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12
Signalling
System
Noe
7
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.
SS7

SIGNALLING
BETWEEN EXCHANGES
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
(INS)
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)
250
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
DUP
0
ISUP
0
TC
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
SS7
SIGNALLING
NETWORKS
251
Exchange
A
I
I
Exchange
B
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
(ST)
function is shown residing within the exchange processor.
12.2
SS7
SIGNALLING
NETWORKS
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.
252
SIGNALLING
SYSTEM
NO.
7
U
I
r
I
H,
I
ExchDange
I
U
Signalling links

'm
Traffic-carrying circuits
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
(STP),
as illustrated in
Figure
1
2.3.
THE STRUCTURE
OF
SS7
SIGNALLING
253
Exchange
n
Exchange Exchange
SP
sp
/////////l
sp
SP
////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
(SPC).
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.
254
SIGNALLING
SYSTEM
NO.
7
OS1
layer Application
[l

7
6
5
L
3
I
SCCP
Ilj
DUP
-
MTP
Message transfer
over signalling
network
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
12.4
THE
MESSAGE TRANSFER PART (MTP)
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
MTPl, MTP2, MTP3) as follows.

Level
1
(datalink functions)
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.)
THE 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. To 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
ISUP).
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).
MTP
information
The flag is followed by a number ofjelds of information, which together ensure the
correct message transfer. These fields include: the
message sequence numbers
that keep
SP SP
SP
STP
SP
A-B
and

A-C
signalling
messages evenly divided
to
route
via
both
D
and
E.
STP
SP
Figure
12.5
Load sharing over signalling.
A-B
and A-C signalling messages evenly divided to
route via
both
D
and E
256
SIGNALLING
SYSTEM
NO.
7
Next message
r
First bit
transmitted

U
bits
l
Message sequence
Check
and ‘user part’
(message substance
1
Flag
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 MTP message is concluded with a
check bit
field. This is the data
(cyclic
redundancy check code
or
CRC)
needed to perform the error detection and correction
mechanism of the MTP level
2.
The
check bits
are followed by the flag at the start of the
next message.
12.5
THE USER PARTS OF
SS7
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
(TUP)
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.
THE TELEPHONE
USER
PART (TUP)
257
12.6
THE TELEPHONE USER PART (TUP)
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
TUP encoded messages using the MTP. Crudely put, an example of a TUP 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. TUP messages occupy the
signalling information Jield
of the underlying MTP message. The messages comprise a
TUP 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 (DPC)
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 (OPC)
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)
CIC OPC
code
DPC
\
/
\
]
/[[
bit
sent
0
/
information field
MTP message
Figure
12.7
TUP
message structure and relation
to
MTP
258

SIGNALLING
SYSTEM
NO.
7
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 DUP 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
(DSSI,
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.
12.9 THE ENHANCED TELEPHONE USER PART (TUP+)
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 part)
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

THE
SIGNAQLLING CONNECTION CONTROL PART (SCCP)
259
0
interrogation of a central database (Chapter 11)
0
updating cellular radio ‘location registers’ (Chapters 11 and 15)
0
remote activation and control of services or exchanges
0
data transfer between network management or network administration and control
centres
The SCCP is
a
user part
which in conjunction with the MTP allows
a
SS7
signalling
network to conform to the OS1 network service (OS1 layers l-3), and to support
protocols designed according to layers
4-7
of the OS1 model. Most importantly, this
allows new user parts to conform with the OS1 model.
SCCP controls the type of connection made available between the two signalling
points in the exchange and the database. In effect it establishes the
signalling relation
in
preparation for one of the higher level
application parts.

