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between a subscriber and his local exchange subscriber signaling, and between the exchanges at the two ends of a trunk interexchange signaling.. Instead, a signaling network, consisting

2

INTRODUCTION TO

SIGNALING

2.1 OVERVIEW

This section presents a brief historical outline of signaling The earliest telephone exchanges were “manual” switchboards, in which all calls were set up and taken down by operators Signaling between subscribers and operators was limited to tivtging To make a call, the subscriber would send a ringing signal This alerted an operator, who would connect her telephone to the calling line, and ask for the called number The operator then would connect her telephone

to the called line, and ring the line After answer by the called party, the operator would establish the connection

Signaling as we know it today started around 1890, with the invention, by Almon B Strowger (a Kansas City undertaker), of an automatic switchboard that could receive the called number dialed by the calling subscriber, and would then automatically set up the connection During the past 100 years, signaling applications and technology have evolved in parallel with the developments in telecommunications

2.1 I Early Signaling

Signaling in the period from 1890 to 1976 had three main characteristics In the first place, its application was limited to plain ordinaly telephone service (POTS): the set-up and release of connections between two subscribers In the second place, the signals were carried by the same circuit (subscriber line, trunk) that carried the speech during the call This type of signaling is known as

38

Signaling in Telecommunication Networks John G van Bosse

Copyright  1998 John Wiley & Sons, Inc ISBNs: 0-471-57377-9 (Hardback); 0-471-22415-4 (Electronic)

between a subscriber and his local exchange (subscriber signaling), and between the exchanges at the two ends of a trunk (interexchange signaling)

Initially, automatic telephony was possible only for calls between subscribers served by the same exchange, which required subscriber signaling only Later on,

it became possible to dial calls between subscribers served by nearby exchanges These calls also required interexchange signaling National long-distance calls needed operator assistance until the 195Os, when direct distance dialing (DDD) was introduced Znternatibnal direct distance dialing (IDDD), which requires signaling on international trunks, became possible in the 1960s [3]

Channel-associated call-control signaling is still widely used today How- ever, beginning in 1976, other forms and applications of signaling have made their appearance in telecommunication networks They are briefly outlined below

2.1.2 Common-channel Signaling

alternative form of call-control signaling for trunks [2,4,5] In CCS, signaling information is not carried by the individual trunks Instead, a signaling network, consisting of signaling data links (SDL) and signal transfer points (STP), transfers digital signaling messages between the exchanges In Fig 2.1-1, trunk groups TG1, TG2, and TG, have common-channel signaling, and the signaling network consists of one signal transfer point (STP), and the signaling data links SDL,, SDL,, and SDk A call-control message from exchange A to exchange

B for a trunk in TG, traverses SDL,, the signal transfer point, and SDL, In this example, each signaling link carries messages for all CCS trunks that are attached to an exchange For example, SDL, carries messages for the trunks in

Exchange Exchange

\

1 SDL, I

cl-

/

\ I

L -I STP

Figure 2.1-l Common-channel call-control signaling TG,,2,3: trunk groups SDLA,B,C: signaling data links STP: signal transfer point

groups TG, and TG, We say that SDL, is the “common” signaling channel for the trunks in these groups

Common-channel signaling became possible after the introduction of stored-program controlled exchanges We shall see that CCS signaling is more powerful, flexible, and also faster than CAS signaling CCS is being introduced rapidly in national telecommunication networks, and in the international network However, the replacement of CAS is not yet complete, and CAS and CCS signaling coexist in many networks

2.1.3 Other Applications of CCS Signaling

Since 1980, CCS is also being used for other applications In Fig 2.1-2, asewice control point (SCP) and an operations, administration, and maintenance center (OAM) also have signaling data links, and exchanges can send messages to, and receive messages from, these network entities Procedures that involve signaling between an exchange and a SCP, or between an exchange and an OAM, are known as transactions

The SCPs in a network support intelligent network (IN) services [5-71 These services require information that cannot be stored conveniently in exchanges

A well-known IN service is 800 calling The 800-numbers are not in the conventional format of national numbers, and cannot be used by the exchanges

to route calls to their destinations Therefore, when an exchange receives a call with an 800 number, it starts a transaction with the SCP, requesting the

Exchange

A

Exchange

B

Exchange

C

\ I SDL,

\ I I

\ 1 SDL, I

‘iii SDL ri

STP \

\

SDL\ OAM

\

\

i

Figure 2.1-2 Common-channel signaling for transactions involving an exchange and a service control point (SCP), or an operations, administration, and maintenance center (OAM)

translation of the received 800 number The SCP then translates the number into the national number of the called party, and returns that number to the exchange, which then proceeds to set up the connection

