13-wide area networks

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Computer Networks with

Internet TechnologyWilliam Stallings

Chapter 13

Wide Area Networks


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Frame Relay Networks

Designed to eliminate much of theoverhead in X.25

Call control signaling on separate logical

connection from user data Multiplexing/switching of logicalconnections at layer 2 not layer !"

#o hop$%y$hop &ow control and error

control'hroughput an order of magnitude higher

than X.25


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Frame Relay Architecture

X.25 has ! layers( physical) lin*) networ* +rame ,elay has 2 layers( physical and

data lin* or -+"

-+ core( minimal data lin* control0reservation of order for frames

01mall pro%a%ility of frame loss

-+ control( additional data lin* or

networ* layer end$to$end functions


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Figure 131 Frame Relay !ser"Network

Inter#ace $rotocol Architecture


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%A$F Core

+rame delimiting) alignment andtransparency

+rame multiplexing/demultiplexing

nspection of frame for length constraints Detection of transmission errors

Congestion control


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Figure 13& %A$F"core Formats


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Frame Relay

!ser 'ata Trans#er

#o control 3eld) which is normally usedfor(0dentify frame type data or control"

01e4uence num%ers

mplication(0Connection setup/teardown carried on

separate channel

0Cannot do &ow and error control


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AT( %ogical Connections

9irtual channel connections 9CC"0nalogous to virtual circuit in X.25

0:asic unit of switching

0:etween two end users

0+ull duplex0+ixed sie cells

0Data) user$networ* exchange control" andnetwor*$networ* exchange networ*

management and routing" 9irtual path connection 9C"

0:undle of 9CC with same end points


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Figure 133

AT( Connection Relationship


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Ad)antages o# *irtual $aths

1impli3ed networ* architecture ncreased networ* performance and

relia%ility

,educed processing 1hort connection setup time

;nhanced networ* services


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*$+*C Characteristics


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Control Signaling " *CC

Done on separate connection 1emi$permanent 9CC

Meta$signaling channel0>sed as permanent control signal channel

>ser to networ* signaling virtual channel0+or control signaling

0>sed to set up 9CCs to carry user data

>ser to user signaling virtual channel

0?ithin pre$esta%lished 9C0>sed %y two end users without networ* intervention to

esta%lish and release user to user 9CC


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Control Signaling " *$C

1emi$permanent Customer controlled

#etwor* controlled


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AT( Cells

+ixed sie 5 octet header

@8 octet information 3eld

1mall cells reduce 4ueuing delay for highpriority cells

1mall cells can %e switched moree=ciently

;asier to implement switching of smallcells in hardware


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Figure 13,

AT( Cell Format


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-eader Format

Aeneric &ow control0Bnly at user to networ* interface

0Controls &ow only at this point

9irtual path identi3er 9irtual channel identi3er

ayload type0e.g. user info or networ* management

Cell loss priority

eader error control


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-eader .rror Control

8 %it error control 3eld Calculated on remaining !2 %its of header

llows some error correction


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/eneric Flow Control 0/FC

Control tra=c &ow at user to networ* interface>#" to alleviate short term overload

'wo sets of procedures0>ncontrolled transmission

0Controlled transmission

;very connection either su%ect to &ow control ornot

1u%ect to &ow control0May %e one group " default

0May %e two groups and :"

+low control is from su%scri%er to networ*0Controlled %y networ* side


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Single /roup o# Connections 01

'erminal e4uipment ';" initialies twovaria%les0',#1M' &ag to E

0ABFC#', credit counter" to 7

f ',#1M'GE cells on uncontrolledconnection may %e sent any time

f ',#1M'G7 no cells may %e sent on

controlled or uncontrolled connections" f -' received) ',#1M' set to 7 and

remains until #BF-'


