n example from
the embedded system
world:
“Health care IT professionals say m
edical device makers prohibit them
from
changing the systems and even from
running anti-virus software in
ThIT
di
it
tf
tft
lt
some cases. These IT adm
inistrators say manufacturers often are slow
to supply softw
are patch updates and routinely claim the Food and D
rug A
dministration (FD
A) requires approval of patch-base changes. H
owever
the FDA
says it has no such rules…”
Data C
omm
unications & Fieldbus S
ystems
399
NE
ED
FOR
PO
SITIV
E C
ON
TRO
L –S
IMP
LE K
NO
WN
/SH
AR
ED
PA
SS
WO
RD
S
►B
ecause of the need for positive access and control, there is a trend toward
psim
ple, known, and shared passw
ords. Users like to avoid situations such as:
“Do you know
the password to turn off the nuclear reactor before it m
elts dow
n?Iforgotm
inetoday…
”dow
n? I forgot mine today…
►B
ut there’s hope: people in the SC
AD
Acom
munity are beginning to talk about
strong authorization systemsD
ata Com
munications &
Fieldbus System
s400
CO
MM
ON
PA
SS
WO
RD
SC
OM
MO
N P
AS
SW
OR
DS
A
CR
OS
S M
ULTIP
LE D
EV
ICE
S
►There’s also the sheer issue of m
anaging passwords for thousands of
devices–
passwords
willtend
tobe
comm
onacross
devicesas
apractical
devices –passw
ords will tend to be com
mon across devices as a practical
matter (this is m
uch like SN
MP
comm
unity strings)
►►A
nd of course those passwords aren’t changed very often (if at all), even
when staff transitions occur or years have gone by…
Data C
omm
unications & Fieldbus S
ystems
401
PLA
IN TE
XT (U
NE
NC
RY
PTE
D) TR
AFFIC
►These days, few
of us would be w
illing to send our passwords over plain
text transmissions paths (as w
e would w
hen using telnet), yet plain text transm
issions are still very comm
on in the SC
AD
Aw
orld.
►In the realtim
ew
orld, encryption overhead and jitter m
ay be the crucial problems to overcom
e…
Data C
omm
unications & Fieldbus S
ystems
402
ALL TR
AFFIC
IS O
N JU
ST O
NE
PO
RT
►In m
any cases, SC
AD
Atraffic w
ill be on just one port such as 502/tcp(e.g., M
odbus/TCP
). This is both good and bad.
►The use of a single port (or just a couple of ports) m
akes it easy to track thattraffic
ortopoke
ahole
infirew
allsto
allowthattraffic
topass
butitthat traffic, or to poke a hole in firew
alls to allow that traffic to pass, but it
also makes it easy for the bad guys to scan for connected devices, and it
makes it im
possible to do port-based selective filtering.
Data C
omm
unications & Fieldbus S
ystems
403
IN TH
E IN
TER
ES
T OF B
ALA
NC
E:
A D
ISS
EN
TING
OP
INIO
N►
“Despite tantalising
accounts of terrorist interest in targeting SC
AD
Anetw
orks and other critical infrastructure, there actually appears to be little interest am
ongthe
hackercomm
unityin
developingtools
andexploits
againstPLC
oram
ong the hacker comm
unity in developing tools and exploits against PLC
or industrial protocols such as M
odbus/TCP
or Ethernet/IP
. Unlike IT products,
tools for automatically "hacking " P
LCs, rem
ote IO devices, robots, or E
thernet-b
dt
dilil
blbased sensors are not readily available.
“Bedroom
hackers with little or no know
ledge of automation system
s are, in reality, unlikely to cause deliberate harm
.”
Data C
omm
unications & Fieldbus S
ystems
404
Day Five
►E
thernet
►H
ost to Host layer protocols
►P
lanning SC
AD
A projects
►G
lossary
Data C
omm
unications & Fieldbus S
ystems
405
ETH
ER
NE
T LAN
CO
NC
EP
T VIE
W
Ethernet w
as the first LAN
technology to be developed -- the pioneering research w
ork being done in the early 1970's.
It was developed by X
erox at a time w
hen support technology w
as not cost-effective, nor was there a
significant market.
The microprocessor developm
ents of the late 1970's changed both issues, m
aking the technology viable and creating a m
arket via the personal computer.
To further develop and promote ethernet technology, X
erox partnered with Intel and D
EC
, creating a strong corporate credibility that has helped m
ake ethernet the most popular LA
N technology.
The IEE
E adopted ethernet as the basis of the IE
EE
802.3 standard (alth
ough w
ith su
btle
diffe
rence
s from
the ve
rsion p
rom
ote
d b
y DE
C/IN
TE
L/X
ER
OX
-- the so
-calle
d "D
IX" e
thern
et).
Ethernet is a bus architecture system
(which
implie
s a co
mm
on, o
r party-lin
e b
roadca
st mediu
m)
which is shared using a contention access control technique know
n as carrier sense multiple
access with collision detection (C
SM
A/C
D).
D-116
D
ata Com
munications &
Fieldbus System
s406
D-276
ETH
ER
NE
T CO
LLISIO
N R
EC
OV
ER
Y P
RO
CE
SS
Whenever there is a collision involving tw
o or more stations, they w
ill each independently determine
that there has been a problem, stop transm
itting, and reattempt the transm
ission later.
The retry waiting tim
e is randomly selected (fro
m w
ithin
a sp
ecifie
d tim
e in
terva
l), so as to minim
ize the risk of a second collision event.
How
ever, when the system
is busy, there is a significant probability of a second collision with som
e other station.
In order to separate (in tim
e) stations that have collided (a
nd to
smooth
out tra
ffic patte
rns th
at p
eak a
t so
me m
om
ent -- p
resu
mably ca
use
d b
y som
e e
xtern
al e
vent), the protocol provides for a station that
encounters more than one collision to double the size of the w
indow w
ithin which it sets its random
tim
er for each subsequent retry interval.
This process makes it increasingly likely that a station that has had a run of bad luck (in
gettin
g its
messa
ge tra
nsm
itted) w
ill encounter increasingly bad luck (i.e. It b
eco
mes a
first in - la
st out q
ueuin
g
pro
cess).
This doubling of the time w
indow continues for 10 events (so
that th
e ra
ndom
time se
lectio
n w
indow
ca
n g
row
to 2
**10 =
1024 tim
es its o
rigin
al va
lue). If the collisions continue beyond 10 events, the
time w
indow stays unchanged up to 16 collisions, at w
hich time an error is reported to the LLC
layer, typically requiring som
e kind of manual intervention. This tim
er control scheme is called the truncated
exponential backoff algorithm.
As a consequence of this m
echanism, it is not possible to guarantee a bound on ethernet access tim
e delay -- such as is often required for real-tim
e applications.
Data C
omm
unications & Fieldbus S
ystems
407
CO
AX
IAL C
AB
LE: C
omprised of a center core conductor, cylindrical insulator and outer cylindrical
shield conductor surrounding the assembly. Typically, an outer insulating jacket is provided
around the composite cable. E
thernet uses a 50 Ohm
cable (simila
r to R
G 2
13
), colo
red
ye
llow
if it is PV
C in
sula
ted
; ora
ng
e if it is p
len
um
-safe
).
PH
YS
ICA
L ME
DIU
M A
TTAC
HM
EN
T
PH
YS
ICA
L (S
IGN
ALIN
G IN
TER
FA
CE
)
ME
DIU
M A
CC
ES
S C
ON
TRO
L
LOG
ICA
L LINK
CO
NTR
OL
UP
PE
R LE
VE
L FU
NC
TIO
NS
ATTA
CH
ME
NT U
NIT
CA
BLE
(9 wire, 15 pin
connector, maxim
um
length is 50 meters)
ATTA
CH
ME
NT U
NIT
INT
ER
FAC
E (A
UI)
CO
NN
EC
TOR
S
ME
DIU
M
ATTA
CH
ME
NT
UN
IT(TR
AN
SC
EIV
ER
)
ATTA
CH
ME
NT
UN
ITIN
TER
FAC
E
(AU
I)
TWO
CO
ND
UC
TOR
PR
ES
SU
RE
C
ON
TAC
T/CO
NN
EC
TION
(V
ampire Tap) LO
CA
TED
AT
MIN
. 2.5 m IN
TER
VA
LS(C
able is marked every 2.5 m
)
OS
IP
HY
SIC
AL
LAY
ER
OS
ILIN
KLA
YE
R
"THIC
K-W
IRE
" ETH
ER
NE
T CA
BLE
ATTA
CH
ME
NT
D-082
NE
TWO
RK
INTE
RFA
CE
CA
RD
(NIC
) FUN
CT
ION
S
D
ata Com
munications &
Fieldbus System
s408
CO
AX
IAL C
AB
LE: C
omprised of a center core conductor, cylindrical insulator and outer cylindrical
shield conductor surrounding the assembly. T
hinwire coax is R
G58 R
/U, 50 O
hm cable,
approximately 1/4 inch diam
eter.
PH
YS
ICA
L (S
IGN
ALIN
G IN
TER
FAC
E)
ME
DIU
M A
CC
ES
S C
ON
TRO
L
LOG
ICA
L LINK
CO
NTR
OL
UP
PE
R LE
VE
L FU
NC
TION
S
OS
IP
HY
SIC
AL
LAY
ER
OS
ILIN
KLA
YE
R
"THIN
-WIR
E" E
THE
RN
ET C
AB
LE A
TTAC
HM
EN
T
D-084
NE
TWO
RK
INTE
RFA
CE
CA
RD
(NIC
) FUN
CTIO
NS
AAAA
AAAA
AAAAAAAAAAAA
AAAAAAAAA
AAAAAAAAAAAA
AAAAAAAAA
BN
C-Type tw
ist-lock connector is used. E
ach workstation has a "T" connection,
thus providing for an onward connection
to other users.
A cable term
ination is required at the last station on a cable run.
Data C
omm
unications & Fieldbus S
ystems
409
���
���
���
D-021
ET
HE
RN
ET U
TP W
IRIN
G C
ON
CE
PT
S
RJ-45 P
ATC
H P
AN
EL
(OP
TIO
NA
L)
WA
LL PLA
TE A
ND
RJ-45 C
ON
NE
CT
OR
PA
TC
H C
OR
D
TH
E W
IRIN
G H
UB T
AK
ES T
HE F
OR
M O
F A M
ULT
I-PO
RT R
EP
EA
TER (O
BS
OLE
TE
), IN
TE
LLIGE
NT O
R S
WIT
CH
ING H
UB
UN
SH
IELD
ED T
WIS
TE
D-P
AIR (U
TP
)H
OU
SE C
AB
LING IS
ALW
AY
S "H
OM
E R
UN
"
Data C
omm
unications & Fieldbus S
ystems
410
RJ-45 D
AT
AC
ON
NE
CT
OR
(RE
CE
PT
AC
LE
VIE
W)
PIN N
UM
BE
R1 2 3 4 5 6 7 8
Ethernet T
ransmit P
air
Ethernet R
eceive Pair
+-+-
D-424
Standard C
able Configurations:
Ethernet (10 and 100 B
ase-T)
12345678
ET
HE
RN
ET R
J-45 PIN
CO
NN
EC
TIO
N C
ON
VE
NT
ION
S
Data C
omm
unications & Fieldbus S
ystems
411
K-074
IND
US
TR
IAL E
TH
ER
NE
T CO
NN
EC
TO
RS
AB
OV
E: C
ON
TA
CT
CO
RR
OS
ION
;B
ELO
W: C
ON
TAC
T WE
AR
DU
E T
O V
IBR
AT
ION
RJ45 C
ON
TAC
T PR
OB
LEM
ST
he normal, office-oriented R
J-45 Ethernet connector is a
source of concern when adapted to industrial
applications.
This is m
ainly due to the light-duty, fragile connections that are m
ade when the connectors are crim
ped onto the cables, and the m
inimal conducting surface area w
hen they are m
ated.
Industrial environmental issues include exposure to dust,
temperature extrem
es, excessive humidity levels,
electromagnetic interference, and vibration -- all of w
hich can significantly jeopardize reliability.
In this context, standard RJ-45 plugs and jacks m
ay corrode, w
ear, clog with debris, and eventually fail.
Failing to take appropriate care w
ith these seemingly
trivial items can lead to serious unreliability problem
s and resulting high m
aintenance costs.
continued ....
Data C
omm
unications & Fieldbus S
ystems
412
K-075
Siem
on's Industrial Max connectors (w
ww
.siemon.com
)
AM
P N
etconnect Etherseal connectors (w
ww
.ampnetconnect.com
)
One approach is to fit the conventional R
J-45 plugs and jacks with an outer housing that w
ill protect them
from dirt, hum
idity and vibration exposure -- see examples show
n in box below.
Variations on this idea typically include screw
and bayonet-style sheaths, or jackets that are designed to protect the encased R
J-45 plug and jack assembly.
IND
US
TR
IAL E
TH
ER
NE
T CO
NN
EC
TO
RS
, cont.
Alternatives to these "hardened"
RJ-45 E
thernet connector designs is a com
pletely different connector approach.
Som
e have used DB
-9 shell connectors (com
monly used on
PC serial ports) -- w
hich can be solder-connected and screw
anchored.
Term
inating individual conductors w
ith screw lugs and barrier strips
is another possibility -- a very reliable and robust solution, but labor-intensive to install.
Data C
omm
unications & Fieldbus S
ystems
413
ETH
ER
NE
T CO
NFIG
UR
ATIO
N LIM
ITATIO
NS
D-085
TH
ICK
-WIR
E E
TH
ER
NE
T (10-B
AS
E-5)
Cable segm
ent maxim
um length: 500 m
etersM
aximum
number of attachm
ents per segment: 100 taps
Minim
um tap separation: 2.5 m
etersM
aximum
number of devices per netw
ork: 1024 (limita
tion
com
mo
n to
all co
nfig
ura
tion
s)
TH
IN-W
IRE
ET
HE
RN
ET
(10-BA
SE
-2)
Cable segm
ent maxim
um length: 200 m
eters (som
e 1
89
me
ters)
Maxim
um num
ber of attachments per segm
ent: 30 attachments
Minim
um connection-point separation: 1 m
eter
TW
IST
ED
-PA
IR E
TH
ER
NE
T (10-B
AS
E-T
)
Star-w
ired cable configuration; most vendors lim
it cable lengths to 100 meters
Each w
ire run supports only one userM
ulti-port repeater hubs aggregate multiple user connections
FIB
ER
OP
TIC
BA
SE
D E
TH
ER
NE
T (10-B
AS
E-F
)
Fiber cable configurations are nom
inally suited to "backbone" roles, linking star couplersM
aximum
fiber cable segment/system
length with passive star couplers: 2,200 m
etersM
aximum
fiber cable system (e
nd
-to-e
nd
) length with active collision detectors: 4,400 m
eters
Data C
omm
unications & Fieldbus S
ystems
414
D-426
FIBE
R O
PTIC
ETH
ER
NE
T CO
NFIG
UR
ATIO
NS
Ethernet based on fiber optic cable segm
ents will deploy in one of three configurations; point-to-point,
passive star coupled and active star coupled.
Po
int-to
-Po
int
Point-to-point use of fiber optic technology is sim
ple, involving a copper-to-fiber repeater connection at each end
(of a
two-fib
er ca
ble
span).
This class of service is typically configured using com
ponents compatible w
ith passive star coupling (se
e b
elo
w). Thus, this connection is called a passive fiber optic segm
ent (PF
OS
).
Passive S
tar Co
up
led
The passive star coupler provides an optically coupled connection (light p
ath
) between all of its cable
ports -- really an optical hub.
Passive-connected ethernet com
ponents (mediu
m a
ttach
ment u
nits a
nd co
uple
rs) use a different data form
at and a different collision detection method than com
parable active components -- and also
different from conventional m
etallic cabled systems.
Passive optical segm
ents (PF
OS
) may be up to 500 m
eters length.
Data C
omm
unications & Fieldbus S
ystems
415
D-427
FIBE
R O
PTIC
ETH
ER
NE
TC
ON
FIGU
RA
TION
S, cont.
Passive S
tar Co
up
led, cont.
Correct functioning of the collision detection logic in this passive configuration is based on a data
reformatting procedure som
ewhat like that used w
ith token ring, whereby an 'illegal' m
anchester data pattern is generated, thus elim
inating the long synchronization preamble in m
etallic ethernet data form
ats.
Active S
tar Co
up
led
The active star coupled configuration provides for complete optical signal regeneration at the coupler,
or hub point -- thus significantly increasing the number of ports that can be feasibly supported, and the
practical maxim
um span distance.
Maxim
um active fiber optic segm
ent length is 2,000 meters
Active hubs support collision detection at the hub itself (w
ith th
e S
QE
collisio
n sig
nal e
nco
ded in
the
modifie
d d
ata
stream
) -- this being the principal feature enabling the now 4,000 m
eter edge-to-edge optical backbone segm
ent.
D
ata Com
munications &
Fieldbus System
s416
K-093
�������������������������������
�������������������������������
W/S
W/S
W/S
W/S
W/S
W/S
W/S
W/S
OH
H H
OH
H
FIE
LD T
ER
MIN
AL
FIE
LD T
ER
MIN
AL
CA
BLE
CO
NN
EC
TIV
ITY B
ET
WE
EN B
UILD
ING
S
FIB
ER O
PT
ICC
AB
LE LIN
K
Where m
ultiple building structures are to be linked by a LAN or other netw
ork connection, it is very im
portant that all inter-building connections be made either w
ith fiber optic cable (rather than a m
etallic medium
like UT
P or coaxial cable), or that careful attention be paid to the electrical
characteristics (i.e. whether properly isolated or properly bonded) of m
etallic grounds.
This is because the electrical and grounding system
s for the buildings will typically be
independent, or poorly bonded -- with the result that netw
ork cables linking the two structures offer
the lowest im
pedance connection. The risk is that electrical fault or lightning current surges w
ould potentially destroy the connected equipm
ent -- and create a serious personnel hazard.
114
Data C
omm
unications & Fieldbus S
ystems
417
TR
AF
FIC
DE
MA
ND
(Megabits per S
econd)2.5
5.07.5
10
D-241
ETH
ER
NE
T CH
AN
NE
L UTILIZA
TION
PR
OFILE
CO
LL
ISIO
N S
AT
UR
AT
ED
RE
GIO
N(T
raffic
Ja
m S
yn
dro
me
)
US
EF
UL
( Net) D
AT
A
TR
AN
SM
ISS
ION
RE
GIO
N
PA
CK
ET
S
(Incl. Overhead
)S
UC
CE
SS
FU
LL
Y T
RA
NS
MIT
TE
D
EFFECTIVE THROUGHPUT EFFICIENCY
0
100 %
0
UN
SA
TIS
FA
CT
OR
Y O
PE
RA
TIN
G R
EG
ION
(due to network saturation)
OP
TIM
AL O
PE
RA
TIN
G
RE
GIO
NH
IGH
CO
ST
OP
ER
AT
ING
R
EG
ION
80 %
60 %
40 %
20 %
Data C
omm
unications & Fieldbus S
ystems
418
ET
HE
RN
ET F
RA
ME LE
NG
TH vs. P
HY
SIC
AL LE
NG
TH
W/S
A
2
W/S
B
W/S
A
3
W/S
BO
NE
-WA
Y TR
AN
SIT TIM
E = t seconds
AW
/S
BO
NE
-WA
Y T
RA
NS
IT TIM
E = t seconds
RE
TUR
N TR
AN
SIT TIM
E = t seconds
1
TH
E W
OR
ST C
AS
E R
OU
ND T
RIP T
RA
NS
IT T
IME (2 x t seconds) D
ET
ER
MIN
ES T
HE
MIN
IMU
M A
LLOW
AB
LE F
RA
ME LE
NG
TH
W/S
D-270
To appreciate the relationship betw
een m
essage (frame) length and overall physical
dimensions of the LA
N, consider the follow
ing:
Step 1: A
ssume the channel is idle, and "A
" is ready to transm
it, and it does. The fram
e from
"A" propagates at finite speed to the extrem
ity of the cable system
, taking a time w
hich is dependent on cable length.
Step 2: A
ssume "B
" now m
akes the (unfortunate) decision to start transm
itting m
omentarily before the fram
e from "A
" arrives, and before it is in a position to know
.that there is other traffic on the channel.
Step 3: "B
" learns imm
ediately that there has been a collision -- but "A" has no know
ledge of the conflict until sufficient tim
e elapses for two round-trip fram
e transits of the network.
Thus, the m
inimum
length of an Ethernet fram
e must be sufficiently long so that "A
" is assured to be still busy transm
itting when the collision inform
ation comes available at "A
" (both parties must
know w
hen there has been a collision).
Therefore, the m
inimum
length message m
ust occupy the channel for 2 x t seconds. In practice, the IE
EE 802.3 standard specifies a 64 byte m
inimum
frame length for this reason.
Data C
omm
unications & Fieldbus S
ystems
419
D-743
DIS
TA
NC
E C
ON
ST
RA
INT
S W
ITH E
TH
ER
NE
T
There are tw
o unrelated constraints on cable length in Ethernet system
s:
Collision D
iameter
It is critical that devices that share a comm
on Ethernet segm
ent are capable of effectively detecting collisions that they have contributed to.
Each of the several 802.3-series of E
thernet standard specifications prescribe a maxim
um
device separation within w
hich distance this capability is assured -- the collision diameter.
This issue is only m
eaningful in classical shared-cable configurations in which collisions can and
do occur because of the half-duplex nature of the cable signalling -- and with full duplex
configurations where collisions are im
possible, drive distance is the only concern.
Drive D
istance
The drive distance is sim
ply the maxim
um distance over w
hich a survivable electrical (or optical) signal can be reliably delivered, considering cable and signal characteristics.
Drive distance is typically a less restrictive constraint than collision diam
eter, and depending on cable type selected can be as large as m
any kilometers.
Thus, full duplex fiber-connected E
thernet systems are feasibly able to operate over m
etropolitan area dim
ensions -- making E
thernet into a credible MA
N technology.
Data C
omm
unications & Fieldbus S
ystems
420
D-501
DR
IVE D
IST
AN
CE
Drive distance is the m
aximum
distance a signal is able to travel over a specified medium
, and still be usable at the receiving end.
Minim
um drive distance is alw
ays specified as part of a LAN standard, although vendors are
sometim
es able to extend these distances as long as only their technology is used (effectively, proprietary extensions of the standards).
100 BA
SE
-T supports a hub-centered LA
N diam
eter of 210 meters (105 m
eters per lobe, or drop cable -- com
pared with 500 m
eters for 10 Mbps E
thernets).
Thus, in upgrading a netw
ork to 100 Mbps operation, it m
ay be necessary to reconfigure the netw
ork to ensure the physical size of the system m
eets these constraints -- by partitioning the netw
ork and replacing repeater-based hubs with bridging or routing sw
itching hubs.
Drive distance constraints are alw
ays much less lim
iting with the use of fiber optic m
edia, because of the superior transm
ission characteristics of the medium
.
Extrem
e Netw
orks (ww
w.extrem
enetworks.com
) pioneered the use of full-duplex Ethernet
(thus avoiding the distance constraints created by collision diameter lim
its) over fiber optic m
edia, adapting Ethernet to m
etropolitan-scale networks (M
AN
s).
Data C
omm
unications & Fieldbus S
ystems
421
FULL D
UP
LEX
LAN
TEC
HN
OLO
GY
,C
ON
FIGU
RA
TION
EX
AM
PLE
D-619
Full duplex LA
N technology elim
inates all problems w
ith access control (esp
ecia
lly significa
nt w
ith
eth
ern
et, w
here
the d
iam
ete
r of th
e co
llision d
om
ain
strongly in
tera
cts with
perfo
rmance
).
Full duplex configurations are by definition point-to-point (with
one d
evice
havin
g u
nch
alle
nged a
ccess
to o
ne co
mm
unica
tion p
ath
).
Application of the concept is typically lim
ited to backbone (hub-to
-hub) connectivity, but m
ay be reasonably extended to include high volum
e end systems, such as enterprise servers.
FU
LL DU
PLE
XB
AC
KB
ON
EH
UB
SY
STE
M(M
AY
SU
PP
OR
TD
IVE
RS
E
TE
CH
NO
LOG
IES
)
FU
LL DU
PLE
XE
XT
EN
SIO
N T
OH
IGH
DE
MA
ND
SE
RV
ER
S/U
SE
RS
D
ata Com
munications &
Fieldbus System
s422
DA
TA
FIE
LD
(LO
GIC
AL
LIN
K
CO
NT
RO
L
INF
OR
MA
TIO
N)
FRA
ME
CH
EC
KS
EQ
UE
NC
E
4 B
YT
ES
14 o
r 22
BY
TE
S
SO
UR
CE
AD
DR
ES
SDESTINATIO
N ADDRESS
7 B
YT
ES
2 o
r 6 B
YT
ES
2 o
r 6 B
YT
ES
1 B
YT
E4 B
YT
ES
D-143
IEE
E 802.3 - E
THE
RN
ET FR
AM
E FO
RM
ATS
VA
RIA
BL
E L
EN
GT
H(M
INIM
UM
46
BY
TE
S)
STA
RT
FRA
ME
D
ELIM
ITER
SY
NC
H.
PR
EA
MB
LE
0 to
15
00
BY
TE
S
LEN
GTH
CO
UN
TER
2 B
YT
ES
PAD (FILL)CHARACTERS
INF
OR
MA
TIO
N(V
AR
IAB
LE
LE
NG
TH
)S
SA
PDSAP
1 B
YT
E4
2 to
14
97
BY
TE
S
CO
NTR
OL
FIELD
1 B
YT
E1
or 2
BY
TE
S
LO
GIC
AL
LIN
K C
ON
TR
OL
LA
YE
R(L
LC
) FR
AM
E S
TR
UC
TU
RE
ME
DIA
AC
CE
SS
CO
NT
RO
L L
AY
ER
(MA
C) F
RA
ME
ST
RU
CT
UR
E
PR
EA
MB
LE
: Consists of a 7 byte string of alternating 1's and 0's.
Purpose is to establish bit-level synchronization at all receiving stations.
ST
AR
T F
RA
ME
DE
LIM
ITE
R: C
onsists of a 10101011 bit pattern (contin
uatio
n
of th
e p
ream
ble
patte
rn, m
arke
d w
ith "1
1" a
t the e
nd) ; P
urpose is to mark
the start of the frame.
AD
DR
ES
S F
IEL
DS
: Message destination and source address specification.
There are tw
o forms; 2 byte form
is "locally administered", 6 byte form
is "universally adm
inistered" (no 2
device
s are
manufa
cture
d w
ith th
e sa
me
unive
rsal a
ddre
ss).
LE
NG
TH
CO
UN
TE
R: S
imple m
essage length specification (byte
count).
DA
TA
FIE
LD
: Contains data content provided by L
LC layer; com
bined with P
AD
F
IELD
, composite m
ust be no shorter than 46 bytes, ensuring com
plete m
essage is never less than 64 byte length.
PA
D F
IEL
D: O
ptional field provides filler characters to ensure compliance w
ithm
essage length requirements.
FR
AM
E C
HE
CK
SE
QU
EN
CE
: Consists of a 4 byte C
RC
error detection code.
DS
AP
and
SS
AP
: Destination and S
ource Service A
ccess Point indicators
(poin
ters o
r links to
upper la
yer p
roto
cols -- d
estin
atio
n a
nd so
urce
resp
ective
ly).
Data C
omm
unications & Fieldbus S
ystems
423
100 BA
SE
-T (FAS
T ETH
ER
NE
T)
D-451
Ethernet users are m
oving rapidly to replace a default 10 BA
SE
-T architecture with 100 B
AS
E-T --
or at least implem
ent new system
s with a sim
ple upgrade path (switch
sele
ctable
NIC
's and
Cate
gory 5
UT
P).
100 BA
SE
-T technology operates over the same U
TP/fiber optic m
edia as 10 BA
SE
-T, and is capable of supporting the conventional 100 m
eters span required for typical workstation access.
This new technology is interoperable w
ith 10 BA
SE
-T, and supports the same shared cable
collision-arbitrated mode of operation.
Sim
ilarities to 10 BA
SE
-T
CS
MA
/CD
access protocolIE
EE
Standard 802.3 and broadly supported by vendor com
munity
100 meter lobe star topology (h
ub b
ase
d)
Cable m
edia: Category 3, 4 or 5 and fiber optic
Differences from
10 BA
SE
-T
10-fold increase in transmission speed
Does not support obsolescent bus topology cable plant
Operating diam
eter of collision domain m
ax. = 210 meters (c.f. 5
00 m
ete
rs for 1
0 B
AS
E-T
)
D
ata Com
munications &
Fieldbus System
s424
D-452
MIX
ED
TE
CH
NO
LOG
Y E
TH
ER
NE
TS
AM
PLE
R
SW
ITC
HIN
GH
UB
SH
AR
ED
M
ED
IA H
UB
100 BA
SE
-TS
WIT
CH
ED
HU
B LA
N(10 B
AS
E-T
DR
OP
S)
100 BA
SE
-TS
HA
RE
D M
ED
IA H
UB
10 BA
SE
-TS
HA
RE
D M
ED
IA LA
N10 B
AS
E-T
SW
ITC
HE
D LA
N
10 BA
SE
-TC
ON
NE
CT
ION
100 BA
SE
-TC
ON
NE
CT
ION
SH
AR
ED
ME
DIA
HU
BS
WIT
CH
ING
HU
B
SW
ITC
HIN
GH
UB
100 BA
SE
-TS
WIT
CH
ED
HU
B LA
N(100 B
AS
E-T
DR
OP
S)
Data C
omm
unications & Fieldbus S
ystems
425
D-659
100 BA
SE
-T E
TH
ER
NE
T
PR
OT
OC
OL S
TA
CK
PH
YS
ICA
L L
AY
ER
PR
OT
OC
OL
(PH
Y)
ME
DIA
AC
CE
SS
CO
NT
RO
L (M
AC
)
IEE
E 8
02
.3P
HY
SIC
AL
LA
YE
R
IEE
E 8
02
.3C
SM
A/C
D
IEE
E 8
02.2
LL
C
UP
PE
R L
AY
ER
S(e
.g. In
tern
et P
roto
co
l)
UP
PE
R L
AY
ER
S(e
.g. In
tern
et P
roto
co
l)
IEE
E 8
02.2
LL
C
IEE
E 8
02.3
CS
MA
/CD
or F
UL
L D
UP
LE
X M
AC
IEEE 802.3 100 Base-T ETHERNET
ANSI X3T9 FDDI IEEE 802.3 ETHERNET
Fast E
thernet (100 B
ase
-T) borrow
s the low
er layer protocols directly from
the AN
SI F
DD
I specifications, and the upper layers of the traditional IE
EE
802-series standards
PH
YS
ICA
L M
ED
IUM
DE
PE
ND
EN
T (P
MD
)
PH
YS
ICA
L L
AY
ER
PR
OT
OC
OL
(PH
Y)
PH
YS
ICA
L M
ED
IUM
DE
PE
ND
EN
T (P
MD
)
D
ata Com
munications &
Fieldbus System
s426
D-500
An im
portant managem
ent challenge with any system
atic upgrade of technology is the logistical issue of m
aintaining operational continuity through the process.
The IE
EE standard specifies an optional speed sensing feature, properly called N
way autosensing
, w
hich allows both 10 B
AS
E-T and 100 B
AS
E-T technology to coexist on the sam
e network.
Autosensing devices are able to sense w
hether or not there are 10 BA
SE
-T devices on the LAN
, or if the netw
ork is 100% 100 B
AS
E-T equipped.
The system
runs as a 10 BA
SE
-T network as long as there are any rem
aining devices with only 10
Mbps capability -- and w
hen the last 10 BA
SE
-T device is replaced, the whole system
automatically
shifts gears and operates at the higher rate.
This parallel 10/100 dual speed feature creates a big operational bonus by enabling a seam
less upgrade/transition path to the m
ore desirable 100 BA
SE
-T mode of operation.
Early versions of this autosensing technology had a problem
with m
arginal cable configurations, ill advisedly shifting to 100 M
bps when connected by inferior cable infrastructure.
New
er autosensing algorithms sense signal quality as w
ell as bit rate, thus making a m
ore informed
decision about operating speed (and some w
ill send a SN
MP T
rap message to the adm
inistrator advising of deficient operating conditions).
AU
TO
SE
NS
ING
Data C
omm
unications & Fieldbus S
ystems
427
GIG
AB
IT ET
HE
RN
ET
Gigabit E
thernet is a viable, popular super-high speed Ethernet technology that is capable of carrying
standard Ethernet fram
es over backbone architected systems (i.e. hub-to-hub connections) at 1.062
gigabit per second.
The application of gigabit E
thernet is principally deployed as a backbone technology, typically with
100 Mbps E
thernet used for access to end devices.
The physical layer standard adopted for gigabit E
thernet is taken from pre-existing F
ibre Channel
standards.
Although contention control procedures are part of the gigabit E
thernet standard, most of the actual
applications operate full duplex, point-to-point between sw
itching hubs.
In order to preserve functional compatibility w
ith earlier versions, gigabit Ethernet m
aintains a true collision controlled (C
SM
A/C
D) E
thernet capability, operating over the same 200 m
eter collision diam
eter that is supported by 100 BA
SE
-T.
This is achieved through a schem
e to modify the E
thernet frame to extend its length as it transits
gigabit Ethernet segm
ents, so as to ensure that collision detection capability is sustained.
Gigabit E
thernet has overwhelm
ed the competitive alternative offered by A
TM
, partly because of slow
progress in resolving outstanding standards details that have hobbled acceptance of AT
M, partly
because of more attractive installed costs, and partly because of the obvious sim
plicity of maintaining
a coherent technology all the way from
the desktop to the backbone.
(20422)
D
ata Com
munications &
Fieldbus System
s428
D-250
PR
ES
EN
TA
TIO
N
SE
SS
ION
TR
AN
SP
OR
T
AP
PLIC
ATIO
N
NE
TW
OR
K
DA
TA LIN
K
PH
YS
ICA
L
HT
ML
JAV
AS
CR
IPT
SE
CU
RE
SO
CK
ET
SLA
YE
R
HTTP
NE
TW
OR
K
AC
CE
SS
LAY
ER (N
AL)
HO
ST
-TO
-HO
ST
LAY
ER
EX
TE
NS
ION
S A
ND
AD
DIT
ION
S T
O T
HE
BA
SE M
OD
EL S
UP
PO
RT
FU
NC
TIO
NS N
OT
PR
OV
IDE
D IN
EA
RLIE
R
DE
FIN
ITIO
NS
INTE
RN
ET
CO
MP
UTIN
GA
RC
HITE
CTU
RE
- BA
SE M
OD
EL
TC
P
PH
YS
ICA
L LA
YE
R
(PH
Y)
LLC S
UB
-LAY
ER
MA
C S
UB
-LAY
ER
IEE
E LA
N S
TA
ND
AR
DS
PR
OV
IDE F
OR
SP
LITT
ING T
HE D
AT
A
LINK LA
YE
R IN
TO T
WO
PA
RT
S:
TH
E LO
GIC
AL LIN
K
CO
NT
RO
L S
UB
-LAY
ER (LLC
)T
HE M
ED
IA A
CC
ES
S
CO
NT
RO
L S
UB
-LAY
ER (M
AC
)
AP
PLIC
ATIO
NLA
YE
R
RE
LAT
ION
SH
IP: O
SI A
ND IC
A LA
YE
RE
D P
RO
TO
CO
L VIE
WS
TRA
DITIO
NA
LO
SI 7-LA
YE
RM
OD
EL
INTE
RN
ET
CO
MP
UTIN
GA
RC
HITE
CTU
RE
- DE FA
CTO
INTE
RN
ET
PR
OTO
CO
LIN
TER
NE
TP
RO
TOC
OL
Data C
omm
unications & Fieldbus S
ystems
429
CO
NN
EC
TIO
NLE
SS vs.
