09 1 isdn and mobile telephony

8/3/2019 09 1 ISDN and Mobile Telephony

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CommunicationSystems9th lecture

Chair of Communication SystemsDepartment of Applied Sciences

University of Freiburg2006


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Communication SystemsAdministrative stuff / Last lecture introduction to telephonynetworks

15.06 is holiday, no lecture, no practical

Telephony networks rather old communication infrastructure

Invention of the telephone in the late 19th century

First manually switched, later on mechanical automaticallyswitched networks

Inband dial signaling first with pulses later on with DTMF (tohandle other media than copper wire too)

Completely other concepts to handle standardization, protocoland definition of interfaces (than in the IP world)

Computerized switching centers and the introduction of ISDNin the 1980s


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Communication SystemsLast lecture introduction to telephony networks

ISDN Integrated Services Digital Network

First fully digital telephony network

PCM for voice digitization

BRI offers two B channels (64kbit/s for either voice or datacommunication) and a separate D channel for out of band dialand control signaling

D channel are defined in the 3 lower OSI layers physicalinterface with different encodings, e.g. 4B3T

DLL represented by LDAP protocol

DSS1 is handling call setup, signaling, call destruction, givesinformation on caller ID, ...


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Communication Systemsplan for this lecture

Signaling in large scale digital networks

Last lecture signaling between terminal endpoints (TE) andthe local switching center

Primary Rate Interface But how is a call setup and routed globally

Signaling system #7, MTP, SCCP, ISUP

QSIG for inter-connecting PBX

First introduction to mobile telephony networks and GSM.


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Communication SystemsPrimary Rate Interface - PRI

Talked on ISDN Basic Rate Interface (BRI) last lecture

Enough for average household or small office

But insufficient to serve larger enterprises and organizations

Primary Rate Interface (PRI) handles large scaleconnectivity

ITU-T specifications G.703, G.704, G.705

includes 30 B channels (each B channel 64kbit/s), a full rate D

channel at 64kbit/s and a framing/synchronization pattern(64kbit/s)

Channels could be bundled, so called H channels: H0384kbit/s, H11 1534kbit/s and H12 1920kbit/s


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Communication SystemsPRI physical specification, interfaces

Brutto bandwidth sums up to 2.048kbit/s connection

Copper wire connections allow up to 250m without refresh, forlonger distances often fiber optics is used

All connections are unidirectional, so no channel separation isneeded as was in BRI

Name of the interface from switching center: UK2

, for fiberoptics: U

G2, user interface is named S

2M

Different channels transmitted in TDM (Time Division

Multiplexing) International E1 (European ISDN standard) systems use

HDB3 (High Density Bipolar 3) line coding


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Communication SystemsPRI - HDB3 line coding

Two kinds of transmission media used to transmit E1 signals

coaxial cable (2,37V peak base)

twisted-pair cable (3V peak base)

HDB3 Line Coding is similar to AMI coding

To avoid co-current flow strings of 4 zeroes are replacedwith one of four bipolar violation codes, example for 8consecutive zeros


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Communication SystemsNetwork signaling

Of course there are options to bundle more than 30channels into a site connection other media, e.g. fibreoptics is used then (underlying transport technology is oftenATM Asynchronous Transfer Mode)

In BRI and PRI connections a separate channel is used forsignaling D / DSS1 multiplexed into the same physicalconnection

In modern large scale telephony networks signaling and real

connections are completely independent Connectivity between switching centers is handled by a

specialized signaling system Signal System Nr. 7 (SS7)


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Communication Systemsindependent networks for signaling and connection

Signaling layer consists of Signaling Points (SP) andSignaling Transfer Points (STP)


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Communication Systemsline switching and signaling network

Signaling layer is a virtual layer ontop the connection layernetwork (coupling network)

SP's are the direct involved switching centers of aconnection, STP just route signaling information

End points of a connection are the the end switching centerswhere the subscribers are connected to

Every switching center should be connected at least to twoSTP for backup

Thus route optimizations and fallback routes implemented

Signaling data itself is transported in usual bearer channels (Bchannel) of the connection layer of the switching centers


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Communication Systemsline switching and signaling network

Signaling network uses a network indicator to distinguishdifferent networks

SP use a Signaling Point Code of 14bit and could beassociated with up to four networks

This allows changeovers between different networks

For international connections special ISTP (InternationalSignal Transfer Points) are operated

SS7 is specified in ITU Q.700 recommendation

Resembles the OSI 7-layer model because mostly packetorientated operation

Major distinction of protocol levels is made into transport andapplication part


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Communication SystemsSignaling System 7

SS7 does notconform exactly toOSI

Primarily madenot for direct userdata exchange buttelephony networkspecificinformation


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Communication SystemsSignaling System 7 layer 1

