Chapter 9 Telephone Network

School of Information Science and Engineering, Shandong University Associate 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 equipments „ Subscriber loop systems „ Switching systems „ Transmission systems „ Signaling 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. Junction lines

Exchange Subscribers C/E C/E M N Power feed & control C/E：concentrator / expander 集中器/扩展器 Issues on Subscriber loop † Subscriber loop interface „ Functions: BORSCHT „ B=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 exchanges „ Trunk group † Topologies adopted in Telephone networks „ Mesh network (网状网) ¾ Fully connected network ¾ Number of Trunk groups is proportional to square of the exchanges interconnected ¾ Suitable for heavy traffic among exchanges E E

EE Topologies adopted in Telephone networks

„ Star network (星型网络) ¾ A tandem exchange (汇接交换机) is employed. ¾ All other exchanges communicate through tandem exchange. ¾ Suitable for low traffic applications. „ Hierarchical network ¾ Multilevel star connection. ¾ The number of trunk groups can be minimized. Star & hierarchy 四级中心

三级中心 高效路有 二级中心

初级中心

本地交换 用户 Routing Methods

† Right-through routing „ The 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

Routing Decision Right-through

Source:A C

Right-through D E Right-through

Destination:B Routing Methods

† Own-exchange/distributed routing „ Alternative 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

Routing Decision Routing Decision

A C

Routing Decision D E Routing Decision

B Routing Methods

† Computer-controlled routing „ Based 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

Routing decisions are made by an independent signaling network.

STP

A B

CD 9.3 Transmission Plan † Transmission 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 budget „ Line 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 Amplifier

ABEcho of A

2/4 † Measures taken Hybrid „ Attenuator: short delay echos (<50ms) „ Echo suppressor: long delay echos (>50ms) „ Echo canceller: long delay echos (>50ms) Echo

† Attenuator: short delay echos (<50ms)

Echo of B Attenuator

ABEcho of A

Attenuator Echo

† Echo suppressor: long delay echos (>50ms)

Controlled Echo of B Attenuator

ABEcho of A

Controlled Attenuator Echo

† Echo canceller: long delay echos (>50ms)

Store and delay

AB

Canceller Singing & Control

† Singing: both talker and listener are disturbed. Amplifier

ABSinging

2/4 Hybrid † Control „ CCITT: a minimum loss of 10dB 9.7 Signaling Techniques

† Terminology „ Subscriber loop signaling „ Intraexchange or register signaling „ Interexchange or interregister signaling † Signaling techniques „ Inchannel signaling (信道内信令方式) ¾ Uses the speech or data path for signaling. „ Common channel signaling (公共信道信令) ¾ Uses a separate common channel for passing control signals for a group of trunks or information paths. Signaling techniques

Signaling Inchannel Common Channel

D.C. Low Voice PCM Associated Nonassociated Frequency Frequency

Inband Outband Inchannel vs. CCS

† Inchannel † CCS „ Trunks are held up „ Trunks are not during signaling required for signaling „ Signal repertorie is „ Possible to be limited expanded „ Interference between „ No interference voice and control between voice and signals control signals „ Misuse by customers „ No misuse „ Slow „ Fast „ Difficult to change „ Flexible „ Reliability is not „ Reliability is critical 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.

STP: Signaling transfer point; SP: Signaling point A B

CD 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. STP

A B

CD CCS Network Nodes

† Types of node: SP & STP † Signaling 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 signaling „ SS2: 600/750Hz signaling „ SS3: 2280Hz single voice frequency signaling „ SS4: 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 multifrequency inband 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

Element Duration (ms) Recognition(ms) Compound 150±30 80±20 Single-short 100±20 40±10 Single-long 350±70 200±40 SS4

† Sample control signals Control signal Code

Terminal seizure PXs

Transit seizure PYs

Clear forward PXl

Forward transfer PYl P=prefix element (2-VF compound) Xs=2040Hz short Xl=2040Hz long Ys=2400Hz short Yl=2400Hz long SS4

† Digits of the dialed number „ Transmitted as binary codes of four elements. „ Binary ‘1’: 2040Hz „ Binary ‘0’: 2400Hz „ Pulse duration: 35±7ms „ Gap 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. † TASI „ Attempt 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 trunkchannel association 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

Switching Signaling Signaling Switching Equipment Terminal Terminal Equipment A A B B M E

E M

M: mouth E: ear Outband signaling

† Outband signaling types „ d.c. signaling „ Low frequency a.c. signaling „ a.c. signaling above speech band „ Inslot PCM † Usage „ Done on link-by-link basis „ End-to-end signaling is precluded Outband signaling with E and M control

d.c.-a.c. a.c.-d.c.

F LPF M

D LPF

LPF D

M LPF F

a.c.-d.c. d.c.-a.c. Built-in PCM signaling † In-slot signaling „ The 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 signaling „ The 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 signaling Speech sample

Frame 1 12345678 12345678 TS 1 TS 24 Frame 2 Bit 1 unused

Frame 3 Bit 1 used for signaling as in Frame 1

Frame 4 Bit 1 used for frame synchronization Built-in PCM signaling

† CEPT 30-channel system outslot signaling „ Totally 32 time slots per frame „ Time slot 0 is used for synchronization „ Time 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

SYN SIG

Frame0 TS0 TS1 TS2 TS16 TS30 TS31 0-15 Frame1 TS0 TS1 TS2 TS16 TS30 TS31 1-16 Frame2 TS0 TS1 TS2 TS16 TS30 TS31

14-29 Frame15 TS0 TS1 TS2 TS16 TS30 TS31 9.9 Common Channel Signaling

† CCS „ Signaling 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

Switching Switching network network Speech circuits group

SPC SPC ST M M ST processor processor Voice channel 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. Signaling Header Circuit label Error Check Information † MUM: multiunit message „ A message with multiple signal units.

Signaling Header Circuit label Error Check Information Sub Length Other sig~ Error Check header Sub Length Address digits Error Check header 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

levels

OA&M ISUP TUP 4

SCCP

3 Signaling network

2 Signaling link MTP NSP

1 Signaling datalink

MTP: message transfer part NSP: network service part TUP: telephone user part ISU: ISDN user part SCCP: signaling connection control part OA&M: operation, administration and maintenance Signaling units of SS7

† Three types of signaling units „ MSU: message signal unit „ LSSU: link status signal unit „ FISU: fill-in signal unit † Flag bit pattern: ‘01111110’ „ Used as delimiter for synchronization „ Only flags can contain six 1’s Signaling units of SS7

MSU F Control SER SIF CRC F 8 248 16~496 16 8 LSSU F Control Status CRC F 8 248 16 8 FISU F Control CRC F 8 24 16 8 Control BSN Bl FSN FI LI U subfield 71 7 1 62 F=flag CRC=cyclic redundancy code SIF=signaling information SER=service information field BSN=backward sequence number BI=backward indicator FSN=forward sequence number FI=forward indicator LI=length indicator U=unused Assignments

† Ex.17 † Ex.18