A common channel signalling system, optimized for digital networks, it allows direct transfer of call information transfer between exchange processors.. The user parts of the system that
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12.1
The ITU-T signalling system number 7, SS number 7, SS7, CCITT7, C7 or number seven signalling system is the most recently developed of telephone network signalling systems It is already widely deployed in digital telephone networks and ISDNs across the world, and will also
be a ‘cornerstone’ of ‘intelligent networks’ and broadband ISDNs (B-ISDN) It is a complex, common channel signalling system, which enables the controlling processors of two digital exchanges or databases to communicate directly and interact with one another in a manner
optimized for digital transmission media SS7 has also formed the basis of a number of further- developed regional signalling systems In the United States, for example, ‘ANSI SS7’ is a
derivative, while the UK national version is ‘C7/BT’ This chapter describes the overall structure and capabilities of SS7
The SS7 signalling system is described in the 4.700 series of ITU-T recommendations
A common channel signalling system, optimized for digital networks, it allows direct transfer of call information transfer between exchange processors Comprising a number of layered and modular parts, each with a different function, it is a powerful general-purpose signalling system capable of supporting a range of applications and administrative functions, including
e ISDN (integrated services digital network)
e intelligent networks ( I N S )
e mobile services (e.g cellular radio)
e network administration, operation and management
249
Networks and Telecommunications: Design and Operation, Second Edition.
Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
Trang 2250 SIGNALLING SYSTEM NO 7
In addition, its modular nature lends itself to the development of new user parts which
may be designed to support almost any new service that can be conceived The user parts of the system that have been developed so far are
0 MTP
0 SCCP
0 TUP
0 D U P
0 ISUP
0 T C
0 TCAP
0 OMAP
0 INAP
0 MAP
message transfer part signalling connection and control part telephone user part
data user part ISDN services user part
transaction capabilities (used by intelligent networks)
transaction capabilities application part operation and maintenance application part intelligent network application part mobile application part
The MTP and SCCP form the ‘foundations’ of the system, providing for carriage of messages The TUP, DUP and ISUP use the MTP and/or SCCP to convey messages relating to call control, for telephone, data, and ISDN networks, respectively The OMAP, MAP and INAP are other application parts for operation and maintenance interaction, mobile network control and intelligent network services, respectively Initially the SS7 system was designed so that the MTP could be used in association
with any or all of the telephone, data and ISDN user parts However, following the
emergence of the OS1 model, the SCCP was developed as an adjunct to the MTP; the two in combination provide the functions of the OS1 network service (layers 1-3)
SS7 signalling can be installed between two exchanges, provided that the necessary signalling functions are available in both exchanges The functions reside in a unit
termed a signalling point This may be a separate piece of hardware to the exchange, but usually it is a software function in the exchange central processor SS7 signalling points
(SPs), basically exchanges, intercommunicate via signalling links and are said to share a signalling relation
A single SS7 signalling link enables information to be passed directly between two exchange processors, allowing the set-up, control, and release of not just one, but a large number of traffic-carrying circuits between the exchanges Messages over the unit take the form ‘connect circuit number 37 to the called customer number 01-234 5678’ The term common channel signalling aptly describes this method of operation, distinguishing it from the channel-associated signalling method, wherein call set-up
signals pertinent to a particular circuit are sent down that circuit SS7 is not the first common channel signalling system to be developed; CCITT 6 (SS6) was also a common
channel system, but CCITT 6 was less flexible than SS7 and not so suitable for digital network use
Having a common channel for conveyance of signalling messages saves equipment at
both exchanges, because only one ‘sender’ and one ‘receiver’ is required at each end of the link, as against the one per circuit required with channel-associated systems The
Trang 3SS7 SIGNALLING NETWORKS 251
ST = signalling terminal
Figure 12.1 Linking two exchanges using SS7 signalling
combination of a SS7 sender and receiver is normally referred to as a signalling
terminal In practice signalling terminals are a combination of a software function in the exchange central processor and some hardware to terminate the line and undertake the basic bit transfer function (OS1 layer 2, datalink)
A label attached to each message as it passes over the signalling link enables the
receiving signalling point to know which of the many circuits it relates to Figure 12.1 illustrates the network configuration of a simple SS7 signalling link It shows calls