The OAM centers allow centralized operation, maintenance and adminis- tration of the network For example, there are transactions in which the OAM requests an exchange to test a particular trunk, and to report the test results Other transactions enable the OAM to verify and change the subscriber and routing data that are stored in the exchanges

Cellular mobile telecommunications were introduced in the 198Os, and are widely used today [8] Figure 2.1-3 shows a cellular mobile network Exchanges

in mobile networks are known as mobile switching centers (MSC) The MSC has one or more trunk groups (TG) to exchanges in the public switched

number of cells Each cell has a base station (BS) that contains microwave radio equipment, and has trunks and data links to the MSC

A mobile station (MS) in a cell communicates with the base station of that cell on a radio-frequency channel, of which there are two kinds Avoice channel (VC) of a base station is permanently connected to a trunk between the BS and the MSC Acontrol channel (CC) of a base station is permanently connected to

a data link between the BS and the MSC

Exchange

rl

r Public Switched Telecommunication Network

me- -c -

Cellular Mobile Network

Figure 2.1-3 Signaling in cellular mobile networks MSC: mobile switching center MS: mobile station VC: voice channel CC: control channel T: trunk DL: data link BS: base station

In Fig 2.1-3, mobile MSA is engaged in a call The MSC has allocated trunk T,, and the associated voice channel VC,, to the call T, and VC, transfer the speech, and the signaling, between the mobile and the MSC

The signaling for all mobiles in a cell that are not involved in a call is carried

on a control channel of the cell, and its associated data link For example, mobile MSB is not involved in a call, and the signaling between the mobile and the MSC takes place on DL, and CC The signaling between the mobiles and the MSC is thus a combination of channel-associated and common-channel signaling

In addition, signaling in cellular mobile networks includes transactions between mobile switching centers and mobile-network databases One group of these transactions allows a mobile to obtain service while it is roaming outside its “home” cellular system

2.1.5 Digital Subscriber Signaling

The public switched telecommunication networks in some countries are being converted into integrated services digital networks (ISDN) [2,9,10] An ISDN serves conventional (analog) subscribers and lSDN users ISDN users can communicate with each other in two modes In circuit-mode communications, the network sets up a dedicated connection for the call, which can be used for voice and data communications In packet-mode communications (not covered in this book), the users communicate with short bursts of data, known

as packets

An ISDN user has 64 kb/s digital terminals TE of several types, for example digital (PCM) telephones, high-speed facsimile terminals, and high-speed computer modems, etc -see Fig 2.1-4 A digital subscriber line (DSL) connects the user’s TEs to his local exchange DSLs are two-wire or four-wire circuits that allow simultaneous information transmission at 144 kb/s in both directions A DSL also carries a number of overhead bits, and the total transmission rate on the line is therefore somewhat higher

The 144 kb/s information bit streams are divided into two 64 kb/s B-

mode communications, allowing two simultaneous calls The D-channel is the

ISDN User Local Exchange

“common” channel for signaling between the user and the local exchange The signaling is message-oriented, and known as digital subscriber signaling system No 1 (DSSl)

2.1.6 Multiple Interexchange Signaling Systems

For economic reasons, exchanges have to remain in service for about 20 years This means that national networks, and the international network, contain exchanges of several vintages The older exchanges are equipped to handle older interexchange signaling systems only-and it is difficult or impossible to upgrade them to accommodate newer forms of signaling

Now, consider a telecom whose network currently uses two interexchange signaling systems (which we denote here as #l and #2), and wants to introduce

a new signaling system (#3) It can purchase a number of exchanges that are equipped to handle system #3, and interconnect them with “system #3” trunk groups However, the new exchanges should also handle signaling systems #l and #2, for use on trunk groups between an old and a new exchange As a result, several signaling systems coexist in the network

A connection routed via one or more intermediate exchanges may therefore involve trunks with different interexchange signaling systems For example, at an intermediate exchange, the connection could involve a trunk with signaling system #2, and a trunk with signaling system #3 The call-control at the exchange has to include procedures for signaZing inter-working between the signaling systems One aspect of inter-working is the conversion of the formats of individual signals and/or messages More difficult problems arise when a signal,

or an information item in a CCS signaling message, exists in the new system #3, but not in the old system #2 Signaling inter-working functions can be quite complex, and have to be designed with care

The equipment in a telecommunication network is usually purchased from several manufacturers In order to ensure that equipment from different suppliers can be interconnected without problems, the telecoms have developed (and continue to develop) standards that are documented in specifications Purchase orders for equipment are accompanied by a set of specifications which have to be met by the manufacturer’s equipment Specifications cover signaling, and many other aspects of telecommunications The information in this book is based largely on signaling standards published by the organizations mentioned below