CO
NN
EC
TIO
N-O
RIE
NT
ED S
ER
VIC
ES
Netw
ork technologies to which
this concept applies:
OS
I standards : The O
SI
standards are mostly
connection-oriented at all layers.
TCP
/IP -- T
CP is connection-
oriented; IP is connectionless.
LAN system
s - typically low-level
LAN protocols are connectionless.
D-220
CO
NN
EC
TION
-OR
IEN
TED S
ER
VIC
ES
Involve a call (session) set-up procedure, a continuing dialog and formal closure procedures.
Sim
ilar to X.25 packet sw
itching call control procedures -- and much like H
DLC
control sequences.
Error recovery is the responsibility of these connection services -- at w
hatever layer they operate.
CO
NN
EC
TION
LES
S S
ER
VIC
ES
Often called unreliable service, m
essage units are called datagram
s.
Messages are sent w
ithout prior notice -- and without any
prior agreement from
the receiving station to participate.
Because there is no form
al custody transfer understanding, the receiver is not accountable for acknow
ledging receipt of messages delivered this w
ay.
Analogous to first class letter services -- the post office
makes a "best efforts" attem
pt at delivery, but neither failure nor success are advised to the sender.
Data C
omm
unications & Fieldbus S
ystems
430
TC
P/IP
LAY
ER
ED
ST
RU
CT
UR
E V
IEW
D-185
AP
PL
ICA
TIO
NL
AY
ER
HO
ST
-TO
-HO
ST
LA
YE
R
INT
ER
NE
TL
AY
ER
NE
TW
OR
K
AC
CE
SS
LA
YE
R
INT
ER
NE
T P
RO
TO
CO
L
TRA
NS
MIS
SIO
N C
ON
TRO
L PR
OTO
CO
LUSER DATAGRAM PROTOCOL
WE
B/W
WW
FILET
RA
NS
FE
RELEC
TRO
NIC
NA
ME
SE
RV
ER
RE
AL TIM
EA
PP
LN'S
ET
HE
RN
ET
LAN
sC
OR
EB
AC
KB
ON
EN
ET
WO
RK
S(e
.g. W
orld
com
)P
HY
SIC
AL
C
ON
NE
CT
ION
LA
YE
R
UN
DE
RLY
ING
PH
YS
ICA
L N
ET
WO
RK
S A
RE
DIV
ER
SE
HT
TP
FT
PS
MT
P
PP
P
RE
GIO
NA
LN
ET
WO
RK
S
EN
TE
RP
RIS
EW
AN
s
RT
PD
NS
INT
ER
NE
T IN
FR
AS
TR
UC
TU
RE
INT
RA
NE
T IN
FR
AS
TR
UC
TU
RE
Data C
omm
unications & Fieldbus S
ystems
431
T-084
RE
LAT
ION
SH
IP B
ET
WE
EN T
CP
/IP LA
YE
RS
HO
ST-TO-H
OST
INT
ER
NE
T
NE
T. A
CC
ES
S
AP
PLIC
AT
ION
INT
ER
NE
T
NE
T. A
CC
ES
S
INT
ER
NE
T
NE
T. A
CC
ES
S
HO
ST-TO-H
OST
INT
ER
NE
T
NE
T. A
CC
ES
S
AP
PLIC
AT
ION
NE
TWO
RK
"A"
NE
TWO
RK
"B"
NE
TWO
RK
"C"
SE
GM
EN
TT
CP
HD
R
ME
SS
AG
E
HO
ST S
YS
TEM
HO
ST S
YS
TEM
RO
UTE
RR
OU
TER
MS
GH
DR
PA
CK
ET
DA
TAG
RA
MIP
HD
R
PK
TH
DR
FR
AM
EH
DR
FR
AM
EF
RA
ME
TLR
PA
CK
ET
DA
TAG
RA
MIP
HD
R
PK
TH
DR
FR
AM
EH
DR
FR
AM
EF
RA
ME
TLR
PA
CK
ET
DA
TAG
RA
MIP
HD
R
PK
TH
DR
FR
AM
EH
DR
FR
AM
EF
RA
ME
TLR
Data C
omm
unications & Fieldbus S
ystems
432
INTE
RN
ET P
RO
TOC
OL
T-355
BIT
PO
SIT
ION
S W
ITH
IN 32-B
IT W
OR
DS
32 BIT WORDS
01
23
45
67
89
01
23
45
67
89
01
23
45
67
89
01
22
22
22
22
22
33
11
11
11
11
11
12345678
IHL
VE
RS
ION
TY
PE
OF
SE
RV
ICE
TO
TA
L L
EN
GT
H
FR
AG
ME
NT
OF
FS
ET
FLAG
SID
EN
TIF
ICA
TIO
N
TIM
E T
O L
IVE
PR
OT
OC
OL
HE
AD
ER
CH
EC
KS
UM
SO
UR
CE
(IP) A
DD
RE
SS
DE
ST
INA
TIO
N (IP
) AD
DR
ES
S
VA
RIA
BL
E L
EN
GT
H O
PT
ION
S F
IEL
DP
AD
DIN
G
HO
ST
-to-H
OS
T L
AY
ER
INF
OR
MA
TIO
N (e.g
. TC
P H
EA
DE
R) B
EG
INS
HE
RE
INT
ER
NE
T P
RO
TO
CO
L H
EA
DE
R D
ET
AIL
The Internet Protocol (IP
) header (a
s illustra
ted in
the
box) is alw
ays found on the front of a datagram
-- a uniform
, and uniformly
interpreted part of an otherw
ise highly variable environm
ent.
The IP header is never
shorter than 5 words (2
0
octe
ts) in length, but options fields allow
it to be as large as 15 w
ords long. The Internet H
eader Length (IH
L) field specifies the
actual header length.
The Versio
n number field specifies the version of IP
to which the header form
at conforms --
here Version 4 is assum
ed (alth
ough V
ersio
n 6
is desig
nate
d a
s the n
ear-te
rm re
pla
cem
ent).
The Typ
e of S
ervice field specifies quality of service (QoS
) requirements; four bits in this
field specify routing priority based on minim
ization of delay, maxim
ization of throughput, optim
ization of reliability or minim
ization of cost (but n
ot a
ll route
rs are
able
to su
pport
multip
le ro
utin
g d
ecisio
n crite
ria).
Data C
omm
unications & Fieldbus S
ystems
433
INTE
RN
ET P
RO
TOC
OL, cont.
T-356
To
tal Len
gth
specifies the total datagram length -- m
easured in octets. In practice, message length is
constrained by the associated network technology, w
hich will define the m
aximum
transmission unit
(MT
U) -- the largest m
essage payload that a given network technology can support.
Frag
men
tation
Issues
IP supports flexible interconnection of netw
orks with differing M
TU lim
itations -- resulting in a need to accom
modate m
essage size conversion at the point of interconnection (route
rs) -- a process described as frag
men
tation.
The Iden
tification field carries a unique num
ber -- effectively a sequence number -- that m
akes it possible for the receiving system
to properly associate all fragments of the original m
essage, making
reassembly feasible.
The Flag
s field carries two single bit flags used as follow
s:
The DF (d
on't fra
gm
ent) bit is used to prevent fragm
entation of a datagramThe M
F (more
fragm
ents fo
llow
ing) bit is used to indicate that this is not the last fragm
ent in a series.
The Frag
men
t Offset is a pointer (ca
libra
ted in
8 o
ctet u
nits) indicating the position that a given
fragment occupied in the original datagram
.
D
ata Com
munications &
Fieldbus System
s434
INTE
RN
ET P
RO
TOC
OL, cont.
T-357
The Tim
e To
Live (T
TL) field is used to prevent the endless circulation of datagram
s that are undeliverable because of routing problem
s. The TTL is a counter which is initialized by the source
station (a
t typica
lly 128), and decrem
ented by one unit by each router that it transits along the way. If
the TTL counter value ever reaches zero, the datagram is judged to be undeliverable and is
destroyed.
The Pro
toco
l field specifies the transport layer protocol, which defines the form
at of the header which
follows the IP
header -- typically TC
P (a
short sa
mple
r of p
roto
cols a
nd th
eir a
ssocia
ted co
de n
um
bers
is pro
vided in
the a
ccom
panyin
g b
ox).
The Head
er Ch
ecksum
is defined by a simple error check calculation, and is used to verify that the IP
header contents are correct.
The 32-bit So
urce an
d D
estinatio
n IP
A
dd
resses define the logical end-to-end source and destination for a given datagram
. It is comm
on to represent these address values using a decim
al value for each octet individually, separated by decim
al points.
The resulting "dotted decimal" address
representation will be in the form
(for
exa
mple
) of 204.174.16.1
●P
RO
TO
CO
L DE
SIG
NA
TIO
NS
(NU
MB
ER
S) A
RE
AD
MIN
IST
ER
ED
BY
TH
E IN
TE
RN
ET
AS
SIG
NE
D N
UM
BE
RS
AU
TH
OR
ITY
(IAN
A)
136891729
ICM
PG
GP
TC
PE
GP
IGP
UD
PIS
O-T
P4
INT
ER
NE
T C
ON
TR
OL M
ES
SA
GE
PR
OT
OC
OL
GA
TE
WA
Y-T
O-G
AT
EW
AY
PR
OT
OC
OL
TR
AN
SM
ISS
ION
CO
NT
RO
L PR
OT
OC
OL
EX
TE
RIO
R G
AT
EW
AY
PR
OT
OC
OL
INT
ER
IOR
GA
TE
WA
Y P
RO
TO
CO
L (AN
Y)
US
ER
DA
TA
GR
AM
PR
OT
OC
OL
ISO
TR
AN
SP
OR
T P
RO
TO
CO
L CLA
SS
4
No
.A
CR
ON
YM
DE
SC
RIP
TIO
N
SA
MP
LE
R O
F P
RO
TO
CO
L C
OD
E A
SS
IGN
ME
NT
S
Data C
omm
unications & Fieldbus S
ystems
435
INTE
RN
ET P
RO
TOC
OL, cont.
T-358
IP H
eader O
ptio
ns S
um
mary
A num
ber of options are supported by IP, four of w
hich are of general interest:
The Strict S
ource Routing option provides a com
plete list of the IP addresses of routers that the
datagram is required to visit along the w
ay (this co
mple
tely p
resp
ecifyin
g a
nd co
ntro
lling th
e ro
ute
th
e d
ata
gra
m is to
follo
w).
The Loose Source R
outing is similar, but only incom
pletely specifies the route to be taken.
The Record R
oute option provides for logging the IP addresses of routers visited along the w
ay (e
ffective
ly the in
verse
of so
urce
routin
g).
The Timestam
p option provides for capturing time-of-day clock inform
ation at routers visited (for
purp
ose
s of n
etw
ork tim
e syn
chro
niza
tion).
IP header options are form
atted as entries in a list, allowing m
ore than one option to be supported in a given datagram
(subje
ct to n
ot e
xceedin
g th
e m
axim
um
allo
wable
IP h
eader size
of 1
5 w
ord
s).
Frequent use of IP header options w
ill degrade router-based network perform
ance, because of the considerably greater com
putational burden which results from
forcing an interpretation of header options at every routing point along the w
ay.
D
ata Com
munications &
Fieldbus System
s436
DO
TT
ED D
EC
IMA
L AD
DR
ES
S R
EP
RE
SE
NT
AT
ION
128.11.5.25
T-071
www.yahoo.com
10000000 00001011 00000101 00011001
The 32 bit binary form
at of an IP address is not a very useful w
ay of representing addresses for hum
an use or recognition.
In order to make these tedious binary sequences m
ore easily remem
bered (that is, more
'human-friendly') a m
ore convenient format based on decim
al representation is comm
only used:
Consider the follow
ing example, a hypothetical 32 bit IP
address:
Using w
hat is known as a dotted decim
al notation (sometim
es called a 'dotted quad'), each 8-bit segm
ent (byte, or octet) of the IP address is decoded as a decim
al value [0 through 255] , and written
with a decim
al point separating each octet of data, as follows:
A still greater convenience to hum
an users of this information the association of these num
eric address values w
ith a descriptive name -- w
hich is practice is an AS
CII character string also delim
ited w
ith decimal points, and organized in a hierarchical form
at (such as the hypothetical example
following) -- know
n as a domain nam
e .
The relationship betw
een the human-friendly A
SC
II domain nam
e and the technically essential IP
binary address is made possible by m
eans of one or more dom
ain name servers -- com
puter system
s provided for just this service.
Data C
omm
unications & Fieldbus S
ystems
437
T-069
INTE
RN
ET A
DD
RE
SS
ING
CO
NV
EN
TION
S
01
23
45
67
89
01
23
45
67
89
01
23
45
67
89
01
22
22
22
22
22
33
11
11
11
11
11
HO
ST
IDN
ET
WO
RK
ID00
12
34
56
78
90
12
34
56
78
90
12
34
56
78
90
12
22
22
22
22
23
31
11
11
11
11
1
HO
ST
IDN
ET
WO
RK
ID1
10
TH
ER
E A
RE
TH
RE
E P
RIN
CIP
AL N
ET
WO
RK
AD
DR
ES
S F
OR
MS
IN U
SE
ON
TH
E IN
TE
RN
ET
, ILLU
ST
RA
TE
D B
ELO
W (additional, less com
mon form
s are given on the next page)
CL
AS
S A
NE
TW
OR
K A
DD
RE
SS
ES, F
eaturing:
7 bits of NE
TW
OR
K ID
(Sufficient to identify 126 m
ajor networks)
24 bits of HO
ST
ID (S
ufficient to identify 16,777,214 hosts on each network)
CL
AS
S B
NE
TW
OR
K A
DD
RE
SS
ES, F
eaturing:
14 bits of NE
TW
OR
K ID
(Sufficient to identify 16,382 interm
ediate sized networks)
16 bits of HO
ST
ID (S
ufficient to identify 65,534 hosts on each network)
CL
AS
S C
NE
TW
OR
K A
DD
RE
SS
ES, F
eaturing:
21 bits of NE
TW
OR
K ID
(Sufficient to identify 2,097,150 sm
all networks)
8 bits of HO
ST
ID (S
ufficient to identify 254 hosts on each network)
01
23
45
67
89
01
23
45
67
89
01
23
45
67
89
01
22
22
22
22
22
33
11
11
11
11
11
HO
ST
IDN
ET
WO
RK
ID1
0
CLA
SS
"A"
AD
DR
ES
SE
SS
TA
RT
WIT
HV
ALU
ES
FR
OM
0 through 127
CLA
SS
"B"
AD
DR
ES
SE
SS
TA
RT
WIT
HV
ALU
ES
FR
OM
128 through 191
CLA
SS
"C"
AD
DR
ES
SE
SS
TA
RT
WIT
HV
ALU
ES
FR
OM
192 through 223
Data C
omm
unications & Fieldbus S
ystems
438
T-070
INT
ER
NE
T AD
DR
ES
SIN
G C
ON
VE
NT
ION
S, cont.
01
23
45
67
89
01
23
45
67
89
01
23
45
67
89
01
22
22
22
22
22
33
11
11
11
11
11
MU
LTICA
ST A
DD
RE
SS
11
10
01
23
45
67
89
01
23
45
67
89
01
23
45
67
89
01
22
22
22
22
22
33
11
11
11
11
11
HO
ST ID
11
10
1
The fifth class of addressing (illustrated below
) is reserved fo unspecified future use.
There are tw
o additional network address form
s provided for in the Internet structure:
Multicast addressing, w
hich provides for a kind of selective broadcast message to all m
embers of
some predefined group.
Features and feasibility of m
ulticasting are dependent on the technology in use (Ethernet, for
example, supports m
ulticast operation).
Multicast can be thought of as m
ore specific than broadcast, and more general than unicast (w
ith conventional, unique addressing).
To im
plement a m
ulticast addressing service it is necessary to perform a m
apping process, taking the IP
address, and translating it into an appropriate address on the destination network (typically
Ethernet) -- not a trivial com
plexity.
Data C
omm
unications & Fieldbus S
ystems
439
T-657
CLA
SS
LES
S IN
TE
RD
OM
AIN R
OU
TIN
G
The original three classes of centrally adm
inistered Internet addresses (i.e. Class "A
", C
lass "B" and C
lass "C" addresses) have proven to be too sim
plistic and too inefficient to support the levels of grow
th experienced (and expected) in the Internet.
This problem
was m
ost directly evident by the exhaustion of available Class "A
" and Class
"B" address assignm
ents
Tw
o practical aspects of this address managem
ent crisis were:
With C
lass "A" and C
lass "B" addresses no longer available, end-user organizations
had to make do w
ith Class "C
" address assignments, w
hich are only capable of supporting 254 host addresses. G
roups of Class "C
" addresses are inconvenient to w
ork with.
A m
ore acute problem has to do w
ith linear increase in the size of core routing tables, since every new
Class "C
" address assignment required another routing table entry
(2 million possible C
lass "C" addresses w
ould mean 2 m
illion routing table entries).
Classless Interdom
ain Routing (C
IDR
) was developed to deal w
ith these problems.
An im
portant part of the CID
R concept is a system
for assigning blocks of addresses that are grouped and adm
inistered in topological (i.e. geographic) order -- so that routers work
with groups of addresses rather than sim
ple lists (i.e. a more hierarchical structure).
Data C
omm
unications & Fieldbus S
ystems
440
T-658
CLA
SS
LES
S A
DD
RE
SS H
IER
AR
CH
Y
APN
ICRIPE N
CC
NIR
LIRLIR
ISPISP
ISP
IAN
A
END USEREND USER
END USEREND USER
END USER
01
23
45
67
89
01
23
45
67
89
01
23
45
67
89
01
22
22
22
22
22
33
11
11
11
11
11
11
0C
lass "C" F
ormat:
Regional Internet R
egistries(P
ossibly allocated bits 3 - 6)
National Internet R
egistries(P
ossibly allocated bits 7 - 12)
Local Internet Registries
(Possibly allocated bits 13 - 17)
Internet Service P
roviders(P
ossibly allocated bits 18 - 22)
End-U
ser Organizations
(Possibly allocated bits 23 - 31)
ARIN
LACN
ICAFR
INIC
Data C
omm
unications & Fieldbus S
ystems
441
INTE
RN
ET C
ON
TRO
L ME
SS
AG
E P
RO
TOC
OL
T-359
Intern
et Co
ntro
l Messag
e Pro
toco
l (ICM
P) is used for a range of applications, prim
arily in the category of error reporting services.
The connectionless, "best efforts" delivery philosophy of IP is m
ade more effective w
ith feed back inform
ation that can be used to modify behavior at the sending site. IC
MP
provides that feedback m
echanism w
ith the following m
essage types:
Destination U
nreachable - Datagram
was undeliverable because of addressing or adm
inistrative prohibition (in
cludin
g 'D
F' b
it restrictio
n)
Time E
xceeded - Time-to-live or fragm
ent reassembly tim
ers expired
Source Q
uench - A flow
control feature designed to prevent data loss at network congestion points.
Redirect - A
routing advisory message (ro
ute
r-to-h
ost) instructing a better starting router choice.
ICM
P also supports tw
o administrative applications:
Timestam
p Request/R
eply - A host-to-host tim
e coordination service.
Echo R
equest/Reply - A
ddress/connectivity/availability test initiative (use
d b
y the P
ING
pro
gra
m).
D
ata Com
munications &
Fieldbus System
s442
T-039
EC
HO
RE
QU
ES
T AN
D R
EP
LY IC
MP
ME
SS
AG
ES
The echo request/reply ICM
P m
essages comprise a transaction pair that is used to verify the
reachability (Is it a va
lid IP
addre
ss, and is th
e n
etw
ork co
nnectivity in
tact ?
) and operational status (Is it tu
rned o
n ?
) of a specified destination router or host.
The echo request/reply is a useful first level network diagnostic tool for verifying feasibility of a
network connection, and for roughly estim
ating network perform
ance.
●●
PIN
G - P
AC
KE
T IN
TE
RN
ET
GR
OP
ER
PIN
G a
pplica
tion-le
vel p
rogra
ms p
rovid
e a
com
mon h
um
an in
terfa
ce to
the e
cho re
quest/re
ply
service
.
Many d
iffere
nt ve
rsions o
f PIN
G h
ave
been d
eve
loped, b
ut th
ey typ
ically a
llow
som
eone to
in
itiate
an e
cho tra
nsm
ission w
ith a
simple
keyb
oard
com
mand like
ping destination_address
In its sim
ple
st form
, PIN
G w
ill resp
ond b
y confirm
ing su
ccess in
reach
ing th
e sp
ecifie
d
destin
atio
n (F
ailu
re is in
dica
ted b
y the a
bse
nce
of a
reply), a
nd d
ispla
ying th
e n
etw
ork ro
und-trip
tra
nsit tim
e d
ela
y that w
as e
nco
unte
red.
Many im
ple
menta
tions o
f PIN
G w
ill perm
it initia
tion o
f multip
le su
ch tra
nsa
ctions, a
nd p
rese
nt
statistica
lly sum
marize
d tra
nsit tim
e d
ela
y resu
lts
●●
Data C
omm
unications & Fieldbus S
ystems
443
T-621
TR
AC
ER
OU
TE
Traceroute records each of the hops along the route from
the local host to the designated remote
host, and therefore provides considerably more inform
ation than ping.
It is useful for diagnosing network-related problem
s (such as router configuration errors) and learning w
hat kind of delays to expect from the routed infrastructure rem
ote from the local subnet.
Traceroute sends a series of U
DP datagram
s, all addressed to the designated remote host -- w
ith the T
ime-T
o-Live (TT
L) value initially set to 1 -- and incremented for each iteration.
With the first datagram
, the first router in the path decrements the T
TL to zero and, returns an IC
MP
TIM
E-E
XC
EE
DE
D type m
essage to the originating host, and in the process, reveals its IP address.
Traceroute then perform
s an inverse DN
S lookup to identify the router's nam
e (Note: D
NS is m
ore com
monly used to perform
a name -to- address translation).
The screen display from
Traceroute show
s the response from each router on a new
line, and the round-trip delay(s) that w
ere observed from typically three tests perform
ed per router.
Many routers do not support this class of IC
MP response, since revealing the router addresses and
configuration of networks is seen to be an unnecessary security liability. N
on-participating routers w
ill time out and appear as an asterisked field -- and for sim
ilar reasons, many router addresses are
not registered with the D
NS and w
ill only present their dotted decimal address value.
Data C
omm
unications & Fieldbus S
ystems
444
TR
AN
SP
OR
T
TRA
DITIO
NA
LO
SI P
RO
TOC
OL
MO
DE
L DE
SC
RIP
TION
NE
TW
OR
K
DA
TA LIN
K
PH
YS
ICA
L
INTE
RN
ET
PR
OTO
CO
L
UP
PE
R LA
YE
RS
UP
PE
R LA
YE
RS
PH
YS
ICA
L
LLC
MA
C
1 2 3 4
TCP
/IP O
VE
RIE
EE 802-S
ER
IES
LAN
S
TC
P
AD
DR
ES
SE
S U
SE
D B
Y T
HE D
AT
A LIN
K LA
YE
R (LA
YE
R 2)
AR
E LO
CA
L.
IN T
HE C
AS
E O
F LA
Ns, T
HE
Y A
RE T
HE 48-B
IT M
AC
AD
DR
ES
SE
S
AD
DR
ES
SE
S U
SE
D B
Y T
HE N
ET
WO
RK LIN
K LA
YE
R
(LAY
ER 3) A
RE S
YS
TE
M-W
IDE (G
LOB
AL).
IN T
HE C
AS
E O
F IP
, TH
EY A
RE T
HE 32-B
IT IP
A
DD
RE
SS
ES
.
AD
DR
ES
SE
S M
AK
EC
ON
NE
CT
IVIT
Y H
AP
PE
N
T-680
LAYER
Bridges, R
outers and Sw
itches are the devices we
use to achieve network connectivity.
Bridges w
ork with the D
ata Link Layer addresses -- here assum
ed to be the 48-bit MA
C addresses.
Routers w
ork with N
etwork Layer addresses -- here
assumed to be the 32-bit IP
addresses.
Sw
itches are hardware-level devices that em
ulate either the role of bridges (Layer 2 sw
itches) or routers (Layer 3 sw
itches).
Data C
omm
unications & Fieldbus S
ystems
445
T-046
AD
DR
ES
S R
ES
OLU
TION
PR
OTO
CO
L
The Address R
esolution Protocol (A
RP
) is a procedure used to determine the physical
(hard
ware
) address corresponding to a given IP address -- a capability that is a critical link in the
process of actually delivering a datagram to the destination host over the physical netw
ork.
AR
P is very effective in resolving addresses on broadcast-oriented netw
orks (such
as sh
are
d m
edia
LA
N's like
eth
ern
et) but w
ould be disastrous on wide area netw
orks like X.25 or fram
e relay.
The router creates and maintains an A
RP
table based on the results of AR
P transactions (a
void
ing
the n
eed to
repetitive
ly AR
P fo
r the sa
me d
ata
).
The A
RP
sequence
of e
vents u
nfo
lds a
s follo
ws:
1. In
com
ing d
ata
gra
ms a
re a
ccepte
d b
y the ro
ute
r.
2. T
he ro
ute
r bro
adca
sts ove
r the lo
cal n
etw
ork w
ith
an A
RP
request, a
sking w
ho o
ut th
ere
reco
gnize
s th
e d
estin
atio
n a
ddre
ss.
3. T
he d
evice
that o
wns th
e sp
ecifie
d IP
addre
ss will
genera
te a
n A
RP
reply m
essa
ge ca
rrying its
loca
l hard
ware
addre
ss -- sufficie
nt to
delive
r the
data
gra
m to
the co
rrect d
evice
.
12
AS
SU
ME
D T
AR
GE
T
DE
VIC
E❉
3
RO
UT
ER
D
ata Com
munications &
Fieldbus System
s446
T-228
WO
RK
SH
EE
T: DA
TAG
RA
M D
ELIV
ER
Y S
EQ
UE
NC
E
RO
UT
ER
RO
UT
ER
ROUTERN
ET
WO
RK
#1N
ET
WO
RK
#3N
ET
WO
RK
#4
NE
TW
OR
K #2
BD
A
RO
UT
ER
INT
ER
NE
TC
HO
ST
C
OM
PU
TE
R S
YS
TE
M
HO
ST
C
OM
PU
TE
R S
YS
TE
M
HO
ST
C
OM
PU
TE
R S
YS
TE
M
HO
ST
#1
HO
ST
#2
HO
ST
#3A
RP
TAB
LE
HO
ST
RO
UT
ING
TA
BLE
AR
P TA
BLE
Data C
omm
unications & Fieldbus S
ystems
447
T-642
IP D
ELIV
ER
Y P
RO
CE
SS R
EV
IEW
SO
UR
CE
NE
TWO
RK
DE
ST
INA
TIO
NN
ETW
OR
KT
RA
NS
ITN
ETW
OR
KS
OU
RC
EH
OS
TD
ES
TIN
AT
ION
HO
ST
EN
TR
YR
OU
TE
RE
XIT
RO
UT
ER
The S
ource Host IP
process initiates the IP
datagram, specifying the source and destination
IP address.
Since the D
estination Host's IP
address identifies a different netw
ork from the source, the S
ource H
ost will consult its H
ost Routing T
able, and identify the IP
address of the Entry R
outer.
Using the E
ntry Router IP
address, the corresponding M
AC address w
ill be discovered w
ith the AR
P process, and the IP
datagram w
ill be encapsulated in a M
AC fram
e using this MA
C
destination address.
The E
ntry Router identifies the IP
address of the E
xit Router from
Routing T
ables constructed by its R
oute Coordination P
rotocol.
It uses AR
P (or w
hatever equivalent process applies to the T
ransit Netw
ork) to discover the M
AC address of the E
xit Router, encapsulates
the IP datagram
in a MA
C fram
e and sends it.
The exit router determ
ines the Destination H
ost IP address from
the IP header, and uses the
AR
P process to discover the corresponding M
AC
address, and sends it to the Destination H
ost.
Data C
omm
unications & Fieldbus S
ystems
448
T-122
PO
INT-TO
-PO
INT P
RO
TOC
OL (P
PP
)
Point-to-P
oint Protocol (P
PP
) was developed as a replacem
ent for SLIP
, with the intent that it
would address the lim
itations which frustrate S
LIP usage
PP
P is a fully functional (a
nd th
ere
fore
radica
lly more
com
ple
x than S
LIP
) implem
entation capable of:
administrative coordination of addresses
supporting multiple netw
ork protocols error detection and correction
PP
P is structurally an H
DLC
link protocol im
plementation, w
ith additional features to enable identification of upper layer protocols (U
LP's) and higher level link
control processes
PP
P w
as developed to support the TC
P/IP
protocol suite in a m
ore generally functional w
ay than SLIP
; the result has been a highly versatile point-to-point services protocol w
hich is becoming increasingly im
portant in enterprise-w
ide WA
N connectivity (S
ee
box)
WA
N R
OU
TE
R-T
O-R
OU
TE
R C
ON
NE
CT
ION
Mo
st existin
g ro
ute
r-con
ne
cted
wid
e a
rea
n
etw
orks a
re se
rved
by p
rop
rieta
ry p
oin
t-to-p
oin
t pro
toco
ls, forcin
g u
sers in
to
a sin
gle
ven
do
r ne
two
rk en
viron
me
nt
Consid
era
ble
pro
gre
ss has b
een m
ade in
u
sing
PP
P a
s a o
pe
n-syste
ms so
lutio
n to
th
is pro
ble
m
In a
dd
ition
to n
ative
IP su
pp
ort, P
PP
ha
s b
ee
n e
xten
de
d to
sup
po
rt IPX
, Ap
ple
talk,
DE
Cnet a
nd O
SI C
LN
P n
etw
ork la
yer
pro
toco
ls -- thu
s sup
po
rting
ge
ne
ral
mu
lti-pro
toco
l WA
N e
nviro
nm
en
ts
●●●
◆◆◆
Data C
omm
unications & Fieldbus S
ystems
449
PO
INT
-TO
-PO
INT
PR
OT
OC
OL F
EA
TU
RE
S
Point-to-P
oint Protocol (P
PP
) is a data link protocol designed to carry IP datagram
s over serial lines, taking a m
uch more sophisticated approach to the problem
than was done w
ith SLIP
.
PP
P is uniquely able to operate over either character-at-a-tim
e (asyn
chro
nous p
roto
col) links and
bit-oriented (synch
ronous) link system
s.
Various extensions to P
PP
have been developed, supporting data compression, inverse
multiplexing (o
pera
ting o
ver m
ultip
le p
ara
llel p
ath
s concu
rrently) and authentication procedures.
PP
P is based on the long-standing H
DLC
(Hig
h L
eve
l Data
Lin
k Contro
l) protocol frame
structure, widely used in X
.25 and other synchronous wide area netw
orks.
T-474
FLA
GAD
DR
ESSC
ON
TRO
LPRO
TOCO
LF
CS
FLA
GIN
FO
RM
AT
ION
(PA
YLO
AD
)
0111111011111111
0000001116 B
ITS
0111111016 B
ITS
PP
P FR
AM
E FO
RM
AT
NO
TES
:(1) A
DD
RE
SS
FIE
LD IS
UN
IFO
RM
LY S
ET
T
O H
EX
FF
(2) CO
NT
RO
L FIE
LD IS
HE
X 03
(Unnum
bered connectionless frame)
(3) TH
RE
E P
RO
TO
CO
L TY
PE
S A
RE
CO
MM
ON
:●
LINK
CO
NT
RO
L PR
OT
OC
OL (F
IELD
SE
T T
O H
EX
C021)
● NE
TW
OR
K C
ON
TR
OL P
RO
TO
CO
L (FIE
LD S
ET
TO
HE
X 8021)
● INT
ER
NE
T P
RO
TO
CO
L (FIE
LD S
ET
TO
HE
X 0021)
D
ata Com
munications &
Fieldbus System
s450
PO
INT
-TO
-PO
INT
PR
OT
OC
OL
FE
AT
UR
ES
, cont.
The protocol field specifies the upper layer protocol being serviced, equivalent to the S
ervices A
ccess Point (S
AP
) codes used with the IE
EE
803.2 LLC standard.
The three protocol field uses com
mon w
ith IP applications provide for link initiation (L
ink C
ontro
l P
roto
col), link control, such as initializing upper layer protocols (N
etw
ork C
ontro
l Pro
toco
l), and Internet P
rotocol itself.
In IP applications, the IP
-specific version of the Netw
ork Control P
rotocol (NC
P) is called In
ternet
Pro
toco
l Co
ntro
l Pro
toco
l (IPC
P).
In addition to these IP-related protocols, P
PP
supports a wide variety of other netw
ork layer protocols, like N
ovell IPX
, XN
S, D
EC
NE
T and A
ppletalk.
Because the control logic of H
DLC
(in syn
chro
nous lin
k applica
tions) is critically dependent on
correctly recognizing the delimiting flags, a special b
it-stuffin
g process is used to break up
happenstance message data that coincidentally replicates the flag pattern (H
EX
7E
).
The problem
of possible flag ambiguity is handled in asynchronous im
plementations of P
PP
by inserting a H
EX
7D (A
SC
II: }) in front of all happenstance flag data patterns, or any octet of value less than H
EX
20 (AS
CII co
ntro
l codes).
Operationally, the LC
P serves to establish a link connection (a
nd m
ain
tain
s resp
onsib
ility for lin
k m
ain
tenance
and te
rmin
atio
n); IP
CP
then takes responsibility for IP m
odule initialization, address coordination and com
pression functions, if any.
T-475
Data C
omm
unications & Fieldbus S
ystems
451
TRA
NS
MIS
SIO
N C
ON
TRO
L PR
OTO
CO
L
T-360
The Transmission C
ontrol P
rotocol (TC
P) header (a
s illu
strate
d in
the b
ox)
follows the IP
header in a TC
P/IP
message segm
ent.
Sim
ilar to the configuration of the IP
header, the TCP
header is also 5 w
ords (20
octe
ts) minim
um length -
and the Offset field
specifies the actual length (in
units o
f 32 b
it word
s).
Unlike the IP
case, TC
P
options (alth
ough p
rovid
ed
for) are of no practical
business interest.
The So
urce and D
estinatio
n P
ort N
um
ber fields specify the upper level protocol (U
LP)
which is being served by TC
P. S
ample P
ort Num
ber codes are presented in the box on the next page.
Seq
uen
ce Nu
mb
ers are used to indicate the progress of transmitting m
essage segments,
where the sequence num
ber identifies the byte position of the first byte in a given segment.
At the receiving site, logical consistency of incom
ing sequence numbers is used to verify that
no segments have gone m
issing.
BIT
PO
SIT
ION
S W
ITH
IN 32-B
IT W
OR
DS
32 BIT WORDS
01
23
45
67
89
01
23
45
67
89
01
23
45
67
89
01
22
22
22
22
22
33
11
11
11
11
11
12345678
DE
ST
INA
TIO
N P
OR
T N
O.
