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A MTP-user passes its outgoing messages to-and receives its incoming messages from- the MTP3 at its signaling point.. Message signal units MSU are messages between the MTP2 and MTPl of a

7

INTRODUCTION TO

CCITI’ began the specification of the second-generation common-channel

signaling system, known as signaling system No.7 (SS7), in the mid-1970s The system was developed initially for telephony call control, and the first recommendations were published in the CCITT Yellow Books of 1981 During the past decade, the applications of SS7 applications have expanded, and now also include call control in the integrated services digital network (ISDN), and operations in the network that aeennot related to individual trunks Most of this work has been published in the CCITT Blue Books of 1989 More recent additions have been issued as individual International Telecommunications

Union (ITU) Recommendations

CCITT No.6 and CCIS were used only in the international network, and in the AT&T long-distance network The objective for SS7 was to develop a signaling system that can be used worldwide However, this objective has not been met completely Several national versions have been defined, notably a version specified by American National Standards Institute (ANSI) and Bellcore (Bell Communications Research) for the U.S networks, and British Telecom’s national userpart (NUP), for the network in the U.K

In what follows, we speak of SS7 when discussing the general aspects of the system that essentially apply to all versions, and refer to the CCITT-defined

system as CCITTNo 7, and to the North American version as ANSI No 7

SS7 Signaling Lidcs The signaling links are carried on the digital transmission channels that were developed in the 1960s for digital (PCM) trunks

In the U.S., these signaling links generally transmit at 56 kb/s In most other countries the transmission is at 64 kb/s [l]

150

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

These bit rates greatly exceed the 2400-4800 bits/s rate of SS6 signaling links SS7 call-control messages usually include more information than their counterparts in SS6, and support new telecommunications services SS7 labels are larger than those in SS6, and contain information that has a network-wide scope SS7 messages can therefore be routed to their destinations without the band-number translations that are necessary in SS6 (Section 6.2)

7.1 SS7 STRUCTURE

In Chapter 6, SS6 signaling has been described in terms of a processor at a signaling point that performs call-control and other functions, andsignaling links

that allow the processor to communicate with processors at other signaling points SS7 is defined in terms of messages and functions; hardware architecture issues are left to tee equipment manufacturers The signaling system is divided into a number ofprotocols (or parts), each of which handles a group of related functions The interfaces between these parts were defined in the early stages of the specification work The various parts could then be specified simultaneously, and independently of each other This has made the overall specification effort more manageable

7.1 l SS7 Hierarchy

The parts of SS7 are organized in a four-level hierarchy [l-4] We say that a higher-level part is a user of services provided by a lower-level part This arrangement is similar to the seven-layer structure of the open systems interconnection (OSI) protocols for data communications that have been specified by the International Standards Organization (ISO)

Several efforts have been made to align the SS7 levels and OS1 layers However, this has been only partially successful OS1 layers 1 and 2 correspond with SS7 levels 1 and 2, but things start to differ in the higher parts of the hierarchies In what follows, the four-level SS7 hierarchy will be used It works well for trunk-related applications, and less well for other applications

7.1.2 SS7 Protocols

The protocols of SS7, and their levels, are shown in Fig 7.1-l

Message Transfer Part (MTP) This provides message transfer services for its

users It is divided into three parts- denoted as MTPl, MTP2, and MTP3-that occupy levels 1,2 and 3 of the SS7 hierarchy

A MTP-user passes its outgoing messages to-and receives its incoming messages from- the MTP3 at its signaling point A signaling point has one MTP3

A combination of a MTP2 and a MTPl represents a signaling link at a

OS1

Layers

L&3

TC-Users

I I

TCAP

MTP3 (Level 3)

t

MTP3 Messages +

MTP2 (Level 2)

,

Points

signaling point (a signaling link between two signaling points consists of a MTPl/MTP2 combination each point) A signaling point that terminates n

signaling links has n of these combinations

Telephone User Part (TUP) (a MTP user) is a protocol for telephony call control, and for trunk maintenance It is very similar to SS6, but includes a number of additional features

Integrated Services User Part (ISUP) ( another MTP user) is a protocol for call-control and trunk-maintenance procedures in both the telephone network and the ISDN

TUP and ISUP messages are trunk-related: they contain information concerning a particular trunk with TUP or ISUP signaling, and are sent by the exchange at on end of the trunk to the exchange at the other end

Signaling Connection Control Part (SCCP) This protocol (a MTP user) provides functions for the transfer of messages that are not trunk-related Its users are ISUP and TCAP SCCP does not fit neatly into the four-level hierarchy, because it is at the same level as ISUP

The combination of MTP and SCCP corresponds to OS1 layers 1, 2, and 3, and is known as the network sewices part of SS7

Transaction Capabilities Application Part (TCAP) Transactions are operations that are not related to individual trunks, and involve two signaling points (2.2.4) The TCAP protocol provides a standard interface to TC-users

(functions at a signaling point that are involved in transactions of various kinds)