2.2.1 Bell System

Prior to its divestiture in 1984, the Bell System was both the dominant telecom

and equipment manufacturer, and set the de facto standards for the North American telecommunication network [ 111

2.2.2 Exchange Carriers Standards Association

After the divestiture, the Exchange Carriers Standards Association (ECSA) was created to ensure that the North American network would continue to operate

as an integrated entity [12] A number of Tl committees within ECSA are responsible for generating drafts of telecommunication standards which are then submitted to the American National Standards Institute (ANSI), and published as American National Standards for Telecommunications

2.2.3 Bellcore

Bellcore, the research and development arm of the former Bell System operat- ing companies, has defined requirements for these companies, documented in

2.2.4 TIA/EIA

Standards for cellular mobile systems in the U.S are established by sub- committees of the TR.45 Committee They are published jointly, by the Tele- communications Industry Association (TIA) and the Electronic Industries Association (EIA), as TZAIEZA Interim Standards

2.2.5 CClTT/lTU-T

Historically, the telecoms that operate individual national networks established standards independently of each other With the advent of international telecommunications, the need for international standards arose, and this has led

to the establishment of international standards organizations

The CCITT (International Telegraph and Telephone Consultative Com- mittee) was established in 1956, as a part of the International Telecommunica- tion Union (ITU), which has its headquarters in Geneva, Switzerland [13] As a result of a major reorganization of ITU, the CCITT ceased to exist in 1993, and became the ITU-T, the telecommunications standardization sector of ITU During its life, CCI’IT activities took place in four-year study periods Individual study groups met several times per year, and presented their results to the plenary assembly, in a meeting held at the end of the study period Approved results became recommendations, which were published in a set of books that were updated and reissued at four-year intervals The last set of books are the CCZTT Blue Books, issued in 1989

During the 1980s it became clear that the four-year study periods were too slow for the rapid developments that were taking place in telecomunications

Therefore, ITU-T draft standards are now reviewed in meetings of the individual study groups, and recommendations are published individually Signaling standards are the responsibility of Study Group 11

While the CCITT/ITU-T recommendations are intended primarily for the international network, many national telecoms have adopted them- sometimes with country-specific modifications-for use in their respective networks

European telecoms established the Conference of European Postal and Tele- communications Administrations (CEPT) in 1959 [ 14,151 Its Coordination and Harmonization Committee (CHH) has issued a number of recommendations The standardizing activities were transferred in 1988 to a new organization, the European Telecommunication Standards Institute (ETSI) The work is carried out by a number of technical committees, one of which deals with signaling protocols and switching (SDS) ETSI standards are published as European Telecommunications Standards

2.3 ACRONYMS

ANSI

CAS

CCIS

CCITT

ccs

CEPT

DDD

ECSA

EIA

ETSI

IDDD

ISDN

ITU

OAM

POTS

PSTN

SCP

SDL

SSP

STP

TG

TIA

American National Standards Institute

Channel-associated signaling

Common-channel interoffice signaling

International Telegraph and Telephone Consultative Committee Common-channel signaling

European Conference of Postal and Telephone Administrations Direct distance dialing

Exchange Carrier Standards Organization

Electronic Industries Association

European Telecommunication Standards Institute

International direct distance dialing

Integrated Services Digital Network

International Telecommunication Union

Operations, administration and maintenance center

Plain old telephone service

Public switched telecommunication network

Service control point

Signaling data link

Service switching point

Signal transfer point

Trunk group

Telecommunications Industry Association

46

Stevenage, U.K., 1981

2 R.L Freeman, Telecommunication System Engineering, Second Edn, John Wiley & Sons, New York, 1989

3 A History of Engineering & Science in the Bell System, Bell Telephone Laboratories, Inc, 1982

4 Special Issue on CCIS, Bell Syst Tech J., 57, No.2, 1978

Mag., 28, No.7, July 1990

6 S.M Boyles, R.L Corn, L.M Mosely, “Common Channel Signaling: The Nexus of

7 R.B Robrock, “The Intelligent Network Evolution in the United States,” Conference Proceedings, Singapore ICCS’90, Elsevier, Amsterdam, 1990

8 W.C.Y Lee, Mobile Cellular Telecommunications, McGraw-Hill, Inc., New York,

1995

10 T.B Bell, “Telecommunications,” IEEE Spectrum, 29, No 2, 1992

11 Notes on the Network, AT&T, New York, 1980

32, No 4, Jan 1994

13 T Irmer, “Shaping Future Telecommunications: The Challenge of Global

Mitteilungen PTT, Switzerland, July 1990

Comm Mag., 32, No 4, Jan 1994