SE
QU
EN
CE
NU
MB
ER
AC
KN
OW
LE
DG
EM
EN
T N
UM
BE
RWIN
DO
W
UR
GE
NT
PO
INT
ER
VA
RIA
BL
E L
EN
GT
H O
PT
ION
S F
IEL
DP
AD
DIN
G
AP
PL
ICA
TIO
N L
AY
ER
INF
OR
MA
TIO
N B
EG
INS
HE
RE
SO
UR
CE
PO
RT
NO
.
FL
AG
SR
ES
ER
VE
DO
FF
SE
T
CH
EC
KS
UM
TR
AN
SM
ISS
ION
CO
NT
RO
L P
RO
TO
CO
L H
EA
DE
R D
ET
AIL
D
ata Com
munications &
Fieldbus System
s452
TRA
NS
MIS
SIO
N C
ON
TRO
L PR
OTO
CO
L, cont.
T-361
Ackn
ow
ledg
emen
t Nu
mb
ers are carried in the opposite direction to data flow
(and id
entify th
e se
quence
num
ber
of th
e n
ext e
xpecte
d m
essa
ge
segm
ent).
Where no data are available for
transmission, the acknow
ledgement
field may be the only m
eaningful content in the entire m
essage.
The Flag
s field carries indicators that validate the acknow
ledgement and
urgent fields, and that are used to logically synchronize (in
itialize
) and close a call.
The Win
do
w field defines an allow
able number of bytes that the receiving TC
P handler is w
illing to accept -- thus TC
P asserts flow
control in a very proactive way. N
o data are ever sent that are not previously agreed to.
The TCP
Checksum
field is similar in objectives and calculation m
ethod to that in the IP header. A
m
ajor difference is the TCP
header includes the segment payload in the calculation -- and regrettably,
portions of the IP header (IP
addre
sses, IP
pro
toco
l code a
nd S
egm
ent L
ength
).
By involving the IP
header in TC
P activity, the desirable separation of layered system
functions is lost, m
aking it impossible to m
ake changes to IP w
ithout simultaneously changing TC
P.
●P
OR
T CO
DE
AS
SIG
NM
EN
TS
AR
E S
ET B
Y T
HE
INT
ER
NE
T
AS
SIG
NE
D N
UM
BE
RS
AU
TH
OR
ITY
(IAN
A) - A
ND
AR
E
CA
LLED
"WE
LL KN
OW
N P
OR
TS
"
5202123257080210
RE
MO
TE JO
B E
NTR
YFILE
TRA
NS
FER
PR
OTO
CO
L - DA
TA C
ON
NE
CTIO
NFILE
TRA
NS
FER
PR
OTO
CO
L - CO
NTR
OL C
ON
NE
CTIO
NTE
LNE
T INTE
RA
CTIV
E TE
RM
INA
L PR
OTO
CO
LS
IMP
LE M
AIL TR
AN
SFE
R P
RO
TOC
OL
GO
PH
ER
PR
OTO
CO
LH
TTP (W
OR
LD-W
IDE
WE
B P
RO
TOC
OL)
Z39.50 (WID
E A
RE
A IN
FOR
MA
TION
SE
RV
ICE
) PR
OTO
CO
L
No
.D
ES
CR
IPTIO
N
SA
MP
LE
R O
F P
OR
T C
OD
E A
SS
IGN
ME
NT
S
Data C
omm
unications & Fieldbus S
ystems
453
KA
RN
JAC
OB
SO
N R
ETR
AN
SM
ISS
ION
TIME
R C
ON
TRO
L
T-362
TCP
operates with a positive acknow
ledgment -- m
eaning that successful delivery of data is explicitly confirm
ed by the receiving station.
There is no response when delivery of a TC
P segm
ent fails, for whatever reason.
The sending TCP
system is forced to m
ake its own decision to retransm
it a message segm
ent if no positive acknow
ledgment is received in a reasonable tim
e.
The complicating consideration is that the duration of an appropriate w
aiting interval is dependent on the type of netw
ork being served; LAN
s typically require waiting tim
es in the order of 10's of m
illiseconds, WA
Ns (e
specia
lly sate
llite-se
rved n
etw
orks) typically require w
aiting times in the
order of seconds.
The Karn Jacobson A
lgorithm specifies a m
oving average calculation, based on observed round-trip acknow
ledgement tim
e delays, as a way of setting this retransm
ission timer.
Getting the process started w
ith a new netw
ork connection typically involves some inefficiency in
defining the retransmission tim
e interval (with
impaire
d re
sponse
times b
ein
g e
vident) -- but the
algorithm very quickly converges to a viable tim
e interval.
The net effect is that TCP
/IP is uniquely capable of delivering effective, efficient results over a
remarkable diversity of netw
orking technology.
D
ata Com
munications &
Fieldbus System
s454
K-0
SC
AD
A P
RO
JEC
TS
continued ....
Managing S
CA
DA projects is really very m
uch like managing any technical/industrial project, w
ith a few
unique features that are traceable to the operational importance of these system
s, and historically at least, the highly proprietary nature of the technology.
The m
ain phases of any project typically include these five elements:
Planning and Identification of R
equirements
This stage typically includes preparing a P
roject Proposal, w
hich is a business and technical plan that defines the scope and characteristics (including functional deliverables and prelim
inary budgets) of the project -- and w
hich is typically the basis on which executive decisions proceed.
A system
atic technical plan should always answ
er the Who? W
hy? What? W
here? When? and
How
Much? questions about the project.
Preparation of Functional S
pecifications
Functional specifications translate the requirem
ents defined in the Project P
roposal guidance to support the detailed design activity, and develop a m
ore precise budget assessment than is
possible in the Project P
roposal.
Data C
omm
unications & Fieldbus S
ystems
455
SC
AD
A P
RO
JEC
TS
, cont.
K
continued ....
Preparation of Functional S
pecifications, cont.
Functional specifications should define the behaviors of the system
s, conceptual architecture and operating characteristics, reliability requirem
ents and environmental fram
ework - typically from
a user's (operator's) perspective.
This stage includes definitions of how
the new project w
ill integrate with existing system
s and provides a plan for operations and m
aintenance (O&
M) through the lifecycle of the project.
Detailed D
esign
This design stage translates the conceptual fram
ework of the functional design into detailed plans,
working draw
ings, specifications and cost estimates based on detailed know
ledge of all com
ponents of the project.
Detail at this point should be sufficient for im
plementation/fabrication to proceed either in-house or
contracted out.
Stakeholders, executives and designers should agree and sign-off on strategic m
ilestones in the detailed design using a system
atic, periodic design review process.
Data C
omm
unications & Fieldbus S
ystems
456
SC
AD
A P
RO
JEC
TS
, cont.
K-0
Implem
entation
Large SC
AD
A projects are typically contracted out to a turn-key system
s vendor, who w
ill be responsible for orchestrating resources sufficient to m
eet project schedule comm
itments.
Implem
entation is a project within a project. T
he classic Critical P
ath or PE
RT (P
rogram E
valuation and R
eview T
echnique) charts and methodologies tend to be m
ost diligently used at this stage of the project.
Acceptance and P
roject Com
pletion
Testing and acceptance m
ay or may not be a staged process (i.e. factory test/acceptance follow
ed by installed test/acceptance procedures) - increasingly com
monly, factory acceptance tests are no
longer considered necessary.
Typically, there is a transition period defined in the contract, w
hereby the contractor deals with
implem
entation defects and the contractor and purchaser share early operations and maintenance
activities.
Data C
omm
unications & Fieldbus S
ystems
457
PR
OJE
CT IS
SU
ES T
HA
T AR
E U
NIQ
UE T
O S
CA
DA
continued ....
SC
AD
A projects have uniquenesses that require special attention in the procurem
ent process:
The Organization's V
ulnerability to Non-P
erformance
SC
AD
A system
s always serve the core business of the enterprise -- it m
anages the "goose that lays the golden eggs", and is therefore the epitom
e of a "mission critical" technical system
.
Failure to perform
with this service is not an option, so a professional, highly structured process
must be follow
ed from concept through to final acceptance.
Validation of P
arts of The Solution is D
ifficult
Often, S
CA
DA system
s are built as "greenfield" projects, or replace an ageing system in its
entirety .
Thus, the m
ajor components of the system
(RT
Us, M
TU
, HM
I, networks) have no convenient w
ay of being tested independent of the other com
ponents.
That is, w
ithout proper attention to this detail, the tendency is to build all the pieces without
comprehensive testing and then discover they don't w
ork together only at the final integration and testing stage.
Data C
omm
unications & Fieldbus S
ystems
458
PR
OJE
CT IS
SU
ES T
HA
T AR
E U
NIQ
UE T
O S
CA
DA
, cont.
SC
AD
A S
ystems H
ave a Large Custom
Com
ponent
No tw
o SC
AD
A system
s are the same -- w
ith the consequence that every system is a custom
design, to som
e extent.
Projects w
ith a large custom com
ponent must be m
anaged with a carefully structured program
that m
easures and validates progress through sometim
es-elaborate test procedures.
SC
AD
A S
ystems H
ave a Long Lifetime
Com
pared with other com
puter-based, electronic systems, S
CA
DA system
s tend to be operate through a m
uch more extended tim
eframe.
Thus typically, system
s being replaced will be very old indeed, and this w
ill result in there being a rather radical change in the w
ay information is handled -- and the w
ay operational tasks are perform
ed.
Anticipating an extended lifetim
e for a new system
, it is very important to ensure that a safe,
continuing supply of spare parts is available. From
the outset, there should be a budget to buy a spares inventory as and w
hen parts availability becomes a jeopardy issue.
314
Data C
omm
unications & Fieldbus S
ystems
459
ALT
ER
NA
TIV
ES IN
PR
OC
UR
ING S
CA
DA S
OLU
TIO
NS
In-house technical staff tends to:
Have m
uch better familiarity w
ith corporate needs and culture issues -- an in-house design tends to "fit" better
Be m
ore limited in their design freedom
-- they do things the way they have alw
ays done them --
for better or for worse.
Like with any procurem
ent process, there are a number of alternate approaches to procuring
SC
AD
A solutions.
These range the gam
ut from doing it all in-house (i.e. contracting for no im
plementation assistance)
through enlisting a consultant's services to manage the process of letting a design-build turnkey
contract.
Considerations that w
ill determine the approach that best suits any particular application include:
The level of fam
iliarity and skills available in-house -- i.e. how capable is the host organization to
do all or part of the work them
selves?
The tim
e-line constraints -- i.e. is the schedule too tight to manage w
ith the presumably lim
ited num
ber of in-house technical resources?
) D
ata Com
munications &
Fieldbus System
s460
IMP
LEM
EN
TA
TIO
N A
ND
CO
MM
ISS
ION
ING IS
SU
ES
There are som
e operational aspects to implem
enting and comm
issioning SC
AD
A system
s that are different from
many kinds of projects.
This includes these context issues:
Often, there are reconfiguration tasks that involve the old system
being replaced - floor space m
ust be freed up, limited telecom
munications circuitry m
ust be reconfigured, etc. Old technology
is often "cranky", and disruption results in unpredictable outcomes.
SC
AD
A system
s operate with live, m
ission-critical infrastructure. How
do you test "Em
ergency S
hutdown" control sequences w
ithout actually doing an emergency shutdow
n?
Both the installation and com
missioning activities are challenged by distance. C
hecking each rem
ote point being supervised and/or controlled typically requires staff positioned at both ends of the netw
ork to validate signals and logical consistency.
A carefully orchestrated strategy for m
anaging the cutover is vital (assuming an in-place system
is being replaced), including som
e parallel operation of old and new technology. A
sudden death cutover is not an option.
41101)
Data C
omm
unications & Fieldbus S
ystems
461
WH
ER
E IN
THE
TEC
HN
OLO
GY
LIFE-C
YC
LE
SH
OU
LD O
NE
BE
A P
LAY
ER
?
PIO
NE
ER
EA
RLY
AD
OP
TER
LATE
AD
OP
TER
LAG
GA
RD
PIO
NE
ER
S G
ET
BU
RIE
DA
T T
HE
SID
E O
F T
HE
R
OA
D
PR
ES
EN
TS
EX
CIT
ING
OP
PO
RT
UN
ITY
TO
BE
A B
RE
AK
TH
RO
UG
HLE
AD
ER
RE
QU
IRE
S V
ER
Y
CA
RE
FU
L, HA
ND
S-O
NM
AN
AG
EM
EN
T
AP
PR
OA
CH
PO
TE
NT
IAL P
AY
OF
F IS
PH
EN
OM
EN
AL, D
OW
N-
SID
E IS
PO
OR
OD
DS
OF
SU
CC
ES
S
OP
PO
RT
UN
ITY
HE
RE
IST
O LE
AD
WIT
H C
AU
TIO
N
BE
A C
AR
EF
UL
SH
OP
PE
R
FO
R M
AN
Y B
US
INE
SS
SIT
UA
TIO
NS
, BE
ING
AN
EA
RLY
FO
LLOW
ER
ISO
PT
IMA
L ST
RA
TE
GY
(manageable m
ixture of risk vs rew
ard)
TH
ER
E IS
ST
ILL A R
ISK
OF
AC
TIN
G B
EF
OR
E T
HE
TE
CH
NO
LOG
Y S
TA
B-
ILIZE
S -- B
UT
WA
ITIN
GLO
NG
ER
IS A
LSO
RIS
KY
BU
Y IN
TO
MA
TU
RE
TE
CH
NO
LOG
Y; T
HU
S:
PR
OD
UC
T C
OS
TS
A
RE
ST
AB
ILIZE
D A
ND
/OR
RE
DU
CE
D
TE
CH
NO
LOG
Y IS
ST
AN
DA
RD
IZE
D
RIS
KS
OF
FA
ILUR
E A
RE
M
INIM
IZE
D -- B
UT
RIS
KO
F B
EIN
G LE
FT
BE
HIN
DIS
HIG
H
LES
S-T
HA
N-O
PT
IMU
MB
US
INE
SS
PA
YO
FF
SP
EN
D T
OO
MU
CH
TIM
EM
AK
ING
EX
CU
SE
S F
OR
BE
ING
SLO
W
CO
ST
S A
RE
NO
T M
UC
HM
OR
E A
TT
RA
CT
IVE
TH
AN
EA
RLIE
R O
PT
ION
SB
UT
TH
ER
E A
RE
MA
JOR
BU
SIN
ES
S P
EN
ALT
IES
FR
OM
BE
ING
NO
N-
PR
OG
RE
SS
IVE
OT
HE
R C
OS
TS
NO
W
DE
FIN
E T
HE
B
US
INE
SS
CA
SE
(Personnel efficiency,
opportunity costs ... )
●●●●
●●●●
●●●
●●●
Data C
omm
unications & Fieldbus S
ystems
462
THE
RIG
HT TIM
E TO
MA
KE
A M
OV
E IS
AN
O
RG
AN
IZATIO
N-S
PE
CIFIC
CO
NS
IDE
RA
TION
INC
RE
AS
ING
(ELA
PS
ED
) TIM
E
LOW
TE
CH
NIC
AL R
ISK
FO
LLOW
ER
DO
MA
INH
IGH
RIS
K, B
LEE
DIN
G E
DG
E D
OM
AIN
INC
RE
AS
ING
QU
ALIT
YO
F D
ELIV
ER
AB
LED
UE
TO
MA
TU
RIT
Y O
FT
HE
TE
CH
NO
LOG
Y
DE
CR
EA
SIN
G C
OS
T,
EF
FO
RT
AN
D R
ISK
- ALS
O D
UE
TO
MA
TU
RIT
Y O
F T
HE
T
EC
HN
OLO
GY
Information and com
puter com
munications technologies are
moving at a pace that challenges
most organizations to keep up.
This effect combined w
ith the increasingly urgent need for all organizations to be m
ore com
petitive (e
ither fo
r busin
ess o
r in
creasin
gly sca
rce p
ublic fu
nds)
makes it a challenging business
decision to correctly identify the cross-over point in the accom
panying diagram.
The consequence of moving too
early is principally excess risk (for a
give
n b
usin
ess cu
lture
).
The consequence of moving too late is principally lost opportunity (to
delive
r busin
ess e
ffective
ness
to th
e o
rganiza
tion).
The cross-over point is sometim
es called the 'ah-h
a' po
int -- because of the inspiration that it is
now tim
e to move is not altogether obvious!
Data C
omm
unications & Fieldbus S
ystems
463
Ap
pe
nd
ice
s
Glossary of T
erms
Suggestions F
or Further R
eading
Data C
omm
unications & Fieldbus S
ystems
464
GL
OS
SA
RY
OF
TE
RM
INO
LO
GY
A-to
-D
Analog to digital conversion. T
he conversion process whereby analog signals
are
con
verte
d in
to a
dig
ital fo
rma
t, or e
nco
de
d in
to a
dig
ital, o
r nu
me
ricrepresentation. Inverse of D
/A process.
AC
CE
SS
ME
TH
OD
In context of LA
N system
s, a means of m
anaging access from individual stations
to the shared cable medium
. Exam
ples of comm
on LAN
access control techniquesare T
oken Passing and C
arrier Sense M
ultiple Access w
ith Collision D
etection(C
SM
A/C
D).
AC
K
A positive acknow
ledgement of m
essage (typically a block or frame m
essagesegm
ent) receipt -- indicating acceptance of the message segm
ent. An A
CK
istyp
ically sig
na
led
with
a sp
ecia
l con
trol ch
ara
cter (w
ith h
alf-d
up
lex lin
kp
roto
cols) o
r a p
re-d
efin
ed
bit-fla
g (w
ith fu
ll-du
ple
x link p
roto
cols). T
he
alternative to an AC
K is a N
AK
, or negative acknowledgem
ent.
AD
AP
TIV
E D
IFF
ER
EN
TIA
L P
UL
SE
CO
DE
MO
DU
LA
TIO
N (A
DP
CM
) A
DP
CM
is a vo
ice sig
na
l en
cod
ing
tech
niq
ue
wh
ich d
elive
rs dig
itally co
de
dtoll-quality signals at 32 kbps. -- com
pared with 64 kbps. using conventional
PC
M. S
everal AD
PC
M standards exist, including the m
ost widely supported
ITU
-T specification.
AD
AP
TIV
E R
OU
TIN
G
A netw
ork routing capability that selects the most appropriate route available
at th
e m
om
en
t of co
nn
ectio
n d
em
an
d. S
om
e d
ata
ne
two
rk ad
ap
tive ro
utin
gp
roce
du
res co
ntin
uo
usly e
valu
ate
this ro
utin
g co
nfig
ura
tion
, an
d re
de
fine
sthe route in m
id-session.
AD
DR
ES
S R
ES
OL
UT
ION
T
he
pro
cess w
he
reb
y an
inte
rne
t (IP) a
dd
ress is in
terp
rete
d (typ
ically b
y arouter/gatew
ay device) and a physical address value deduced. See "address
resolution protocol".
AD
DR
ES
S R
ES
OL
UT
ION
PR
OT
OC
OL
(AR
P)
The A
RP
is a capability required on many T
CP
/IP system
s which enable the
ga
tew
ay ro
ute
r to a
ssocia
te a
n in
com
ing
IP a
dd
ress w
ith a
loca
l pie
ce o
fhardw
are -- such as an Ethernet w
orkstation.
AM
ER
ICA
N N
AT
ION
AL
ST
AN
DA
RD
S IN
ST
ITU
TE
(AN
SI)
AN
SI is a
Ne
w Y
ork, N
Y b
ase
d o
rga
niza
tion
tha
t bo
th d
efin
es sta
nd
ard
sa
pp
licab
le to
US
A in
tere
sts, an
d re
pre
sen
ts the
US
A o
n a
nu
mb
er o
fin
tern
atio
na
l foru
ms, su
ch a
s the
ISO
. AN
SI d
efin
es sta
nd
ard
s ove
r a w
ide
ran
ge
of te
chn
olo
gie
s; in co
nte
xt of co
mp
ute
r com
mu
nica
tion
s the
y are
curre
ntly p
layin
g a
ma
jor ro
le is p
articip
atin
g in
the
pro
cess o
f de
finin
gstandards for high speed W
AN
and MA
N technologies.
AM
ER
ICA
N S
TA
ND
AR
D C
OD
E F
OR
INF
OR
MA
TIO
N IN
TE
RC
HA
NG
E
Th
e A
SC
II cod
e is a
7-b
it cod
e u
sed
to re
pre
sen
t 12
8 d
ata
an
d co
ntro
lch
ara
cters. E
xten
de
d A
SC
II is an
8-b
it versio
n o
f the
cod
e w
ith 1
28
no
rma
lA
SC
II code values, and 128 additional codes accessed using the "Alt" keyboard
key. S
eve
ral in
tern
atio
na
l ad
ap
tatio
ns o
f AS
CII h
ave
be
en
de
velo
pe
d to
enable representation of "non-English" characters, such as accented vow
els.T
he
orig
ina
l AS
CII is so
me
time
s refe
rred
to a
s US
AS
CII to
diffe
ren
tiate
itfrom
these derivative versions. AS
CII w
as defined by the Am
erican National
Standards Institute, or A
NS
I.
AM
PL
ITU
DE
MO
DU
LA
TIO
N (A
M)
Am
plitude modulation is one of three m
odulation techniques (See definition of
"mo
du
latio
n"). W
ith A
M, th
e b
ase
ba
nd
sign
al in
form
atio
n is e
nco
de
d, o
rim
pressed on the carrier signal by modifying its am
plitude to instantaneouslyrespond to variations in the baseband signal.
AN
AL
OG
LO
OP
BA
CK
A
test procedure used to effectively short-circuit the remote end of an analog
da
ta lin
e to
verify th
e e
nd
-to-e
nd
con
ne
ction
. Th
e a
ctua
l loo
pb
ack is n
ot
actu
ally a
wire
d sh
ort-circu
it, bu
t rath
er a
log
ical e
cho
of th
e o
utg
oin
gtest signal.
AN
AL
OG
SIG
NA
L
An analog signal m
odels or imitates the behavior of som
e (generally naturallyoccurring) process, such as hum
an speech. The analog signal literally can be
reg
ard
ed
as a
na
log
ou
s to th
e p
roce
ss be
ing
mo
de
led
, an
d re
pre
sen
ts tha
tp
roce
ss by m
ea
ns o
f a co
ntin
uo
usly va
rying
ph
ysical q
ua
ntity su
ch a
s an
ele
ctrical vo
ltag
e. In
such
a a
case
, the
volta
ge
will ch
an
ge
in th
e sa
me
pro
po
rtion
s, an
d a
t the
sam
e ra
te a
s the
spe
ech
itself. T
hu
s the
ele
ctrical
signal is an analog of the naturally occurring, continuously variable (speech)process.
AN
AL
OG
TR
AN
SM
ISS
ION
A
na
log
sign
als a
re co
ntin
uo
usly va
riab
le sig
na
ls, an
d th
ere
is info
rma
tion
con
veye
d in
the
con
tinu
um
of sig
na
l valu
es. C
on
trast w
ith d
igita
l sign
als
wh
ich d
em
on
strate
on
ly sign
al va
lue
s, an
d co
nve
y info
rma
tion
on
ly by th
eir
va
lue
s a
t the
se
dis
cre
te le
ve
ls. A
na
log
tran
sm
iss
ion
fac
ilities
an
de
qu
ipm
en
t thu
s are
resp
on
sible
for fa
ithfu
lly rep
rod
ucin
g th
e in
finite
lyvariable continuum
of signal states. The com
plexity of analog signals makes
it fun
da
me
nta
lly im
po
ss
ible
to d
ev
elo
p tra
ns
mis
sio
n e
qu
ipm
en
t tha
tu
nd
ers
tan
ds
an
d in
terp
rets
the
sig
na
l va
lue
s (a
s is
do
ne
with
dig
ital
systems), thus m
aking it necessary to amplify and m
anipulate the signals with
a m
inim
um
of d
istortio
n, b
ut a
lwa
ys (be
cau
se e
qu
ipm
en
t is imp
erfe
ct) in a
nim
perfect way.
AN
SI
See definition of "A
merican N
ational Standards Institute".
Data C
omm
unications & Fieldbus S
ystems
465
AP
PL
ICA
TIO
N P
RO
GR
AM
A
pp
licatio
n p
rog
ram
s in co
nte
xt of co
mp
ute
r system
s are
the
pro
gra
ms
de
ve
lop
ed
(ofte
n b
y th
e e
nd
-us
er o
rga
niz
atio
n) th
at d
ea
l with
the
org
an
izatio
n’s re
aso
n fo
r ow
ing
the
com
pu
ter in
the
first pla
ce. A
pp
licatio
nprogram
s include payroll, accounting, text processing, spread sheet systems.
In co
ntra
st to a
pp
licatio
n p
rog
ram
s, sup
erviso
ry pro
gra
ms a
re typ
ically
pro
vide
d b
y the
com
pu
ter ve
nd
or, a
nd
inclu
de
op
era
ting
system
s (such
as
MA
C-O
S and the various W
indows variants).
AP
PL
ICA
TIO
N P
RO
GR
AM
INT
ER
FA
CE
(AP
I) T
he software interface betw
een an application program and the com
puter andn
etw
ork o
pe
ratin
g syste
ms w
hich
sup
po
rt it. Th
e A
PI e
ffective
ly spe
cifies
the technical envelope within w
hich a programm
er must operate. O
n Unix-based
systems, the A
PI is referred to as a "socket".
AR
PA
NE
T
AR
PA
NE
T w
as the first operational packet switching netw
ork, from w
hich evolvedb
oth
X.2
5 p
ac
ke
t sw
itch
ing
sta
nd
ard
s (m
id 1
97
0’s
) an
d th
e T
CP
/IPinternetw
orking protocols (early 1980’s). AR
PA
NE
T’s research/developm
entrole has becom
e largely unnecessary due to OS
I standards development, and its
operational role has been replaced by the DD
N (D
efense Data N
etwork).
AR
Q
Autom
atic Repeat reQ
uest. AR
Q system
s operate by providing for retransmission
of data whenever errors are encountered (w
hich would occur in response to a
"NA
K" m
essage sent back to the source). Thus, w
ith AR
Q, the sending station
rem
ain
s acco
un
tab
le (a
nd
thu
s mu
st ma
inta
in a
ll da
ta in
a b
uffe
r), rea
dy fo
ra
po
ss
ible
ne
ed
to re
tran
sm
it it, un
til the
rec
eiv
ing
sta
tion
ac
ce
pts
resp
on
sibility w
ith a
po
sitive "A
CK
" me
ssag
e. S
ee
"Sto
p-a
nd
-Wa
it AR
Q",
"Go-B
ack-N A
RQ
" and "Selective A
RQ
".
AS
CII
See definition of "A
merican S
tandard Code for Inform
ation Interchange".
AS
YN
CH
RO
NO
US
TR
AN
SF
ER
MO
DE
(AT
M)
AT
M has em
erged as the preferred switching technology for im
plementing cell
relay fast packet switching. A
TM
is currently viewed as the m
ost appropriatew
ay o
f de
liverin
g fu
nctio
na
l, use
r con
trolla
ble
inte
rcon
ne
ction
be
twe
en
hig
hsp
ee
d lo
cal a
ccess a
nd
wid
e a
rea
ne
two
rk facilitie
s. AT
M is sca
lab
le --
op
era
ting
at u
p to
mu
lti-Gig
ab
it rate
s. AT
M’s m
ain
un
iqu
en
ess is its
flex
ibility
-- it is c
ap
ab
le o
f su
pp
ortin
g th
e u
niq
ue
req
uire
me
nts
of
vo
ice
, vid
eo
, gra
ph
ica
l an
d c
on
ve
ntio
na
l da
ta a
pp
lica
tion
s. A
TM
iscu
rren
tly the
favo
red
switch
ing
tech
no
log
y with
bo
th w
ide
are
a n
etw
ork
planners (telephone companies), m
etropolitan area network designers and is
fea
sible
(bu
t no
t po
pu
lar) a
s a h
igh
spe
ed
loca
l are
a n
etw
ork (H
SL
AN
)technology -- a rem
arkable range of applications.
AS
YN
CH
RO
NO
US
TR
AN
SM
ISS
ION
A
sy
nc
hro
no
us
tran
sm
iss
ion
utiliz
es
a c
ha
rac
ter-b
y-c
ha
rac
ter d
ata
synch
ron
izatio
n m
eth
od
in w
hich
ea
ch in
form
atio
n ch
ara
cter is in
divid
ua
llyh
an
dle
d, b
y the
use
of sta
rt an
d sto
p b
its. Th
e tim
e in
terva
l be
twe
en
ea
chch
ara
cter is n
ot co
ntro
lled
. Se
e d
efin
ition
of S
ynch
ron
ou
s tran
smissio
n fo
ra
com
pa
rison
. Asyn
chro
no
us tra
nsm
ission
is com
mo
nly ca
lled
"start-sto
p"
transmission.
AT
M
See "A
synchronous Transfer M
ode".
AT
TE
NU
AT
ION
A
ttenuation is the loss or decrease in analog signal strength (as measured by
voltage, current or power) caused by som
e component of a com
munications
system
-- typica
lly the
me
diu
m itse
lf. Atte
nu
atio
n is u
sua
lly me
asu
red
an
dexpressed in decibels.
AU
DIO
FR
EQ
UE
NC
IES
A
udio frequencies are those components of speech, m
usic, etc. (sound) that area
ud
ible
to a
pe
rson
with
no
rma
l he
arin
g, typ
ically ta
ken
to b
e in
the
ran
ge
from about 30 to 20,000 hertz.
AU
TH
EN
TIC
AT
ION
A
uthentication deals with the question of confidence that a m
essage’s claimed
orig
ina
tor w
as in
fact th
e a
uth
en
tic orig
ina
tor o
f the
me
ssag
e. C
om
mo
nly,
pu
blic ke
y en
cryptio
n te
chn
olo
gy is u
sed
to a
ssure
ele
ctron
ic me
ssag
ea
uth
en
tica
tion
. Th
e p
rob
lem
is s
imila
r to v
erify
ing
tha
t a tra
ditio
na
ld
ocu
me
nt’s sig
na
ture
is au
the
ntic -- a
nd
ind
ee
d, d
igita
l sign
atu
res a
re o
ne
tool used in network-oriented authentication procedures.
AU
TO
MA
TIC
NU
MB
ER
IDE
NT
IFIC
AT
ION
(AN
I) T
he
ab
ility for th
e te
lep
ho
ne
nu
mb
er o
f the
callin
g sta
tion
to b
e m
ad
ea
vaila
ble
to th
e ca
lled
loca
tion
. Th
ere
are
nu
me
rou
s ap
plica
tion
s of A
NI;
me
ssag
e a
ccou
ntin
g, in
teg
ratio
n w
ith b
usin
ess co
mp
ute
r ap
plica
tion
s an
dpersonal caller identification to nam
e a few.
AU
TO
MA
TIC
RE
PE
AT
RE
QU
ES
T E
RR
OR
CO
RR
EC
TIO
N
See definition of "A
RQ
".
AU
TO
MA
TIC
RO
UT
E S
EL
EC
TIO
N (A
RS
) A
RS
is a b
usin
ess te
lep
ho
ne
system
fea
ture
use
d to
sele
ct the
mo
stcost-effective outgoing service or route for each telephone connection.
BA
CK
BO
NE
NE
TW
OR
K
A high speed netw
ork system that links other typically low
er speed networks
together. The m
ost comm
on usage is associated with a connectivity solution
for a
nu
mb
er o
f loca
l are
a n
etw
orks, su
ch a
s is en
cou
nte
red
in a
typica
lcam
pus environment. M
etropolitan area networks (M
AN
’s) may be configured as
backbone networks.
BA
LA
NC
ED
CIR
CU
IT
A b
ala
nce
d co
mm
un
icatio
n circu
it or in
terfa
ce is co
nfig
ure
d so
tha
t the
two
con
du
ctor re
qu
ired
to su
pp
ort th
e e
lectrica
l con
ne
ction
are
ide
ntica
llyco
nfig
ure
d. T
he
two
con
du
ctors (n
orm
ally w
ires) w
ill the
refo
re b
e o
f the
sam
e typ
e, h
ave
eq
ua
l ele
ctrical ch
ara
cteristics a
nd
ne
ithe
r will b
e d
irectly
connected to ground or earth.
BA
ND
A
frequency band is a range of analog frequencies that are normally associated
with
the
sam
e a
pp
licatio
n, su
ch a
s the
AM
rad
io b
roa
dca
st ba
nd
, wh
ichoccupies the range of frequencies from
about 550 khz to 1600 khz.
Data C
omm
unications & Fieldbus S
ystems
466
BA
ND
WID
TH
T
he range of frequencies present in a signal (signal bandwidth), or the range
of fre
qu
en
cies su
pp
orte
d b
y a co
mm
un
icatio
n syste
m (syste
m o
r eq
uip
me
nt
bandwidth). B
andwidth is a m
easure of the frequency difference between the
highest and lowest frequencies in this range. T
he usable telephone channelbandw
idth is nominally 3 khz; the highest frequency on a standard telephone
channel is approximately 3,300 hz; the low
est 300 hz.
BA
SE
BA
ND
SIG
NA
L
The native form
at of a signal as derived from the source apparatus, before any
modulation or m
ultiplexing procedures are effected. Thus, in telephony, the
ba
se
ba
nd
sig
na
l is th
e s
pe
ec
h s
ign
al its
elf; in
da
ta a
pp
lica
tion
s th
ebaseband signal is the digital voltage square w
ave waveform
.
BA
UD
T
he baud rate of a signal is the rate at which recognizable changes of coded
sign
al sta
tus (sig
na
l ele
me
nts) a
re tra
nsm
itted
. In a
pp
licatio
ns w
he
re o
ne
bit of information is encoded as one recognizable signal elem
ent, or symbol,
the baud rate is the same num
erical value as the bit rate. In most m
odems,
da
ta is e
nco
de
d w
ith m
ultip
le b
its pe
r sign
al e
lem
en
t, an
d th
us th
e b
it rate
is a
lmo
st a
lwa
ys
gre
ate
r tha
n th
e b
au
d ra
te. It is
a w
ide
ly m
ista
ke
na
ssum
ptio
n th
at b
it rate
an
d b
au
d ra
te a
re th
e sa
me
; thu
s it is com
mo
n to
refe
r to th
e b
au
d ra
te o
f a m
od
em
in a
n a
ttem
pt to
de
sc
ribe
its d
ata
transmission capability.
BIN
AR
Y C
OD
E
A b
ina
ry cod
e is sim
ply th
e re
pre
sen
tatio
n o
f info
rma
tion
in th
e sim
ple
st of
units, being combinations of the binary pair of digits (the "1" and "0" values)
. Th
e e
xecu
tab
le fo
rm o
f a co
mp
ute
r pro
gra
m is in
bin
ary co
de
; effe
ctively
unintelligible to humans but necessary for com
puter execution.
BIN
D
Berkeley Internet N
ame D
omain - a specific im
plementation of the D
omain N
ame
Server function on B
erkeley Unix system
s.
BIT
B
IT is a
con
tractio
n o
f "BIn
ary d
igiT
," the
sma
llest u
nit o
f info
rma
tion
in a
bin
ary syste
m. A
bit o
f info
rma
tion
pro
vide
s the
ab
ility to d
iffere
ntia
tebetw
een a "1" or a "0" condition.