In turn, TCAP is a user of SCCP

In Fig 7.1-1 we disinguish messages of several types in a signaling point User

messages are messages between a level-4 protocol and the MTP3, and are named after the level-4 protocol: we speak of TUP, ISUP and SCCP messages 1MTp3

messages are messages between the MTP3 and a MTP2 Message signal units

(MSU) are messages between the MTP2 and MTPl of a signaling link at a signaling point, and between the MTPls at both ends of a signaling link A MSU contains a message originated by MTP3, or by a MTP3 user

User messages are transferred between two “peer” level-4 protocols at two signaling points Figure 7.1-2 illustrates the transfer of a TUP message from TUP-A to TUP-C (at signaling points A and C, respectively) that is routed via signal transfer point B

At signaling point A, TUP-A passes the TUP message downwards to its MTP3, which expands it into a MTP3 message, and passes it to the MTP2 of the signaling link to B MTP2 expands the MTP3 message into a message signal unit (MSU) and passes it to its MTPl

The MSU traverses the signaling link, and arrives at the MTPl of signaling point B, where MTP2 extracts the MTP3 message, and passes it to its MTP3 MTP3 transfers the MTP3 message to the MTP2 of the signaling link to C The second leg of the message transfer is similar: is passed and expanded downwards in signaling point B, traverses the signaling link between B and C (as

a MSU), and is passed upwards in signaling point C It finally arrives as a TUP message at TUP-C

I

# (1)

MTP3

I

f (3) ,

,

(3)

l l

MTP3

MTP2 1 1 MTP2

(3)

C (Exchange)

TUP-c (Of MTP3 (2)t

MTP2 (Vf

Figure 7.1-2 Transfer of aTUP message (1); TUP message (2): MTP3 message (3): message signal unit

7.2 IDENTIFICATION OF SIGNALING POINTS AND TRUNKS

Signaling system No.7 identifies signaling points (exchanges, service

points, etc.) and trunks with parameters that have a nationwide scope

control

7.2.1 Point Codes

Each SS7 signaling point in a network is identified by a point code (PC) Most signaling points have only one (national) point code However, an international switching center (ISC) is identified in the network of its country by a national PC, and in the international network by an international PC

7.2.2 ANSI No.7 Point Codes

The format of ANSI No.7 point codes (used in the U.S only) are shown in Fig 7.2-l(a) The PC has three eight-bit fields that contain the parameters N, C, and

M [l] Parameter N identifies the network of a particular telecom in the U.S (Ameritech, Sprint, AT&T, etc.) Parameter C represents a cluster of signaling points within a network For example, a particular value of C may identify the exchanges with signaling links to a particular pair of STPs Parameter M identifies a signaling point within a cluster The ANSI No.7 point codes are assigned by Bellcore

7.2.3 International Point Codes

Point codes for the international network are assigned by CCITT [5], and consist of parameters Z and V (3 bits each), and U (8 bits)-see Fig 7.2-l(b)

Z identifies six major geographical world zones In decimal representation:

2 Europe

3 North America

4 Mid-east and most of Asia

5 Australia and part of Asia (Indonesia, Malaysia, Thailand, Guam, etc)

6 Africa

7 Latin America

0 a

q

@I

Figure 7.2-l Point code formats (a): ANSI No 7 (b): CCIlT No 7 (From Rec Q.708 Courtesy of ITU-T.)

These world zones are different from the world zones represented by the initial digits in country codes (1.2.3)

Parameter U identifies an area of a national telecommunication network within a world zone Most countries have only one network that covers the entire country, and are therefore represented by one value of U For example, the combination 2 = 2, U = 168 represents the national network of Bulgaria

V identifies a particular international switching center in the network-or network area-specified by Z and U In countries with one national network, up

to seven ISCs can be identified

Countries with more than one national network have several U codes For example, the U.K national networks operated by British Telecom and Mercury Telecommunications are identified by Z = 2, U = 068, and Z = 2, U = 072

In the U.S., values of U are assigned to particular areas of the long-distance networks operated by the vatious international carriers (AT&T, MCI, Sprint, etc.) For example, a value of U identifies the east-coast area of AT&T’s long- distance network, and V identifies an ISC in that area of the network

CCITT has allocated the range U = 020-059 in zone Z = 3 to the U.S

7.2.4 National Point Codes in Other Countries

Countries other than the U.S use 14.bit national point codes The code assignments are made by the individual national telecoms

7.2.5 Identification of Trunks

A trunk group with TUP or ISUP signaling is identified uniquely in a national network (or in the international network) by the point codes of the exchanges that are interconnected by it

The circuit identification code (CIC) identifies a trunk within a trunk group The CIC field has a length of 12 bits, and thus can identify trunks in groups of up

to 4095 trunks

7.3 SS7 SIGNAL UNITS AND PRIMITIVES

SS7 signal units (SU) have different lengths, but always occupy an integral number of octets (groupings of eight bits) A SU consists of an ordered set of fields with parameters In the documentation of the early SS7 parts (MTP and TUP), the SUs are shown as in Fig 7.3-l(a) Par.l, Par.2, etc denote the first, second, etc parameter fields of the SU In the more recently defined parts of SS7, a SU is shown as a stack of octets-see Fig 7.3-l(b) Bits in the octets are numbered from right to left, and bit 1 of octet 1 is the first bit sent out For uniformity, this representation is used throughout this book