flowing over a large number of traffic-carrying circuits which are connected to the switch matrix part of the exchange Meanwhile all these circuits are controlled according to the information passed directly between the exchange processors The
signalling terminal ( S T ) function is shown residing within the exchange processor
Networks employing SS7 signalling comprise two separate subnetworks One subnet-
work is the network of traffic-carrying circuits interconnecting the exchanges The
second subnetwork is that of the signalling links In Figure 12.1 we saw this separation
of traffic-carrying circuits from signalling link as it would apply on a single connection between two exchanges Figure 12.2 now shows a more complicated example to illustrate another powerful feature of SS7: the fact that signalling networks and traffic- carrying networks may be designed and implemented almost in isolation from one
another Just because there are direct traffic-carrying circuits between two exchanges (they have a direct trafic-carrying relation) it does not follow that the signalling information (or signalling trafJic) has to travel over direct signalling links, though
clearly a signalling relation of some sort is needed
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U
I
r I H ,
I ExchDange I
U
Signalling links
'm T r a f f i c - c a r r y i n g c i r c u i t s
Figure 12.2 Traffic-carrying and signalling networks in SS7
Figure 12.2 shows the traffic-carrying networks and signalling networks inter-
connecting four exchanges, A, B, C and D The traffic circuits directly connect A-C,
A-B, B-C and B-D All traffic to or from exchange D passes via exchange B and all
traffic to or from exchange A passes either via B or C, and so on The signalling
network, however, is different Signalling links only exist between A-B, B-C and B-D,
so that signalling trafic has to be routed differently from the actual traffic In the case of
the actual traffic from A to B, there exist both direct traffic circuits and a direct
signalling link In effect, this is the same as Figure 12.1, so that both signalling messages
and traffic can be passed directly between the two Similarly exchange B may pass
signalling messages and traffic directly either to exchange C or exchange D, and may
also act as a normal transit exchange for two-link routing of traffic from exchange A to
either of exchanges C or D These are all examples of associated mode signalling, in
which signalling links and traffic circuits have a similar configuration, and signalling
messages and traffic both route in the same manner In short, there is a signalling link
associated with each link of direct traffic-carrying circuits
By contrast, although exchange A is directly connected to exchange C by traffic-
carrying circuits, there is no direct signalling link Signalling information for these
circuits must be passed on another route via exchange B This is known as the quasi-
associated mode of signalling, and the signalling point (SP) in exchange B is said in
this instance to perform the function of a signal transfer point ( S T P ) , as illustrated in
Figure 1 2.3
Trang 5THE STRUCTURE OF SS7 SIGNALLING 253
Exchange
n
Exchange Exchange
/ / / / U
Associated mode Ouosi - associated mode
slgnalling link SP = signalling point
.m traffic- carrying circuits STP = signal transfer point
Figure 12.3 Modes of SS7 signalling
Signalling information is passed over SS7 signalling links in short bursts; indeed a SS7 signalling network is like a powerful packet-switched data network To identify each of the signalling points for the purpose of signalling message delivery around the network, each is assigned a numerical identifier, called a signalling point code ( S P C )
This code enables an SP to determine whether received messages are intended for it, or
whether they are to be transferred (in STP mode) to another SP The codes are allocated
on a network by network basis Thus the code is only unique within, say, national network A, national network B or the international network
12.3 THE STRUCTURE OF SS7 SIGNALLING
Thanks to the modular manner in which the SS7 system has been designed, it
encourages the development of new modules in support of future telecommunications services and functions Figure 12.4 illustrates the functional structure of the SS7 system,
relative to the layers of the Open Systems Interconnection (OSI) model (see Chapter 9)
In the same way as the OS1 model has a number of functional layers, each an
important foundation for the layers above it, so SS7 signalling is designed in a number
of functional levels Note in Figure 12.4, that the component levels and parts of SS7 do
not align with the OS1 layered model The lack of alignment of signalling levels with OSZ layers is unfortunate and it arises from the fact that the two models were developed concurrently but for different purposes The lack of alignment of levels with layers
means that not all higher layer OS1 protocols are currently suitable for use in
conjunction with the lower levels of SS7 signalling The various standards development bodies are trying to rationalize the component parts of SS7 to conform with the OS1
model The signalling connection and controlpart (SCCP), for example, delivers the OS1