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Single /roup o# Connections 0&

f ',#1M'GE and no cell to transmit onany uncontrolled connection(0f ABFC#',H7) '; may send cell on controlled

connection

Cell mar*ed as %eing on controlled connection ABFC#', decremented

0f ABFC#',G7) '; may not send on controlledconnection

'; sets ABFC#', to ABF9->; uponreceiving 1;' signal0#ull signal has no eIect


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!se o# -A%T

'o limit eIective data rate on 'M 1hould %e cyclic

'o reduce data rate %y half) -' issued to

%e in eIect 57J of time Done on regular pattern over lifetime of

connection


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AT( Ser)ice Categories

,eal time0Constant %it rate C:,"

0,eal time varia%le %it rate rt$9:,"

#on$real time0#on$real time varia%le %it rate nrt$9:,"0vaila%le %it rate :,"

0>nspeci3ed %it rate >:,"

0Auaranteed frame rate A+,"


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Real Time Ser)ices

mount of delay 9ariation of delay itter"


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rt"*2R

'ime sensitive application0'ightly constrained delay and delay variation

rt$9:, applications transmit at a rate thatvaries with time

e.g. compressed video0roduces varying sied image frames

0Briginal uncompressed" frame rate constant

01o compressed data rate varies

Can statistically multiplex connections


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nrt"*2R

May %e a%le to characterie expectedtra=c &ow

mprove


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!2R

May %e additional capacity over anda%ove that used %y C:, and 9:, tra=c0#ot all resources dedicated

0:ursty nature of 9:,

+or application that can tolerate some cellloss or varia%le delays0e.g. 'C %ased tra=c

Cells forwarded on ++B %asis :est eIorts service


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A2R

pplication speci3es pea* cell rate C,"and minimum cell rate MC,"

,esources allocated to give at least MC,

1pare capacity shared among all ,:sources

e.g. -# interconnection


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/uaranteed Frame Rate 0/FR

Designed to support %ac*%one su%networ*s :etter service than >:, for frame %ased tra=c

0ncluding and ;thernet

Bptimie handling of frame %ased tra=c passing from-# through router to 'M %ac*%one

0>sed %y enterprise) carrier and 1 networ*s0Consolidation and extension of over ?#

:, di=cult to implement %etween routers over 'Mnetwor*

A+, %etter alternative for tra=c originating on ;thernet

0#etwor* aware of frame/pac*et %oundaries0?hen congested) all cells from frame discarded

0Auaranteed minimum capacity

0dditional frames carried of not congested


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Cellular Wireless Networks

>nderlying technology for mo%ile phones)personal communication systems) wirelessnetwor*ing etc.

Developed for mo%ile radio telephone0,eplace high power transmitter/receiver

systems'ypical support for 25 channels over 87*m

0>se lower power) shorter range) moretransmitters


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Cellular Network rgani4ation

Multiple low power transmitters0E77w or less

rea divided into cells0;ach with own antenna

0;ach with own range of fre4uencies

01erved %y %ase station'ransmitter) receiver) control unit

0dacent cells on diIerent fre4uencies to avoidcrosstal*


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Shape o# Cells

14uare0?idth dcell has four neigh%ors at distance dand four at

distance d

0:etter if all adacent antennas e4uidistant 1impli3es choosing and switching to new antenna

exagon0rovides e4uidistant antennas

0,adius de3ned as radius of circum$circle Distance from center to vertex e4uals length of side

0Distance %etween centers of cells radius R is R

0#ot always precise hexagons'opographical limitations

-ocal signal propagation conditions

-ocation of antennas

2

3


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Figure 135 Cellular /eometries


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Fre6uency Reuse

ower of %ase transceiver controlled0llow communications within cell on given fre4uency

0-imit escaping power to adacent cells

0llow re$use of fre4uencies in near%y cells

0>se same fre4uency for multiple conversations

0E7 K 57 fre4uencies per cell

;.g.0N cells all using same num%er of fre4uencies

0K total num%er of fre4uencies used in systems

0;ach cell has K/N fre4uencies

0dvanced Mo%ile hone 1ervice M1" KG!L5) NGgiving 5 fre4uencies per cell on average


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Characteri4ing Fre6uency

Reuse

D G minimum distance %etween centers of cells that usethe same %and of fre4uencies called cochannels"

, G radius of a cell

d G distance %etween centers of adacent cells d G ,"