Layer 1 defines the (physical) access to the couplingnetwork

Protocol name: MTP (Message Transfer Part) specified in inITU G.701-710

Defines a bi-directional signaling link

Uses the standard 64kbit/s connections

In PRI normally channel 16 is defined for that purpose


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Layer 2 main task is frame synchronization (more taskscompared to OSI data link layer)

Distinction of blocks (signal units) through flags

Flag consists of 8 bits: 0111 1110 Securing of block ordering via numbering

Transparent transmission

Flow and congestion control

Error detection with the help of checksums in every signal unit Error correction through retransmission and

acknowledgements preventive cyclic retransmission andARQ


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There are three different message types

Message Signaling Unit (MSU) is meant for transmission ofstandard signaling information

Link Status Signaling Unit (LSSU) transmits detailedinformation on the current status of the signaling link

Fill-in Signaling Unit (FISU) synchronisation if no signalinginformation is transmitted - kind of keep-alive


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Layer 3 composed of MTP and SCCP

Decides if a message is to be routed or not

General routing decisions for signaling messages

Distribution of messages to application layer (different types ofuser parts)

Network management and monitoring

Four types of messages

MTP management information Messages for the Telephone User Part (TUP), ISDN User Part

(ISUP)

SCCP (Signaling Connection Control Part) messages


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All messages contain Destination Point Code (DPC) andOrigination Point Code (OPC) (comparable to addresses inTCP/IP)

Routing in SS7 follows these principles

Minimal pathes, pass minimum number of SP, STP

If more then one link, distribute load equally

Every signaling information should take the same path

The Signaling Link Selection (SLS) is for load balancing

All messages with same SLS are sent through the samechannel


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Communication SystemsSignaling System 7 layer 3, SCCP part

SCCP is an extension to MTP for OSI conformance, definedin Q.711-716

Transports ISUP and TCAP messages

End-to-end routing (MTP handles the hop-to-hop routing) Extends the Signaling Point Code for GT (Global Title)

addressing

GT is worldwide unique for international routing of signalinginformation

SCCP defines four protocol classes:

class 0: connection less transmission, segmentation (max. 16),distribution of messages over several signaling links


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Communication SystemsSignaling System 7 SCCP protocol classed

SCCP defines four protocol classes:

class 1: connection less, keeps sequence of messages, sameSLS code (no load distribution)

class 3: simple connection orientated transmission, end-to-end,flow control

class 4: connection orientated, option to bypass flow control,end-to-end connection

There are several message types defined for the different

classes


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Communication SystemsSignaling System 7 layer 4-7, application layer protocols

Transaction Capabilities Application Part (TCAP) defined forthe services of the Intelligent Network (IN, advancednetwork for management, configuration and mobiletelephony services)

Message exchange through structured or unstructureddialogs for several actions within network

TUP and DUP are the old style User Parts for non ISDNtelephony and data

The ISDN User Part (ISUP) handles the ISDN signaling stuff End-to-end, so that intermediate switching centers do not have

to decode signaling information


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Communication SystemsSignaling System 7 ISUP

ISUP is used for:

Setup and destruction of B channel connections

Handling of signaling of the several ISDN

characteristics (call deflection, call on hold,conference, ID signaling, ...)

Connection of different logical connections, e.g.when passing network borders (internationalconnections)

ISUP consists of a header, required parameters of fixedlength, required parameters of variable length andoptional part


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Communication SystemsSignaling System 7 ISUP

Header contains

Routing label for all signaling messages needed byparticipating switching centers (OPC, DPC, ...)

Circuit Identification Code for addressing of a certain Bchannel used

Message type defines the kind of signaling and messageformat

Construction of body (the parameters are the message)

differs dependent of the message type End octet: 0000 0000


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Communication SystemsSignaling in private branch exchanges

The telephony providers often do not handle internalcommunication of (large) firms, organizations, ...

They use private branch exchanges (PBX) under theirown jurisdiction

Digital PBX are connected to the public network mostcommonly via ISDN BRI or PRI interfaces

Internally they use not SS7 but DSS1 (D channelprotocol) in small scale exchanges and QSIG in large

scale PBX Q.931 protocol was intended to be used for signaling

within PBX but every manufacturer created his ownprotocol (there is much money in the market and thusmuch interest that a customer does not uses different

equipment)


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Communication SystemsSignaling in private branch exchanges

QSIG was specified by the ECMA (European ComputerManufacturers Association)

Q in the name refers to the Q reference point in the PBXs

At layer 1&2 QSIG is identical to the DSS1 EURO ISDNprotocol

The layer 3 is split into three sublayers: Basic Call (BC),Generic Function (GF) Protocol and QSIG Procedures forSupplementary Services (SS)

BC implements ISDN standard functionality, GF shouldallow the inter-connect of devices of different vendors,SS allows for transparent services extensions(automatic call completion, display of tolls, ...)