network service (OS1 layer 3 service), so that a communication system can use the SCCP (and MTP below it) to support layers 4-7 OSI-based protocols The levels in SS7
signalling provide a convenient separation of signalling functions, and in the remainder
of the chapter the signalling level model is used in explanation
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OS1 layer Application
[l
7
6
5
L
3 I SCCP Ilj DUP
-
MTP
Message transfer over signalling
n e t w o r k over single link
data link
Signalling level
L User level
Network level Link
* level
, Oatalink level
Figure 12.4 The structure of SS7 signalling ASE, Application service element; TCAP, Trans-
action capability; ISP, Intermediate service part; ISUP, ISDN services user part; TUP, Telephone user part; DUP, Data user part; SCCP, Signalling connection and control part; MTP, Message transfer part
The foundation level of the SS7 signalling system is the message transfer part defined by ITU-T Recommendations Q.701-4.707 The message transfer part takes care of the conveyance of messages, fulfilling signalling level functions 1 to 3 (sometimes labelled
M TP l, MTP2, MTP3) as follows
The first level defines the physical, electrical and functional requirements of the signal- ling data link The level one function is attuned to the particular transmission medium
as laid down by ITU-T G series recommendations The level 1 function allows for an unstructured bit stream to be passed between SPs over an isolated signalling data link
Level 2 (signalling link junctions)
This level defines the functions and procedures relating to the structure and transfer of a signal Message flow control, and error detection and correction are included (Flow control prevents the over-spill and consequent loss of messages that result if a message
is sent when the receiving end was not ready to receive it; error detection and correction procedures eliminate message errors introduced on the link.)
Trang 7THE MESSAGE TRANSFER PART 255
Level 3 (signalling network functions)
This level defines the functions and procedures for conveying signalling messages
around an entire signalling network It provides for the routing of messages around the
signalling network In this role it has a number of ‘signalling network management’
capabilities including ‘load sharing’ of signalling traffic between different signalling
links and routes (illustrated in Figure 12.5) and re-routing around signalling link
failures Link sharing on the same route between signalling points (SPs) guards against
lineplant failure (Figure 12.5(a)) Route sharing may additionally provide protection
against failure of STPs Thus in Figure 12.5 the signalling traffic from SP A to SPs B
and C is shared over the two STPs, D and E In the event of a failure of any of the
routes shown, signalling messages could be re-routed
MTP is useless on its own for setting up telephone or other connections T o perform
these functions MTP needs to be used in association with one of the SS7 user parts
which are level 4 or user functions Examples are the telephone user part (TUP) and the
integrated services digital network user part (ISDN-UP or I S U P ) These define the
content and interpretation of the message, and they provide for connection control
The structure of an MTP message is shown in Figure 12.6 It comprises four parts,
transmitted in the following order
Flag
TheJag is the first pattern of bits sent This is an unmistakeable pattern to distinguish
the beginning of each message, and delimit it from the previous message It is
comparable to the synchronization (SYN) byte in data communications (Chapter 9)
The flag is followed by a number o f j e l d s of information, which together ensure the
correct message transfer These fields include: the message sequence numbers that keep
SP SP
A - B and A - C signalling messages evenly divided
t o route via both D and E
Figure 12.5 Load sharing over signalling A-B and A-C signalling messages evenly divided to
route via both D and E
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Next message
r
First bit transmitted
l
Message sequence Check
and ‘user part’
(message substance 1 F l a g
numbers, length Signalling information field
type information
(Inserted by appropriate
‘level I ’ ‘user part’)
Figure 12.6 CCITT 7 MTP message structure
the messages in the correct order on receipt, and allow lost messages to be resent; and
information about the type and length of the information held in the main ‘signalling
information field’; it might say which user part is in operation and record the length of
the message
Signalling information jield
This is the main information field or the ‘substance’ of the message The information
is inserted by one of the user parts, as appropriate for the particular application
(e.g telephone user part (TUP), or integrated services user part (ISUP)) The structure
of this$eld depends on which user part is in use
Check bits
Finally, each M TP message is concluded with a check bit field This is the data (cyclic
redundancy check code or C R C ) needed to perform the error detection and correction
mechanism of the M TP level 2 The check bits are followed by the flag at the start of the
next message
The various user parts of SS7 are alternative functions meeting the requirements of level