Four classes of
transjer service
can be made available as defined by the OS1 model. These can be grouped into
connectionless
and
connection-oriented
types, as we learned in Chapters
1
and
7.
These
are shown in Table 12.1.
In the context of the SCCP classes shown in Table 12.1,
connection-oriented
data
transfer (classes 2 and 3) is that in which an association is established between sender
and receiver before the data are sent. In reality this means the establishment of a
virtual
or
packet-switched
connection between the ends (as we discussed in Chapter
9),
and not
a
circuit-switched
connection. Thus before the application part signalling commences
operation over the
SS7
signalling link, a
virtual connection

is created by the SCCP. In
the alternative
connectionless mode
of data transfer (SCCP classes
0
and l), messages
are despatched onto the
SS7
signalling network without first ensuring that the recipient
is ready to receive them.
Connection-oriented
procedures are useful when a large
amount of data needs to be transferred. The
connectionless mode
is better suited to
small and short messages, because it avoids the burden of extra messages to establish
the
connection.
By their nature, connectionless messages always include address
information.
Table
12.1 The classes
of
SCCP
Class Type
Class 0 Connectionless
Message sequence not guaranteed
Class
1
Connectionless

Message sequence guaranteed
Class
2
Basic connection-oriented
Message segmentation and reassembly
Class
3
Connection oriented
Message segmentation and reassembly
Flow control
Detection
of
message loss and mis-sequence
260
SIGNALLING
SYSTEM
NO.
7
1
SCCP message
SCCP user data
DPC
OPC
SLS
\
/
/
\
0
information field

M
TP message
Figure
12.8
SCCP message structure and relation to
MTP.
SLS, signalling link selection; OPC,
originating point code; DPC, destination point code
The structure of SCCP messages is similar to that of the TUP, as we can see from
Figure
12.8,
except that the SCCP includes no
circuit identzjcation code
(CIC).
The
CIC
is superfluous because no circuit will be established.
Figure 12.9 shows an example of the use of SCCP and TC for a database query
during the call set-up phase
of
a Freephone (800) call. The caller (who happens to be an
ISDN subscriber) dials the
0800
number, which is conveyed to the ISDN exchange by
the D-channel signalling protocol, DSSl (Q.931). Following analysis of the number, the
ISDN
exchange realizes that it must refer to the intelligent network databases for a
number translation. It does
so
using the SCCP and TC. Meanwhile the circuit set-up is

@
Exchange refers
to
Intelligent database for number
network translation
database
@
I
SCCP and
TC
I(+MTP)
I
-
Y-*
__
-
_.
ISDN
m
-

DO
ISUP
(+MTP)
@
lSDN
/%
B
exchange
Traffic clrcuit

exchange
Dials
0800
12345
@)
Circuit extended uslng
ISUP
signalling
to
next exchange
circuit establlshed

signalling relation
Figure
12.9
A
database
query
using SCCP and
TC
TRANSACTION CAPABILITIES (TC)
261
suspended. When the database interaction is over and the
ISDN
exchange has the
appropriate information, the circuit can be connected using the standard ISUP
signalling.
The SCCP is defined by ITU-T Recs
Q.711-Q.714.
12.11

TRANSACTION CAPABILITIES (TC)
Building on the foundation of MTP and SCCP the
transaction capabilities (TC)
are that
part of the SS7 signalling system which conveys
non-circuit-related information.
Its
development has been intertwined with the development of
intelligent networks
(Chapter
11).
The
transaction capabiZity
is ideally suited to supervising short ‘ping-
pong’ style dialogue between signalling points, typically between an exchange and an
intelligent network
database.
Transaction capabilities (TC)
break down into three
component parts, and undertake the functions of
OS1
layers
4-7.
The underlying
foundation
(OS1
layers
1-3)
is the SCCP and MTP. The
intermediate service part

(ZSP)
c3
Application
Signalling level
TC
User
(Application entity,
AE)
TC
Component
Application sublayer
part
(TCAP)
Transaction
sublayer
H
Transaction
capabilities
(TC)
I
I
Intermediate
service part
(ISPI
Switching connection
control part
(
SCCP)
I
I