BIT
RA
TE
T
he
rate
at w
hich
bits o
f info
rma
tion
are
tran
smitte
d, u
sua
lly exp
resse
d in
bits p
er se
con
d (b
ps), a
nd
mu
ltiple
s the
reo
f: kbp
s, mb
ps, g
bp
s, tbp
s, etc.
Com
pare with the definition of baud.
BL
OC
K
One com
plete message segm
ent, or data unit on a data comm
unications link. Ab
lock is stru
cture
d b
y pre
-ag
ree
me
nt to
con
vey co
ntro
l info
rma
tion
such
as
source/destination addresses, message num
ber, retransmission requests ("N
AK
"codes), and error checking features. T
he message units know
n as blocks tendto
be
as
so
cia
ted
with
ha
lf du
ple
x s
ys
tem
s; th
e c
om
pa
rab
le u
nit o
finform
ation in full duplex systems is typically know
n as a frame.
BR
IDG
E
A bridge is one of the sim
plest LAN
connection devices based on examination of
physical, or MA
C-layer addresses only. B
ridges do not examine netw
ork layeraddresses,or m
ake any changes to the contents of a message -- in contrast to
the
role
of ro
ute
rs. Brid
ge
s mu
st en
cou
nte
r the
sam
e te
chn
olo
gy (e
.g.
Eth
ern
et) o
n a
ll seg
me
nts, a
nd
are
ide
ally su
ited
to a
rela
tively sm
all
number of locally connected LA
N segm
ents.
BR
OA
DB
AN
D
(1) Com
munication facilities w
hich have analog bandwidth values greater than
tha
t of a
voice
-gra
de
tele
ph
on
e ch
an
ne
ls, an
d a
re th
us a
ble
to su
pp
ort d
ata
tran
smissio
n ra
tes in
exce
ss of th
ose
fea
sible
on
tele
ph
on
e ch
an
ne
ls. (2)
Ge
ne
rally h
igh
cap
acity d
ata
com
mu
nica
tion
s system
s. An
exa
mp
le is
"Bro
ad
ba
nd
ISD
N", w
hic
h is
a fu
lly d
igita
l (an
d th
us
ha
s n
o lite
ral
"bandwidth" features at all) system
offering very high data transmission rate
capability.
BR
OA
DC
AS
T
A m
essage delivery concept where all m
embers of a com
munity receive exactly
the same m
essage. Exam
ples are television broadcasting and Ethernet LA
N’s.
BR
OU
TE
R
A hybrid bridge-router. F
eatures of brouters vary amongst vendors, but they
typica
lly offe
r rou
ting
service
s in so
me
circum
stan
ces (e
.g. w
he
re th
ey
reco
gn
ize th
e n
etw
ork p
roto
col in
use
) an
d a
ct as b
ridg
es in
oth
ers (e
.g.
where they don’t recognize the netw
ork protocol in use).
BU
FF
ER
A
bu
ffer is a
pla
ce to
store
info
rma
tion
tem
po
rarily w
hile
som
e (u
sua
lly)related activity can be perform
ed. Buffers are used to save data w
hile errorch
eckin
g a
nd
retra
nsm
ission
can
be
pe
rform
ed
, the
y are
ne
cessa
ry wh
ere
interfaces occur between data transm
ission systems that operate at different
da
ta ra
tes, o
r op
era
te w
ith d
iffere
nt p
roto
cols. B
uffe
rs will g
en
era
llyin
trod
uce
a tim
e d
ela
y, an
d re
qu
ire ca
refu
l ma
na
ge
me
nt so
as to
avo
idoverflow
(and consequent loss of data).
BU
S
A b
us is a
ph
ysical m
ed
ium
wh
ere
the
re is n
o sig
nifica
nce
to th
e p
ositio
n o
rco
nn
ectio
n p
oin
t of d
evice
s. A co
mp
ute
r ba
ckpla
ne
is a b
us (o
ne
can
plu
ga
ccesso
ries a
nd
au
xiliary e
qu
ipm
en
t into
an
y ava
ilab
le "slo
t") an
d a
coa
xial
cable ethernet LAN
is a bus system (one can connect at m
ost any point alongth
e ca
ble
). Bu
s system
s are
alw
ays o
pe
rate
d co
ncu
rren
tly -- tha
t is tosa
y, sign
als a
re d
elive
red
to a
ll po
ints o
n th
e b
us a
t effe
ctively th
e sa
me
time.
BY
TE
In
mo
st con
texts, a
byte
is an
8-b
it un
it of in
form
atio
n, w
hich
is mo
ree
xactly, a
nd
mo
re te
chn
ically re
ferre
d to
as a
n "o
ctet". A
byte
typica
llyrepresents one character of text inform
ation. Since com
puters process datain
pa
ralle
l form
at, th
e b
yte b
eco
me
s the
sma
llest u
nit o
f info
rma
tion
in m
ost
da
ta m
an
ipu
latio
n a
ctivities. S
ince
som
e e
arlie
r com
pu
ters u
sed
oth
er th
an
mu
ltiple
s of 8
bit w
ord
s sizes, th
ere
ha
ve b
ee
n situ
atio
ns w
he
re th
e te
rmb
yte w
as u
sed
to re
fer to
oth
er th
an
8 b
its of in
form
atio
n. F
or e
xam
ple
, the
old DE
C P
DP
-8 computer w
as a 12-bit mem
ory system; the characters w
ere
Data C
omm
unications & Fieldbus S
ystems
467
encoded into 6 bits of data -- thus a byte was 6 bits length.
BY
TE
MU
LT
IPL
EX
ING
B
yte m
ultip
lexin
g, o
r byte
inte
rlea
ved
mu
ltiple
is a fo
rm o
f time
-divisio
nm
ultiplexing in which w
hole bytes of data are captured from each sub-channel,
an
d se
nt a
s a u
nit. B
ytes o
f da
ta fro
m d
iffere
nt su
b-ch
an
ne
ls follo
w e
ach
oth
er in
seq
ue
ntia
l time
slots. A
ltern
ative
me
cha
nism
s are
bit in
terle
avin
g,
and message interleaving; byte interleaved m
ultiplexing is the most com
mon.
CA
BL
E
A cable is an assem
bly of one or more conductors (or optical fibers) w
ithin ab
un
dle
(wh
ich typ
ically in
clud
es a
n e
nve
lop
ing
pro
tective
she
ath
). Ca
ble
sa
re typ
ically co
nstru
cted
so a
s to a
llow
the
use
of co
nd
ucto
rs sep
ara
tely o
rin
gro
up
s. Th
e te
rm is o
ften
use
d to
sug
ge
st the
coa
xial ca
ble
tha
t is the
ke
y e
lem
en
t in c
ab
le te
lev
isio
n s
ys
tem
s -- b
ut is
no
t an
ac
cu
rate
generalization.
CA
RR
IER
A
carrie
r sign
al is a
simp
le co
ntin
uo
us fre
qu
en
cy sign
al cre
ate
d fo
r the
purpose of being modulated by another inform
ation carrying signal.
CA
RR
IER
FR
EQ
UE
NC
Y
The carrier frequency is the operating frequency of the carrier signal used in
the
mo
du
latio
n p
roce
ss. In ra
dio
com
mu
nica
tion
, the
carrie
r freq
ue
ncy
corre
spo
nd
s to th
e fre
qu
en
cy to w
hich
on
e tu
ne
s the
rad
io to
rece
ive a
desired program or m
essage.
CA
TV
C
omm
unity Antenna T
elevision, or CA
TV
refers to the cable television servicea
re o
ffere
d in
mo
st com
mu
nitie
s. Un
like th
e o
ver-th
e-a
ir bro
ad
cast sig
na
lsu
sed
in co
mm
ercia
l tele
vision
an
d ra
dio
, CA
TV
sign
als a
re co
nfin
ed
to a
coa
xial ca
ble
distrib
utio
n n
etw
orks. A
dd
ition
ally, m
icrow
ave
an
d sa
tellite
links, may be used as part of these distribution netw
orks.
CC
IR
See "IT
U-R
", which is the current identity for the form
er CC
IR.
CC
ITT
S
ee "ITU
-T", w
hich is the current identity for the former C
CIT
T.
CC
TV
C
losed Circuit T
elevision, or CC
TV
systems use coaxial cable or other "closed
circuit" (ie
. no
n-b
roa
dca
st) me
dia
to ca
rry tele
vision
sign
als. E
xam
ple
s of
CC
TV
applications include CA
TV
(cable television entertainment services) and
security video systems.
CE
LL
RE
LA
Y
Ce
ll rela
y is a p
acke
tized
ne
two
rking
tech
no
log
y ba
sed
on
the
use
of sm
all,
fixed
len
gth
pa
ckets ca
lled
cells. T
he
rou
ting
log
ic an
d p
acke
t con
trol
functions are implem
ented largely in hardware, m
aking it possible to handleth
e ca
ll switch
ing
pro
cess a
t very h
igh
spe
ed
s -- mu
ch m
ore
qu
ickly tha
nsoftw
are-driven technology (as is used in conventional X.25 packet sw
itchinga
nd
fram
e re
lay syste
ms). A
synch
ron
ou
s Tra
nsfe
r Mo
de
(AT
M) sw
itchin
gim
ple
me
nta
tion
s are
the
curre
ntly fa
vore
d te
chn
olo
gy fo
r de
liverin
g ce
ll
relay services -- for both WA
N, M
AN
and high speed LAN
applications.
CE
NT
RA
L O
FF
ICE
T
he
tele
ph
on
e ce
ntra
l office
, or e
xcha
ng
e is th
e p
lace
wh
ere
loca
l acce
sslines (local loops), and inter-exchange com
munications lines are term
inated,a
nd
wh
ere
sw
itch
ing
eq
uip
me
nt (w
hic
h s
erv
es
to in
terc
on
ne
ct th
es
ecom
munication facilities) is located.
CE
NT
RE
X
A business telephone service w
here the features and functions of on-premises
private branch exchange (PB
X), or "sw
itchboard" equipment are delivered from
the
tele
ph
on
e co
mp
an
y’s cen
tral o
ffice. C
en
trex te
nd
s to b
e fa
vore
d b
ylarge, m
ulti-office corporations and government departm
ents.
CE
PT
C
ouncil of European P
ostal and Telecom
munications adm
inistrations. CE
PT
is ab
od
y wh
ich a
ssum
es re
spo
nsib
ility for th
e sp
ecia
l inte
rests o
f Eu
rop
ea
ntelecom
munications providers by m
aking policy and standards recomm
endationsfor telecom
munications practice w
ithin Europe. C
EP
T’s role has been largely
superseded by the European T
elecomm
unications Standards Institute (E
TS
I),w
hich
inclu
de
s in its m
em
be
rship
the
gro
win
g n
um
be
r of n
on
-go
vern
me
nt
European telecom
munications providers.
CG
I S
ee definition of "Com
mon G
ateway Interface".
CH
AN
NE
L
A channel is a path for com
munication of inform
ation, which m
ay be derivedfro
m a
co
mm
un
ica
tion
circ
uit o
r tran
sm
iss
ion
fac
ility. M
ultip
lex
ing
tech
niq
ue
s ma
ke it p
ossib
le fo
r mu
ltiple
cha
nn
els to
be
crea
ted
on
a sin
gle
comm
unication facility such as a microw
ave radio system. A
channel can bethought of as having the sam
e relationship to the comm
unication facility as ap
ain
ted
lan
e o
n a
hig
hw
ay h
as to
the
hig
hw
ay itse
lf. In so
me
usa
ge
s, the
term channel im
plies a unidirectional comm
unication capability.
CH
AR
AC
TE
R
Characters m
ay be human-readable alphanum
eric characters, such as the letterso
f the
alp
ha
be
t, nu
me
ric an
d p
un
ctua
tion
symb
ols. In
con
text o
f com
pu
ter
systems, the term
"character" is often taken to mean the 7 or 8 bit A
SC
II (oro
the
r cod
e) re
pre
sen
tatio
ns o
f da
ta. T
he
se "ch
ara
cters" in
clud
e th
e fu
ll set
of a
lph
an
um
eric ch
ara
cters d
escrib
ed
ab
ove
, bu
t also
inclu
de
a n
um
be
r of
"con
trol ch
ara
cters", w
hich
invo
ke syste
m co
ntro
l fun
ction
s. Th
is latte
rcategory includes, for exam
ple, the Carriage R
eturn Character (A
SC
II 0DH
) andthe E
scape Character (A
SC
II 1BH
).
CH
RO
MIN
AN
CE
T
he
colo
r pa
rt of a
vide
o sig
na
l, wh
ich p
erta
ins to
the
hu
e a
nd
satu
ratio
n(but not the brightness) of the signal.
CIR
CU
IT
A co
mm
un
icatio
n circu
it is a p
hysica
l tele
com
mu
nica
tion
facility w
hich
pro
vide
s a w
ay o
f tran
smittin
g in
form
atio
n b
etw
ee
n tw
o o
r mo
re p
oin
ts. An
electronic circuit consists of an assembly of com
ponents connected togetherto form
some signal processing or other electronic function.
Data C
omm
unications & Fieldbus S
ystems
468
CIR
CU
IT - F
OU
R W
IRE
A
4-w
ire co
mm
un
icatio
n p
ath
use
s fou
r wire
s (two
pa
irs) to su
pp
ort fu
llys
ep
ara
ted
info
rma
tion
flow
in e
ac
h o
f two
dire
ctio
ns
. Th
at is
, an
ind
ep
en
de
nt, d
ed
icate
d p
air o
f wire
s is use
d fo
r ea
ch d
irectio
n. T
his te
rmhad its origins in an era w
hen all comm
unication facilities were derived from
me
tallic w
ire; th
e te
rm is n
ow
use
d to
de
scribe
the
fun
ction
al e
qu
ivale
nt
of 4
-wire
s, wh
ich m
igh
t be
actu
ally su
pp
orte
d b
y gla
ss fibe
r op
tic cab
le, o
rperhaps m
icrowave radio technology.
CIR
CU
IT - T
WO
WIR
E
A 2
-wire
com
mu
nica
tion
pa
th u
ses tw
o w
ires (ie
. on
e p
air) to
sup
po
rtc
om
mu
nic
atio
n s
erv
ice
s. A
2-w
ire fa
cility
ca
n s
up
po
rt bid
irec
tion
al
com
mu
nica
tion
if the
cab
le is sh
ort e
no
ug
h so
as to
no
t req
uire
an
y sign
al
amplification (w
hich is an inherently one-way process) and does not include
an
y mu
ltiple
or o
the
r sign
al p
roce
ssing
fun
ction
s (wh
ich a
re a
lsou
nid
irec
tion
al a
ctiv
ities
). Oth
erw
ise
, a 2
-wire
se
rvic
e w
ill be
unidirectional only.
CIR
CU
IT S
WIT
CH
ING
C
ircuit switching system
s provide an imperm
anent connection between tw
o (orm
ore
) loca
tion
s, an
d p
rovid
e th
e syste
m u
sers w
ith e
xclusive
use
of a
continuously open channel, over which inform
ation may be exchanged. C
ircuitsw
itchin
g co
sts are
pro
po
rtion
al to
ho
ldin
g tim
e, a
nd
are
un
rela
ted
to th
evolum
e of information transm
itted. Com
pare with P
acket Sw
itching.
CL
IEN
T
The softw
are supporting user-oriented computing functions in a client/server
com
pu
ting
en
viron
me
nt. C
lien
t softw
are
typica
lly (at le
ast) co
nce
rns itse
lfw
ith m
an
ag
ing
the
hu
ma
n in
terfa
ce; fo
r this re
aso
n clie
nt syste
ms a
reco
mm
on
ly asso
ciate
d w
ith th
e g
rap
hica
l use
r inte
rface
(GU
I). Th
e te
rm’client’ is often (som
ewhat casually) associated w
ith the hardware com
putingsystem
which supports the client as defined here.
CL
IEN
T/S
ER
VE
R P
AR
AD
IGM
T
he Client/S
erver computing m
odel has been accepted as the de facto standardo
f the
ind
ustry b
eca
use
it sup
po
rts en
riche
d h
um
an
inte
rface
s (e.g
. the
gra
ph
ical u
ser in
terfa
ce, o
r GU
I), an
d is a
me
na
ble
to d
istribu
ted
pro
cessin
g(co
nsiste
nt w
ith d
ow
nsize
d h
ard
wa
re co
mp
ute
r arch
itectu
res), a
mo
ng
oth
er
thin
gs
. Clie
nt/s
erv
er s
ys
tem
s te
nd
to b
e in
ten
siv
e u
se
rs o
f ne
two
rkresources.
CL
OC
K
A p
recise
ly time
d, re
pe
titive d
igita
l pu
lse sig
na
l seq
ue
nce
wh
ich is u
sed
tocontrol a data transm
ission and reception procedures.
CL
OC
K R
EC
OV
ER
Y
Th
e p
roce
ss of d
erivin
g a
loca
l clock sig
na
l at a
rece
iving
loca
tion
by
extra
cting
the
em
be
dd
ed
timin
g in
form
atio
n fro
m th
e sig
na
l rece
ived
on
asynchronous channel.
CO
AX
IAL
CA
BL
E
A com
munications cable w
hich consists of central conductor surrounded by anco
nce
ntric, o
r co-a
xial o
ute
r con
du
ctor. T
he
two
con
du
ctors a
re se
pa
rate
dfro
m e
ac
h o
the
r by
a c
ylin
dric
al s
ha
pe
d in
su
lato
r. Co
ax
ial c
ab
le is
technically very much superior to (tw
isted) paired telephone cable.
CO
DE
C
A codec, or C
oder-Decoder is a device that converts analog signals to digital,
an
d d
igita
l sign
als to
an
alo
g. C
od
ec’s a
re u
sed
to co
nve
rt an
alo
g sig
na
lssu
ch a
s spe
ech
to a
dig
ital fo
rma
t as re
qu
ired
for tra
nsm
ission
ove
r dig
ital
me
dia
-- an
d th
e co
mp
an
ion
fun
ction
at th
e re
ceivin
g e
nd
ba
ck to th
eoriginal analog form
at.
CO
LO
R B
UR
ST
A
few cycles (typically 8 to 10) of the 3.58 M
hz. color subcarrier which occur
du
ring
the
"ba
ck po
rch" in
terva
l in th
e lu
min
an
ce vid
eo
sign
al. T
he
colo
rb
urst p
rovid
es fre
qu
en
cy an
d p
ha
se re
fere
nce
valu
es fo
r the
rece
iver co
lor
oscillator.
CO
LO
R S
UB
CA
RR
IER
In
NT
SC
tele
vision
, the
3.5
8 M
Hz sig
na
l wh
ich is m
od
ula
ted
with
colo
rinform
ation, and superimposed (am
plitude modulated) onto the lum
inance signald
efin
es th
e co
lor su
bca
rrier. T
he
am
plitu
de
of th
e m
od
ula
ted
sub
carrie
r isproportional to color saturation, and phase is proportional to hue.
CO
MM
ON
CH
AN
NE
L IN
TE
RO
FF
ICE
SIG
NA
LIN
G (C
CIS
) C
CIS
is a te
lep
ho
ne
sign
alin
g te
chn
iqu
e in
wh
ich ca
ll con
trol sig
na
ling
da
tais transm
itted over a separate comm
unication system from
that used to supportthe m
essage traffic. Typical C
CIS
systems use packet com
munication m
ethods.C
urrent interest focuses on Signaling S
ystem N
umber S
even (SS
#7) -- which is
a CC
IS service being deployed w
ith ISD
N.
CO
MM
ON
GA
TE
WA
Y IN
TE
RF
AC
E (C
GI)
The C
omm
on Gatew
ay Interface, or CG
I technology creates an interface between
web servers and other inform
ation systems, enabling access to databases and
legacy computing system
s, for example. W
ithout CG
I (or a similar function),
web servers are lim
ited to presenting fixed format, prepackaged inform
ation.W
ith C
GI (o
r on
e o
f the
pro
prie
tary e
xten
sion
s) it is po
ssible
to co
nstru
ctfully interactive client/server business applications.
CO
MM
ON
MA
NA
GE
ME
NT
INF
OR
MA
TIO
N P
RO
TO
CO
L
CM
IP is th
e O
SI sta
nd
ard
ne
two
rk ma
na
ge
me
nt p
roto
col. It is le
ss wid
ely
supported and more com
plex to implem
ent than the TC
P/IP
equivalent, Sim
pleN
etwork M
anagement P
rotocol.
CO
MM
UN
ICA
TIO
NS
SA
TE
LL
ITE
C
om
mu
nica
tion
s sate
llites a
re e
arth
orb
iting
sate
llites d
esig
ne
d to
fun
ction
as telecomm
unications microw
ave radio relay devices. Typical com
munications
sate
llites a
re p
ositio
ne
d in
ge
osyn
chro
no
us o
rbits, w
hich
are
22
,30
0 m
iles
(35,888 km) above the equator, and traveling eastw
ardly, so that they appearfro
m e
arth
to b
e s
tatio
na
ry in
sp
ac
e -- th
us
the
ir orb
ital p
os
ition
isreferred to as a "geostationary orbit".
Data C
omm
unications & Fieldbus S
ystems
469
CO
MP
AN
DO
R
The w
ord "compandor" is a hybrid term
for a device that performs a com
pressionfunction on outgoing signals (w
hich reduces the dynamic range, or am
plitudesca
le o
f an
an
alo
g sig
na
l (in o
rde
r tha
t the
full ra
ng
e o
f sign
al stre
ng
thva
riatio
n m
ay b
e re
pre
sen
ted
at a
hig
he
r ave
rag
e vo
lum
e le
vel), a
nd
acom
panion expansion function performed on incom
ing signals (that have beencom
pressed at the other end of the comm
unication channel). The purpose of
com
pa
nd
or o
pe
ratio
n is to
red
uce
the
vuln
era
bility o
f we
ak sig
na
ls to n
oise
distortion on the comm
unication link. The D
olby tape recording process is anexam
ple of a compandor system
.
CO
MP
ON
EN
T D
IGIT
AL
SIG
NA
L
A d
igita
l vide
o fo
rma
t in w
hich
sep
ara
te vid
eo
sign
als a
re u
sed
to re
pre
sen
tth
e lu
min
an
ce
(Y), a
nd
co
lor d
iffere
nc
e s
ign
als
(R-Y
an
d B
-Y), o
ralternatively, in R
GB
format.
CO
MP
OS
ITE
VID
EO
A
mixed signal com
prised of the a luminance (m
onochrome) com
ponent and ach
rom
ina
nce
(colo
r) com
po
ne
nt, a
lon
g w
ith syn
chro
nizin
g sig
na
ls (such
as
sy
nc
hro
niz
atio
n p
uls
es
, bla
nk
ing
pu
lse
s a
nd
co
lor b
urs
t). Sta
nd
ard
over-the-air television broadcast signals are composite.
CO
NC
EN
TR
AT
OR
A
con
cen
trato
r is a d
evice
wh
ich co
nn
ects a
nu
mb
er o
f circuits, o
r term
ina
lc
on
ne
ctio
ns
to a
sin
gle
hig
he
r sp
ee
d, s
ha
red
co
mm
un
ica
tion
circ
uit.
Co
nce
ntra
tors a
re fu
nctio
na
lly very sim
ilar to
mu
ltiple
xers -- e
xcep
t tha
tconcentrators are equipped w
ithout a companion concentrator at the host end
of a
link, sin
ce th
e h
ost syste
m itse
lf pe
rform
s the
com
pa
nio
n ro
le. S
ince
the
ir op
era
tion
is inte
gra
lly linke
d to
ho
st system
op
era
tion
(con
trast
with
mu
ltiple
xer d
evice
s, wh
ich a
re tra
nsp
are
nt to
ho
st system
s), the
y are
ge
ne
rally p
rop
rieta
ry pro
du
cts cap
ab
le o
f wo
rking
on
on
ly on
e e
qu
ipm
en
tvendor’s netw
orks.
CO
ND
ITIO
NIN
G
Co
nd
ition
ing
is a
n o
ptio
na
l pro
ce
du
re a
pp
lied
to le
as
ed
line
an
alo
gc
om
mu
nic
atio
n c
ircu
its, w
ith th
e in
ten
t to c
on
trol th
e q
ua
lity o
ftransm
ission. Conditioning procedures require custom
analysis and mitigation
of syste
m im
pa
irme
nts o
n a
n e
nd
-to-e
nd
ba
sis -- an
d a
re th
ere
fore
no
tapplicable to dial services. C
onditioning is not a comm
on service in currentpractice, since m
ost leased line services are provided with digital technology.
Where used, there are tw
o types of conditioning: C and D
conditioning.
CO
NT
RO
L C
HA
RA
CT
ER
C
on
trol ch
ara
cters a
re ch
ara
cter co
de
s (wh
ich a
lon
g w
ith a
lph
an
um
eric
characters comprise the character code sets like A
SC
II and EB
CD
IC) w
hoseo
ccurre
nce
in a
da
ta se
qu
en
ce w
ill resu
lt in th
e in
itiatio
n, m
od
ificatio
n, o
rin
terru
ptio
n o
f op
era
tion
al d
ata
flow
. An
exa
mp
le is th
e A
SC
II "Esca
pe
"character.
CR
OS
S T
AL
K
Cro
ss talk is th
e u
nin
ten
de
d tra
nsfe
r of e
ne
rgy fro
m o
ne
com
mu
nica
tion
sch
an
ne
l to a
no
the
r. It is mo
st com
mo
nly e
nco
un
tere
d a
s the
low
volu
me
occurrence of one voice telephone conversation on top of another -- caused byboth connections sharing w
ires in a comm
on cable bundle, and simultaneously,
some m
is-configuration of equipment.
CS
MA
/CA
C
arrie
r Se
nse
Mu
ltiple
Acce
ss with
Co
llision
De
tectio
n is th
e co
nte
ntio
na
ccess p
roto
col u
sed
with
eth
ern
et L
AN
system
s. Th
e "C
SM
A" p
art o
f the
pro
toco
l pro
vide
s for a
statio
n first liste
nin
g fo
r activity o
n th
e ca
ble
system
be
fore
com
me
ncin
g tra
nsm
ission
tha
t wo
uld
be
de
structive
to th
ecu
rren
t use
ap
plica
tion
. Th
e "C
D" fu
nctio
n p
rovid
es fo
r con
tinu
ing
tolis
ten
for a
ctiv
ity fro
m o
the
r sta
tion
s o
n th
e c
ab
le s
ys
tem
afte
rtra
ns
mis
sio
n is
initia
ted
-- to d
ete
ct c
ollis
ion
s w
hic
h re
su
lt from
the
tran
smissio
n a
ttem
pt. In
the
eve
nt o
f a co
llision
, bo
th a
ffecte
d sta
tion
sw
ill be required to back off and attempt the com
munication later.
CU
G
Clo
sed
Use
r Gro
up
. A G
UG
is an
ad
min
istrative
ly de
fine
d virtu
al n
etw
ork
service, such that mem
bers of a GU
G are able to com
municate freely am
ongstthem
selves, but are isolated from the larger public com
munity that shares the
comm
on comm
unications infrastructure.
CU
ST
OM
ER
PR
EM
ISE
S E
QU
IPM
EN
T (C
PE
) C
PE
is tele
com
mu
nica
tion
s term
ina
l eq
uip
me
nt co
nn
ecte
d to
the
en
d o
f atelecom
munications netw
ork termination link. T
he comm
on telephone is CP
E --
on a customers prem
ises, and connected to the public telephone network. A
we
ll de
fine
d d
em
arca
tion
po
int w
ill esta
blish
the
bo
un
da
ry of re
spo
nsib
ilitybetw
een CP
E and the netw
ork service provider. In the U.S
.A., clear policies
have been defined prohibiting ownership of C
PE
by the telecomm
unicationsnetw
ork service company.
D-to
-A
Dig
ital to
an
alo
g co
nve
rsatio
n. T
he
con
versio
n p
roce
ss wh
ere
by d
igita
lsig
na
ls are
con
verte
d in
to a
n a
na
log
form
at, o
r de
cod
ed
from
a d
igita
l, or
numeric representation. T
he inverse of the A-to-D
process.
DA
TA
D
ata is the encoded representation of information, w
hich may be in the form
ofn
um
be
rs, cha
racte
r-orie
nte
d te
xts, tab
ula
ted
facts, in
structio
ns, e
xecu
tab
lep
rog
ram
s, etc. D
ata
is a p
lura
l term
(da
tum
is the
form
al sin
gu
lar te
rm) --
however com
mon usage ignores this distinction, using the "data" term
for bothsingular and plural.
DA
TA
CO
MM
UN
ICA
TIO
NS
EQ
UIP
ME
NT
(DC
E)
In th
e co
nte
xt of a
da
ta co
mm
un
icatio
ns, it is th
e e
qu
ipm
en
t wh
ich is
interposed between the D
ata Term
inal Equipm
ent (DT
E) and the com
munication
circuit. The purpose of the D
CE
is to translate signals presented by the DT
Ein
to a
form
at su
itab
le fo
r pa
ssing
acro
ss the
com
mu
nica
tion
link. T
hu
s, the
DC
E m
ust be selected to operate within the constraints of the m
edium (based
on analog/digital and 2-wire/4-w
ire options, for example).
DA
TA
DIC
TIO
NA
RY
A
document or database file that specifies the precise content characteristics
of d
ata
ele
me
nts, w
hich
are
in tu
rn a
re in
divid
ua
l item
s of in
form
atio
n, like
perhaps a telephone number.
Data C
omm
unications & Fieldbus S
ystems
470
DA
TA
EN
CR
YP
TIO
N S
TA
ND
AR
D (D
ES
) T
he public domain data encryption m
ethodology developed by the US
National
Institute of Standards and T
echnology (NIS
T, form
erly the National B
ureau ofS
tan
da
rds
). DE
S s
ys
tem
s a
ll op
era
te u
sin
g th
e s
am
e 5
6 b
it ke
ye
ncryp
tion
/de
cryptio
n a
lgo
rithm
-- an
d m
ain
tain
the
secu
rity of d
ata
thro
ug
ha private key code know
n only to the two com
municating entities.
DA
TA
SE
T
The data set is a largely disused telephone industry term
for the analog modem
.T
his u
sag
e co
ntin
ue
s with
resp
ect to
circuit d
esig
na
tion
s on
the
EIA
-23
2-C
interface standard.
DA
TA
TE
RM
INA
L E
QU
IPM
EN
T (D
TE
) In the context of a data com
munications, it is the data processing equipm
entw
hich is attached to, and terminates the netw
ork connection, and which is the
en
tity be
ing
serve
d b
y the
com
mu
nica
tion
facility a
nd
the
asso
ciate
d d
ata
comm
unications equipment, or D
CE
.
DA
TA
GR
AM
T
he term for the m
essage unit which is supported by the Internet P
rotocol, orIP
in TC
P/IP
networks. A
lthough technically distinct, a datagram is som
ewhat
like a packet. (The term
’datagram’ im
plies an uncoordinated, unannouncedco
mm
un
icatio
n; ’p
acke
t’ imp
lies a
pre
-esta
blish
ed
log
ical co
nn
ectio
n p
rior
to sending data.)
DB
S (D
IRE
CT
BR
OA
DC
AS
T S
AT
EL
LIT
E)
Dire
ct bro
ad
cast sa
tellite
s a g
eo
synch
ron
ou
s orb
it sate
llites d
esig
ne
d to
de
liver a
sufficie
ntly h
igh
po
we
red
sign
al so
as to
be
reco
vere
d b
y a sm
all
rece
iving
dish
an
ten
na
. By ra
dica
lly red
ucin
g th
e co
st of th
e re
ceivin
ge
arth
term
ina
l in th
is ma
nn
er, D
BS
tech
no
log
y is com
pe
titive a
ltern
ative
for
delivery of entertainment broadcast services.
DC
E
See definition of "D
ata Com
munication E
quipment".
DE
CIB
EL
T
he
de
cibe
l is form
ally th
e te
nth
pa
rt of a
"Be
l" -- na
me
d in
ho
no
r of
Ale
xan
de
r Gra
ha
m B
ell. In
pra
ctice, th
e "B
el" u
nit is n
eve
r use
d. T
he
de
cibe
l is a u
nit (a
bb
revia
ted
"dB
") for m
ea
surin
g a
nd
de
scribin
g th
ere
lative
stren
gth
of tw
o sig
na
ls -- an
d is typ
ically b
ase
d o
n u
nits o
f po
we
ror voltage. T
he decibel is defined as ten times the logarithm
(to the base 10)o
f the
ratio
of th
e p
ow
er o
f two
sign
als. C
om
mo
n p
ractice
ofte
n su
bstitu
tes
on
e o
f the
se tw
o sig
na
ls with
an
arb
itrary re
fere
nce
leve
l, ma
king
the
de
cibe
l into
an
ab
solu
te m
ea
sure
of sig
na
l stren
gth
. Wh
ere
so u
sed
, the
reference level must alw
ays be indicated, such as a 1 watt pow
er reference --in w
hich case the unit would becom
e "dBW
".
DE
MO
DU
LA
TIO
N
Dem
odulation is the process of recovering the encoded information or data from
a re
ceive
d, m
od
ula
ted
carrie
r wa
ve. D
em
od
ula
tion
is the
inve
rse o
f the
inverse of modulation.
DIG
ITA
L L
OO
PB
AC
K
Digital loopack capability is provided by data com
munications equipm
ent (DC
E)
-- such as modem
s and DS
U’s -- w
hich involves taking an incoming data signal
and sending it back out over the return data path to the origination location.T
he
pu
rpo
se is to
test a
com
mu
nica
tion
s circuit, in
clud
ing
DC
E. S
ee
"Analog Loopback".
DIG
ITA
L M
OD
UL
AT
ION
D
igita
l mo
du
latio
n in
vo
lve
s a
pro
ce
ss
of e
nc
od
ing
(an
d s
ub
se
qu
en
td
em
od
ula
tion
, or d
eco
din
g) o
f bin
ary d
igita
l da
ta o
nto
an
an
alo
g ca
rrier o
fso
me
type
. Wh
eth
er u
sing
an
au
dio
or R
F ca
rrier, th
e d
evice
tha
t pe
rform
sthis function w
ill be a modem
.
DIS
TR
IBU
TE
D C
OM
PU
TIN
G E
NV
IRO
NM
EN
T (D
CE
) T
he
DC
E is a
n o
pe
n sta
nd
ard
s-orie
nte
d co
llectio
n o
f softw
are
tech
no
log
ies
(primarily related to data base m
anagement and hum
an interface aspects ofclient/server system
s) supported by the Open S
ystems F
oundation, or OS
F. A
ke
y fe
atu
re o
f the
DC
E is
tha
t it is in
ten
de
d to
en
ab
le fu
nc
tion
al
client/server interoperability between heterogeneous hardw
are and software
elements of a com
puting environment.
DIS
TR
IBU
TE
D M
AN
AG
EM
EN
T E
NV
IRO
NM
EN
T (D
ME
) D
ME
is a collection of network m
anagement specifications and standards w
hichact as a kind of m
anagement um
brella to the DC
E, or D
istributed Com
putingE
nvironment -- both of w
hich are defined and supported by the Open S
oftware
Foundation, or O
SF
.