4 of the signalling level model The user parts may be used in isolation, or sometimes
may be used together Thus the telephone user part ( T U P ) and the MTP together are
sufficient to provide telephone signalling between exchanges The data user part (DUP),
ISDN user part (ISUP) and other user parts need not be built into a pure telephone
exchange An example where more than one user part is employed is the combination of
SCCP (signalling connection and control part), ISP (intermediate service part) and
TCAP (transaction capability application part) These are all necessary for the support
of the intelligent networks described in Chapter 11) The remainder of the chapter
describes the capabilities of each of the level 4 user parts of SS7
Trang 9THE TELEPHONE USER PART (TUP) 257
The telephone user part comprises all the signalling messages needed in a telephone network to set up telephone calls (we described the sequence of call set-up in Chapter 7) Thus an exchange using the SS7 signalling system carries out the normal process of digit
analysis and route selection, seizes the outgoing circuit and sends the dialled digit train
onto the next exchange in the connection by using the SS7 signalling link, conveying
TU P encoded messages using the MTP Crudely put, an example of a T U P message might be ‘connect the call on circuit number 56 to the destination directory number 071-234 5678’ Backward messages such as ‘destination busy’ are also included in the telephone user part
The structure of TUP messages is shown in Figure 12.7 T U P messages occupy the
signalling information Jield of the underlying MTP message The messages comprise a
T UP signalling information field which is used to convey ‘dialled digits’, ‘line busy’,
‘answer’ signals, and other circuit-related information, together with four adminis- trative fields as follows
Destination point code ( D P C )
This code identifies the signalling point to which the signalling message is to be delivered by the MTP (The destination of a signalling message is not necessarily the same as the final destination of the call.) The signalling point is in the exchange that forms the next link of the connection (for forwardmessages) or in the previous exchange
(for backward messages)
Originating point code ( O P C )
This code identifies the signalling point which originated the message (again not necessarily the origination point of the call)
Circuit identijication code (CIC)
This is a number that indicates to the exchange at the receiving end of the signalling link which traffic circuit each message relates to
The telephony user part is defined in ITU-T Recs Q.721-Q.725
TUP messoge
>
TUP signalling information
CIC= Circuit identificatlon (others as SCCP fields)
\
0
/
information field MTP message
Figure 12.7 TUP message structure and relation to MTP
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12.7 THE DATA USER PART (DUP)
The Data user part is similar to the telephone user part, but it is optimized for use on circuit-switched data networks The message structure of the DUP is very similar to that of the TUP, illustrated in Figure 12.7 The D U P is defined by ITU-T recommenda- tions 4.741 but was hardly ever used It has been largely superseded by the ISUP
12.8 THE INTEGRATED SERVICES USER PART (ISUP)
Used in conjunction with the MTP, the SS7 integrated services digital network user part,
ISDN-UP or ISUP, is the signalling system designed for use in ISDNs In effect it is a
combination of capabilities similar to TUP and DUP, which allow voice and data switched services to be integrated within a single network The message structure is similar to that of TUP and DUP, but the messages used are incompatible with both of the other systems ISUP is defined by CCITT Rec Q.761-Q.764
The ISDN user part (ISUP) interacts as necessary with the ISDN D-channel, signalling ( D S S I , digital subscriber signalling 1, as defined by recommendation Q.931)
to convey end-to-end information between ISDN user terminals Such information
includes the terminal compatibility checking procedure which ensures that a compatible
receiving terminal is available at the location dialled by the caller As we learned in
Chapter 10, the procedure prevents, for example, the connection of a group 4 facsimile machine to a videoconference at the receiving end
The TUP+ is an enhanced version of the TUP, though incompatible with it It was developed by CEPT as recommendation TjSPS 43-02 for use as an interim ISDN-like signalling system supporting an early pan-European ISDN It is used in Europe by France Telecom for international ISDN signalling, but is likely to be superseded by ISUP
12.10 THE SIGNALLING CONNECTION CONTROL PART (SCCP)
The SCCP is used to convey non-circuit-related information between exchanges or databases, between an exchange and a database or between two exchanges (for certain
types of ISDN supplementary services) By non-circuit-related we mean that although a
signalling relation is established between an exchange and a database, no traffic circuit
is intended to be set up In essence the SCCP (in conjunction with the TC and relevant
application p a r t ) provides a means for querying a reference store of information, as is
necessary during call set-up on intelligent networks It is an ideal data transfer mechanism for