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Communication Systemsmobile telephony networks - history

With the development of digital telephony networks anddefined signaling standard between switching centersthe preconditions for digital mobile telephony were met

First mobile phone networks started around end of1940s in the US

St. Louis, Missouri, single cell system

A-Netz in Germany operated from 1957 to 1977

Analogous network in the frequency range between

156MHz and 174MHz

15 manually switched channels (in the final versionover 100 wireless sector areas with together over 300channels)


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Communication Systemsmobile telephony networks German A-Netz

50 kHz distance betweenchannels

Use of frequencymodulation

10.500 participants

Mostly used in cars (size!!)

Main problems were themanual operation, noautomated handover whenmoving and limited possiblenumber of participants


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Communication Systemsmobile telephony networks - history

The deployment of German B-Netz started in 1972 andwas the automated successor of the A-Netz

Main advantage: calls could origine both in the mobileand the wired network but for calling a B-Netz

participant you had to know the area (diameter of 27km)where located

The old federal republic was split into 150 zones with adiameter of up to 150km (a zone could host more then onebase station)

Frequency ranges 148,4 MHz - 149,13MHz and 153,0MHz -153,73MHz, later 157,61 MHz -158,33MHz / 162,2MHz -162,93MHz


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Communication Systemshistory B1/B2 mobile network

B-Netz, B1/B2 net from 1982

Offered 38 voice channels up to 1980

75 channels after recycling of the A-Netz frequencies

Bandwidth per channel 14kHz, channel distance of 20kHz Frequency modulation with a 4kHz frequency deviation

The network even offered a limited roaming with Austria,Luxemburg and the Netherlands

In the beginning 16.000 participants, after the extension27.000 participants would be the maximum

Peak usage was in the middle of the 80s: 850 frequencychannels and 158 base stations


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Communication Systemshistory of mobile telephony networks cellular networks

Main problem of the analogous networks was the limitednumber of participants

Challenge was how to accommodate much more userswithin a mobile phone network

Cellular concepts were developed and tested from the lateseventies: technology advances enable affordablecellular telephony

entering of the modern cellular era started 1974-1978 withfirst field Trial for Cellular System by AMPS in Chicago

Cellular concepts reuse frequencies and do not try touse the same frequency over a wide area (to avoidhandovers of moving participants)


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First generation of digitally switched mobile networksstarted in the 1980s

Several competing standards in different countries

NMT (Nordic Mobile Telephone) scandinavian standard;adopted in most of Europe

First european system (Sweden, 1981)

TACS (Total Access Communication Systems), starts in1985

UK standard; A few of Europe, Asia, Japan

AMPS (Advanced Mobile Phone Service)

US standard


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Radiocom 2000 (Only in France)

Analog transmission of voice (no data services) usingfrequency modulation

Various bands were defined in different countries,areas:

NMT: 450 MHz first, 900 MHz later

TACS: 900 MHz and 1230 bidirectional channels (25KHzeach)

AMPS: 800 Mhz

Most of the first generation mobile networks areswitched of, but the frequencies still partly inpossession of the several network operators (newservices like Wireless DSL ...)


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Communication Systemsfirst generation cellular networks - C-Netz in Germany

C-Netz in Germany (used too in Portugal and SouthAfrica) started in 1985 and offered a lot of advantages

Common prefix: 0161for all participants

Interruption-less handover between cells Scrambling of the analogous radio signal

exacerbated the eavesdropping of connections

Not only car systems but real portable devices

huge" capacity of up to 850.000 participants inGermany

Frequency range 451-455,74MHz, 461-466,74MHz

Operated up to the 31th of December 2000


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Communication Systemscellular networks - planning

Planning of cellular networks is a complex procedure

cover the same area with a larger number of base stations(BS)

Partitioning of an area into radio cells idealized ashexagons, the hexagon is a rather good approximation ofa circle

Frequency channels could not be reused in neighboring cellsbecause of interference

modelling: setup of clusters Cluster contains: k cells, which use together the complete

frequency range

k size of the cluster


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Communication Systemscellular networks - clustering of areas

Cell radius willdiffer in sizedepending onexpected density

of users Real coverage of

a cell is oftendifferent fromidealized mode

Ideal coveragepattern wouldgenerate no holesand no cellsuperposition


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Communication Systemscellular networks - planning

Example of cluster size 3

Generic formula

k = i + i*j+ j

Thus cluster sizes of 1, 3,4, 7, 9, 12, 13, 16, 19, 21,... are possible

With decreasing ofcluster size the capacity

of the network increases But the interference will

increase (tradeoff)


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Communication Systemscellular networks of second generation

General formula for reuse distance: D=R * sqrt(3K)