# G num%er of cells in repetitious pattern

0 ,euse factor

0 ;ach cell in pattern uses uni4ue %and of fre4uencies

exagonal cell pattern) following values of # possi%le

0 # G 2O P2 O x P") ) P G 7) E) 2) !) Q

ossi%le values of # are E) !) @) ) L) E2) E!) E6) EL) 2E) Q D/,G

D/d G

N3

N


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Figure 137

Fre6uency Reuse $atterns


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Increasing Capacity 01

dd new channels0#ot all channels used to start with

+re4uency %orrowing0'a*en from adacent cells %y congested cells

0Br assign fre4uencies dynamically Cell splitting

0#on$uniform distri%ution of topography and tra=c

01maller cells in high use areas

Briginal cells 6.5 K E! *m

E.5 *m limit in general

More fre4uent handoI

More %ase stations


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Increasing Capacity 0&

Cell 1ectoring0Cell divided into wedge shaped sectors

0! K 6 sectors per cell

0;ach with own channel set 1u%sets of cellRs channels

0Directional antennas Microcells

0Move antennas from tops of hills and large %uildings totops of small %uildings and sides of large %uildings

;ven lamp posts

0+orm microcells0,educed power

0Aood for city streets) along roads and inside large%uildings


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Figure 138

Fre6uency Reuse .9ample


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peration o# Cellular Systems

:ase station :1" at center of each cell0ntenna) controller) transceivers Controller handles call process

0#um%er of mo%ile units may in use at a time

:1 connected to mo%ile telecommunications switching

o=ce M'1B"0Bne M'1B serves multiple :10M'1B to :1 lin* %y wire or wireless

M'1B(0Connects calls %etween mo%ile units and from mo%ile to 3xed

telecommunications networ*

0ssigns voice channel0erforms handoIs

0Monitors calls %illing"

+ully automated

Fi 13 :


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Figure 13:

)er)iew o# Cellular System


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Channels

Control channels01etting up and maintaining calls

0;sta%lish relationship %etween mo%ile unit andnearest :1

'ra=c channels0Carry voice and data

T i l C ll i


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Typical Call in

Single (TS Area 01

Mo%ile unit initialiation01can and select strongest set up control channel0utomatically selected :1 antenna of cell

>sually %ut not always nearest propagation anomalies"

0andsha*e to identify user and register location

01can repeated to allow for movement

Change of cell0Mo%ile unit monitors for pages see %elow"

Mo%ile originated call0Chec* set up channel is free

Monitor forward channel from :1" and wait for idle

01end num%er on pre$selected channel

aging0M'1B attempts to connect to mo%ile unit

0aging message sent to :1s depending on called mo%ile num%er

0aging signal transmitted on set up channel

T i l C ll i


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Typical Call in

Single (TS Area 0&

Call accepted0Mo%ile unit recognies num%er on set up channel0,esponds to :1 which sends response to M'1B

0M'1B sets up circuit %etween calling and called :1s

0M'1B selects availa%le tra=c channel within cells and

noti3es :1s0:1s notify mo%ile unit of channel

Bngoing call09oice/data exchanged through respective :1s and M'1B

andoI

0Mo%ile unit moves out of range of cell into range ofanother cell

0'ra=c channel changes to one assigned to new :1 ?ithout interruption of service to user


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Figure 13;

.9ample o#

(o


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ther Functions

Call %loc*ing0 During mo%ile$initiated call stage) if all tra=c channels %usy)mo%ile tries again

0 fter num%er of fails) %usy tone returned

Call termination0 >ser hangs up

0 M'1B informed0'ra=c channels at two :1s released

Call drop0 :1 cannot maintain re4uired signal strength

0'ra=c channel dropped and M'1B informed

Calls to/from 3xed and remote mo%ile su%scri%er0 M'1B connects to 1'#

0 M'1B can connect mo%ile user and 3xed su%scri%er via 1'#

0 M'1B can connect to remote M'1B via 1'# or via dedicated lines

0 Can connect mo%ile user in its area and remote mo%ile user


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Re6uired Reading

1tallings chapter E!

13-wide area networks

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