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For example, when two exchanges, say A and B, are interconnected by a group of CCS trunks, there is a signaling relation between these signaling points.. When messages for relation A,B a

5

INTRODUCTION TO COMMON-CHANNEL

SIGNALING

trunk is carried by the trunk itself In common-channel signaling (CCS), a common signaling link (SL) carries signaling messages for a number of trunks Just as multi-frequency (MF) signaling became feasible with the introduction of the second-generation (common-control) switching systems, CCS was developed for the third-generation (stored program controlled, SPC) exchanges that were introduced in the 1960s

There are several reasons for the move from multi-frequency signaling to ccs [l]:

1 It is often less costly to interface the processing equipment of SPC exchanges with a relatively small number of signaling links than to provide pools of MF registers and line-signaling hardware for the individual trunks

2 Common-channel signaling is much faster than multi-frequency signaling Early CCS systems already reduced post-dialing delays on long-distance calls from lo-15 s to around 3 s

3 New telecommunication technology and services require the transfer of additional signaling information for processing a call In Chapter 4, we have already encountered some additional information items (echo suppressor information, calling line category, etc.) In CCITI’ No.5 and CCITT-R2 signaling, this information is in the form of digits with special meanings (see Section 4.3.3 and Tables 4.4-4 and 4.4-5) Common- channel signaling messages provide a more flexible way to transfer both

109

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)

the classical supervision and address signals, and other types of call- control information

4 Subscribers cannot access the CCS signaling links This avoids the “blue- box” fraud problems that have plagued many frequency-division

with in-band signal frequencies (Section 4.4.6)

5 In channel-associated signaling, the signals on a trunk necessarily relate to that trunk, and are used for call control In CCS the messages can-but do not have to be-related to individual trunks Call control on trunks was the original application of CCS signaling, and still is the predominant one However, CCS signaling links have now become a common transport facility for call control and other applications (see Sections 2.2.3-2.2.5) The first-generation common-channel signaling system, known as signaling

system No.6 (SS6), was introduced in the 1970s It exists in two versions

been deployed in the U.S network, but is now being replaced An international version (CCITT No.6) is still used in the international network

SS6 was followed, about 10 years later, by signaling system No 7 (SS7) This system also exists in several versions The version specified by CCITT is in service on a number of trunk groups in the international network and, with country-specific modifications, in a number of national networks A version defined by the American National Standards Institute and Bellcore is in operation in the U.S

This chapter introduces a number of basic CCS concepts, setting the stage for more detailed discussions on individual CCS systems in later chapters

Telecommunication networks that employ CCS signaling require, in addition to the network of trunks and exchanges, asignaling network This network consists

few definitions

Signaling Point (SP) A signaling point is an entity in the network to which CCS signaling links are attached For example, an exchange that serves CCS trunk groups has CCS signaling links, and is therefore a signaling point Likewise, a network database that is accessed via CCS signaling links is a signaling point

Signaling Link (SL) A signaling link is a bidirectional transport facility for CCS signaling messages between two signaling points

SIGNALING NETWORKS 111

Figure 5.1-l Signaling routes for relation (A, B) (a): associated signaling (b): quasi-

0 a

associated signaling

Signaling Relation A signaling relation exists between any pair of signaling points that need to communicate by CCS signaling For example, when two exchanges, say A and B, are interconnected by a group of CCS trunks, there is a signaling relation between these signaling points In what follows we denote a signaling relation between points A and B by (A,B)

Signaling Route A signaling route is a predetermined path for the CCS messages of a particular relation Usually, there is a signaling route set, consist- ing of several routes, for each signaling relation

Associated and Quasi-associated Signaling When messages for relation (A,B) are carried on a signaling route that consists of a direct signaling link (SL) between A and B, we speak of associated signaling-Fig 5.1-l(a) When

A and B have a signaling relation, but are not directly interconnected by a SL,

a signaling route for relation (A,B) consists of two or more SLs in tandem- Fig 5.1- 1 (b) This signaling mode is called quasi-associated signaling

Types of Signaling Points In Fig 5.1-l(a) and 5.1-l(b), signaling points A and B, which originate and receive (and process) signaling messages for relation (A,B), are known as the signaling endpoints (SEP) for that relation In Fig 5.1-1(b), signaling point C transfers messages for relation (A,B), but does not originate or process messages for that relation We say that signaling point

C is a signal transferpoint (STP) for signaling relation (A,B)

0 cl El

0 E

Figure 5.1-2 Signaling points SEP: signaling end point STP: signal transfer point STEP: combined end/transfer point E: signaling point and exchange