DO
MA
IN N
AM
E S
YS
TE
M (D
NS
) A
’human-friendly’ site nam
ing system used on the Internet to avoid the need
to remem
ber numeric address values. T
he DN
S uses a syntax w
here strings ofcharacters are separated by ’decim
al points’ or ’periods’, as in "ee.mit.edu".
DO
TT
ED
DE
CIM
AL
AD
DR
ES
S
A num
eric TC
P/IP
(Internet) address which uses the syntax of breaking the 32
bit b
ina
ry IP a
dd
ress in
to fo
ur b
ytes, re
pre
sen
ting
ea
ch b
yte a
s a d
ecim
al
value, and separating each byte segment w
ith a decimal point. A
hypotheticalexam
ple could be "128.34.222.197".
DP
CM
D
iffere
ntia
l pu
lse co
de
mo
du
latio
n, o
r DP
CM
is a va
rian
t of p
ulse
cod
em
odulation (PC
M) in w
hich successive signal sample values are encoded on the
basis of differences between these sam
ple values -- instead of their absolutevalue, as is done w
ith simple P
CM
. See definition of "P
CM
".
DS
-0
Ind
ividu
al vo
ice ch
an
ne
ls are
en
cod
ed
usin
g in
du
stry stan
da
rd p
ulse
cod
em
odulation (PC
M), w
ith a resulting 64 kbps. data sequence, and are known as
DS
-0 channels. This 64 kbps. data stream
doesn’t necessarily carry encodedvo
ice in
form
atio
n, b
ut is re
ga
rde
d a
s voice
cha
nn
el-e
qu
ivale
nt. N
orth
Am
erican telephone networks aggregate 24 D
S-0 channels into one D
S-1 (or T
-1)se
rvice a
t 1,5
44
,00
0 b
ps. u
sing
time
divisio
n m
ultip
lex te
chn
iqu
es. IT
U-T
standards used in Europe aggregate 30 D
S-0 channels into w
hat is widely know
nas "E
-1", or First O
rder Multiplex channels at 2,048,000 bps.
Data C
omm
unications & Fieldbus S
ystems
471
DS
-1
Dig
ital S
ign
al - 1
is a
ge
ne
ric te
rm fo
r the
"T-1
" hig
h s
pe
ed
da
tacom
munications service available com
mercially in N
orth Am
erica and Japan. Itis a
1,5
44
,00
0 b
ps p
oin
t-to-p
oin
t da
ta se
rvice. T
he
"T-1
" de
sign
atio
n h
as
its orig
ins a
s a ve
nd
or-sp
ecific (A
T&
T) p
rod
uct n
am
e -- th
e D
S-1
eq
uiva
len
tis se
en
to b
e a
mo
re g
en
eric re
fere
nce
. Sim
ilarly, D
S-3
an
d T
-3 a
reequivalent. T
he ITU
-T equivalent to D
S-1 used in E
urope is properly referredto
as
"Firs
t Ord
er M
ultip
lex
", bu
t is v
ery
wid
ely
refe
rred
to a
s "E
-1",
parallel to the "T-1" designation.
DS
AP
D
estination Service A
ccess Point -- the upper layer protocol specification for
the network layer protocol w
hich is to receive incoming packets.
DS
I D
igita
l spe
ech
inte
rpo
latio
n is a
dig
ital im
ple
me
nta
tion
of tim
e a
ssign
ed
speech interpolation (TA
SI). S
ee definition of "TA
SI".
DT
E
See definition of "D
ata Term
inal Equipm
ent".
DU
PL
EX
TR
AN
SM
ISS
ION
T
wo-w
ay transmission capability or process. U
sage varies; to be unambiguous,
one should specify full duplex (simultaneous bidirectional com
munication), or
ha
lf du
ple
x (seq
ue
ntia
l bid
irectio
na
l com
mu
nica
tion
) in b
oth
dire
ction
s.Less form
al usage may associate full duplex w
ith "duplex", and differentiateonly half duplex configurations.
EB
CD
IC
Extended B
inary Coded D
ecimal Interchange C
ode: an IBM
proprietary 8-bitalphanum
eric information code. E
BC
DIC
is comparable to, but incom
patiblew
ith AS
CII code.
EC
HO
SU
PP
RE
SS
OR
U
sed
to co
ntro
l the
sub
jective
imp
airm
en
t cau
sed
by e
cho
es o
n te
lep
ho
ne
com
mu
nica
tion
by re
du
cing
the
am
plitu
de
of e
cho
ed
sign
als. T
he
ech
osu
pp
resso
r sen
ses w
hich
spe
ech
pa
th (in
a 4
-wire
, or d
ua
l spe
ech
pa
thco
nfig
ura
tion
) is active
at a
ny p
articu
lar m
om
en
t an
d fu
nctio
ns b
y red
ucin
gth
e a
mp
litud
e in
the
dire
ction
of co
mm
un
icatio
n o
pp
osite
to th
at w
ith th
eo
bs
erv
ed
sp
ee
ch
ac
tivity
. Sin
ce
hu
ma
n s
pe
ec
h is
fun
da
me
nta
lly a
half-duplex process, and because the echo suppressor is capable of quicklyse
nsin
g n
orm
al re
versa
ls in d
irectio
n o
f spe
ech
activity, th
is red
uctio
n in
signal amplitude w
ill only affect the unwanted echo signal. E
cho suppressorsa
re o
f limite
d e
ffective
ne
ss wh
ere
lon
g tim
e d
ela
y (such
as e
nco
un
tere
d o
nsa
tellite
-de
rived
com
mu
nica
tion
cha
nn
els, a
nd
lon
g in
tern
atio
na
l terre
strial
channels) impairs sw
itching responsiveness. Suppressor sw
itching times of up
to 1
00
millise
con
ds w
ill (alo
ng
with
oth
er fa
ctors) lim
it line
turn
aro
un
dtim
es on half-duplex dial telephone data connections.
ED
I S
ee definition of "Electronic D
ata Interchange".
EF
T
Electronic F
unds Transfer involves the electronic com
munication of financial
assets, either between trading partners directly, or m
ore comm
only, between
one organization and a bank.
EIA
S
ee definition of "Telecom
munications Industries A
ssociation".
EIA
INT
ER
FA
CE
T
he
Ele
ctron
ic Ind
ustrie
s Asso
ciatio
n h
as sp
ecifie
d th
e p
hysica
l, ele
ctrical,
functional and procedural aspects of interconnection of equipment (D
TE
and DC
E)
in internationally recognized standards. The m
ost comm
only encountered EIA
interface standard is the EIA
-232-C, form
erly known as the R
S-232-C
.
EL
EC
TR
OM
AG
NE
TIC
INT
ER
FE
RE
NC
E (E
MI)
EM
I is inte
rfere
nce
in th
e fo
rm o
f ele
ctrom
ag
ne
tic en
erg
y rad
iate
d b
ye
lec
trica
l po
we
r sy
ste
ms
(su
ch
as
po
we
r tran
sm
iss
ion
line
s, p
ow
er
transformers, industrial m
otors), natural phenomena (such as lightning strikes)
an
d e
lectro
nic a
pp
ara
tus (su
ch a
s rad
io tra
nsm
itters, ra
da
r eq
uip
me
nt,
cathode ray tube display devices). Vulnerability is very m
uch affected by thekin
d o
f com
mu
nica
tion
s cab
le syste
ms u
sed
(un
shie
lde
d tw
isted
pa
ir cab
le,
or UT
P, shielded tw
isted pair cable, or ST
P, coaxial cable or fiber optic m
edia), a
nd
in e
xtrem
e ca
ses, b
uy sh
ield
ing
me
asu
res ta
ken
to p
rote
ct term
ina
lequipm
ent as well.
EL
EC
TR
ON
IC D
AT
A IN
TE
RC
HA
NG
E
The com
puter-to-computer exchange of standard business docum
entation inm
achine processable form. E
DI is/w
as an early form of e-business and has
largely been overtaken by this class of solution.
EL
EC
TR
ON
IC M
AIL
T
he electronic comm
unication of generally free-formatted inform
ation over acom
puter network. T
he largest e-mail system
in the world is supported by the
global Internet, comprising hundreds of m
illions of users world-w
ide.
EL
EC
TR
ON
IC M
AIL
BO
X
A com
puter file system m
anaged by a network provider w
here incoming e-m
ail fora
give
n re
cipie
nt is lo
dg
ed
, pe
nd
ing
the
ir con
ven
ien
ce in
takin
g d
elive
ry --very analogous to a physical postal m
ail box.
EN
CR
YP
TIO
N
Encryption technology provides for system
atically and thoroughly encoding acom
municated m
essage so that any unintended recipient will be functionally
inca
pa
ble
of co
rrectly in
terp
retin
g its co
nte
nt. T
he
re a
re se
vera
l wid
ely
used encryption systems, m
any of which are developed in the U
.S.A
. and arere
stricted
to u
se in
tha
t cou
ntry. M
ilitary u
se o
f en
cryptio
n is g
en
era
llyre
ga
rde
d to
be
con
side
rab
ly mo
re so
ph
isticate
d a
nd
effe
ctive th
an
the
systems available for civilian applications.
ER
RO
R C
ON
TR
OL
E
rror co
ntro
l is a te
rm u
sed
to d
escrib
e a
ny o
f a n
um
be
r of te
chn
iqu
es fo
rco
ntro
lling
erro
rs on
a d
ata
com
mu
nica
tion
link. E
rror co
ntro
l alw
ays
req
uire
s s
om
e c
ap
ab
ility o
f de
tec
ting
the
pre
se
nc
e o
f erro
rs. U
po
nd
ete
ctio
n, th
ere
are
ba
sic
ally
two
pro
ce
du
ral o
ptio
ns
: (1) in
vo
ke
a
D
ata Com
munications &
Fieldbus System
s472
retra
nsm
ission
req
ue
st (Se
e d
efin
ition
of "A
RQ
"), or (2
) pe
rform
ad
ditio
na
lcalculations on the com
posite message received, and m
ake a determination of
error location(s) without further retransm
ission (See definition of "F
EC
").
ER
RO
R C
OR
RE
CT
ING
CO
DE
D
ata
pro
tecte
d b
y erro
r corre
cting
cod
es is tra
nsm
itted
with
ad
ditio
na
l bits
of d
ata
-- erro
r corre
cting
cod
e fie
lds. E
rror co
rrectin
g co
de
s de
fine
the
sea
dd
ition
al d
ata
ele
me
nts
in s
uc
h a
wa
y th
at it is
po
ss
ible
to v
alid
ate
rece
ived
da
ta b
y che
cking
the
erro
r corre
cting
cod
e b
its, since
the
sen
din
ga
nd
rec
eiv
ing
sta
tion
s b
oth
follo
w th
e s
am
e id
en
tica
l erro
r ch
ec
kin
gcalculation procedure.
ER
RO
R D
ET
EC
TIN
G C
OD
E
A m
eans for encoding data signals, such that they conform to specific rules of
con
structio
n -- so
tha
t de
pa
rture
s from
tho
se ru
les a
t the
rece
iving
loca
tion
can be automatically detected. S
uch codes require additional, or redundantd
ata
to b
e tra
nsm
itted
tha
n a
re n
ece
ssary to
me
rely co
nve
y the
de
sired
information.
ER
RO
R R
AT
E
Da
ta e
rror ra
tes a
re ca
lcula
ted
an
d e
xpre
ssed
as th
e ra
tio o
f erro
red
da
tab
its to th
e to
tal n
um
be
r tran
smitte
d. E
rror ra
tes a
re typ
ically e
xpre
ssed
interm
s of numbers like "1 in 1,000,000) -- m
eaning that for every million data
bits transmitted, one bit is statistically likely to be errored, or incorrect.
ET
HE
RN
ET
E
thernet LAN
technology was developed initially by X
erox, who subsequently
pa
rtne
red
with
DE
C a
nd
Inte
l to fu
rthe
r its ma
rket a
ccep
tan
ce. T
he
IEE
E8
02
.3 sta
nd
ard
versio
n is slig
htly d
iffere
nt fro
m e
the
rne
t. Eth
ern
et is a
concurrent (messages are delivered to all stations at one tim
e), bus orientedcontention LA
N technology. T
he original ethernet was a 1 m
bps. design; allcurrently supported versions are 10 m
bps. (with the exception of developing
100 mbps. "pseudo-ethernet" versions. E
arly implem
entations were all based
on "Fat E
thernet" (10-Base-5) cable designs; developm
ent of "Thin E
thernet"(10-B
ase-2) and "10-Base-T
" telephone cable versions have changed the cableenvironm
ent of typical installations.
ET
SI
European T
elecomm
unications Standards Institute, an independent and broadly
based standards organization which has assum
ed the carrier coordination rolepreviously held by C
EP
T.
EV
EN
PA
RIT
Y C
HE
CK
(OD
D P
AR
ITY
CH
EC
K)
Even and O
dd parity checking are examples of error detection codes w
hich areu
se
d to
va
lida
te d
ata
at a
rec
eiv
ing
loc
atio
n. P
arity
pro
vid
es
for
con
structin
g d
ata
seq
ue
nce
s (typica
lly cha
racte
r-sized
seg
me
nts) in
such
aw
ay th
at th
ere
will b
e a
pre
dicta
bly e
ven
(or o
dd
) nu
mb
er o
f "1" b
its in th
esequence.
EX
CH
AN
GE
, PR
IVA
TE
BR
AN
CH
(PB
X)
A private business telephone sw
itching system (located on user’s prem
ises)w
hich
is con
ne
cted
to th
e p
ub
lic tele
ph
on
e n
etw
ork via
trun
k circuits. In
time past it w
as operated by a switchboard attendant. M
ore recent usage ofth
e te
rm P
BX
pre
sup
po
ses th
at th
e e
qu
ipm
en
t will b
e fu
lly au
tom
atic. In
an
era
wh
en
this a
uto
ma
tic role
cou
ld n
ot b
e a
ssum
ed
, the
term
"PA
BX
", with
the
"A" u
sed
to in
dica
te "a
uto
ma
tic". Th
e a
cron
ym "P
BX
" is the
mo
stcom
monly used form
in current practice.
EX
TE
ND
ED
SU
PE
RF
RA
ME
(ES
F)
ES
F is a D
S-1/T
-1 enhancement w
hich makes all 192 data bits in each of the 24
rep
ea
ting
fram
e p
atte
rns a
vaila
ble
for cu
stom
er d
ata
. With
ES
F in
pla
ce,
each DS
-0 subchannel on the T-1 or D
S-1 service can deliver a "clear channel"
64
kbp
s. con
ne
ction
. Th
is is in co
ntra
st to th
e p
recu
rsor ve
rsion
of T
-1know
n as T-1/D
-3. With the T
-1/D-3 configuration, each D
S-0 subchannel is
req
uire
d to
sa
crific
e o
cc
as
ion
al "ro
bb
ed
sig
na
ling
bits
", limitin
geffective clear channel use to 56 kbps. E
SF
is an enhancement to T
-1 network
systems to m
ake them capable of supporting IS
DN
services.
FA
CS
IMIL
E
Fa
cs
imile
is a
sy
ste
m fo
r tran
sm
itting
info
rma
tion
in g
rap
hic
al fo
rm,
typica
lly ove
r the
pu
blic sw
itche
d te
lep
ho
ne
ne
two
rk. Th
e im
ag
e is ra
ster
scanned at medium
resolution, resulting scanning information com
municated by
da
ta m
od
em
, an
d re
con
structe
d b
y the
rece
iving
eq
uip
me
nt -- o
ften
on
thermally sensitive paper.
FA
ST
PA
CK
ET
SW
ITC
HIN
G
Fa
st pa
cket is a
ge
ne
ral te
rm u
sed
to d
escrib
e p
acke
t com
mu
nica
tion
stechnology based on a m
ore dynamic packet sw
itching methodology than that
permitted by the X
.25 international standard. The tw
o principal technologiesin
clu
de
d in
this
ca
teg
ory
are
"Fra
me
Re
lay
" an
d "C
ell R
ela
y".
Imp
lem
en
tatio
ns o
f cell re
lay a
re co
mm
on
ly asso
ciate
d w
ith A
synch
ron
ou
sT
ransfer Mode (A
TM
) -- a currently important technical developm
ent.
FD
DI
See definition of "F
iber Distributed D
ata Interface".
FD
MA
S
ee definition of "Frequency-division m
ultiple access".
FE
C
See definition of "F
orward E
rror Correction".
FIB
ER
DIS
TR
IBU
TE
D D
AT
A IN
TE
RF
AC
E (F
DD
I) O
ne of two leading high speed standard netw
orking technologies (Alternative is
DQ
DB
, see
de
finitio
n o
f "Distrib
ute
d Q
ue
ue
, Du
al B
us") su
itab
le fo
r eith
er
MA
N a
pp
licatio
ns (sh
are
d b
y a n
um
be
r of u
sers, a
nd
ph
ysically re
ach
ing
ap
ub
lic me
trop
olita
n a
rea
use
r com
mu
nity) o
r HS
LA
N (h
igh
spe
ed
LA
Na
pp
licatio
ns), w
he
re th
e p
hysica
l scale
of th
e n
etw
ork is u
sua
lly limite
d to
a sin
gle
, con
tigu
ou
s pie
ce o
f rea
l esta
te (e
g. a
un
iversity ca
mp
us o
rm
ilitary b
ase
), an
d p
rovid
es a
ba
ckbo
ne
service
to typ
ically a
larg
e n
um
be
rof low
er capacity LAN
systems. F
DD
I operates at 100 Mbps as a dual token ring
system w
ith a total circumference of up 200km
. Extensions to F
DD
I includeC
DD
I, and FD
DI-II w
hich extends the data rate capability into the Gigabit per
se
co
nd
ran
ge
. FD
DI e
njo
ys
the
be
ne
fit of b
ein
g a
n A
NS
I sta
nd
ard
technology, but is not included in the almost universally em
braced IEE
E/IS
OL
AN
stan
da
rds -- th
us it’s lo
ng
term
futu
re co
mp
are
d w
ith o
the
r hig
h-sp
ee
dLA
N/M
AN
technologies is limited.
Data C
omm
unications & Fieldbus S
ystems
473
FIB
ER
OP
TIC
SY
ST
EM
S
Fib
er o
ptic syste
ms co
mm
un
icate
with
(ge
ne
rally in
frare
d) lig
ht p
ulse
stra
nsm
itted
thro
ug
h e
xtrem
ely th
in, h
igh
ly pu
re g
lass fib
ers th
at tra
p, o
rco
nta
in th
e lig
ht e
ne
rgy so
tha
t it follo
ws th
e ca
ble
aro
un
d b
en
ds a
nd
em
erg
es fro
m th
e fa
r en
d w
ith su
fficien
t en
erg
y as to
drive
an
op
to-e
lectric
de
tecto
r de
vice. S
ho
rt dista
nce
ap
plica
tion
s som
etim
es u
s a p
lastic "fib
er"
op
tic cab
le m
ed
ium
. In a
dd
ition
the
ir incre
dib
le d
istan
ce a
nd
da
ta ra
teca
pa
bilitie
s (com
me
rcially a
vaila
ble
system
s op
era
te o
ver m
an
y kilom
ete
rsw
itho
ut re
pe
ate
rs at d
ata
rate
s of m
an
y Gb
ps) -- th
ere
is con
side
rab
leb
en
efit in
usin
g fib
er o
ptic te
chn
olo
gy w
he
re im
mu
nity to
ele
ctrom
ag
ne
ticinterference (E
MI), w
here industrial hazard voltages are present and/or where
security issues are of concern.
FIE
LD
O
ne
ha
lf of a
n in
terla
ce
d te
lev
isio
n p
ictu
re, o
r fram
e. In
52
5 lin
etelevision fram
e systems (N
orth Am
erican NT
SC
), the field consists of 262.5lines and is repeated at a 59.94 H
z. rate.
FIL
E T
RA
NS
FE
R P
RO
TO
CO
L (F
TP
) T
he
orig
ina
l, an
d m
ost co
mm
on
ly use
d m
ea
ns o
f mo
ving
files o
ver T
CP
/IPnetw
orks (and therefore the Internet). See ’anonym
ous FT
P’.
FIN
GE
R
Fin
ge
r is a
clie
nt/s
erv
er c
on
figu
red
ap
plic
atio
n p
roc
es
s fo
r TC
P/IP
inte
rne
two
rke
d s
ys
tem
s th
at m
ak
e it p
os
sib
le to
en
qu
ire a
s to
the
ava
ilab
ility of re
mo
te h
ost syste
ms a
nd
/or in
divid
ua
ls. By in
vokin
g th
e"finger" com
mand, a m
essage is send over the TC
P/IP
network to the specified
ho
st system
, wh
ich re
turn
s statistica
l da
ta re
ga
rdin
g cu
rren
t usa
ge
or
personal information of individual users.
FL
OW
CO
NT
RO
L
In data comm
unication systems or netw
orks which m
ake use of buffer storage,th
e p
ossib
ility of co
ng
estio
n p
reve
ntin
g n
orm
al d
ata
flow
s wo
uld
, if ign
ore
d,
cause a risk of buffer overflow. T
hus, information system
s connected to suchsyste
ms m
ust b
e ca
pa
ble
of re
spo
nd
ing
to sig
na
ls req
uirin
g te
mp
ora
ry(typ
ically m
om
en
tary) ce
ssatio
n o
f tran
smissio
n. T
he
re a
re tw
o typ
es o
fflo
w co
ntro
l: (1) co
ntro
l via th
e in
terfa
ce re
ad
y sign
als (typ
ically D
SR
),and (2) through the use of flow
control characters, such as the AS
CII X
OF
F and
XO
N control characters.
FO
RW
AR
D E
RR
OR
CO
RR
EC
TIO
N
FE
C system
s operate by providing sufficient redundant information accom
panyinge
ach
orig
ina
l me
ssag
e tra
nsm
ission
tha
t it is po
ssible
to n
ot o
nly id
en
tifyth
e p
rese
nce
of e
rrors o
n d
ata
, bu
t to a
lso lo
cate
the
m -- th
us e
na
blin
g a
correction decision to be made w
ithout any reverse transmissions. T
hus then
am
e. S
ince
FE
C syste
ms o
pe
rate
with
ou
t an
y fee
db
ack to
the
po
int o
fo
rigin
, the
re is n
o d
efin
itive cu
stod
y tran
sfer a
ction
, as th
ere
is with
AR
Q(S
ee definition of "AR
Q") system
s. Consequently, F
EC
systems for business
ap
plica
tion
s are
typica
lly con
figu
red
with
an
acco
mp
an
ying
ba
ckstop
AR
Qcapability -- or m
ore correctly, in a hybrid AR
Q/F
EC
configuration.
FO
UR
WIR
E C
IRC
UIT
S
ee definition of "Circuit, F
our Wire".
FR
AM
E
The com
plete representation of a single video image, w
hich is reproduced atthe field refresh rate (29.97 H
z. in NT
SC
equipment).
FR
AM
E
One com
plete message segm
ent, or data unit on a data comm
unications link. Afra
me
is structu
red
by p
rea
gre
em
en
t to co
nve
y con
trol in
form
atio
n su
ch a
ssource/destination addresses, m
essage number, retransm
ission requests ("NA
K"
codes), and error checking features. The m
essage units known as fram
es tendto
be
as
so
cia
ted
with
full d
up
lex
sy
ste
ms
; the
co
mp
ara
ble
un
it of
information in half duplex system
s is typically known as a block.
FR
AM
E R
EL
AY
F
rame relay is a sim
plified, expedited equivalent of conventional X.25 packet
network sw
itching technology -- such that the delivered service has a higherth
rou
gh
pu
t cap
ab
ility -- an
d is th
us m
ore
suita
ble
to L
AN
con
ne
ctivitya
pp
licatio
ns. F
ram
e re
lay’s g
rea
test sim
plifica
tion
(com
pa
red
to X
.25
pa
cket sw
itchin
g) is th
at it d
ispe
nse
s with
link-b
y-link d
ata
valid
atio
n,
making it possible to com
mence forw
arding a packet onto its outbound routeb
efo
re th
e tra
iling
po
rtion
ha
s a
rrive
d (in
co
ntra
st to
the
store-validate-forward process inherent in packet sw
itching). Thus the term
"relay" rather than "switching".
FR
EQ
UE
NC
Y
The rate at w
hich signal values alternate. Thus, frequency is the m
easure ofh
ow
freq
ue
ntly a
na
log
sign
al va
lue
s rep
ea
t the
ir cyclic pa
ttern
of a
mp
litud
eva
riatio
n. F
req
ue
ncy w
as o
nce
me
asu
red
in te
rms o
f "cycles p
er se
con
d",
which w
as a useful, descriptive term. C
urrent practice describes frequency inthe H
ertz unit. Variations are the K
ilohertz, Megahertz, G
igahertz, etc.
FR
EQ
UE
NC
Y D
IVIS
ION
MU
LT
IPL
E A
CC
ES
S (F
DM
A)
FD
MA
is a technique for frequency, or bandwidth sharing an analog com
mon
comm
unication channel -- typically a geographically dispersed channel, sucha
s a sa
tellite
service
. De
vices a
t diffe
ren
t ge
og
rap
hica
l loca
tion
s can
fea
sibly sh
are
such
a ch
an
ne
l if the
y are
discip
line
d to
coo
rdin
ate
the
irin
div
idu
al tra
ns
mis
sio
ns
so
as
to n
ot tra
ns
mit in
ex
ac
tly th
e s
am
efrequency bandw
idth segment as any other user. F
DM
A is in som
e ways like a
distributed frequency division multiplexer system
. Com
pare with T
DM
A.
FR
EQ
UE
NC
Y D
IVIS
ION
MU
LT
IPL
EX
(FD
M)
Frequency-division m
ultiplexer or frequency division multiplexing. F
DM
is ap
roc
es
s b
y w
hic
h d
iffere
nt a
na
log
sig
na
l so
urc
es
ca
n c
o-e
xis
t on
aco
mm
un
icatio
n circu
it or fa
cility by a
lloca
ting
ea
ch sig
na
l a u
niq
ue
fractio
nof the com
posite system bandw
idth.
FR
EQ
UE
NC
Y M
OD
UL
AT
ION
(FM
) F
requency modulation is one of three m
odulation techniques (See definition of
"mo
du
latio
n"). W
ith F
M, th
e b
ase
ba
nd
sign
al in
form
atio
n is e
nco
de
d, o
rim
pressed on the carrier signal by modifying its frequency to instantaneously
respond to variations in the baseband signal.
Data C
omm
unications & Fieldbus S
ystems
474
.
FR
EQ
UE
NT
LY
AS
KE
D Q
UE
ST
ION
S (F
AQ
) F
requently Asked Q
uestion -- or FA
Q -- docum
ents are comm
only created andd
istribu
ted
on
the
Inte
rne
t pe
rtain
ing
to th
ou
san
ds o
f diffe
ren
t top
ics. FA
Qd
ocu
me
nts a
re a
vaila
ble
de
alin
g w
ith m
ost o
f the
top
ics of th
e U
SE
NE
TN
ewsgroups, and m
any other technical and non-technical topics of interest tothe Internet com
munity. F
AQ
documents are typically organized in question
an
d a
nsw
er fo
rma
t, an
d a
lmo
st alw
ays a
re d
esig
ne
d to
give
the
rea
de
r an
en
try leve
l exp
osu
re to
the
title su
bje
ct. A co
mp
reh
en
sive F
AQ
arch
ive site
is maintained at M
IT, at an anonym
ous FT
P server designated as "rtfm
.mit.edu"
in the "pub" directory.
FU
LL
DU
PL
EX
(FD
X)
Full D
uplex refers to a data flow sequences, or com
munications system
s which
op
era
te (o
r pe
rmit o
pe
ratio
n) in
bo
th d
irectio
ns a
t the
sam
e tim
e. C
om
pa
rew
ith "Half D
uplex".
GO
-BA
CK
-N A
RQ
G
o-Back-N
AR
Q system
s provide for a sequence numbered m
essage stream (such as
supported by HD
LC) to identify a m
essage segment, or fram
e that is found tobe flaw
ed by transmission errors, and provide for retransm
ission of the flawed
or errored frame, and all m
essage frames that have been transm
itted since thee
rrore
d fra
me
. Th
us, th
e se
nd
ing
statio
n sim
ply "b
acks u
p" to
the
po
int
wh
ere
the
erro
r occu
rred
an
d sta
rts ove
r. Th
us it w
ou
ld b
e m
ore
litera
l torefer to the procedure as "G
o Back T
o N", rather than "G
o Back N
". Contrast
with "S
elective AR
Q", w
hich provides for only retransmitting the errored fram
e.S
ee definition of "AR
Q".
GO
PH
ER
Largely obsolete: G
opher is a menu-based hierarchical inform
ation search andre
co
ve
ry to
ol w
ide
ly u
se
d o
n th
e In
tern
et. It w
as
de
ve
lop
ed
at th
eU
niversity of Minnesota, w
here many gopher search sequences still originate.
Go
ph
er is a
clien
t/serve
r structu
red
en
viron
me
nt th
at o
pe
rate
s with
eith
er a
text or graphical presentation.
GR
OU
ND
RE
TU
RN
CIR
CU
IT
A g
rou
nd
retu
rn circu
it is an
ele
ctrical co
nn
ectio
n co
nn
ectin
g tw
o p
oin
ts,w
hich
is ge
ne
rally a
me
tallic co
nn
ectio
n (a
ltho
ug
h it m
ay b
e a
n e
arth
conduction path) that (1) provides an electrically comm
on, neutral connectionm
edium, and (2) creates an electrical return path for signals passed betw
eentw
o syste
ms -- su
ch a
s with
the
pin
no
. 7 co
nn
ectio
n o
n th
e E
IA-2
32
-Cinterface.
HA
LF
DU
PL
EX
(HD
X)
Half duplex system
s (and circuits) demonstrate bi-directional com
munication
cap
ab
ility -- bu
t (du
e to
com
mu
nica
tion
eq
uip
me
nt o
r pro
toco
l limita
tion
s)a
re o
nly a
ble
to co
mm
un
icate
in o
ne
dire
ction
at a
time
. Th
ey a
re th
us
sequential bi-directional systems or circuits. H
uman speech is an exam
ple ofa half duplex process.
HA
MM
ING
CO
DE
T
he Ham
ming code is an early, sim
ple, and relatively ineffective forward error
corre
cting
cod
e th
at u
ses a
un
iqu
e p
arity ca
lcula
tion
to d
ete
ct an
d iso
late
transmission errors in data.
HD
LC
S
ee definition of "High Level D
ata Link Protocol".
HD
TV
H
igh
De
finitio
n T
ele
vision
; a d
eve
lop
me
nta
l dig
ital te
levisio
n b
roa
dca
sting
tech
no
log
y de
sign
ed
to re
pro
du
ce m
an
y mo
re lin
es o
f reso
lutio
n th
an
are
ava
ilab
le w
ith e
xisting
an
alo
g te
levisio
n b
roa
dca
sting
. Th
e U
.S. d
efin
ition
is based on 1080 lines, 1920 horizontal samples (pixels) per line distributed
as an MP
EG
-2 compressed digital signal.
HE
RT
Z
Th
e "H
ertz" is th
e u
nive
rsally u
sed
un
it of fre
qu
en
cy, eq
uiva
len
t to th
e n
ow
dis
us
ed
"cy
cle
s p
er s
ec
on
d" u
nit. L
arg
er u
nits
inc
lud
e k
iloh
ertz
(abbreviated khz, and meaning thousands of hertz), m
egahertz (abbreviated Mhz,
and meaning m
illions of hertz) and gigahertz (abbreviated Ghz, and m
eaningbillions of hertz).
HIG
H L
EV
EL
DA
TA
LIN
K C
ON
TR
OL
(HD
LC
) H
DLC
is the most w
idely used synchronous, serial bit oriented link protocol.H
DLC
is an ISO
standard patterned after IBM
’s proprietary Synchronous D
ataLink P
rotocol, or SD
LC. T
he U.S
. AN
SI equivalent of H
DLC
is called AD
CC
P -
Advanced D
ata Com
munications C
ontrol Procedure.
HIG
H S
PE
ED
LA
N (H
SL
AN
) T
he HS
LAN
term is used to describe the application of high speed technology to
a (campus) backbone service. H
SLA
N and M
AN
systems share the sam
e technology-- see definition of "M
etropolitan Area N
etwork" for discussion of technology
alternatives.
HO
ME
PA
GE
A
ho
me
pa
ge
is a sta
rting
po
int fo
r exp
lorin
g a
Wo
rld W
ide
We
b site
. Th
edefault file nam
e for the home page is ’index.htm
’.
HO
RIZ
ON
TA
L P
AR
ITY
ER
RO
R D
ET
EC
TIO
N
Horizontal parity calculations are based on defining a block check character
(BC
C), th
e in
divid
ua
l bits o
f wh
ich a
re d
efin
ed
on
the
ba
sis of m
ain
tain
ing
an
eve
n (o
r od
d) n
um
be
r of "o
ne
" bits th
rou
gh
ou
t all o
f the
bits in
tha
tp
ositio
n in
all ch
ara
cters o
f a d
ata
blo
ck. Th
us, fo
r exa
mp
le, th
e th
ird b
ito
f the
BC
C w
ill be
de
fine
d so
tha
t it, alo
ng
with
all o
f the
da
ta b
its inth
e th
ird b
it po
sitio
n in
all c
ha
rac
ters
in th
e b
loc
k w
ill co
llec
tive
lydem
onstrate even (or odd) parity.
HO
T S
TA
ND
BY
H
ot sta
nd
by e
qu
ipm
en
t is eq
uip
me
nt re
du
nd
an
tly pro
vision
ed
so th
at it is
powered up and instantly ready to assum
e an operational role -- in the eventth
at a
n o
pe
ratin
g co
mp
an
ion
un
it sho
uld
fail. H
ot sta
nd
by e
qu
ipm
en
tco
nfig
ura
tion
s are
ge
ne
rally sim
ple
r to im
ple
me
nt th
an
full co
ncu
rren
tre
du
nd
an
t op
era
tion
s, in w
hich
no
rma
l system
activity is sh
are
d b
y two
pro
du
ctive d
evice
s -- an
d in
wh
ich th
e o
pe
ratio
na
l role
is assu
me
d b
y the
survivor when one fails. T
he idle equipment in hot standby configurations is
ne
ver p
rod
uctive
with
resp
ect to
the
role
for w
hich
it is stan
din
g b
y --a
ltho
ug
h it m
ay p
erfo
rm so
me
low
prio
rity tasks w
hich
are
sub
ject to
be
ing
preempted.
Data C
omm
unications & Fieldbus S
ystems
475
HU
B
A h
ub
is a d
evice
wh
ich is lo
cate
d a
t som
e ce
ntra
l po
int in
a n
etw
ork, a
nd
provides a means of term
inating a number of incom
ing connections. In LAN
system
s, the
hu
b is typ
ically lo
cate
d in
a te
lep
ho
ne
wirin
g clo
set, a
nd
isequipped w
ith a direct wire connection to each device on the LA
N. H
ubs area
vaila
ble
in va
rying
de
gre
es o
f sop
histica
tion
, som
e ca
pa
ble
of su
pp
ortin
ga num
ber of different LAN
technologies and centralized network m
anagement
systems.