The lengths of SS7 parameter fields are not limited to integral multiples of octets For example, Par.2 and Par.3 in Fig 7.3-l(b) have a length of 12 bits

A working signaling link carries a continuous SU stream in each direction

- Bits (_ LF

I 2

I

I

I I

0 a

@I

7.3.1 Signal Unit Types

In addition to the message signal unit, SS7 includes two other SU types [6] The

link status signal units (LSSU) and fill-in signal units (FISU) originate in the MTP2 at one end of the signaling link, and are processed by the MTP2 at the other end (they are the SS7 equivalents of the “link” SUs described in Section 5.2) LSSUs are used for the control of a signaling link, and FISUs are sent when

no MSUs or LSSUs are waiting to be sent out

Figure 7.3-2 shows a MSU in some detail It consists of a MTP3 message, surrounded by MTP2 data Length indicator (LI) has a dual role In the first place, it indicates number of octets, measured from octet 4 through octet (n - 2)

In addition, the value of LI implies the SU type In MSUs, LI exceeds 2, LSSUs have LI = 1 or 2, and FISUs have LI = 0

The MTP2 data are added by the MTP2 of the signaling point where the message originates, and are processed, and then removed, by the MTP2 at the signaling point on the other end of the signaling link

The A4TP3 message consists of the service information octet (SIO) and the

signaling information field (SIF) The maximum length of SIF is 272 octets SIO identifies the MTP-user (TUP, ISUP, or SCCP) that has originated the message, and is used by the MTP3 in the destination signaling point to deliver the message to the “peer” user

- Bits

MTP3

Signal Unit

The SIF consists of the routing label (RL) and the user message (UM) The routing label contains parameters that are used by the MTP3s in the signaling points along the message path to route the MSU to its destination signaling point The user message contains information for the MTP-user at the desti- nation signaling point

The user message, and the information in SIO and RL, are supplied by the originating MTP-user The parameters in SIO and RL are interface parameters, and are used by the MTP3s, and by the MTP-user at the destination signaling point The user message is passed transparently (not examined by MTP3)

7.3.2 Primitives

In the CCITT model of SS7, the messages between protocols in a signaling point are passed in standardized interface elements calledprimitives [7] There are four types of primitives: requests and responses pass information from

a higher level protocol to a lower level protocol, and indications and confir- mations pass information in the opposite direction

Primitives between two protocols are named after the lower level protocol For example, the primitives for the message transfer between MTP3 and the MTP-users are known as MTP-transfer primitives

CCITT has defined the information to be included in each primitive Since primitives pass information inside a signaling point only, their (software) implementation is left to the equipment manufacturers

Figure 7.3-3 shows the message-transfer primitives between MTP3 and a MTP-user at a signaling point On outgoing messages, the user passes a MTP- transfer request that includes the user message, and the parameters in SIO and

MTP-user (TUP, ISUP, SCCP)

MTP-transfer Request

l User Message

0 Parameters

in SIO and RL

v

MTP3

MTP-transfer Indication

l User Message

0 Parameters

in SIO and RL,

Figure 7.3-3 Message transfer primitives (From Rec Q.701 Courtesy of ITU-T.)

RL On incoming messages, MTP3 delivers these data to the MTP-user, in a MTP-transfer indication that contains the same information

ANSI

CB

CCITT

CIC

F

FISU

ISC

IS0

ISDN

ISUP

ITU

kb/s

LI

LSSU

MSU

MTP

OS1

PC

PCM

RL

SCCP

SIF

SIO

American National Standards Institute

Check bits

International Telegraph and Telephone Consultative Committee Circuit identification code

Flag

Fill-in signal unit

International switching center

International Standards Organization

Integrated Services Digital Network

ISDN user part

International Telecommunication Union

Kilobits per second

Length indicator

Link status signal unit

Message signal unit

Message transfer part

Open systems interconnection

Point code

Pulse code modulation

Routing label

Signaling connection control part

Signaling information field

Service information octet

SL Signaling link

SS6 Signaling system No.6

ss7 Signaling system No.7

STP Signal transfer point

TCAP Transaction capabilities application part

1 A.R Modaressi, R.A Skoog, “Signaling System No.7: A Tutorial,” IEEE Comm Afag., 28, No.7, July 1990

2 R Manterfield, Common Channel Signalling, Peter Peregrinus Ltd, London, 1991

3 KG Fretten, C.G Davies, “CCITT Signalling System No.7: Overview,” Br Telecomm Eng., 7, April 1988

4 Specifications of Signalling System No 7, Rec Q.700, CCITT Blue Book, VI.7, ITU, Geneva, 1989

5 Ibid., Rec Q.708

6 Ibid., Rec Q.703

7 Ibid., Rec Q.701