Some signaling networks include signaling points that function as end points for some relations, and as transfer points for other relations These dual purpose signaling points are called signal transfer and end points (STEP)

In documents on signaling networks, the various types of signaling points are usually shown as in Fig 5.1-2 The letter “E” is used in this section to indicate signaling points (SEP or STEP) that are exchanges with CCS trunks

5.1 l Basic Signaling Networks

As a start, we explore some alternative signaling networks to handle CCS signaling between exchanges A, B, and C that are interconnected by CCS trunk groups TG1, TG2, and TG3 -see Fig 5.1-3 The signaling network therefore requires routes for relations (A,B), (B,C), and (CA)

Associated Network Figure 5.1-3(a) shows a network in which each route consists of one signaling link that is associated with one signaling relation For example, route SL, is associated with signaling relation (A,B), and transports the signaling messages for the trunks in trunk group TG, only All exchanges in the figure are signaling end points

Signaling links in associated operation are often poorly used, because a link

TGl

Figure 5.1-3 Signaling networks (a) for associated operation and (b) for quasi-associated

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operation

SIGNALING NETWORKS 113

can handle the signaling messages for several thousand trunks, while most trunk groups consist of fewer than 100 trunks

Quasi-associated Network Figure 5.1-3(b) shows a configuration for quasi-associated operation of the signaling links None of the exchanges are directly connected by a signaling link Instead, each exchange has a link to signal transfer point S Each link carries messages for several relations All signaling routes are indirect, traversing two links in tandem, and passing through signal transfer point S All exchanges are again signaling end points The structures of associated and quasi-associated signaling networks resemble respectively the mesh- and star-configurations of trunk groups in telecommunications networks

In the networks of Fig 5.1-3 there is one signaling route for each relation A failure of a signaling link disables the signaling route(s) for which it carries CCS messages, and this severely affects the service in a telecommunications network For example, a failure of SL1 in Fig 5.1-3(a) stops all signaling for relation (A,B), and thus shuts down all trunks in trunk group TG1 Also, on failure of SL,

in Fig 5.1-3(b), the trunks in groups TGI and TG2 are disabled, and signaling point A becomes isolated

Actual signaling networks are therefore designed with redundancy, such that signaling for all relations remains possible when a link failure occurs

Redundancy can be obtained in several ways For example, the signaling routes in the associated signaling network of Fig 5.1-3(a) can be replaced by route sets containing two direct routes each (see Fig 5.1-4) Then, if say SL, fails, signaling for relation (A,B) is still possible, using SL,

In the quasi-associated configuration of Fig 5.1-3(b), redundancy can be obtained by a adding a second signal transfer point with links to each signaling end point This creates a network in which the route sets for each signaling relation consist of two quasi-associated routes (Fig 5.1-5) Under normal

Figure 5.7 -4

signaling)

Signalin g network with two routes for each signal ,ing relation (associated

Figure 5.1-5 Signaling network with two routes for each signaling relation (quasi-associated signaling)

conditions, the signaling traffic for a trunk group is divided across the signaling routes in a route set For example, signaling for the odd numbered trunks in TGI

is on route SL,-Sb, and signaling for the even numbered trunks is on route SL,- SL, When SL, fails, route SL,-SL, is disabled, and all signaling traffic for TG, is carried by the other route

A third alternative is shown in Fig 5.1-6 This arrangement differs from Fig 5.1-3(a) in that exchanges A, B and C are now signaling transfer and end points

(STEP) Under normal conditions, all signaling is associated However, when for example SL, fails, the messages related to trunks of group TG, are sent via

C, which the.n acts as the STP for the signaling traffic of relation (A,B)

We conclude this section by exploring two well known signaling networks

Figure Signaling network with combined signal transfer and end points

SIGNALING NETWORKS 115

Figure 5.1-7 Bell System signaling network * Signaling links to other regions (From HE Comm Mag 28.7 Copyright 0 1990 IEEE.)

51.3 The Bell System Signaling Network

Figure 5.1-7 shows part of the quasi-associated signaling network deployed by the Bell System for CCIS signaling [2,3] The basic structure has been retained for SS7 signaling in AT&T’s present long-distance network [4]

The territory of the U.S is divided into a number of regions, and each region

is equipped with a pair of STPs (only two regions are shown in Fig 5.1-7) Each exchange with CCS trunks is a signaling end point, and has anA-link (SLA) to the two STPs in its region The B-links (SLB) interconnect STPs of different regions, and the C-links (SLC) interconnect STP pairs of individual regions The STPs in the network are thus interconnected by a complete mesh of signaling links The C-links normally do not carry signaling traffic, and are used only under certain failure conditions