HY
PE
RT
EX
T
Hypertext docum
ents are specially formatted text w
hich has embedded codes that
hig
hlig
ht p
ortio
ns o
f the
text, a
nd
pro
vide
a lo
gica
l linka
ge
to o
the
r files
or in
form
atio
n re
sou
rces. B
y view
ing
hyp
erte
xt do
cum
en
ts with
ap
pro
pria
teso
ftwa
re, it is p
ossib
le to
po
int to
a p
iece
of h
igh
ligh
ted
Hyp
erte
xt text,
click the
mo
use
, an
d e
stab
lish th
e lin
kag
e to
the
em
be
dd
ed
refe
ren
ceinform
ation. Thus, hypertext m
akes it possible to move betw
een documents by
keying on internal content of these documents, rather than using an external
menu system
.
HY
PE
RT
EX
T M
AR
KU
P L
AN
GU
AG
E (H
TM
L)
HT
ML is an industry standard syntax for m
arking up, or annotating text so asto
ma
ke
it into
Hy
pe
rtex
t. Se
e d
efin
ition
of ’H
yp
erte
xt’.
Extensions/alternatives to H
TML include S
GM
L (Standard G
eneral Markup Language)
, XM
L (Extensible M
arkup Language) and WM
L (Wireless M
arkup Language).
HY
PE
RT
EX
T T
RA
NS
FE
R P
RO
TO
CO
L (H
TT
P)
HT
TP
is the specified protocol for exchanging hypertext encoded documents (and
graphics, audio and video information) on the W
orld Wide W
eb.
IEE
E
See definition of ’Institute of E
lectrical and Electronics E
ngineers’.
IEE
E 8
02.3
IE
EE
80
2.3
is the
mo
st wid
ely u
sed
LA
N te
chn
olo
gy, b
ein
g th
e fo
rma
lized
,sta
nd
ard
de
rived
from
the
orig
ina
l eth
ern
et. E
the
rne
t as d
efin
ed
by th
eX
ero
x/DE
C/In
tel co
nso
rtium
is sub
tly diffe
ren
t from
the
de
rived
IEE
E 8
02
.3version. S
ee "Ethernet" definition.
IEE
E 8
02.4
T
he IEE
E 802.4 is a specification for a token bus LA
N technology developed at
Ge
ne
ral M
oto
rs as a
de
livery syste
m fo
r ind
ustria
l au
tom
atio
n a
pp
licatio
ns.
Th
e s
ys
tem
ch
ara
cte
ristic
s a
re n
ot e
as
ily a
da
pte
d to
typ
ica
l offic
ea
pp
licatio
ns -- m
akin
g th
e IE
EE
80
2.4
larg
ely irre
leva
nt to
no
n-in
du
strial
uses.
IEE
E 8
02.5
S
ee definition of "Token R
ing" LAN
technology.
IEE
E 8
02.6
T
he IEE
E standard specification for M
etropolitan Area N
etworks (M
AN
) technology.T
he IEE
E 802.6 standard is based on the D
istributed Queue D
ual Bus (D
QD
B)
MA
N technology. D
QD
B is essentially a dead technology.
IN-B
AN
D S
IGN
AL
ING
In-band signaling is an analog telephone netw
ork signaling scheme w
herebyessential netw
ork signaling information is conveyed by m
eans of tone codedsig
na
ls tha
t ha
ve fre
qu
en
cy valu
es su
ch th
at th
ey ca
n b
e ca
rried
with
in th
ea
na
log
ba
nd
wid
th n
orm
ally u
sed
for vo
ice tra
nsm
ission
. (Co
mp
are
with
definition of ’Com
mon C
hannel Interoffice Signaling’).
INS
TIT
UT
E O
F E
LE
CT
RIC
AL
AN
D E
LE
CT
RO
NIC
S E
NG
INE
ER
S (IE
EE
) T
he
IEE
E is th
e w
orld
’s larg
est p
rofe
ssion
al o
rga
niza
tion
, wh
ich o
ver its
histo
ry ha
s esta
blish
ed
a re
pu
tatio
n fo
r pu
blish
ing
a re
ma
rkab
le a
rray o
fh
igh
ly reg
ard
ed
jou
rna
ls an
d o
the
r pe
riod
icals th
at d
efin
e th
e sta
te-o
f-the
art in
mo
st dim
en
sion
s of e
lectrica
l an
d e
lectro
nics te
chn
olo
gy. In
rece
nt
time they have becom
e involved in the computer com
munications standards
settin
g p
roce
ss -- mo
st no
tab
ly with
resp
ect to
the
80
2-se
ries o
f LA
Nstandards.
INT
EG
RA
TE
D S
ER
VIC
ES
DIG
ITA
L N
ET
WO
RK
(ISD
N)
ISD
N is a standards-based technology keyed to the delivery of digital-form
ats
ign
als
ov
er p
ub
lic a
cc
es
s n
etw
ork
s. T
his
is in
co
ntra
st to
ex
istin
gtelephone system
s which are based on analog signals and an analog interface
at the demarcation point betw
een telephone company and custom
er premises
equipment (C
PE
).
INT
ER
FA
CE
A
n interface is the formal point of interconnection -- or boundary -- betw
eentw
o pieces of equipment. T
he functions of each signals which are exchanged,
the
ir ele
ctrical ch
ara
cteristics a
nd
the
ph
ysical fe
atu
res o
f con
ne
ctors
are
ge
ne
rally th
e ke
y com
po
ne
nts o
f inte
rface
stan
da
rds. T
he
stan
da
rdE
IA-232-C
interface is an example.
INT
ER
NA
TIO
NA
L O
RG
AN
IZA
TIO
N F
OR
ST
AN
DA
RD
IZA
TIO
N (IS
O)
Th
e IS
O (o
ften
inco
rrectly re
ferre
d to
as th
e In
tern
atio
na
l Sta
nd
ard
sO
rganization) is an international agency that develops and endorses standards-- fo
r a w
ide
varie
ty of te
chn
olo
gie
s. Th
e IS
O h
ad
a m
ajo
r role
in cre
atin
ga
un
ified
wo
rld vie
w o
f info
rma
tion
tech
no
log
y thro
ug
h th
e O
pe
n S
ystem
sInterconnection M
odel (OS
I) -- and related protocols.
INT
ER
NA
TIO
NA
L T
EL
EC
OM
MU
NIC
AT
ION
S U
NIO
N (IT
U)
The IT
U is an agency of the U
nited Nations, w
hich was established to provide
glo
ba
l stan
da
rdiza
tion
of co
mm
un
icatio
ns p
roce
du
res a
nd
pra
ctices --
including coordination of telephony practices and radio technology usage. Key
operating branches of the ITU
are the ITU
-T and the IT
U-R
. (Refer to these
individual definitions).
INT
ER
NE
T
Th
e te
rm In
tern
et h
as tw
o ke
y inte
rpre
tatio
ns: (1
) Wh
en
use
d w
ith a
n u
pp
er
case "I", Internet refers to the world-w
ide comm
unity of interconnected TC
P/IP
ne
two
rks -- all o
f wh
ich n
ece
ssarily su
pp
ort th
e In
tern
et P
roto
col, o
r "IP".
(2) W
he
n u
se
d w
ith a
low
er c
as
e "I", in
tern
et re
fers
to th
e g
en
eric
interconnection of networks -- m
ore properly referred to as an "internetwork"
to avoid any confusion with (1).
D
ata Com
munications &
Fieldbus System
s476
INT
ER
NE
T E
NG
INE
ER
ING
TA
SK
FO
RC
E (IE
TF
) T
he IET
F is a largely volunteer group responsible for guiding the developm
entof T
CP
/IP and Internet technical standards and practices.
INT
ER
NE
T P
RO
TO
CO
L (IP
) IP
is the
ba
sic da
ta d
elive
ry service
an
d N
etw
ork L
aye
r pro
toco
l for T
CP
/IPnetw
orks. IP-level addressing and routing logic m
akes it possible to deliverm
essage traffic (datagrams) through large, com
plex networks like the Internet.
INT
ER
NE
T S
ER
VIC
E P
RO
VID
ER
(ISP
) A
com
me
rcial e
nte
rprise
tha
t pro
vide
s Inte
rne
t acce
ss in typ
ically a
limite
dg
eo
gra
ph
ic service
are
a -- fo
r a m
od
est fe
e. S
om
e IS
P’s o
ffer n
atio
na
lse
rvice -- a
nd
a w
ide
ran
ge
of q
ua
lity, com
pe
ten
cy, fea
ture
s an
d co
st of
service is evident.
INT
RA
NE
T
Th
ere
are
diffe
ring
view
s of ju
st wh
at a
n In
tran
et is. A
ll wo
uld
ag
ree
tha
tit in
volve
s ad
op
ting
the
inte
rne
t com
pu
ting
arch
itectu
re a
nd
tech
no
log
y for
com
pa
ny-p
rivate
use
. So
me
de
fine
it is an
infra
structu
re, a
nd
thu
s issynonym
ous with a private T
CP
/IP netw
ork environment. O
thers argue that itis synonym
ous with w
eb technology, and thus describes a private web service
-- an
ap
plic
atio
n d
efin
ition
. In p
rac
tice
it is b
oth
; it wo
uld
be
un
thin
kab
le to
crea
te a
priva
te in
tran
et w
itho
ut u
sing
bo
th T
CP
/IP a
nd
we
btechnology.
IP S
WIT
CH
ING
T
here are two m
ain categories of LAN
switching technology deployed in support
of V
irtua
l LA
N se
rvice -- sw
itche
s ba
sed
on
ad
ap
ting
brid
gin
g te
chn
olo
gy
(Layer 2 Sw
itches) and switches based on adapting routing technology (Layer 3
switch
es). O
f the
La
yer 3
switch
alte
rna
tives, IP
Sw
itchin
g is b
y far th
em
ost p
op
ula
r. It take
s ad
van
tag
e o
f the
ne
ar-u
nive
rsal u
se o
f Inte
rne
tP
rotocol in business networks -- and m
akes switching decisions based on IP
network and/or sub-netw
ork addresses. See "T
ag Sw
itching" for a contrastingLayer 2 sw
itch discussion.
ISO
S
ee definition of "International Organization for S
tandardization".
ITU
-R
The International T
elecomm
unications Union - R
adio Com
mittee (form
erly theC
on
sulta
tive C
om
mitte
e o
n In
tern
atio
na
l Ra
dio
, or C
CIR
) is resp
on
sible
for
coordinating the usage of radio comm
unication technology around the world.
Th
is inclu
de
s com
mu
nica
tion
sate
llite se
rvices, m
icrow
ave
rad
io se
rvices
an
d b
roa
dca
sting
service
s. Th
e IT
U-R
is on
e o
f the
two
key o
pe
ratin
gb
ran
che
s of th
e In
tern
atio
na
l Te
leco
mm
un
icatio
ns U
nio
n (IT
U) -- th
e o
the
rm
ain branch is the ITU
-T.
ITU
-T
The International T
elecomm
unications Union - T
elephony Com
mittee (form
erly theC
onsultative Com
mittee on International T
elephony and Telegraphy or C
CIT
T) is
responsible for coordinating the development of technology and standards in
ma
tters
rela
ting
to te
lep
ho
ny
an
d te
leg
rap
hy
-- from
wh
ich
da
taco
mm
un
icatio
ns te
chn
olo
gy h
as e
volve
d. T
he
ITU
-T/C
CIT
T h
as h
istorica
llyb
ee
n m
os
t influ
en
tial in
Eu
rop
e, a
ltho
ug
h th
eir ro
le is
be
co
min
g
incre
asin
gly a
ccep
ted
glo
ba
lly. Th
e IT
U-T
is on
e o
f the
two
key o
pe
ratin
gb
ran
che
s of th
e In
tern
atio
na
l Te
leco
mm
un
icatio
ns U
nio
n (IT
U) -- th
e o
the
rm
ain branch is the ITU
-R.
JA
VA
Java is a S
un MicroS
ystems developm
ent that adds executable software to the
info
rma
tion
reso
urce
s cap
ture
d in
a w
eb
tran
sactio
n. T
hu
s, in a
dd
ition
toa
cqu
iring
text a
nd
gra
ph
ics info
rma
tion
wh
en
we
b b
row
sing
, Java
en
ab
les
acce
ss to e
xecu
tab
le so
ftwa
re w
hich
can
an
ima
te a
we
b p
rese
nta
tion
or b
ecu
stom
ized
to su
it spe
cific bu
sine
ss ap
plica
tion
s, such
as d
ata
en
try. On
es
tron
g b
en
efit o
f Ja
va
is its
pla
tform
-ind
ep
en
de
nc
e; it ru
ns
with
ou
tm
odification on any recent web brow
ser on any machine.
LA
N
See definition of "Local A
rea Netw
ork".
LE
AS
ED
CIR
CU
IT
Te
lep
ho
ne
lea
sed
(or "d
ed
icate
d", "p
rivate
" or "fu
ll pe
riod
") line
s are
tele
ph
on
e co
mp
an
y pro
vision
ed
on
a 2
4 h
ou
r a d
ay, 7
da
ys a w
ee
k ba
sis.Leased lines bypass all sw
itching equipment. Leased lines are unsuitable for
oc
ca
sio
na
l us
e (to
o c
os
tly), o
r for a
pp
lica
tion
s re
qu
iring
va
riab
leco
nn
ectio
n p
oin
ts (no
t con
ven
ien
tly reco
nfig
ura
ble
). Th
ey a
re su
itab
le fo
rh
igh
cap
acity, co
ntin
uo
us a
pp
licatio
ns -- like
pe
rha
ps h
ea
dq
ua
rters to
branch office data connections.
LE
O S
AT
EL
LIT
ES
S
ee definition of "Low E
arth Orbit" S
atellites.
LO
AD
ING
CO
IL
Lo
ad
ing
coils a
re co
ils of w
ire (in
du
ctan
ce) a
dd
ed
in se
ries w
ith lo
ng
metallic local loop telephone cables for the purpose of m
inimizing am
plituded
istortio
n. L
oa
din
g co
ils em
be
dd
ed
in th
e te
lep
ho
ne
cab
le p
lan
t com
plica
tedeploym
ent of high capacity digital technologies like AD
SL.
LO
CA
L A
CC
ES
S A
ND
TR
AN
SP
OR
T A
RE
A (L
AT
A)
Sp
ecific to
U.S
.A. re
gu
lato
ry po
licy, the
LA
TA
’s are
the
ge
og
rap
hic a
rea
sw
ithin
wh
ich th
e re
gu
late
d te
lep
ho
ne
com
pa
nie
s op
era
te. C
om
pe
titivelong-distance carriers are not allow
ed to offer service between points w
ithinone LA
TA
; only between LA
TA
’s. The physical size of LA
TA
’s vary from the
scop
e o
f a sin
gle
city (e.g
. Wa
shin
gto
n D
.C.) in
de
nse
ly po
pu
late
d a
rea
s of
the country to entire states (e.g. North D
akota and New
Mexico).
LO
CA
L A
RE
A N
ET
WO
RK
A
LAN
is a somew
hat loosely defined network w
hich covers a relatively small
ge
og
rap
hic a
rea
, such
as a
sing
le o
ffice b
uild
ing
, an
ind
ustria
l cam
pu
s, au
nive
rsity cam
pu
s, or a
milita
ry ba
se. L
AN
tech
no
log
y typica
lly sup
po
rtsh
igh
da
ta ra
tes (m
an
y Mb
ps), re
lative
ly low
erro
r rate
s, an
d a
ll reso
urce
sare ow
ned and administered by the user organization.
LO
CA
L L
OO
P
The telephone netw
ork connection to individual customer prem
ises is called thelo
cal lo
op
. It is a co
pp
er tw
isted
pa
ir cab
le co
nn
ectio
n, th
e le
ng
th o
fw
hich
is de
term
ine
d b
y the
dista
nce
sep
ara
ting
custo
me
r pre
mise
s from
tele
ph
on
e e
xcha
ng
e site
s. Th
e u
nive
rsality o
f the
cop
pe
r wire
loca
l loo
p
Data C
omm
unications & Fieldbus S
ystems
477
infra
structu
re is b
ein
g ch
alle
ng
ed
by w
irele
ss an
d ca
ble
tele
vision
-ba
sed
alternative technologies.
LO
OP
T
he
term
"loo
p" is a
n a
ltern
ate
term
for "lo
cal lo
op
". Se
e d
efin
ition
of
"local loop".
LO
W E
AR
TH
OR
BIT
SA
TE
LL
ITE
S
Lo
w e
arth
orb
it sate
llite te
chn
olo
gy d
iffers fro
m cla
ssical g
eo
synch
ron
ou
ss
ate
llite c
om
mu
nic
atio
ns
sy
ste
ms
in th
e lite
rally
low
altitu
de
orb
it of
operation. Low orbit im
plies a satellite that appears to move across the sky
-- thu
s req
uirin
g (1
) a n
um
be
r of sa
tellite
s servin
g th
e sa
me
ap
plica
tion
,and (2) non-directional antenna system
s (almost alw
ays). LEO
technology isthe basis of a num
ber of mobile telephone and data access netw
orks, such asthe financially challenged Iridium
project.
LU
MIN
AN
CE
T
he monochrom
e, or brightness part of a video signal. Whenever a m
onochrome
video signal is encountered, it consists entirely of luminance inform
ation.
MA
IL G
AT
EW
AY
A
host computer system
that is connected to two or m
ore different networks,
an
d se
rves to
pa
ss e-m
ail b
etw
ee
n th
em
. Be
cau
se th
ere
is a d
iffere
nce
inte
chn
olo
gy su
pp
orte
d o
r a n
ee
d fo
r secu
rity pa
rtition
ing
be
twe
en
the
ma
ilsystem
s being ’gatewayed’ (otherw
ise a gateway w
ould not be required), thereare protocol conversion and/or proxy functions required at a gatew
ay.
MA
ILIN
G L
IST
A
distrib
utio
n list to
wh
ich b
ulk e
-ma
il ma
y be
de
livere
d. M
ailin
g lists a
reused as a w
ay of supporting ’listserv’ newsgroups, or discussion groups.
MA
IN D
IST
RIB
UT
ION
FR
AM
E (M
DF
) T
he MD
F is a rack system
in telephone exchanges where all incom
ing cableconnections are physically term
inated. From
the MD
F, connections are m
ade totelephone sw
itching apparatus, or other cable connection termination point by
means of cable "jum
pers".
MA
N
See definition of "M
etropolitan Area N
etwork".
MA
RK
SIG
NA
L
The "m
ark" signal is the opposite, or complem
entary signal to a "space" signal.S
ee the definition of "space signal" for a definition of both.
MA
RK
-TO
-SP
AC
E T
RA
NS
ITIO
N
A m
ark-to
-spa
ce tra
nsitio
n in
da
ta (i.e
. the
tran
sition
from
the
"1" to
the
"0" s
ign
al s
tate
) is p
artic
ula
rly c
ritica
l as
a tim
ing
refe
ren
ce
po
int in
asynchronous transmission system
s -- an most references to this topic are in
context of achieving bit level and character synchronization in asynchronoussystem
s.
ME
AN
TIM
E B
ET
WE
EN
FA
ILU
RE
(MT
BF
) A
pplied to either systems or com
ponents of systems, M
TB
F specifies the m
ean(o
r ave
rag
e) le
ng
th o
f time
the
de
vice o
r eq
uip
me
nt w
ill be
exp
ecte
d to
operate between failures. U
sually expressed in hours, such as "expected MT
BF
= 9
,00
0 h
rs." -- imp
lying
an
exp
ecte
d fa
ilure
rate
of a
bo
ut o
nce
pe
r yea
r of
24 hour operation.
ME
AN
TIM
E T
O R
EP
AIR
(MT
TR
) M
TT
R specifies the expected m
ean or average time required to restore a failed
system
to o
pe
ratio
na
l statu
s -- me
asu
red
from
the
po
int in
time
wh
en
afailure occurred.
ME
DIU
M
The com
munications resource used to actually carry inform
ation; It may be a
cop
pe
r cab
le, a
fibe
r op
tic cab
le, a
micro
wa
ve ra
dio
sign
al o
r a sa
tellite
comm
unications system.
ME
DIU
M A
CC
ES
S C
ON
TR
OL
(MA
C)
The M
AC
corresponds to the firmw
are-based logic on a LAN
Netw
ork InterfaceC
ard (NIC
) that supports the LAN
access protocol (e.g. ethernet’s CS
MA
/CD
).T
he
OS
I stan
da
rd sp
ecifica
tion
of L
aye
r 2 (D
ata
Lin
k La
yer) is sp
lit into
two
components in the IE
EE
LAN
standards; the upper Logical Link Control (LLC
)su
b-la
yer, w
hich
is ide
ntica
l for a
ll IEE
E-sta
nd
ard
LA
N’s, a
nd
the
low
er
Me
diu
m A
ccess C
on
trol (M
AC
) sub
-laye
r, wh
ich is d
iffere
nt fo
r ea
ch L
AN
technology.
ME
SS
AG
E S
WIT
CH
ING
M
essage switching system
s (obsolete) were com
puter-based applications thataccepted incom
ing text messages, store them
temporarily (in (typically som
ek
ind
of ro
tatin
g m
em
ory
) un
til the
ap
pro
pria
te o
utg
oin
g c
ircu
it wa
sa
vaila
ble
, an
d th
en
retra
nsm
itted
, or fo
rwa
rde
d th
e m
essa
ge
on
to th
eaddressed destination. T
hese systems replaced torn tape m
essage centers(W
W-II era) -- and have in turn been replaced by electronic m
ail systems.
ME
TR
OP
OL
ITA
N A
RE
A N
ET
WO
RK
(MA
N)
A M
etropolitan Area N
etwork is a netw
ork service that has the data rate andd
istan
ce fe
atu
res th
at m
ake
it suita
ble
for in
tern
etw
orkin
g L
AN
system
s (or
oth
er h
igh
sp
ee
d a
pp
lica
tion
s) a
cro
ss
the
dim
en
sio
ns
typ
ica
l of a
me
trop
olita
n a
rea
-- a d
ime
nsio
n o
f pe
rha
ps 1
00
km. in
dia
me
ter. M
AN
tech
no
log
y is see
n to
req
uire
sufficie
nt d
ata
rate
cap
ab
ility so a
s to o
ffer
virtu
al p
riva
te n
etw
ork
se
rvic
es
to a
nu
mb
er o
f un
rela
ted
us
er
organizations -- on a comm
on MA
N backbone. D
ata rates of Gigabits per second
and higher are currently regarded as necessary for this role. MA
N technology,
when deployed as a private cam
pus backbone service, is generally referred toas a H
igh Speed LA
N (H
SLA
N) service. M
ost MA
N solutions are either A
TM
orhigh-speed sw
itched Ethernet based.
MIC
RO
WA
VE
SIG
NA
LS
R
adio signals are classified as microw
ave signals if their wavelength is less
tha
n 1
0 c
m. -- c
orre
sp
on
din
g to
a fre
qu
en
cy
of 8
90
MH
z. o
r gre
ate
r.C
omm
ercial microw
ave systems operate in the frequency range betw
een about2,000 M
Hz (2 G
Hz.) and 15,000 M
Hz. (15 G
Hz.), although em
erging wireless
systems (such as LM
DS
) will operate at m
uch higher frequencies.
D
ata Com
munications &
Fieldbus System
s478
MO
DE
M
The w
ord "modem
" is a contraction of the words "m
odulator" and "demodulator",
reco
gn
izing
the
two
fun
ction
s tha
t the
de
vice p
erfo
rms. T
he
mo
du
lato
rfu
nc
tion
of th
e m
od
em
ap
plie
s to
the
en
co
din
g o
f dig
ital in
form
atio
np
rese
nte
d to
it by th
e a
ttach
ed
DT
E, su
ch th
at th
e p
hysica
l cha
racte
risticsand lim
itations of the comm
unication channel can be accomm
odated, and the dataca
n b
e su
ccessfu
lly con
veye
d a
cross th
e ch
an
ne
l -- carrie
d o
n a
n a
ud
ible
an
alo
g "ca
rrier sig
na
l". Th
e d
em
od
ula
tion
fun
ction
is simp
ly the
inve
rseprocess. M
odems thus are used to pass digital signals across analog circuits.
MO
DU
LA
TIO
N
Mo
du
latio
n is a
pro
cess w
he
reb
y on
e sig
na
l’s varia
tion
s (typica
lly a so
urce
info
rma
tion
sign
al like
voice
, an
d re
ferre
d to
as th
e b
ase
ba
nd
sign
al) a
rem
od
ula
ted
on
to, o
r imp
ose
d u
po
n a
seco
nd
sign
al, ca
lled
the
carrie
r sign
al.
Ca
rrier sig
na
ls are
synth
etica
lly ge
ne
rate
d, a
nd
the
ir cha
racte
ristics are
se
lec
ted
on
the
ba
sis
of th
eir a
bility
to ca
rry th
e d
es
ired
info
rma
tion
acro
ss the
com
mu
nica
tion
ga
p o
f inte
rest. A
typica
l exa
mp
le is a
rad
iotra
nsm
itter’s ca
rrier sig
na
l. Se
e d
efin
ition
s of a
mp
litud
e m
od
ula
tion
,frequency m
odulation and phase modulation.
MO
DU
LA
TIO
N, P
UL
SE
CO
DE
(PC
M)
See definition of "P
ulse Code M
odulation".
MO
DU
LA
TO
R
A m
odulator is a device which takes baseband signals (such as voice or digital
da
ta), a
nd
use
s the
m to
mo
dify th
e ch
ara
cteristics o
f a syn
the
tic carrie
rsig
na
l for p
urp
ose
s of d
istan
ce co
mm
un
icatio
n, o
r to sh
are
hig
h ca
pa
cityanalog com
munication facilities (frequency division m
ultiplexing).
MU
LT
IPL
EX
ER
M
ultiplexers are devices which m
ake it possible for more than one signal to be
simu
ltan
eo
usly ca
rried
on
a sin
gle
(typica
lly hig
h ca
pa
city) com
mo
n circu
it.In
da
ta co
mm
un
icatio
n a
pp
licatio
ns, m
ultip
lexe
rs are
typica
lly eith
er "tim
edivision m
ultiplexers" or "statistical time division m
ultiplexers".
MU
LT
IPO
INT
LIN
E
Mu
ltipo
int, o
r mu
ltidro
p lin
es o
r circuits p
rovid
e fo
r the
inte
rcon
ne
ction
of
mu
ltiple
acce
ss po
ints o
n a
sing
le, co
mm
on
da
ta circu
it -- a d
aisy ch
ain
configuration.
MU
LT
IPR
OC
ES
SIN
G
Mu
ltipro
cessin
g in
volve
s the
simu
ltan
eo
us a
pp
licatio
n o
f mo
re th
an
on
eprocessor (in either a m
ulti-processor or distributed processing system) to a
sing
le jo
b o
r task. M
ultip
roce
ssing
is com
mo
nly h
an
dle
d w
ith p
ara
llel o
rarray processors.
MU
LT
IPR
OG
RA
MM
ING
A
mu
ltipro
gra
mm
ing
com
pu
ter o
pe
ratin
g syste
m e
na
ble
s the
(ap
pa
ren
t)sim
ultaneous execution of more than one task by tim
e-slicing its sequence ofo
pe
ratin
g in
structio
ns. M
ultip
rog
ram
min
g syste
ms a
re a
ble
to d
em
on
strate
ap
pa
ren
t simu
ltan
eo
us e
xecu
tion
of ta
sks by ra
pid
ly switch
ing
con
text
be
twe
en
ap
plica
tion
s, exe
cutin
g a
few
instru
ction
s for o
ne
task a
nd
the
nm
oving on and executing a few instructions for another.
MU
LT
IST
AT
ION
AC
CE
SS
UN
IT (M
AU
) T
he MA
U is the hub device specified for T
oken Ring LA
N system
s. Originally,
the
no
n-in
tellig
en
t IBM
82
20
MA
U w
as th
e o
nly o
ptio
n -- re
qu
iring
spe
cificconnector types and m
ounting arrangements. M
ore recently, numerous vendors
offe
r a w
ide
ran
ge
of p
hy
sic
al c
on
figu
ratio
ns
, co
nn
ec
tor ty
pe
s a
nd
intelligence in MA
U products.
MU
LT
ITA
SK
ING
S
ee definition of "Multiprogram
ming".
MU
LT
ITH
RE
AD
ING
M
ultithreading is the simultaneous processing of m
ultiple service requests bya
n a
pp
lica
tion
pr
og
ra
m -
- ty
pic
ally
a s
er
ve
r p
ro
ce
ss
.M
ultitasking/Multiprogram
ming are supported by the operating system
while
Mu
ltithre
ad
ing
is s
up
po
rted
by
the
ap
plic
atio
n p
roc
es
s (a
ltho
ug
hm
ultith
rea
din
g typ
ically re
qu
ires a
mu
ltipro
gra
mm
ing
op
era
ting
system
).M
ultithreading is associated with ’reentrancy’.
NA
K
A negative acknow
ledgement of m
essage (typically a block of frame m
essagesegm
ent) receipt -- indicating a request to retransmit the m
essage segment. A
NA
K is typ
ically sig
na
led
with
a sp
ecia
l con
trol ch
ara
cter (w
ith h
alf-d
up
lex
link p
roto
cols) o
r a p
re-d
efin
ed
bit-fla
g (w
ith fu
ll-du
ple
x link p
roto
cols).
The alternative to a N
AK
is an AC
K, or positive m
essage acknowledgem
ent.
NA
TIO
NA
L IN
ST
ITU
TE
OF
ST
AN
DA
RD
S A
ND
TE
CH
NO
LO
GY
(NIS
T)
Form
erly, the National B
ureau of Standards (N
BS
), NIS
T has historically been
responsible for maintaining U
SA
physical standards, such as length and time.
Th
ey h
ave
be
en
resp
on
sible
for d
efin
ing
stan
da
rds fo
r da
ta e
ncryp
tion
technology (such as the public domain D
ata Encryption S
tandard or DE
S) --
in cooperation with the N
ational Security A
gency (NS
A).
NE
TB
IOS
N
ET
BIO
S (N
etwork B
asic Input/Output S
ystem) w
as the essential core of the "IBM
LAN
Program
", which w
as the first widely used LA
N operating system
. NE
TB
IOS
is still sup
po
rted
by M
icroso
ft ne
two
rking
pro
du
cts, alth
ou
gh
it attra
ctslittle continuing interest.
NE
TS
CA
PE
N
etsca
pe
is a g
rap
hica
l bro
wse
r clien
t too
l wid
ely u
sed
for a
ccess to
the
World W
ide Web. N
etscape was inspired by the success of M
osaic (and developedb
y the
orig
ina
l au
tho
r of M
osa
ic) -- the
first com
mo
nly u
sed
pa
ckag
e o
f this
type
. Ne
tscap
e w
as th
e d
om
ina
nt w
eb
bro
wse
r for a
time
, bu
t lost th
isclaim
to Microsoft’s Internet E
xplorer.
NE
TW
OR
K
An interconnected com
munication facility w
hich provides services to a number
of service points, or users.
NE
TW
OR
K F
ILE
SY
ST
EM
(NF
S)
NF
S w
as originally a proprietary data base access technology developed by Sun
Mic
ros
yste
ms, c
ap
ab
le o
f op
era
ting
ov
er T
CP
/IP n
etw
ork
s. It is s
tillp
rop
rieta
ry, bu
t Su
n h
as m
ad
e N
FS
spe
cificatio
ns p
ub
licly ava
ilab
le, a
nd
itis now
regarded as an "open" technology. A key part of the acceptance of N
FS
Data C
omm
unications & Fieldbus S
ystems
479
ha
s be
en
the
con
ven
ien
ce o
f inte
rpro
cess co
mm
un
icatio
n (e
.g. clie
nt to
server) enabled by the associated Rem
ote Procedure C
all (RP
C) feature.
NE
TW
OR
K M
AN
AG
EM
EN
T C
EN
TE
R (N
MC
) A
cen
tral fa
cility eq
uip
pe
d w
ith n
etw
ork m
on
itorin
g e
qu
ipm
en
t cap
ab
le o
fd
ete
cting
ne
two
rk failu
res a
nd
ab
erra
tion
s, an
alyzin
g th
eir sig
nifica
nce
an
dd
ispa
tchin
g re
sou
rces to
de
al w
ith th
e p
rob
lem
. Typ
ical N
MC
cap
ab
ilities
inc
lud
e c
olle
ctin
g o
pe
ratin
g s
tatis
tics
, ma
inte
na
nc
e s
ch
ed
ulin
g a
nd
configuration control.
NIS
T
See "N
ational Institute of Standards and T
echnology".
NO
DE
A
node is a junction point in a network, w
here comm
unication links converge.T
he
term
ha
s also
com
e to
imp
ly a p
oin
t of in
tellig
en
ce in
a n
etw
ork,
particularly in the context of packet switching.
NO
ISE
In
con
text o
f com
mu
nica
tion
s system
s, no
ise is a
ny u
nw
an
ted
sign
al th
at
dis
turb
s o
r imp
airs
the
ab
ility to
co
rrec
tly in
terp
ret d
es
ired
inco
min
gsig
na
ls. No
ise m
ay b
e ca
use
d b
y exte
rna
l distu
rba
nce
s (e.g
. fluo
resce
nt
ligh
ting
) or co
mm
un
icatio
ns e
qu
ipm
en
t itself. T
he
con
cep
t of n
oise
an
dn
ois
e im
pa
irme
nt re
late
to th
e a
na
log
vie
w o
f sig
na
ls a
nd
sig
na
linterpretation.
NT
SC
N
atio
na
l Te
levisio
n S
tan
da
rds C
om
mitte
e, w
hich
wa
s the
orig
ina
l tele
vision
tech
no
log
y spe
cificatio
n. N
TS
C is th
e te
levisio
n sta
nd
ard
use
d in
No
rthA
merica, and carries an em
bedded dependency on a 60 Hz. electrical pow
erinfrastructure.
OC
TE
T
An
8 b
it un
it of in
form
atio
n, e
qu
ivale
nt to
the
wid
ely re
cogn
ized
"byte". T
he
octet term is favored in international usage (such as standards specifications)
an
d o
the
r form
al a
pp
licatio
ns -- n
om
ina
lly be
cau
se o
f the
histo
rically
ambiguous byte (w
hich hasn’t always referred to 8 bits -- som
etimes 6 bits),
an
d p
ron
un
ciatio
n d
ifficultie
s with
diffe
ren
tiatio
n "b
it" an
d "b
yte" in
the
French language.
OF
F H
OO
K
The condition of a telephone w
hich occurs when the handset is rem
oved from its
crad
le, th
us e
ng
ag
ing
the
ho
ok sw
itch. O
ff ho
ok sta
tus th
us m
ea
ns th
at a
circuit sw
itche
d te
lep
ho
ne
con
ne
ction
is eith
er in
use
, or in
pro
cess o
fb
ein
g a
ctivate
d. A
n o
ff ho
ok te
lep
ho
ne
con
ditio
n w
ill ren
de
r the
line
"bu
sy"to any in-calling attem
pts.
ON
HO
OK
T
he condition of a telephone which occurs w
hen the handset is placed on itscradle, thus disengaging the hook sw
itch. On hook status thus m
eans that acircuit sw
itched telephone connection is available for use, or otherwise idle.
An
on
ho
ok
tele
ph
on
e c
on
ditio
n w
ill ren
de
r the
line
av
aila
ble
for
in-calling attempts.