Message transfer
part
(MTP)
Figure
12.10
The transaction capabilities
262
SIGNALLING
SYSTEM
NO.
7
carries out the functions of
OS1
layers
4-6,
while the
component sublayer
and
transaction sublayer
exist within
OS1
layer
7
and together form the
transaction
capabilities application part (TCAP).
Transaction capabilities
exist to serve a
TC-user,

normally called an
application entity
(AE).
An AE contains the necessary functions to serve
a
particular application. In
addition, every application entity also contains the transaction capabilities application
part (TCAP). TCAP is, in essence, a copy of the ‘rules’ which enable the messages to be
interpreted. (For example an
AE
may support VPN service. Another might support
freephone.)
Figure
12.10
illustrates the architecture of TC.
The ISP is required only when large amounts of data are to be transferred, using one
of the SCCP
connection-oriented
classes.
The TCAP controls the dialogue between the exchange and the database, overseeing
requests (questions or instructions) and making sure that corresponding responses are
generated. The content of the request (the question itself) is prepared by the TC user
(i.e. the
application entity (AE)).
An
application entity
is
the logical set of questions, responses and instructions which
constitute the dialogue necessary to support an application. An
AE

comprises one or a
number of simple functions, called
application service elements (ASEs).
The idea is that
a small set of multi-purpose ASEs can be combined together in different permutations
to serve different applications. The intelligent network
(IN)
architecture, for example,
defines a set of primitive network actions which
it
calls
functional components
or
service
independent building blocks.
These are examples of ASEs. Figure
12.11
illustrates the
concept of
application service elements.
Three well known
application entities
defined by ITU-T are the
mobile application
part (MAP),
used to support
roaming
mobile telephone networks,
the operation and
maintenance application part (OMAP),

used for control and maintenance of remote
equipment and exchanges and the
intelligent network application part (INAP),
used in
intelligent networks. MAP comprises one complex ASE, OMAP comprises two: MRVT
and SRVT (the MTP and SCCP Routing Verification Tests).
Returning to the TCAP itself, let
us
briefly describe the functions of the
transaction
and
component
sublayers. The
transaction sublayer
is responsible for initiating,
Application
1
=
ASE
1
+
ASE
2
Application
2
=
ASE
2
+AS€
3

-
Application
3
=
ASE
L
1
Application
Service
(
ASEs
1
Elements
(Within
OS1
layer
7
I
AE
=
Application
entity
Figure
12.11
Permutation
of
ASEs
to
serve different applications
THE MOBILE APPLICATION PART (MAP)

263
maintaining and closing the dialogue between the signalling points. Classification of
messages into one of the four types listed below helps the two end signalling point
devices to relate each message back to the previous dialogue and to check that the
communication is occurring in an orderly fashion. Thus each message also has a
transaction identity code.
e
Begin (dialogue or transaction)
e
Continue
e
End
e
Abort
The
component sublayer
provides machine discipline to the dialogue, controlling the
invocation
of requests and making sure that they receive proper responses. The ASE
information within the requests and responses is thus classified into one of five types
e
invoke an action
e
return the final response (to a sequence)
0
return an intermediate (but not final) response
e
return a message to signal an error
e
reject a message (if a request is not understood, or is out of sequence)

Although the information content of the requests and responses is not known by the
component layer (it is understood only by the ASE or ASEs), the component sublayer is
able to make sure that commands are undertaken and responses are given.
The transaction capabilities are defined in ITU-T Recs
Q.771-4.775.
12.12 THE MOBILE APPLICATION PART (MAP)
The
mobile application part
is an example of an
application entity
of
SS7
signalling,
developed to serve a particular application. It is used between a mobile telephone
network exchange and an
intelligent network
database, called a
home
(HLR)
or
visitor
location register
(VLR).
The database is kept informed of the current location of the
mobile telephone handset. Thus the mobile telephone customer’s incoming and
outgoing calls can be handled at any time. Chapter
15
on cellular telephone networks
describes this application more fully.
12.13 OPERATION AND MAINTENANCE