In normal operation, the signaling network has two signaling routes R for relations between two SEPs located in the same region For example, the route set for relation (X,Y) consists of:

R(XY)*: SLA,-SLA, Also, in normal operation, the signaling route set for a relation between SEPs located in different regions consists of four routes For example, the route set for relation (Y,Z) consists of:

R(Y,Z),: SLA,-SLB,-SLA, R(Y,Z),: SLA,-SLB,-SLA, R(Y,Z),: SLA,-SLB,-SLA,

In this arrangement, the routes of a route set again share the message load for a signaling relation For example, trunk group TG2 can be divided (for signaling purposes only) into four subgroups When all routes for relation (Y,Z) are operational, the message traffic for a particular subgroup is carried

by one of the four routes When a route is disabled, the signaling traffic for the affected subgroup is diverted to one of the remaining routes

Simultaneous failures of signaling links in a route set can disable all normal signaling routes for a relation In Fig 5.1-7, simultaneous failures of SLA, and SLA4 disable both signaling routes for (X,Y) In this case, signaling is maintained by using the route SLA2-SLC-SLA3, which does not belong to the normal route set for (X,Y)

5.1.4 Mixed Signaling Networks

Some networks use associated signaling for some relations, and quasi-associated signaling for the others relations Figure 5.1-8 shows an example that is often used between international exchanges (ISC) in two countries Exchanges X and

Y in country A are connected to exchanges Z and U in country B by international CCS trunk groups TG,, ,TG,, and by two international signaling links (SL,&) Each pair of in-country exchanges is also interconnected a by national signaling link (Sb,SLJ All exchanges are combined signal transfer and end points The normal (non-failure) signaling routes can be assigned as follows:

R(X,Z): SL,

R(Y,U): Sr,

R(X,U): S&-SL,

R(Y,Z): Sb-SL,

(Associated signaling) (Associated signaling) (Non-associated signaling) (Non-associated signaling)

In this example, there is only one normal route for each signaling relation However, signaling for all relations can be maintained on failure of a signaling

Figure 5.1-8 Mixed-mode signaling network

117

link, by using routes that are not used for signaling relations under normal conditions For example, when SL, fails, the message traffic for relation (X,Z)

can be diverted to the route Sb-Sb-SL,, and the traffic for relation (Y,Z) can

be diverted to the route Sb-SL,

This section introduces some fundamental aspects of CCS signaling links and signal units The signaling links are described using the hardware-oriented terms

of the literature on SS6 [3,4,5] The characteristics of the signaling links in SS7 are quite similar, but are described in a more abstract manner (see Chapter 8)

Figure 5.2-l shows a signaling link between signaling points A and B It consists

transfers digital data The primary function of the signaling link is to provide a reliable transfer of signaling messages between processors P, and P,

In a signaling point, there are four interfaces between the processor and a signaling terminal The processor enters its outgoing messages M0 into-and retrieves its incoming messages MI from- the ST In addition, the processor can send commands (COM) to the ST-for example, to activate or deactivate the link A ST sends indications (IND) to alert the processor about certain conditions on the link (excessive errors in received messages, overload, etc.)

Information is transferred across the signaling data link in signal units (SU) (groups of consecutive bits) We distinguish two SU types-see Fig 5.2-2

information originated by the ST at one end of the signaling link, and intended for the ST at the other end

A processor message is an ordered set of digitally coded parameters

PA

MO

COM

e

Transmission Channels (Signaling Data Link) Figure 5.2-l Signaling link

Signaling Point A Signaling Point B

O-9

-m w

\

0 a L -) \

(W

L - - - )

@I

0 a ,+ -

09 / ,* -‘/

N -d

0 C

-w

0 C

* -

Figure 5.2-2 Signal units and processor messages (a): message signal unit (b): processor message (c): link signal unit

(information elements) The initial parameters identify the message type, and imply the meanings and locations of the later parameters in the message The length (number of bits) of SUs is fixed in SS6, and variable in SS7 The contents of SUs are shown in Fig 5.2-3 All SUs have an information field (INF) and a check-bit field (CB) In SS6, the INF fields of message SUs holds message parameters only Most messages fit in one message SU, but some messages require two or more SUs In SS7, the INF field of a message SU holds all parameters of a message, and several link parameters

In SS6 and SS7, the INF fields of link SUs contain link parameters only

Message

Parameter

Fields

Check Bits

0 a

CB 1 Check Bits 1

Figure 5.2-3 Contents of signal units (a): SS6 message signal unit (b): SS7 message signal unit (c): SS6 and SS7 link signal unit