ON
LIN
E T
RA
NS
AC
TIO
N P
RO
CE
SS
ING
(OL
TP
) O
n L
ine
Tra
nsa
ction
Pro
cessin
g is a
ge
ne
ral te
rm fo
r tran
sactio
n-o
rien
ted
bu
sine
ss com
pu
ting
system
s tha
t offe
r on
-line
, or re
al-tim
e a
ccess fro
mrem
ote users. OLT
P system
s are typically configured as client/server systems.
OP
EN
SY
ST
EM
S IN
TE
RC
ON
NE
CT
ION
(OS
I) T
he OS
I model and related standards w
ere developed by the ISO
(in cooperationw
ith other, regional standards groups, like AN
SI and E
TS
I. The O
SI m
odel was
de
ve
lop
ed
with
the
ob
jec
tive
of d
efin
ing
an
inte
rna
tion
al, o
pe
n (ie
.non-proprietary) set of standards for com
puter comm
unications applicationsthat w
ill permit interoperability betw
een different vendor systems. A
ny hopeo
f actu
ally b
uild
ing
ne
two
rks in co
nfo
rma
nce
to th
e O
SI sta
nd
ard
s wa
sp
retty m
uch
killed
by th
e co
mp
lexity o
f the
fina
l spe
cificatio
ns a
nd
the
concurrent success of TC
P/IP
and the Internet.
OP
TIC
AL
FIB
ER
O
ptica
l fibe
r is a th
in fila
me
nt o
f tran
spa
ren
t ma
teria
l; typica
lly gla
ss or
pla
stic. Op
tical fib
er is co
mp
rised
of a
very th
in co
re (w
hich
actu
ally
ca
rries
ligh
t pu
lse
s o
ve
r the
len
gth
of th
e fib
er), a
nd
a m
ore
bu
lky
surrounding material know
n as cladding. See "F
iber Optic C
able" definition.
OU
T-O
F-B
AN
D S
IGN
AL
ING
A
method of telephone netw
ork signaling which com
municates the necessary
sign
alin
g a
nd
con
trol in
form
atio
n o
ver a
ne
two
rk con
ne
ction
oth
er th
an
the
message channel. C
omm
on Channel Interoffice S
ignaling (CC
IS) and S
ignalingS
ystem N
umber 7 (S
S7) are exam
ples of out-of band signaling technologies.
PA
BX
P
rivate automatic branch exchange - (S
ee definition of "Exchange").
PA
CK
ET
A
(typically) brief message segm
ent which includes both user data and m
essagea
dd
ressin
g a
nd
con
trol in
form
atio
n w
hich
is rou
ted
, or sw
itche
d a
s acom
posite message. T
he data and related control information are arranged in a
stan
da
rd, p
rescrib
ed
form
at, th
us e
na
blin
g p
acke
ts from
oth
er u
sers to
intermingle on com
mon netw
ork facilities.
PA
CK
ET
SW
ITC
HIN
G
Pa
cket sw
itchin
g syste
ms su
pp
ort th
e tra
nsm
ission
of d
ata
by m
ea
ns o
fsp
ecifica
lly form
atte
d a
nd
ad
dre
ssed
pa
ckets, re
sultin
g in
a co
mm
un
icatio
nc
ap
ab
ility th
at o
cc
up
ies
a tra
ns
mis
sio
n fa
cility
for th
e d
ura
tion
of
tran
smissio
n o
f the
pa
cket o
nly. T
he
com
mu
nica
tion
facility, o
r ne
two
rk isth
us a
vaila
ble
to d
elive
r pa
ckets m
an
y diffe
ren
t use
rs on
a sh
are
d b
asis.
Note: Large m
essages will be subdivided in to a num
ber of discrete packetsfor transm
ission across the network, and reassem
bled at receiving end of thelink -- by m
eans of a packet assembly/disassem
bly function. Packet sw
itchingse
rvice co
sts are
pro
po
rtion
al to
the
volu
me
of in
form
atio
n tra
nsm
itted
,and are unrelated to holding tim
e. Com
pare with C
ircuit Sw
itching.
PA
CK
ET
SW
ITC
HIN
G N
ET
WO
RK
A
packet switching netw
ork is designed to carry packet-formatted data. W
ithinth
e p
acke
t ne
two
rk, all u
sers’ tra
ffic is pre
sen
ted
in a
n id
en
tical fo
rma
t(p
acke
tized
), diffe
ren
tiate
d o
nly b
y a sta
nd
ard
ized
he
ad
er -- a
na
log
ou
s tothe address on an envelope. P
acket routing decisions are made at a num
ber of
D
ata Com
munications &
Fieldbus System
s480
switching, or node sites on the basis of header contents.
PA
L
Ph
ase
Alte
rna
te L
ine
s, refe
rring
to th
e p
ha
se a
ltern
atio
n u
sed
in th
e co
lor
sub
carrie
r, with
the
pu
rpo
se o
f crea
ting
mo
re sta
ble
hu
e re
pro
du
ction
. PA
Lis th
e te
levisio
n sta
nd
ard
use
d th
rou
gh
ou
t mo
st of w
este
rn E
uro
pe
(exce
pt
Fra
nce
), an
d in
mo
st cou
ntrie
s tha
t use
a 5
0 H
z. ele
ctrical in
frastru
cture
.P
AL is the m
ost widely used analog television technology standard.
PA
RA
LL
EL
TR
AN
SM
ISS
ION
P
arallel transmission involves the provisioning of m
ultiple data comm
unicationp
ath
s so th
at th
e d
ata
bits p
erta
inin
g to
a u
nit o
f da
ta (typ
ically a
byte
,or data character) can be passed across different circuits. P
arallel systems
are
typica
lly 8 b
its (or so
me
inte
ge
r mu
ltiple
of 8
) wid
e, e
na
blin
g a
ll da
tab
its of o
ne
byte
to b
e se
nt in
on
e clo
ck time
un
it. Pa
ralle
l tran
smissio
nis fa
ster th
an
com
pa
rab
le se
rial a
ltern
ative
s, bu
t is serio
usly tro
ub
led
by
timin
g co
ntro
l an
d sig
na
l cross-ta
lk pro
ble
ms if th
e lin
k len
gth
exce
ed
s afew
meters. C
ompare w
ith Serial T
ransmission.
PA
RIT
Y
Pa
rity che
cking
invo
lves th
e a
dd
ition
of (re
du
nd
an
t) no
n-in
form
atio
n b
its tod
ata
(typ
ica
lly, b
ut n
ot a
lwa
ys
the
se
ve
n o
r eig
ht-b
it ch
ara
cte
rre
pre
sen
tatio
ns), in
such
a w
ay a
s to fo
rce th
e n
um
be
r of o
ne
bits in
the
gro
up
to b
e e
ithe
r alw
ays a
n e
ven
(or a
lwa
ys an
od
d) n
um
be
r. Th
us p
arity
imp
lem
en
tatio
ns a
re d
escrib
ed
as b
ein
g e
ithe
r eve
n p
arity o
r od
d p
arity.
Pa
rity imp
lem
en
tatio
ns p
erm
it de
tectio
n o
f sing
le e
rror o
ccurre
nce
s in th
ed
ata
wh
ich is "co
vere
d" b
y the
pa
rity calcu
latio
n. (S
ee
"Ho
rizon
tal P
arity"
and "Vertical P
arity".
PB
X
Private branch exchange. A
telephone exchange on the user’s premises w
ithaccess to the public netw
ork. (See definition under "E
xchange").
PH
AS
E M
OD
UL
AT
ION
P
ha
se m
od
ula
tion
is on
e o
f thre
e m
od
ula
tion
tech
niq
ue
s (Se
e d
efin
ition
of
"mo
du
latio
n"). W
ith P
M, th
e b
ase
ba
nd
sign
al in
form
atio
n is e
nco
de
d, o
rim
pre
ss
ed
on
the
ca
rrier s
ign
al b
y m
od
ifyin
g its
ph
as
e v
alu
e to
instantaneously respond to variations in the baseband signal.
PIN
G
The "P
acket Internet Groper", or P
ING
application program is a utility program
ava
ilab
le o
n T
CP
/IP-e
na
ble
d syste
ms. P
ING
pe
rform
s the
simp
le ta
sk of
rep
etitive
ly tran
smittin
g d
ata
gra
ms to
a d
esig
na
ted
ne
two
rk ad
dre
ss, an
drecovering the replies (using the Internet M
essage Protocol, or IC
MP
). PIN
Gv
erifie
s th
e v
alid
ity o
f the
ch
os
en
ad
dre
ss
, av
aila
bility
of a
ne
two
rkco
nn
ectio
n to
the
spe
cified
de
stina
tion
, an
d th
e o
pe
ratio
na
l statu
s of th
at
system.
PO
INT
OF
PR
ES
EN
CE
(PO
P)
Th
e p
oin
t (in a
cou
ntry, sta
te/p
rovin
ce o
r reg
ion
) wh
ere
a n
etw
ork se
rvicep
rov
ide
r offe
rs a
cc
es
s to
the
ir ne
two
rk. It is
typ
ica
lly a
cu
sto
me
r’sproblem
(expense) to provide the connection between the nearest P
OP
and theirbusiness location.
PO
INT
TO
PO
INT
PR
OT
OC
OL
P
oin
t-to-P
oin
t Pro
toco
l -- a lin
k con
trol p
roto
col ca
pa
ble
of e
nca
psu
latin
gany of a num
ber of network layer protocols; m
ost importantly IP
. Num
erousva
riatio
ns a
nd
/or fe
atu
re e
xten
sion
s ha
ve b
ee
n a
dd
ed
to th
e o
rigin
al P
PP
spe
cificatio
ns, e
na
blin
g P
PP
to b
e u
sed
in a
wid
e va
riety o
f ap
plica
tion
s,such as V
PN
’s.
PO
INT
-TO
-PO
INT
A
com
mu
nica
tion
circuit th
at e
stab
lishe
s a co
nn
ectio
n b
etw
ee
n o
nly tw
olocations (in contrast to a m
ulti-point circuit).
PO
LL
ING
P
ollin
g syste
ms o
pe
rate
as a
wa
y of co
ntro
lling
the
sha
red
acce
ss tom
ultip
oin
t com
mu
nica
tion
line
s. Th
e ce
ntra
l site syste
m u
nd
erta
kes to
be
a"m
aster of ceremonies", directing each station to speak in a prearranged order.
Ind
ividu
al re
mo
te sta
tion
s tha
t ha
ve n
o kn
ow
led
ge
of o
the
r statio
ns’ n
ee
ds
to co
mm
un
icate
are
inh
ibite
d fro
m tra
nsm
itting
at th
e sa
me
time
-- an
d th
us
interfering with one another.
PO
RT
A
logical (software) or physical (hardw
are) point of interaction with a system
.Id
en
tificatio
n o
f ap
pro
pria
te p
orts to
use
for a
ccessin
g syste
ms (g
en
era
llyin
con
text o
f seve
ral a
ltern
ative
po
rts be
ing
ava
ilab
le) is o
ften
ne
cessa
ry;see D
SA
P and S
SA
P definitions.
PP
P
See definition of "P
oint to Point P
rotocol".
PR
OP
AG
AT
ION
TIM
E D
EL
AY
P
rop
ag
atio
n tim
e d
ela
y is the
time
req
uire
d fo
r a sig
na
l to tra
vel fro
m o
ne
po
int in
a s
ys
tem
circ
uit to
an
oth
er -- th
at is
, the
time
req
uire
d to
trave
rse th
e n
etw
ork. C
om
mo
nly, it is th
e ro
un
d-trip
time
de
lay th
at a
ffects
system perform
ance (eg. the waiting tim
e between m
essage transmission and
rece
ipt o
f ackn
ow
led
ge
me
nt). O
pe
n a
ir rad
io sig
na
ls (such
as sa
tellite
co
mm
un
ica
tion
sig
na
ls) tra
ve
l at (v
ery
clo
se
to) th
e s
pe
ed
of lig
ht;
co
ns
train
ed
sig
na
ls o
pe
ratin
g w
ithin
ca
ble
s (m
eta
llic o
r op
tica
l) are
co
mm
on
ly e
stim
ate
d a
t on
e h
alf o
f the
sp
ee
d o
f ligh
t. It tak
es
ligh
ta
pp
roxim
ate
ly 5 m
illiseco
nd
s to tra
nsit 1
,00
0 m
iles -- o
r 3 m
illiseco
nd
s totra
nsit 1
,00
0 km
. Pro
pa
ga
tion
de
lay is p
artly co
mp
rised
of th
e tra
nsit
time
limita
tion
s of th
e sp
ee
d o
f ligh
t, an
d p
artly d
ue
to sig
na
l pro
cessin
gdelays in equipm
ent along the way.
PR
OT
OC
OL
A
form
ally d
efin
ed
pro
ced
ure
wh
ich d
escrib
es th
e fo
rma
t, an
d p
roce
du
ral
seq
ue
ncin
g o
f me
ssag
es b
etw
ee
n tw
o co
mm
un
icatin
g e
ntitie
s. req
uire
d to
initiate and maintain com
munication. T
he term "P
rotocol" has become m
oreg
en
era
lized
in re
cen
t pra
ctice. A
t on
e tim
e it re
ferre
d to
wh
at is n
ow
kno
wa
s "L
ink
lay
er p
roto
co
l", wh
ich
de
als
ex
plic
itly w
ith th
e s
truc
turin
g(b
lockin
g o
r fram
ing
) of b
ina
ry da
ta se
qu
en
ces, th
eir va
lidity ch
eckin
g(error control) and addressing. T
he more general (current) usage is extended
to a
ll seve
n la
yers o
f the
OS
I stan
da
rd a
rchite
cture
de
finitio
n -- in
wh
ich"protocol entities" exchange inform
ation in "protocol data units".
Data C
omm
unications & Fieldbus S
ystems
481
PT
&T
P
ostal, telephone and telegraph organization which acts as a nation’s public
tele
ph
on
e se
rvice p
rovid
er. T
he
term
PT
&T
is no
t use
d in
con
text o
fin
vesto
r-ow
ne
d te
lep
ho
ne
com
pa
nie
s -- PT
&T
’s are
go
vern
me
nt-co
ntro
lled
monopoly organizations w
hich acts as an arm of national telecom
munication
policy. The trend to global deregulation has very m
uch reduced the number of
countries where this class of telephone operation exists.
PU
BL
IC S
WIT
CH
ED
TE
LE
PH
ON
E N
ET
WO
RK
(PS
TN
) T
he
PS
TN
is the
ub
iqu
itou
s, glo
ba
l pu
blic te
lep
ho
ne
system
tha
t pro
vide
sg
en
era
l con
ne
ctivity to th
e a
pp
roxim
ate
ly 1 b
illion
tele
ph
on
e su
bscrib
ers
wo
rldw
ide
. Th
e in
teg
ratio
n o
f the
trad
ition
al w
ired
tele
ph
on
e in
frastru
cture
with
rap
idly g
row
ing
wire
less se
rvices is g
rea
tly exp
an
din
g th
e ro
le o
f the
PS
TN
.
PU
LS
E
A p
uls
e is
a s
ho
rt ch
an
ge
in th
e e
lec
trica
l or o
ptic
al s
ign
al s
tate
of
comm
unication -- or electronic -- system. P
ulse values are typically encodedto
rep
rese
nt b
ina
ry da
ta, a
nd
are
allo
we
d to
exist fo
r very sp
ecific p
erio
ds
of time, consistent w
ith some uniform
, and mutually accepted m
easure of time.
(See definition of "C
lock").
PU
LS
E-C
OD
E M
OD
UL
AT
ION
(PC
M)
PC
M is a m
ethod of representing analog signals (such as speech) by takingp
erio
dic sa
mp
les o
f the
time
-varyin
g a
na
log
sign
al (th
e in
du
stry stan
da
rdsam
pling rate is 8000 samples per second), and converting each sam
ple valuein
to a
bin
ary co
de
(the
ind
ustry sta
nd
ard
is an
eig
ht-b
it cod
e, su
fficien
tto
rep
rese
nt 2
56
distin
ct sam
ple
valu
es). P
CM
is no
t a tru
e m
od
ula
tion
process (See definition of "M
odulation").
PU
SH
TE
CH
NO
LO
GY
In contrast to typical inform
ation recovery or retrieval systems w
hich requireth
e u
ser to
exp
licitly see
k, or "p
ull" in
form
atio
n to
the
ir wo
rkspa
ce, p
ush
technology automates the outw
ard dissemination of inform
ation. This is done
by
de
finin
g a
us
er in
tere
st p
rofile
in s
om
e w
ay
, an
d p
rov
idin
g a
ninform
ation delivery mechanism
.
RA
ID
Redundant A
rrays of Inexpensive Disks, or R
AID
systems use sophisticated disk
con
trolle
rs an
d a
ssocia
ted
softw
are
to cre
ate
hig
hly re
liab
le a
gg
reg
ate
d,
fault-tolerant disk systems from
readily available, inexpensive disk hardware.
Th
ere
are
five le
vels o
f RA
ID so
ph
isticatio
n ra
ng
ing
from
simp
le re
du
nd
an
tm
irroring through hot-swappable dynam
ically redundant systems.
RE
DU
CE
D IN
ST
RU
CT
ION
SE
T C
OM
PU
TE
R (R
ISC
) A
com
pu
ter w
hich
op
era
tes a
simp
lified
instru
ction
set (co
mp
are
d w
ith a
traditional "Com
plex Instruction Set C
omputer", w
hich supports a large number
of o
ften
len
gth
y an
d co
mp
lex b
uilt-in
instru
ction
s as re
qu
ired
by h
igh
leve
lla
ng
ua
ge
s) wh
ich is sp
ecia
lly de
sign
ed
to o
pe
rate
efficie
ntly o
n h
ard
wa
red
esig
ne
d o
r this p
urp
ose
. RIS
C syste
ms th
ere
fore
pe
rform
the
ma
jority o
fsim
ple
rep
etitive
com
pu
ting
tasks ve
ry efficie
ntly, a
nd
com
bin
e th
ese
simp
lified
pro
ced
ure
s into
softw
are
rou
tine
s wh
en
req
uire
d to
pe
rform
mo
recom
plex functions.
RE
DU
ND
AN
CY
R
ed
un
da
ncy in
volve
s the
con
cep
t of d
up
licatio
n, o
r (typica
lly) un
de
rutilize
dca
pa
city. Th
ere
are
two
diffe
ren
t con
texts: (1
) Info
rma
tion
red
un
da
ncy,
which involves data representations (codes) that are less efficient than they
mig
ht th
eo
retica
lly be
. Th
is is com
pa
rab
le to
red
un
da
ncy in
lan
gu
ag
e -- a
typ
ica
l se
nte
nc
e is
still c
om
pre
he
ns
ible
wh
en
a fe
w c
ha
rac
ters
are
inco
rrectly p
rese
nte
d -- a
s with
spe
lling
erro
rs. (2) E
qu
ipm
en
t red
un
da
ncy
invo
lves p
rovisio
nin
g o
f spa
re co
mp
on
en
ts in a
critical syste
m, so
tha
tfa
ilure
of o
ne
will n
ot te
rmin
ate
service
. Du
al a
uto
mo
bile
he
ad
lam
ps a
re a
ne
xam
ple
-- on
e o
nly co
uld
pro
vide
sufficie
nt illu
min
atio
n, b
ut its fa
ilure
would be too serious to be an acceptable risk.
RE
PE
AT
ER
, RE
GE
NE
RA
TIV
E
(1) Repeaters are used in telephone netw
ork connections to link high capacity,lo
ng
dis
tan
ce
sy
ste
ms
tog
eth
er. T
he
dis
tan
ce
limita
tion
s o
f sp
ec
ifictechnologies (fiber optic cables, m
icrowave radio system
s, etc.) are overcome
by ca
scad
ing
mu
ltiple
ma
ximu
m le
ng
th sp
an
s of th
ese
tech
no
log
ies w
ithrepeater interconnections. (2) R
epeaters are used in much this sam
e role incontext of local area netw
orks as a way of extending the physical rang of LA
Nsegm
ents, or to interface different cable media types on a single LA
N.
RE
SID
UA
L B
IT E
RR
OR
RA
TE
, UN
DE
TE
CT
ED
ER
RO
R R
AT
E
Th
e re
sidu
al e
rror ra
tes (S
ee
de
finitio
n o
f "erro
r rate
") is the
rate
at w
hich
erro
rs are
ab
le to
esca
pe
erro
r de
tectio
n a
nd
rem
ain
un
de
tecte
d a
fter
wh
ate
ver m
ea
ns o
f erro
r de
tectio
n h
as b
ee
n u
tilized
-- an
d th
us re
sidu
al in
data that is thought to be correct.
RE
SP
ON
SE
TIM
E
Response tim
e is the time that a system
requires to respond to a given input.T
his is issue is comm
only associated with enquiry-response transactions on
mu
ltipo
int co
mm
un
icatio
n syste
ms. W
he
n a
n e
nq
uiry (o
r oth
er tra
nsa
ction
initia
tive) is ke
yed
into
a te
rmin
al b
y an
op
era
tor th
ere
is ge
ne
rally a
nexpected reply from
the controlling computer system
. There is a short w
indowo
f time
follo
win
g th
is initia
tive w
ithin
wh
ich th
e te
rmin
al u
ser p
erce
ives
favorable responsiveness from the rem
ote host system. B
eyond this critical,and subjective tim
e window
, the user becomes quickly frustrated w
ith processd
ela
ys. Co
mp
on
en
ts of re
spo
nse
time
de
lay w
hich
mu
st be
ind
ividu
ally
addressed in system planning are: (1) tim
e required to transmit enquiry data
inbound to the computer; (2) tim
e required to process the service request atthe com
puter; and (3) time required to transm
it the response data back to thete
rmin
al. T
he
pe
rson
al co
mp
ute
r revo
lutio
n h
as ch
an
ge
d typ
ical u
ser
response time expectations from
a few seconds (perhaps as m
uch as 5 or 8seconds in tim
e past) to sub-second times.
RF
C
Request for C
omm
ent -- a series of documents w
hich specify TC
P/IP
and numerous
rela
ted
pro
toco
l stan
da
rds, a
nd
pro
vide
a fo
rum
for d
eve
lop
ing
con
sen
sus
rela
tive to
em
erg
ing
tech
nica
l de
velo
pm
en
ts for T
CP
/IP a
nd
the
Inte
rne
t --such as the N
ext Generation versions of IP
and TC
P. T
he Internet Engineering
Ta
sk Fo
rce (IE
TF
) ma
inta
ins a
con
ven
ien
t we
b site
for a
ccessin
g th
e la
test
RF
C and draft docum
ents (ww
w.ietf.org).
Data C
omm
unications & Fieldbus S
ystems
482
RG
B
Re
d, G
ree
n, B
lue
-- the
thre
e p
rima
ry c
olo
rs o
f ligh
t rep
res
en
ted
inte
levisio
n syste
ms. O
nce
colo
r sign
als a
re fu
lly de
cod
ed
in a
tele
vision
receiver, they are presented to the color display in RG
B form
at.
RO
UT
ER
A
rou
ter is a
da
ta n
etw
ork co
nn
ectio
n d
evice
tha
t ma
kes d
ecisio
ns a
bo
ut
passing data between connected netw
orks based on network layer addresses (O
SI
La
yer 3
). Ro
ute
rs are
con
figu
red
to d
ea
l with
spe
cific ne
two
rk laye
rp
roto
cols, a
nd
thu
s mu
st be
con
figu
red
to co
pe
with
all p
roto
cols u
sed
on
con
ne
cted
ne
two
rks. Ro
ute
rs are
req
uire
d to
coo
rdin
ate
"rea
cha
bility"
info
rma
tion
with
oth
er ro
ute
rs with
spe
cific rou
te e
xplo
ratio
n p
roto
cols.
See contrasting definition of "B
ridge".
RO
UT
ING
INF
OR
MA
TIO
N P
RO
TO
CO
L (R
IP)
A ro
ute
r-to-ro
ute
r coo
rdin
atio
n, o
r rou
te e
xplo
ratio
n p
roto
col u
sed
on
sma
llT
CP
/IP netw
orks, or used internally on small T
CP
/IP sub-netw
orks.
RS
-232-C
T
he TIA
interface standard comm
only known as the R
S-232-C
is actually more
corre
ctly spe
cified
as th
e T
IA-2
32
-C. It is a
20
kbp
s. ma
ximu
m d
ata
rate
,unbalanced electrical interface suitable for connecting D
CE
’s to DT
E’s (e.g.
modem
s to computers). A
"D" version of the interface has been defined but is
not yet widely supported.
SE
CA
M
Sequential C
ouleur Mem
oire, a television standard introduced by France, w
ithth
e w
ide
ly acce
pte
d (b
ut u
no
fficial) p
urp
ose
of b
ein
g in
com
pa
tible
with
the
rest o
f the
wo
rld -- n
om
ina
lly to p
rote
ct the
Fre
nch
ma
nu
factu
ring
ind
ustry.
A variant of S
EC
AM
was adopted by the E
astern Bloc countries to discourage
reception of Western E
uropean PA
L signals off-air.
SE
LE
CT
IVE
AR
Q
Se
lective
AR
Q syste
ms p
rovid
e fo
r the
exp
licit ide
ntifica
tion
of a
n e
rrore
dfra
me
of d
ata
, an
d fo
r the
retra
nsm
ission
of o
nly th
at fra
me
. Co
ntra
st with
"Go-B
ack-N A
RQ
", which provides for sim
ply backing up to the point of erroroccurrence, and starting over a that point. H
DLC
link protocol supports thistechnique as an option.
SE
RIA
L L
INE
INT
ER
NE
T P
RO
TO
CO
L (S
LIP
) S
LIP
is a
sim
ple
pro
toc
ol fo
r ex
ten
din
g a
cc
es
s to
IP n
etw
ork
s o
ve
rp
oin
t-to-p
oin
t con
ne
ction
s, such
as d
ial te
lep
ho
ne
links. S
LIP
is reg
ard
ed
as o
bso
lete
, ha
ving
be
en
rep
lace
d b
y PP
P a
s the
IP a
ccess p
roto
col o
fpreference.
SE
RIA
L T
RA
NS
MIS
SIO
N
Se
rial tra
nsm
ission
system
s pro
vide
for tra
nsm
itting
all o
f the
da
ta b
its inth
e in
form
atio
n s
trea
m a
cro
ss
a s
ing
le c
om
mu
nic
atio
n c
ha
nn
el -- in
bit-se
qu
en
tial fo
rma
t. Th
us, it re
qu
ires 8
clock tim
e u
nits to
tran
smit a
sing
le 8
-bit b
yte o
f info
rma
tion
. Effe
ctively a
ll imp
lem
en
tatio
ns o
f da
tacom
munication system
s which operate beyond a few
meters are based on serial
transmission. C
ompare w
ith Parallel T
ransmission.
SE
RV
ER
T
he
so
ftwa
re s
up
po
rting
the
ho
st-o
rien
ted
co
mp
utin
g fu
nc
tion
s in
aclie
nt/se
rver co
mp
utin
g e
nviro
nm
en
t. Se
rver so
ftwa
re typ
ically co
nce
rns
itself w
ith m
an
ag
ing
sha
red
da
ta re
sou
rces; fo
r this re
aso
n d
ata
ba
sem
an
ag
em
en
t system
s are
ofte
n a
key p
art th
e se
rver. T
he
term
’serve
r’ isoften (som
ewhat casually) associated w
ith the hardware com
puting system w
hichsupports the server as defined here.
SH
IEL
DE
D T
WIS
TE
D P
AIR
(ST
P)
ST
P cable is configured in m
uch the same form
at as unshielded twisted pair
(Se
e d
efin
ition
for co
mp
ariso
n), e
xcep
t tha
t a b
raid
ed
exte
rior cylin
drica
lshield is constructed surrounding each individual pair of w
ires on the bundle.S
TP
cable has technically superior performance (com
pared with equivalent U
TP
), b
ut th
e a
dd
ition
al co
st an
d b
ulk m
ake
it un
po
pu
lar co
mp
are
d to
UT
P. T
he
ad
ditio
n o
f the
shie
ld m
ake
s it ne
cessa
ry to p
acka
ge
few
er p
airs p
er ca
ble
bu
nd
le (th
us o
ccup
ying
sign
ifican
tly mo
re d
uct sp
ace
) -- an
d th
e h
elica
lca
ble
twist m
ake
s the
exte
rior ja
cket irre
gu
lar -- a
nd
difficu
lt to m
an
ipu
late
through cable ducts.
SIG
NA
L
A sig
na
l con
sists of e
lectro
ma
gn
etic im
pu
lses w
hich
pro
pa
ga
te a
cross a
tran
smissio
n ch
an
ne
l, an
d a
re e
nco
de
d, o
r mo
du
late
d in
such
a w
ay a
s toconvey inform
ation. Signals can be broadly classified as being either analog
or digital in character.
SIG
NA
L-to
-NO
ISE
RA
TIO
(SN
R)
Th
e ra
tio (typ
ically e
xpre
ssed
in d
ecib
els), o
f the
stren
gth
of a
sign
al to
the
stren
gth
of th
e n
oise
en
erg
y pre
sen
t on
the
cha
nn
el. A
go
od
qu
ality
analog telephone channel would dem
onstrate a 1000:1 (30 decibel) signal tonoise ratio.
SIG
NA
LIN
G
Signaling is the process by w
hich network connection requests (such as dialed
telephone numbers) and control signals (such as "hang up") are com
municated
acro
ss a n
etw
ork. S
ign
alin
g ca
n b
e e
ithe
r "in-b
an
d" (ca
rried
in th
e sa
me
ba
nd
wid
th a
s me
ssag
e tra
ffic) or "o
ut-o
f-ba
nd
" (carrie
d o
ver a
sep
ara
techannel or system
).
SIM
PL
E N
ET
WO
RK
MA
NA
GE
ME
NT
PR
OT
OC
OL
(SN
MP
) S
NM
P is the netw
ork managem
ent coordination protocol used on TC
P/IP
-basedn
etw
orks. S
NM
P su
pp
orts a
n e
ffective
an
d w
ide
ly use
d se
t of n
etw
ork
managem
ent services, and is very popular as a basis for managing enterprise
networks.
SIM
PL
EX
S
implex com
munication system
s have the capability of carrying information in
on
ly on
e d
irectio
n. A
n e
xam
ple
is a p
ocke
t pa
ge
r system
-- wh
ere
the
re is
never any capability of transmitting a signal from
the pager back to the basestation to confirm
receipt of a message.
Data C
omm
unications & Fieldbus S
ystems
483
SIN
GL
E C
HA
NN
EL
PE
R C
AR
RIE
R (S
CP
C)
SC
PC
configured equipment is used w
ith satellite comm
unications equipment that
serve
s limite
d ca
pa
city or sm
all e
arth
statio
ns. T
ypica
lly, VS
AT
ea
rthte
rmin
als o
pe
rate
in S
CP
C m
od
e. T
ech
nica
lly, SC
PC
me
an
s tha
t on
ly on
em
essage channel (such as a voice conversation or a data session) is carried one
ach
sate
llite ra
dio
carrie
r. Hig
h ca
pa
city system
s, on
the
oth
er h
an
d,
mu
ltiple
x a
nd
mo
du
late
a n
um
be
r of m
es
sa
ge
s o
nto
a s
ing
le c
arrie
rsim
ultaneously.
SL
IP
See "S
erial Line Internet Protocol".
SO
NE
T
Synchronous O
ptical Netw
ork (Know
n in Europe as the S
ynchronous Digital
Hierarchy, or S
DH
) is an international standard for high speed WA
N (i.e. long
distance telephone company netw
orks) applications. SO
NE
T is an open-ended
sp
ec
ifica
tion
, in th
at h
igh
er s
pe
ed
da
ta ra
tes
will b
e d
efin
ed
as
the
need/capability emerges. C
urrent SO
NE
T system
s are defined up to the OC
-768rate (approxim
ately 40 Gbps), typical im
plementations operate at the O
C-192
(10 Gbps) rate.
SP
AC
E
Th
e "sp
ace
" con
ditio
n o
n a
da
ta co
mm
un
icatio
n circu
it (op
po
site a
nd
com
ple
me
nta
ry to a
"ma
rk" con
ditio
n) is e
qu
ivale
nt to
a lo
gica
l bin
ary 0
sign
al sta
te. T
he
ma
rk is eq
uiva
len
t to a
log
ical b
ina
ry 1. T
he
term
s ma
rka
nd
spa
ce a
re b
orro
we
d fro
m th
e d
ays o
f Mo
rse co
de
tele
gra
ph
y, an
d th
eearliest autom
atic code reading equipment, w
hich left marks or spaces on a
strip of paper tape corresponding to the presence or absence of Morse code
pulse. The term
s mark and space are m
uch more com
monly associated w
ithasynchronous than synchronous transm
ission.
SP
AC
E C
HA
RA
CT
ER
In m
ost character codes, the requirement to leave a physical space betw
eenp
rintin
g ch
ara
cters is h
an
dle
d th
rou
gh
the
use
of a
"spa
ce" ch
ara
cter. T
his
is the character code transmitted w
hen the keyboard space bar is struck.
SS
AP
S
ource Service A
ccess Point -- the upper layer protocol specification for the
ne
two
rk laye
r pro
toco
l from
wh
ich d
ata
are
de
rived
to co
nstru
ct ou
tgo
ing
packets.
ST
OP
AN
D W
AIT
AR
Q
A p
roce
ss of tra
nsm
itting
da
ta a
nd
wa
iting
for a
resp
on
se fro
m re
ceivin
gsta
tion
be
fore
pro
cee
din
g w
ith tra
nsm
itting
mo
re d
ata
-- or re
tran
smittin
gp
revio
usly se
nt d
ata
-- as re
qu
ired
. Sto
p-a
nd
-wa
it system
s ne
cessa
rilyoperate in a half-duplex m
ode. Binary synchronous link protocol and X
MO
DE
Mare exam
ples of stop-and-wait protocols.
ST
OR
E A
ND
FO
RW
AR
D
A m
essage forwarding m
echanism w
here an incoming m
essage is fully buffered ata
ne
two
rk n
od
e s
witc
hin
g p
oin
t (rou
ter), d
ata
va
lidity
co
nfirm
ed
, the
po
ssibility o
f retra
nsm
ission
con
tem
pla
ted
(be
cau
se o
f erro
rs), an
d a
message-by-m
essage dynamic routing analysis perform
ed. The m
essage is thenforw
arded to the next way-point on route to the destination. T
his procedure
is time consum
ing, but is suitable to wide area netw
orks where data rates are
low-to-m
oderate and errors in transmitted data are frequent enough to w
arrantthe overhead. T
he first store and forward system
s were m
essage switching
system
s (rep
lacin
g to
rn-ta
pe
me
ssag
e ce
nte
rs); latte
rly, X.2
5 p
acke
tsw
itching are described with this term
.
ST
P
See definition of "S
hielded Tw
isted Pair" cable.
SU
PE
RS
ER
VE
R
Superservers are high-end m
ultiprocessor computer system
s designed to functiona
s serve
r pla
tform
s for d
em
an
din
g a
pp
licatio
ns. S
up
erse
rvers a
re typ
ically
RIS
C a
rchite
cture
system
s with
op
timize
d I/O
cap
ab
ility an
d a
re fre
qu
en
tlyequipped w
ith RA
ID m
ass storage systems.
SY
NC
HR
ON
OU
S
Synchronous system
s operate on a comm
on, or uniform tim
e base, creating aconstant and uniform
measure of tim
e for purposes of data transmission and
recovery.
