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Chapter 9Telephone Network

School of Information Science and Engineering, Shandong UniversityAssociate Prof., Deqiang Wang

Outline

Subscriber loop system 用户环路系统

Switching hierarchy and Routing 交换体系与路由

Transmission Plan 传输方案

Transmission Systems 传输系统

Signaling Techniques 信令技术

Introduction

Major systems involved in telecommunication networks

Subscriber end equipmentsSubscriber loop systemsSwitching systemsTransmission systemsSignaling systems

9.1 Subscriber Loop Systems

Cable hierarchy for subscriber loops

MDF:主配线架MF：主馈送电缆FP： 馈送点BF： 分支馈送电缆DP： 分配点DC： 分配电缆DW：入户线

Issues on Subscriber loop

How long the subscriber loop could be?Signaling limits: Current delivered should be high enough.Attenuation limits: The resistance of the cables increase in proportion to length.

Issues on Subscriber loop

How to cover subscribers too far away?

Use of higher diameter wire.Use of equalized telephone set.Use of higher supply voltage.

Issues on Subscriber loop

How to cover disperse subscribers?Party lines: two subscriber share one line.Carrier systems: (FDM or TDM)Concentrators: A large number of subscribers share a small number of lines.

C/E C/ESubscribersN

ExchangeM

Power feed& control

Junction lines

C/E：concentrator / expander 集中器/扩展器

Issues on Subscriber loop

Subscriber loop interfaceFunctions: BORSCHTB=Battery feed 馈电功能

O=Overvoltage protection 过压保护

R=Ringing 振铃

S=Supervision 监控

C=Coding 编码（PCM）

H=Hybrid 2/4线混合

T=Test 测试

9.2 Switching Hierarchy and Routing

Interconnection of switching exchangesTrunk group

Topologies adopted in Telephone networksMesh network (网状网)

Fully connected networkNumber of Trunk groups is proportional to square of the exchanges interconnectedSuitable for heavy traffic among exchanges

E E

E E

Topologies adopted in Telephone networks

Star network (星型网络)A tandem exchange (汇接交换机) is employed.All other exchanges communicate through tandem exchange.Suitable for low traffic applications.

Hierarchical networkMultilevel star connection.The number of trunk groups can be minimized.

Star & hierarchy

四级中心

三级中心

二级中心

初级中心

本地交换

用户

高效路有

Routing Methods

Right-through routingThe original exchange determines the complete route from source to destination.There are a number of predefined routes.A route is selected based on certain criteria, such as time of the day, distribution of the traffic etc.. No routing decisions are taken at the intermediate exchanges/nodes.

Right-through routing

Source:A

Destination:B

C

D E

Routing Decision Right-through

Right-through

Right-through

Routing Methods

Own-exchange/distributed routingAlternative routes can be chosen at the intermediate nodes.Capable of responding to changes in traffic loads and network configurations.Minimal modifications are required when new exchanges are added.

Own-exchange/distributed routing

A

B

C

D E

Routing Decision Routing Decision

Routing Decision

Routing Decision

Routing Methods

Computer-controlled routingBased on the use of common channel signaling (CCS) systems.In CCS, there is a separate computer-controlled signaling network.A number of routing methods can be implemented.

Computer-controlled routing

A B

C D

STP

Routing decisions are made by an independent signaling network.

9.3 Transmission PlanTransmission quality and efficiency of operating of signaling impose limit on number of circuits connected in tandem.CCITT Q40:

The maximum number of circuit to be used in an international call is 12.No more than four international circuits be used in tandem between the originating and the terminating international switching centres.In excepted cases and for a low number of calls, the total number of circuits may be 14, but even in this case, the international circuits are limited to a maximum of four.