Valid for hexagonal geometry

D = reuse distance

R = cell radius q = D/R = frequency reuse factor

For the example k=3, the reuse factor is 3, for k=12 is 6

Frequency reuse implies that remote cells interfere with

tagged one Co-Channel Interference (CCI)

Sum of interference from remote cells

For computation see literature

D R 3 K


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Communication Systemscellular networks of second generation

Four systems in use today

Global System for Mobile Comm. (GSM)

Digital AMPS (D-AMPS), US

Code Division Multiple Access (IS-95) Qualcomm,US Personal Digital Cellular (PDC), Japan

GSM by far the dominant one

Originally pan-european

Deployed worldwide in around 200 countries (even incountries without any working administration), more then500 mobile operators

By now available in US too


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Communication Systemscellular networks of second generation GSM history

1982 the Groupe Speciale Mobile was founded by theCEPT (Conference Europeene de Postes etTelecommunication) to develop a common standard foreuropean mobile networks of the second generation

With the increasing popularity of the GSM standardworldwide, the name was changed to Global System forMobile Communication

1987 introduction of the transfer method which is still in usetoday (with sligth modifications)

1991 first testbed networks started in 5 European countries

1992 the two so called D-Netze in Germany started (D asthe successor of the C-Netz)


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Communication Systemscellular networks of second generation GSM history

Other than in the monopoly wired world in manycountries started competing providers of the newservices

First variant of GSM operated in the 900MHz (890-

915MHz for uplink and 935-960MHz for downlink)frequency band available most countries in Africa, Asia,(partly the US), Europe, Australia

1993 one Million participants in Germany

1993 a secondary frequency band of 1800MHz(DCS1800) was defined (1710-1785MHz up, 1805-1880MHz down)

1995 completion of so called Phase2 of GSM with theintroduction of new features like fax transfer, extended

cell choosing mechanism, ...


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Communication SystemsGSM services and applications

As GSM was developed on the core of IN (IntelligentNetwork) and ISDN, so it inherited a list of features like

Tele- and bearer services like voice and data (SMS, MMS,Internet, Fax) services

comfort services like call deviation and deflection, caller ID,automatic call back on busy, blocking of numbers (outgoing andincoming)

Additional services like telephone answering machines(Mailbox, several information desks on Hotels, itineraries,traffic congestion, ...)

Location based services, e.g. for emergency calls, areadependent tolls (O2 homezone and other similar offers), areadependent tourist information, navigation, tracking of containersand fright trucks


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Communication SystemsGSM system


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Communication SystemsGSM subsystem hierarchy

A mobile switching center (MSC) not much different from a normalswitching center handles several location areas

MSC region -> N Areas -> M BSC -> K BTS -> I MS

with: N < M < K


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A Location Area (LA) is covered by some BTS (Base TransceiverStation) which are managed by some fewer BSC (Base StationControllers)

Thus a provider of about 15 Million subscribers (which are

using Mobile Stations (MS, simply the mobile phoneequipped with a SIM card) will have to setup up to 30.000radio cells

The cells are covered by up to 12.000 BTS, which areoperated by some hundred BSC, which are handled by

up to 50 MSC The MSC uses the services of a Visitor Location

Register (VLR) holding copies of user data from theHome Location Register (HLR)


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The Home Location Registers operated by each provideris the place where subscriber information is kept

Which services (voice, data, fax, roaming, ...) the user issubscribed to

Data is copied temporarily to VLR where the MS of a useris registered

The Authentication Center keeps the user access andauthorization information

The Equipment Identification Center keeps track ofmobile equipment (unique serial of Mobile Terminals(MT) the MS without the SIM)

The mobile network is supervised by the Operation andMaintenance Center


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Communication SystemsISDN, IN, GSM literature

E. Pehl, Digitale und analoge Datenbertragung

ISDNhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN/seminararbeitBorstHartges.pdf

ISDN IIhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN-erweitert/seminarbThurner.pdf

GSMhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/G2-GSM/HA_GSM2_Mohry_1.pdf
http://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN/seminararbeitBorstHartges.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN-erweitert/seminarbThurner.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/G2-GSM/HA_GSM2_Mohry_1.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/G2-GSM/HA_GSM2_Mohry_1.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/G2-GSM/HA_GSM2_Mohry_1.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/G2-GSM/HA_GSM2_Mohry_1.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/G2-GSM/HA_GSM2_Mohry_1.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/G2-GSM/HA_GSM2_Mohry_1.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN-erweitert/seminarbThurner.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN-erweitert/seminarbThurner.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN-erweitert/seminarbThurner.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN-erweitert/seminarbThurner.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN-erweitert/seminarbThurner.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN/seminararbeitBorstHartges.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN/seminararbeitBorstHartges.pdfhttp://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN/seminararbeitBorstHartges.pdf

09 1 isdn and mobile telephony

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