SY
NC
HR
ON
OU
S D
IGIT
AL
HIE
RA
RC
HY
(SD
H)
The S
DH
is the European designation for S
ON
ET
-- the Synchronous O
pticalN
etwork -- standard for high speed W
AN
network m
ultiplexing. See definition
of "SO
NE
T".
SY
NC
HR
ON
OU
S T
RA
NS
MIS
SIO
N
A m
eth
od
of tra
nsm
itting
da
ta in
wh
ich b
oth
the
sen
din
g e
qu
ipm
en
t an
dreceiving equipm
ent are driven by a comm
on clocking system -- typically from
the
tran
sm
itter e
nd
of th
e lin
k. F
urth
erm
ore
, the
clo
ck
ing
sy
ste
m is
con
tinu
ou
sly op
era
ted
, an
d p
rovid
es fo
r a u
nifo
rm m
ea
sure
of tim
e a
t bo
thends of the link -- so that data pulses can be accurately decoded.
SY
ST
EM
S N
ET
WO
RK
AR
CH
ITE
CT
UR
E (S
NA
) S
NA
has been IBM
’s principal computer com
munications architecture, initially
de
fine
d in
19
74
. It is s
ign
ifica
nt a
s th
e in
du
stry
’s firs
t stru
ctu
red
,la
yere
d a
rchite
cture
de
finitio
n, a
nd
as su
ch, se
rved
as th
e m
od
el fo
rd
eve
lop
ing
the
no
w-u
nive
rsal vie
w o
f laye
red
com
pu
ter co
mm
un
icatio
ns
arch
itectu
res. T
he
pro
prie
tary ch
ara
cteristics o
f SN
A ve
ry mu
ch lim
it isusefulness in current tim
e.
TA
G S
WIT
CH
ING
T
here are two m
ain categories of LAN
switching technology deployed in support
of V
irtua
l LA
N se
rvice -- sw
itche
s ba
sed
on
ad
ap
ting
brid
gin
g te
chn
olo
gy
(Layer 2 Sw
itches) and switches based on adapting routing technology (Layer 3
switch
es). O
f the
La
yer 2
switch
alte
rna
tives, T
ag
Sw
itchin
g is th
e m
ost
po
pu
lar. It in
volve
s ap
pe
nd
ing
a ta
g (a
dd
ition
al m
essa
ge
field
) to th
ed
ata
fram
e th
at p
rovid
es a
nu
me
ric ide
ntifica
tion
of th
e virtu
al L
AN
to w
hich
the
orig
ina
tion
statio
n is a
ssocia
ted
. Se
e "IP
Sw
itchin
g" fo
r La
yer 3
switch
discu
ssion
. Th
e IE
EE
80
2.1
p sp
ecifica
tion
mo
ved
this te
chn
olo
gy fro
m its
initially proprietary category.
Data C
omm
unications & Fieldbus S
ystems
484
TA
RIF
F
Tariffs are published, standard rates for com
munications services or equipm
ent.R
eg
ula
ted
tele
ph
on
e co
mp
an
ies h
ave
no
flexib
ility in p
ricing
the
ir service
so
nce
the
ir reg
ula
tor h
as a
pp
rove
d a
give
n ta
riff pa
ckag
e. T
he
term
isa
pp
licab
le to
oth
er in
du
stries a
s we
ll, such
as th
e ca
se o
f freig
ht ta
riffsused by railw
ays. This term
is less widely used now
that the industry works
with com
petitive pricing models driven by deregulation.
TC
P/IP
T
ran
smissio
n C
on
trol P
roto
col/In
tern
et P
roto
col. T
CP
/IP is re
ally tw
odistinct protocols; T
CP
operating at layer 4 of the OS
I reference model, and
IP operating at Layer 3. In com
bination with m
any other less widely discussed
pro
toco
ls, TC
P/IP
ha
s com
e to
be
use
d to
de
scribe
the
wh
ole
fam
ily of
ne
two
rking
pro
toco
ls, wh
ich co
llective
ly en
ab
le in
tero
pe
ratio
n b
etw
ee
nd
issimila
r ho
st system
s an
d b
etw
ee
n d
issimila
r ne
two
rks - a h
ete
rog
en
eo
us
interoperability capability that is unique in the industry.
TE
LE
CO
MM
UN
ICA
TIO
NS
IND
US
TR
IES
AS
SO
CIA
TIO
N (T
IA)
Th
e T
IA (F
orm
erly, th
e E
lectro
nic In
du
stries A
ssocia
tion
, or E
IA) is a
Wa
shin
gto
n, D
C b
ase
d o
rga
niza
tion
tha
t ha
s pro
vide
d a
lea
de
rship
role
indefining technology standards for the U
S com
munity.
TE
LE
PR
INT
ER
A
n o
bso
lete
asyn
chro
no
us te
rmin
al d
evice
tha
t con
sists of a
keyb
oa
rd a
nd
integrated low-speed printer. A
n alternate term is teletypew
riter.
TE
LN
ET
T
he Teletype N
etwork, or T
elnet is an interactive terminal service specific to
TC
P/IP
ne
two
rk ap
plica
tion
s. Te
lne
t (very sim
ilar to
"rlog
in") a
llow
s arem
ote network access user to operate w
ith the same privileges and functions
as a
loca
lly con
ne
cted
term
ina
l use
r. Th
e p
oo
r resp
on
se tim
e a
nd
prim
itiveh
um
an
inte
rface
fea
ture
s of T
eln
et m
ake
it un
favo
red
for u
ser-a
ccessib
leapplications.
TE
RM
INA
L
In general, a terminal is a point in a com
munication system
where inform
ationca
n e
nte
r or le
ave
the
system
. In co
mm
on
pra
ctice, a
term
ina
l is a d
evice
(typica
lly op
era
ted
by h
um
an
s) wh
ich tra
nsm
its an
d/o
r rece
ives in
form
atio
nover a com
munication channel. A
lthough computers connected at the edge of a
ne
two
rk are
tech
nica
lly ’term
ina
ls’, com
mo
n u
sag
e re
stricts the
term
tonon-intelligent devices.
TIA
S
ee definition of "Telecom
munication Industries A
ssociation".
TIM
E A
SS
IGN
ED
SP
EE
CH
INT
ER
PO
LA
TIO
N (T
AS
I) T
ime assignm
ent speech interpolation, or TA
SI system
s take advantage of thefact that hum
an speech on any given telephone circuit in any given directionw
ill ne
ve
r av
era
ge
mo
re th
an
50
% b
us
y. In
fac
t, du
e to
pa
us
es
inc
on
ve
rsa
tion
, it is m
ore
like
40
% b
us
y. It is
ofte
n e
ffec
tive
to e
qu
ipd
evice
s at b
oth
en
ds o
f a te
lep
ho
ne
ne
two
rk to d
yna
mica
lly allo
cate
the
bu
rsts of vo
ice m
essa
ge
activity to
a p
oo
l of a
vaila
ble
on
e-w
ay circu
its --s
uc
h th
at th
e o
the
rwis
e in
ac
tive
, an
d n
on
-pro
du
ctiv
e h
alf o
f ea
ch
con
versa
tion
pa
th ca
n b
e u
tilized
. TA
SI syste
ms ca
n im
pro
ve u
tilizatio
n o
f
ocean cables, for example, by up to 80%
. New
technology TA
SI system
s arealso called D
igital Speech Interpolation, or D
SI system
s.
TIM
E D
IVIS
ION
MU
LT
IPL
E A
CC
ES
S (T
DM
A)
TD
MA
is a te
chn
iqu
e fo
r time
sha
ring
a co
mm
on
com
mu
nica
tion
cha
nn
el --
typica
lly a g
eo
gra
ph
ically d
ispe
rsed
cha
nn
el, su
ch a
s a sa
tellite
service
.D
evice
s at d
iffere
nt g
eo
gra
ph
ical lo
catio
ns ca
n fe
asib
ly sha
re su
ch a
sc
ha
nn
el if th
ey
are
dis
cip
line
d to
co
ord
ina
te th
eir s
ho
rt bu
rsts
of
info
rma
tion
tran
smissio
n w
ith a
ll oth
er u
sers -- o
n a
time
sha
red
, or tim
ed
ivision
ba
sis. TD
MA
is in so
me
wa
ys like a
distrib
ute
d tim
e d
ivision
multiplexer system
. Com
pare with F
DM
A.
TIM
E D
IVIS
ION
MU
LT
IPL
EX
(TD
M)
Tim
e Division M
ultiplexers collect data from a num
ber of typically low speed
data sources, and interleave the incoming data stream
s onto a single, highersp
ee
d co
mm
un
icatio
n lin
k to a
com
pa
nio
n m
ultip
lexe
r de
vice, w
he
re th
ein
terle
ave
d d
ata
is allo
cate
d b
ack to
a n
um
be
r of lo
w sp
ee
d d
istribu
tion
po
rts -- com
mo
nly p
orts o
n a
com
pu
ter syste
m. If th
e a
lloca
tion
of tim
eslots on the com
mon high speed inter-m
ultiplexer channel is fixed, the deviceis
reg
ard
ed
as
a s
imp
le, o
r trad
ition
al T
DM
. If the
allo
ca
tion
isd
yna
mica
lly alte
red
ba
sed
on
statistica
lly varia
ble
de
ma
nd
from
term
ina
ld
evice
s -- such
as w
ith h
um
an
inte
ractive
term
ina
ls -- it is said
to b
e a
Statistical T
ime D
ivision Multiplexer -- or "statm
ux".
TO
KE
N
Th
e to
ken
use
d w
ith to
ken
pa
ssing
LA
N syste
ms is a
spe
cial m
essa
ge
tha
tcirculates over the sam
e path as message traffic. T
he purpose of the tokenis to
gra
nt a
uth
ority to
the
statio
n th
at h
old
s the
toke
n to
tran
smit. T
he
system design is configured to assure that there is only one token present on
the system at any tim
e (thus insuring there will be no contention for access)
and that it is passed quickly around the comm
unity of attached devices (thusin
surin
g th
at n
o sta
tion
wa
its an
ina
pp
rop
riate
ly lon
g tim
e fo
r a ch
an
ce to
transmit). T
he complexity of m
anaging token behavior in the absence of anycentralized authority m
akes token passing LAN
systems m
ore complex than
ethernet.
TO
KE
N B
US
Token bus LA
N system
s are the most com
plex of the comm
on LAN
technologies, dueto the challenge of m
aintaining an addressing regime for passing the token to
all a
ttach
ed
de
vices. B
eca
use
a b
us is a
bro
ad
cast m
ed
ium
, all d
evice
sre
ceive
all m
essa
ge
s, an
d a
dd
ress d
iffere
ntia
tion
is the
on
ly wa
y to id
en
tifythe intended destination for m
essages and tokens. The addressing technique
use
d fo
r toke
n d
istribu
tion
is kno
wn
as a
"log
ical rin
g". T
he
IEE
E 8
02
.4 is
an example of token bus technology.
TO
KE
N R
ING
T
he standard token ring LAN
technology was developed by IB
M in the m
id-1980’s,and accepted by the IE
EE
as a standard (IEE
E 802.5). It is the second m
ostcom
monly used LA
N technology, after E
thernet, with perhaps 5%
market share.
Data C
omm
unications & Fieldbus S
ystems
485
TR
AC
ER
OU
TE
T
race
rou
te (a
com
mo
n u
tility pro
gra
m) p
rovid
es a
listing
of th
e ro
ute
rstraversed in establishing a connection over a T
CP
/IP internet.
TR
AN
SM
ISS
ION
CO
NT
RO
L P
RO
TO
CO
L (T
CP
) T
CP
is the
prin
cipa
l sessio
n co
ntro
l, so ca
lled
"ho
st-to-h
ost" p
roto
col w
ithT
CP
/IP syste
ms. T
CP
pro
vide
s con
ne
ction
con
trol a
nd
po
sitive d
elive
ryacknow
ledgements that are not available w
ith the underlying IP.
TR
AN
SP
ON
DE
R
An electronic subsystem
(typically associated with com
munication satellites)
wh
ich in
clud
es a
com
bin
ed
rece
iver-tra
nsm
itter fu
nctio
n. T
ran
spo
nd
ers
rece
ive, fre
qu
en
cy-shift, a
mp
lify an
d re
tran
smit u
ser’s sig
na
ls, op
era
ting
on
the signals as an analog processor.
TR
IVIA
L F
ILE
TR
AN
SF
ER
PR
OT
OC
OL
(TF
TP
) T
rivial F
ile T
ran
sfer P
roto
col is a
TC
P/IP
ap
plica
tion
pro
cess th
at o
ffers a
mo
re ru
dim
en
tary file
tran
sfer ca
pa
bility th
an
the
mo
re co
mm
on
FT
P. A
nadvantage of T
FT
P is that it doesn’t require a m
ultitasking operating systemfor support, as does F
TP
, and can be implem
ented in firmw
are.
TR
UN
K
Trunks are telephone netw
ork connections which interconnect sw
itching equipment.
(Co
mp
are
with
"loca
l loo
ps" w
hich
exte
nd
con
ne
ctivity to sw
itche
s from
non-switching equipm
ent, such as telephones).
TT
Y
Ab
bre
viatio
n fo
r "Te
letyp
e". T
ele
type
ha
s be
en
histo
rically a
pro
prie
tary
trade name (A
T&
T), but has evolved to refer to m
echanical teleprinters of allm
an
ufa
cture
, an
d th
e m
ea
ns o
f com
mu
nica
tion
tha
t the
y sup
po
rted
. TT
Ycom
munication can be regarded as a prim
itive link protocol, associated with
asynchronous timing, A
SC
II or Baudot coded data, and character-m
ode terminal
systems. T
he serial port of a PC
is a TT
Y-capable interface.
TW
O-W
IRE
CIR
CU
IT
A circu
it form
ed
by a
pa
ir of co
nd
ucto
rs, or w
ires, w
hich
are
insu
late
d fro
me
ach
oth
er, a
nd
typica
lly twiste
d to
ge
the
r. Th
e tw
o m
eta
llic con
du
ctors
crea
te a
nd
sup
po
rt the
simp
lest p
ossib
le co
mm
un
icatio
n m
ech
an
ism, w
hich
typica
lly sup
po
rts tran
smissio
n o
f info
rma
tion
in e
ithe
r dire
ction
(with
ou
ta
ny d
irectio
na
l pre
fere
nce
). Te
lep
ho
ne
loca
l loo
p co
nn
ectio
ns to
the
pu
blic sw
itche
d n
etw
ork a
re a
lwa
ys con
ne
cted
as a
two
-wire
service
(ie.
using one pair of wires). S
ee definition of "Unshielded T
wisted P
air".
UH
F
Ultra high frequency. V
HF
is a term used to describe radio frequencies in the
range 300 MH
z to 3 GH
z. UH
F frequencies are characterized as "line-of-sight"
service
s (no
t cap
ab
le o
f use
sign
ifican
tly be
yon
d th
e h
orizo
n), a
nd
are
wid
ely u
sed
in p
erso
na
l com
mu
nica
tion
(cellu
lar), te
levisio
n , lo
w-ca
pa
citypoint-to-point and radar services.
UN
IFO
RM
RE
SO
UR
CE
LO
CA
TO
R (U
RL
) T
he UR
L concept provides for a comm
on syntax for specifying the applicableaccess protocol, the host system
address, path on the specified host system,
an
d file
spe
cificatio
n fo
r info
rma
tion
reso
urce
s on
TC
P/IP
inte
rne
ts. Th
eU
RL form
at is most com
monly encountered in context of specifying H
ypertextT
ransport Protocol (H
TT
P) inform
ation resources on the World W
ide Web.
UN
INT
ER
RU
PT
AB
LE
PO
WE
R S
UP
PL
Y (U
PS
) U
PS
systems provide for pow
er continuity in event of failure of primary pow
er.T
his is typ
ically a
ccom
plish
ed
usin
g b
atte
ries a
nd
po
we
r inve
rter circu
its togenerate the required prim
ary power replacem
ent. Where long duration backup
cap
ab
ility is req
uire
d, so
me
alte
rna
te so
urce
of p
ow
er is re
qu
ired
, such
as
a diesel generator.
UN
SH
IEL
DE
D T
WIS
TE
D P
AIR
(UT
P)
UT
P is the term
used to describe any telephone-type cable, usually in contexto
f ho
use
cab
ling
, or b
uild
ing
wirin
g syste
ms. U
TP
is com
prise
d o
f a p
air o
fin
su
late
d w
ires
, twis
ted
tog
eth
er in
to a
he
lix. U
TP
ca
ble
pa
irs a
retyp
ically p
acka
ge
d in
to ca
ble
bu
nd
les co
nsistin
g o
f mu
ltiple
pa
irs con
tain
ed
within a com
mon exterior jacket. T
here a number of (m
ost vendors agree on 5)d
iffere
nt c
las
se
s o
f UT
P, d
iffere
ntia
ted
on
the
ba
sis
of w
ire g
au
ge
,in
sula
tion
type
an
d d
iam
ete
r, an
d th
e n
um
be
r of tw
ists pe
r foo
t in th
e ca
ble
con
structio
n. T
he
se d
iffere
nt "le
vels", o
r classe
s of ca
ble
are
de
sign
ate
da
s be
ing
suita
ble
for vo
ice o
nly, lo
w o
r hig
h sp
ee
d d
ata
, ISD
N a
pp
licatio
ns,
etc. See definition of "T
wo-W
ire Circuit".
US
EN
ET
U
SE
NE
T (U
ser’s Netw
ork) is the interconnection of a collection of UN
IX system
s-- a
ll of w
hich
run
the
"ne
tne
ws" so
ftwa
re, w
hich
sup
po
rts a n
ew
s exch
an
ge
service between users of the netw
ork. It has evolved into a remarkable m
eansof exchanging inform
ation about any of hundreds of special interest subjects.
US
ER
DA
TA
GR
AM
PR
OT
OC
OL
(UD
P)
A connectionless protocol w
hich operates parallel to TC
P in T
CP
/IP system
s --and provides support for very sim
ple application processes (such as PIN
G and
TF
TP
), more sophisticated processes w
hich provide their own session control
(su
ch
as
Su
n M
icro
sy
ste
ms
NF
S) o
r time
-critic
al s
erv
ice
s lik
e th
os
esupported by R
eal Tim
e Protocol.
UT
P
See definition of "U
nshielded Tw
isted Pair".
UU
CP
U
UC
P, or the "U
NIX
-to-UN
IX C
opy Program
" is a comm
on application program used
for file
tran
sfer in
the
UN
IX e
nviro
nm
en
t. Th
ere
is a clo
se a
ssocia
tion
between electronic m
ail and file transfer (UU
CP
performs both tasks), and as
a result, UU
CP
has come to be associated w
ith mail transfer in U
NIX
-basedsystem
s.
VE
RO
NIC
A
Ob
sole
te: V
ero
nica
is a g
op
he
r-initia
ted
inte
rne
t sea
rch to
ol w
hich
loca
tes
files (or file directories) based on keyword specification.
Data C
omm
unications & Fieldbus S
ystems
486
VE
RT
ICA
L P
AR
ITY
DE
TE
CT
ION
V
ertica
l pa
rity calcu
latio
ns a
re b
ase
d o
n d
efin
ing
a sin
gle
pa
rity bit w
hich
acco
mp
an
ies e
ach
cha
racte
r tha
t is tran
smitte
d -- su
ch th
at th
e co
mp
osite
gro
up
of b
its (Typ
ically 7
cha
racte
r da
ta b
its plu
s 1 p
arity b
it) will h
ave
ap
red
ictab
le e
ven
(or o
dd
) nu
mb
er o
f "on
e" b
its. (Se
e d
efin
ition
s of "P
arity"
and "Horizontal P
arity").
VE
RY
SM
AL
L A
PE
RT
UR
E T
ER
MIN
AL
(VS
AT
) V
SA
T co
mm
un
icatio
ns sa
tellite
ea
rth te
rmin
als a
re su
ited
to re
lative
ly low
cap
acity re
mo
te lo
catio
ns -- typ
ically u
sed
to d
elive
r a sm
all n
um
be
r of
(perhaps up to 4) channels for low-to-m
oderate data rate applications (up toabout 64 kbps. per channel) or voice applications.
VH
F
Very high frequency. V
HF
is a term used to describe radio frequencies in the
ran
ge
30
MH
z to
30
0 M
Hz
. VH
F fre
qu
en
cie
s a
re c
ha
rac
teriz
ed
as
"line
-of-s
igh
t" se
rvic
es
(no
t ca
pa
ble
of u
se
sig
nific
an
tly b
ey
on
d th
eh
orizo
n), a
nd
are
wid
ely u
sed
in p
erso
na
l com
mu
nica
tion
(mo
bile
) an
dtelevision services.
VIR
TU
AL
CIR
CU
IT
A term
used to describe the addressable connection that exists across a packetsw
itche
d n
etw
ork. V
irtua
l circuits b
eh
ave
in so
me
wa
ys like re
al p
hysica
lcircu
its, bu
t the
y are
"virtua
l" in th
at th
e o
nly lin
k acro
ss such
a n
etw
ork
is susta
ine
d th
rou
gh
a p
acke
t de
livery ca
pa
bility. P
erm
an
en
t virtua
lc
ircu
its a
re c
on
figu
red
an
d a
va
ilab
le fo
r se
rvic
e 2
4 h
ou
rs a
da
yco
ntin
uo
usly -- a
nd
offe
r no
flexib
ility reg
ard
s diffe
ren
t con
ne
ction
op
tion
s.S
witch
ed
virtua
l circuits re
qu
ire u
ser in
itiatio
n o
f ea
ch se
ssion
, an
d o
ffer
flexibility with regard to connection options.
VO
ICE
FR
EQ
UE
NC
Y
An
y (au
dib
le) fre
qu
en
cy with
in th
at p
ortio
n o
f the
au
dio
freq
ue
ncy sp
ectru
mw
hich
is con
sisten
t with
the
tran
smissio
n o
f tele
ph
on
e q
ua
lity spe
ech
. Se
e"V
oice Grade C
hannel").
VO
ICE
GR
AD
E C
HA
NN
EL
A
telephone-grade comm
unication channel which is designed for, and is suitable
for tra
nsm
ission
of sp
ee
ch, m
od
em
-inte
rface
d d
igita
l da
ta, a
nd
facsim
ilein
form
atio
n. In
mo
st cou
ntrie
s, freq
ue
ncy b
an
dw
idth
, or ra
ng
e o
f ab
ou
t 30
0to 3000 H
z. characterizes the voice grade channel.
WA
N
See definition of "W
ide Area N
etwork".
WID
E A
RE
A N
ET
WO
RK
(WA
N)
A W
AN
has traditionally been viewed as a low
speed network service -- a view
no
lon
ge
r accu
rate
. WA
N se
rvices a
re p
rop
erly co
nsid
ere
d to
be
ne
two
rkservices that extend beyond the im
mediately local environm
ent (as supportedb
y LA
N syste
ms) -- a
nd
exce
pt fo
r the
larg
est sca
le u
sers, w
ill be
ba
sed
on
facilities leased or rented from a public netw
ork service provider, or comm
onca
rrier. S
om
e d
efin
ition
s of th
e W
AN
inclu
de
me
trop
olita
n a
rea
ne
two
rk, or
MA
N access facilities -- in w
hich case a LAN
-to-LAN
internetwork connection
would be a LA
N-W
AN
-LAN
configuration. Other definitions differentiate, such
tha
t in in
ter-c
ity L
AN
-to-L
AN
se
rvic
e w
ou
ld b
e s
ee
n a
s a
LAN
-MA
N-W
AN
-MA
N-LA
N configuration.
WIR
EL
ES
S L
AN
W
ireless LAN
systems use radio or infrared technology to avoid or m
inimize the
ne
ed
for h
ard
wire
d ca
ble
system
s. Hig
h co
st, limite
d ca
pa
city an
d se
curity
concerns initially limited w
idespread acceptance of wireless LA
N technology.
Sta
nd
ard
izatio
n e
fforts (IE
EE
80
2.1
1 a
nd
Hip
erla
n) a
re ch
an
gin
g th
is,prom
ising greatly increased acceptance.
WO
RD
H
istorica
lly, a w
ord
ha
s be
en
de
fine
d a
six cha
racte
rs (typica
lly com
prisin
gfive
cha
racte
rs plu
s on
e sp
ace
). Mo
re typ
ically, a
wo
rd is co
nsid
ere
d to
be
a g
rou
p o
f bits (typ
ically co
mp
rising
an
inte
ge
r nu
mb
er o
f octe
ts), wh
ich is
treated as a unit and is capable of being stored or comm
unicated as a unit ofinform
ation within a com
puter. A com
mon w
ord size is 32 bits.
WO
RK
ST
AT
ION
W
orkstations are high-end, powerful m
icrocomputer system
s. These kinds of
eq
uip
me
nt a
re o
ften
asso
ciate
d w
ith g
rap
hics-in
ten
sive te
chn
ical d
esig
na
pp
licatio
ns a
nd
are
also
freq
ue
ntly d
ep
loye
d a
s hig
h-e
nd
serve
r pla
tform
s.W
orksta
tion
s are
ofte
n co
nfig
ure
d w
ith R
ISC
pro
cesso
rs an
d a
re o
ften
supported by the Unix operating system
s.
WO
RL
D W
IDE
WE
B (W
WW
) T
he
WW
W is th
e co
llectio
n o
f Inte
rne
t-ba
sed
reso
urce
s tha
t sup
po
rt the
Hypertext T
ransport Protocol (H
TT
P), m
aking it possible to exchange documents
that are encoded with the H
ypertext Markup Language (H
TM
L). Using a client
software package (such as Internet E
xplorer), the WW
W becom
es a simple,
convenient vehicle for locating information resources.
X.2
5
X.2
5 is a
n o
bso
lete
ITU
-T sta
nd
ard
tha
t de
fine
s the
cha
racte
ristics of
first-g
en
era
tion
pa
ck
et s
witc
he
d n
etw
ork
s. T
he
X.2
5 s
tan
da
rds
are
historically important because they inspired the m
ore current frame relay and
AT
M netw
ork standards.
XE
RO
X N
ET
WO
RK
SY
ST
EM
(XN
S)
XN
S w
as th
e first fu
nctio
na
l ne
two
rk op
era
ting
system
de
sign
ed
for L
AN
systems, and established m
any of the features and functions that have come to
be associated with this class of system
.
Data C
omm
unications & Fieldbus S
ystems
487
SU
GG
ES
TIO
NS
FO
R F
UR
TH
ER
RE
AD
ING
1.0
GE
NE
RA
L C
OM
PU
TE
R C
OM
MU
NIC
AT
ION
S
1.1 Fred H
alsallD
ata
Com
munica
tions
Com
pute
r Netw
orks a
nd O
pen
Syste
ms
Wokingham
UK
Addison W
esley1996 (IS
BN
020142293X).
1.2 Gilbert H
eldU
ndersta
ndin
g D
ata
Com
munica
tions (S
ixthE
ditio
n)R
iders Publishing
1999 (ISB
N 0735700362)
1.3 Joseph A. P
ecar and David A
. Garbin
The M
cGra
w-H
illTe
leco
mm
unica
tions F
actb
ook
New
York
NY
McG
raw H
ill2000(IS
BN
0071351639)
1.4 William
Stallings
Data
and C
om
pute
r Com
munica
tions (S
ixth E
ditio
n)
Upper S
addle RiverN
JP
rentice Hall1999 (IS
BN
0130643709)
1.5 William
Stallings
Handbook o
f Com
pute
r Com
munica
tions S
tandard
sU
pper Saddle R
iverNJ
Prentice H
all1990 (ISB
N 0024155217)
1.6 Andrew
S. Tanenbaum
Com
pute
r Netw
orks
Seco
nd E
ditio
nU
pperS
addle RiverN
JP
rentice Hall1996 (IS
BN
0133499456)
2.0
TR
AN
SM
ISS
ION
TE
CH
NO
LO
GY
2.1 Roger L. F
reeman
Tele
com
munica
tion T
ransm
ission H
andbook
(Fourth
Editio
n)N
ew Y
orkN
YJohn W
iley & S
ons1998 (IS
BN
0471240164)
2.2 Stam
atios V. Kartalopoulos
Intro
ductio
n to
DW
DM
Tech
nolo
gy; D
ata
in a
Rain
bow
Piscataw
ayN
JIE
EE
Press
2000 (ISB
N 0780353994)
3.0
LO
W-S
PE
ED
NE
TW
OR
K T
EC
HN
OL
OG
Y
3.1 George C
. Clark
Jr. and J. Bibb C
ainE
rror-C
orre
ction C
odin
g fo
rD
igita
l Com
munica
tion
New
York
NY
Plenum
Press
1981 (ISB
N0306406152)
4.0
AT
M A
ND
BR
OA
DB
AN
D T
EC
HN
OL
OG
IES
4.1 George A
beR
esid
entia
l Bro
adband
IndianapolisIN
Cisco P
ress1997 (IS
BN
1578700205)
4.2 Uyless B
lackE
merg
ing C
om
munica
tions Te
chnolo
gie
s (Seco
nd
Editio
nU
pper Saddle R
iverNJ
Prentice H
all1997 (ISB
N0137428340)
4.3 Martin P. C
larkA
TM
Netw
orks - P
rincip
les a
nd U
seC
hichesterUK
John Wiley &
Sons
1996 (ISB
N 0471967017)
4.4 Ranier H
ändelManfred H
uber and Stefan S
chröderA
TM
Netw
orks ..
Conce
pts
Pro
toco
lsA
pplica
tions
Reading
Mass.A
ddison Wesley
1998 (ISB
N 0201178176)
4.5 David E
. McD
ysan and Darren L. S
pohnA
TM
Theory a
nd A
pplica
tion
New
YorkN
YM
cGraw
Hill1998 (IS
BN
0070453462)
4.6 Stam
atios V. Kartalopoulos
Understa
ndin
g S
ON
ET
/SD
H a
nd A
TM
Piscataw
ayN
JIE
EE
Press
1999 (ISB
N 0780347455)
4.7 Tim
othy Kw
okA
TM
: The N
ew
Para
dig
m fo
r Inte
rnetIn
tranet a
nd
Resid
entia
l Bro
adband S
ervice
s & A
pplica
tions
Upper S
addle River
NJ
Prentice H
all1998 (ISB
N 0131072447)
4.8 Byeong G
i LeeM
inho Kang and Jonghee Lee
Bro
adband
Tele
com
munica
tions Te
chnolo
gy (S
eco
nd E
ditio
n)N
orwood
MA
Artech H
ouseInc.1996 (IS
BN
0890068666)
4.9 Mark A
. Sportack
Frank C
. Pappas
Em
il Rensing
et alH
igh-
Perfo
rmance
Netw
orkin
gIndianapolis
INH
oward W
. Sam
s &C
ompany (D
ivision of Macm
illanInc.)1997 (IS
BN
1575211874)
4.10 William
Stallings
ISD
N a
nd B
roadband IS
DN
With
Fra
me R
ela
y and
AT
M (F
ourth
Editio
n)U
pper Saddle R
iverNJ
Prentice H
all1998(IS
BN
0139737448)
Data C
omm
unications & Fieldbus S
ystems
488
5.0
TC
P/IP
AN
D T
HE
INT
ER
NE
T
5.1 Paul A
lbitz and Cricket Liu
DN
S a
nd B
IND
SebastopolC
AO
’Reilly &
Associates
Inc.1998 (ISB
N 1565925122)
5.2 David A
ngell and Brent H
eslopT
he In
tern
et B
usin
ess C
om
panio
nR
eadingM
ass.Addison W
esley1995 (IS
BN
0201408503)
5.3 Robin B
urkM
artin Bligh
Thom
as Leeet al
TC
P/IP
Blu
eprin
tsIndianapolis
INH
oward W
. Sam
s & C
ompany (D
ivision of Macm
illanInc.)1997 (IS
BN
0672310554)
5.4 Douglas C
omer
Inte
rnetw
orkin
g W
ith T
CP
/IP - P
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Pro
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nd E
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SebastopolC
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d IP
Service
s for C
isco N
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IndianapolisIN
Cisco P
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5.15 William
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TM
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5.18 Kevin W
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CP
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Netw
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nd E
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6.0
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6.1 Jennifer Niederst and R
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7.4 Stephen A
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CP
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unications & Fieldbus S
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esig
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ple
menta
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Opera
tion &
Managem
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hird
Editio
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8.3 Neil Jenkins and S
tan Schatt
Understa
ndin
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(Fifth
Editio
n)Indianapolis
INH
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s & C
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f IEE
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8.5 James M
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hapman and Joe Leben
Loca
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aN
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rchite
cture
s and Im
ple
menta
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nd E
ditio
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JP
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8.6 Wendy M
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Cronin Jr. and K
arl Pieper
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DI: A
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Fib
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d D
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Inte
rface
Burlington
MA
Digital
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8.7 Martin A
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zowF
ast E
thern
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ple
menta
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nd M
igra
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Solu
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McG
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9.0
CL
IEN
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VE
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PU
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lient/S
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itectu
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nd E
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cGraw
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9.2 D. T
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lient/S
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Understa
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9.4 Robert O
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ssentia
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hird
Editio
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9.5 Paul E
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Intro
ductio
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Clie
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erve
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ms: A
Pra
ctical
Guid
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r Syste
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rofe
ssionals
New
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John Wiley &
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IRE
LE
SS
AN
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OB
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MP
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10.1 Ira Brodsky
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Manager’s G
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Wire
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Netw
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10.2 Editors
Officia
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less A
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addle RiverN
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ss LA
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tero
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ble
Netw
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New
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acmillan T
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Tele
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McG
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Mobile
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Harlow
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Addison W
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Prentice H
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10.9 Mark S
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Mobility; T
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Upper S
addle RiverN
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10.10E
llen Kayata W
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Reading
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11.1 Uyless B
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pper Saddle R
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11.2 Jonathan Davidson
Voice
Ove
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undam
enta
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INC
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N 1578701686)
11.3 Bill D
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Tele
phony
Upper S
addle RiverN
YP
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2000 (ISB
N 0130141186)
11.4 Oliver H
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etitIP
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d M
ultim
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sH
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ddison Wesley Longm
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12
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UR
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R. C
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Steven M
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g th
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ddison Wesley
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12.3 Sim
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SebastopolC
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esig
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INC
iscoP
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12.6 Charlie K
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Radia P
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ike Spencer
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Com
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addle RiverN
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12.7 Thom
as W. M
adronN
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he 9
0’s - Issu
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nd
Solu
tions F
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John Wiley &
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BN
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12.8 Deborah R
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13.2 Michael H
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13.3 Allan Leinw
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ditio
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13.4 Kornel Terplan
Com
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13.6 Chuck H
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AR
RL
Handbook fo
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New
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Weavin
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nd U
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Destin
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he W
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Wid
e W
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14.2 George G
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Infin
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idth
Will R
evo
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Our W
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New
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N 0684609303)
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. LuckyS
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14.4 Cliff S
tollT
he C
ucko
o’s E
gg; T
rackin
g a
Spy T
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Com
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.Pocket B
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Data C
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unications & Fieldbus S
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15
.0 M
AG
AZ
INE
SJ
OU
RN
AL
S A
ND
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RIO
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AL
S
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evie
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etw
ork M
agazin
e; Published m
onthly by CM
P(w
ww
.networkm
agazine.com) (Toll free telephone 800-577-5356)
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agazin
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