9.3 Transmission Plan

Factors in Transmission loss budgetLine loss (线路损耗)Switch loss (开关损耗)Echo level (回声电平)Singing (啸叫)

Echo & Measures taken

Echo: talker is disturbed

Measures takenAttenuator: short delay echos (<50ms)Echo suppressor: long delay echos (>50ms)Echo canceller: long delay echos (>50ms)

A BEcho of A

Amplifier

2/4Hybrid

Echo

Attenuator: short delay echos (<50ms)

A BEcho of A

Echo of BAttenuator

Attenuator

Echo

Echo suppressor: long delay echos(>50ms)

A BEcho of A

Echo of BControlledAttenuator

ControlledAttenuator

Echo

Echo canceller: long delay echos(>50ms)

A B

Store and delay

Canceller

Singing & Control

Singing: both talker and listener are disturbed.

ControlCCITT: a minimum loss of 10dB

A BSinging

Amplifier

2/4Hybrid

9.7 Signaling Techniques

TerminologySubscriber loop signalingIntraexchange or register signalingInterexchange or interregister signaling

Signaling techniquesInchannel signaling (信道内信令方式)

Uses the speech or data path for signaling.Common channel signaling (公共信道信令)

Uses a separate common channel for passing control signals for a group of trunks or information paths.

Signaling techniquesSignaling

Inchannel Common Channel

D.C. LowFrequency

VoiceFrequency

PCM Associated Nonassociated

Inband Outband

Inchannel vs. CCSInchannel

Trunks are held up during signalingSignal repertorie is limitedInterference between voice and control signalsMisuse by customersSlowDifficult to changeReliability is not critical

CCSTrunks are not required for signalingPossible to be expandedNo interference between voice and control signalsNo misuseFastFlexibleReliability is critical

Modes of CCS

Channel associated mode (信道关联模式)The signaling path passes through the same set of switches as does the speech path.Topologies of the signaling network are the same as that of speech network.

A B

C D

STP: Signaling transfer point;SP: Signaling point

Modes of CCS

Channel nonassociated mode (~非关联模式)The signaling information may follow a different route from that of speech.The topologies of signaling network are different from that of speech network.

A B

C D

STP

CCS Network Nodes

Types of node: SP & STPSignaling Points (SP) 信令点

A SP is capable of handling control messages directly addressed to it, but is incapable of routing messages.

Signaling Transfer Points (STPs) 信令转接点

A STP capable of routing messages and could also perform the functions of a SP.

9.8 In-channel Signaling CCITT Inchannel Signaling Systems

SS1: 500/20Hz signalingSS2: 600/750Hz signalingSS3: 2280Hz single voice frequency

signalingSS4: 2040 and 2400Hz two voice

frequency compound end-to-end analog signaling

SS5: 2400 and 2600Hz two VF compound analog line signaling and 2/6 multifrequencyinband analog interregister signaling with TASI

TASI: Time assigned speech interpolation

SS4 (inband signaling)

SS4 adopts inband signaling using a combination of two voice frequencies or a single voice frequency.Timings for SS4 signaling elements

200±40350±70Single-long40±10100±20Single-short80±20150±30Compound

Recognition(ms)Duration (ms)Element

SS4

Sample control signals

PXlClear forward

PYlForward transfer

PYsTransit seizure

PXsTerminal seizure

CodeControl signal

P=prefix element (2-VF compound)Xs=2040Hz shortYs=2400Hz short

Xl=2040Hz longYl=2400Hz long

SS4

Digits of the dialed numberTransmitted as binary codes of four elements.Binary ‘1’: 2040HzBinary ‘0’: 2400HzPulse duration: 35±7msGap between neighbor digits: 35±7ms

SS5 (inband signaling)Line signaling

Compound of the two voice frequencies or a continuous single frequency.

Interregister signaling:2-out-of-6 MF (multiple frequency) code.

TASIAttempt to improve trunk utilization by assigning a circuit to a speech channel only when there is speech activity.A technique to support more speech channels with a number of trunks.Leads to speech/signaling clipping.

SS5

Techniques used to maintain trunkchannelassociation during the signaling period:

The address information is transmitted as a block after gathering all the address digits, and the gaps are ensured to be less than the speech detector hangover time.Address digits are transmitted as and when they arrive and a lock tone is transmitted during the gaps.

E and M signaling control

A standard method of transferring signaling information between the switching equipment and the signaling equipment.

M-lead: carries signals from the switching equipment to the signaling equipment.E-lead: carries signals from the signaling equipment to the switching equipment.

E and M signaling control

SwitchingEquipment

A

SignalingTerminal

A

SwitchingEquipment

B

SignalingTerminal

B

M

E M

E

M: mouth E: ear

Outband signaling

Outband signaling typesd.c. signalingLow frequency a.c. signalinga.c. signaling above speech bandInslot PCM

UsageDone on link-by-link basisEnd-to-end signaling is precluded

Outband signaling with E and M control

LPF

LPF

d.c.-a.c.

M

D

a.c.-d.c.

FLPF

LPF

d.c.-a.c.

M

D

a.c.-d.c.

F

Built-in PCM signaling

In-slot signalingThe signaling information pertaining to a particular speech channel is carried in the same time slot as the speech.Example: Bell 24-channel system.

Out-slot signalingThe signaling information pertaining to a particular speech channel is carried in a separate time slot.Example: CEPT 30-channel system.

Built-in PCM signaling

Bell D2 24-channel multiframe PCM signaling structure

Bit 1 unused

Bit 1 used for signaling as in Frame 1

Bit 1 used for frame synchronization

Frame 1

Frame 2

Frame 3

Frame 4

12345678 12345678

signalingSpeech sample

TS 1 TS 24

Built-in PCM signaling

CEPT 30-channel system outslotsignaling

Totally 32 time slots per frameTime slot 0 is used for synchronizationTime slot 16 is used for signaling, carrying signaling information for two speech channels each time.A multiframe structure (16 frames) is adopted for signaling purpose.

CEPT 30-channel system outslot signaling

TS0 TS1 TS2 TS16 TS30 TS31

TS0 TS1 TS2 TS16 TS30 TS31

TS0 TS1 TS2 TS16 TS30 TS31

TS0 TS1 TS2 TS16 TS30 TS31

Frame0

Frame1

Frame2

Frame15

SYN SIG

0-15

1-16

14-29

9.9 Common Channel Signaling

CCSSignaling is completely separate from switching and speech transmission.Dedicated channels for signaling are used to support a group of circuits.The CCS network is basically a store and forward (S&F) network where signaling information travels on a link-by-link basis along the route.

Basic scheme for CCS

Switchingnetwork

Switchingnetwork

Speech circuitsgroup

SPCprocessor

SPCprocessorST STM M

Voicechannel

Data channel

Signaling channel

CCS system

ST: Signaling Terminal M: Modem

CCS signaling message formats

SU: signaling unit of fixed length.SUM: single unit message

A message of one signal unit length.

MUM: multiunit messageA message with multiple signal units.

Header SignalingInformation Circuit label Error Check

Header SignalingInformation Circuit label Error Check

Subheader Length Other sig~ Error Check

Subheader Length Address digits Error Check

SS7

First defined in1980, revised in1984 and 1988.Can be used over a variety of digital circuit switched networks.The functions in SS7 are defined assuming packet switched operation.Primarily optimized to work with digital SPC exchanges utilizing 64kbps digital channels.

Architecture of SS7

Signaling datalink

Signaling link

Signaling network

SCCP

ISUP TUPOA&M

4

3

2

1

MTP NSP

levels

MTP: message transfer part NSP: network service partTUP: telephone user part ISU: ISDN user partSCCP: signaling connection control partOA&M: operation, administration and maintenance

Signaling units of SS7

Three types of signaling unitsMSU: message signal unitLSSU: link status signal unitFISU: fill-in signal unit

Flag bit pattern: ‘01111110’Used as delimiter for synchronizationOnly flags can contain six 1’s

Signaling units of SS7

F Control SER SIF CRC F

F Control Status CRC F

F Control CRC F

BSN Bl FSN FI LI U

MSU

LSSU

FISU

Controlsubfield

8

8

8

8 8

8

8

8

24 16

24 16

24 16

7 1 7 1 6 2

F=flag CRC=cyclic redundancy code SIF=signaling informationSER=service information field BSN=backward sequence numberBI=backward indicator FSN=forward sequence numberFI=forward indicator LI=length indicator U=unused

16~496

Assignments

Ex.17Ex.18