APPLICATION PART (OMAP)
OMAP
is another
application entity
of
SS7
signalling. It provides for network
maintenance as well as other network operations and management functions of remote
exchanges and equipment.
OMAP
contains
2
ASEs: the
MTP
routing verijication test
264
SIGNALLING
SYSTEM
NO.
7
(MVRT)
and the
SCCP
routing verijication test
(SRVT).
These are procedures
designed to enable the network operator to test the integrity of the signalling networks
and identify faults.
12.14 INTELLIGENT NETWORK APPLICATION PART (INAP)
The

intelligent network application part
(ZNAP)
comprises a set of application
functions or
service building blocks
from which complex intelligent network services can
be built. The initial functionality defined by the best standardized INAP (that
of
ETSI) comprises functions defined in its
capability set
1
(CSI).
These are a range
of relatively simple
intelligent network services,
but will be very important because they
will be standardized across many different manufacturers’ switch and service control
point equipment.
12.15 THE USE AND EVOLUTION
OF
CCITT7 SIGNALLING
SS7 is an adaptable and continuously evolving signalling system that has been designed
to meet the challenging and ever-changing service needs of public and major private
network exchanges making up the ISDN, the B-ISDN and the
intelligent network.
Network operators may choose to implement the subset of user parts which most
matches their needs, adopting new user parts as they become available.
Depending on their particular circumstances, some network operators may choose to
implement an adapted version of some of the
SS7

standards, taking up some
of
the
permitted signalling
options.
The options allow the operator
to
‘tailor’ the system to
particular national requirements (e.g. C7/BT is the
UK
national version of TUP/ISUP
and T1-ISUP is the version of ISUP used between public networks in the United States.
Because of the considerable capital investment already committed to SS7, there is a
pressure for use of a common system, and there is pressure also from established SS7
users to ensure that new developments are
backward compatible
with previous versions
of the system.
One of the ways in which backward compatibility is ensured is by building into all
SS7 implementations a mechanism for handling unrecognized information. Such
information is bound to be sent occasionally from a more advanced exchange to an
exchange with an older version of
SS7.
The common methods for dealing with this
information are
0
to discard it
0
to ignore it
0

to assume that some other
expected
response (or
default)
was actually received
0
to reply with a message
of
‘confusion’
0
to terminate the call and reset
0
to raise an alarm to a human
SIGNALLING NETWORK PLANNING AND TESTING
265
Such a
backward compatibility
mechanism obviates any need to stop the development of
SS7,
or the extension of its services.
If
backward compatibility is not built into any new
signalling standard then joint discussions between the operators interconnected
networks using different versions will be necessary to agree amendments allowing
compatible operation.
12.16
SIGNALLING NETWORK PLANNING AND TESTING
The signalling links of a
SS7
signalling network

need careful planning and
implementation just like any other data network. The links need to be sufficient in
number to handle the overall signalling traffic demand, and to be oriented in a topology
that gives good resilience
to
network failures.
Two possible topologies are illustrated in Figure 12.12. Figure 12.12(a) shows a
meshed network in which exchanges are capable of both SP and STP functions and rely
on one another for the resilience of their signalling relations. In contrast, Figure 12.12(b)
shows a topology commonly used in North America, where dedicated and duplicated
computers perform the STP function alone; the exchanges are not capable of the STP
function.
Because of the very complex nature of
SS7
signalling, and because of the heavy
network reliance on it, it is normal to undertake a comprehensive validation testing
programme prior to the introduction of each new link and exchange. Exchanges built
0
Exchange
-
performs
SP
and
STP
functions
-
Signalling link
(a)
Meshed signalling network
SP

STP
only
-
not
a
normal exchange
Normal exchange
-
SP
only
-
Signalling link
(traffic circuits not shown)
0
(b)
Use
of
STPs
Figure
12.12
Typical
SS7
signalling networks
266
SIGNALLING
SYSTEM
NO.
7
by different manufacturers can sometimes be incompatible at first, and
a

testing
programme is invaluable in ensuring that their problems are ironed-out before being
brought into service.
12.17
INTERCONNECTION
OF
SS7
NETWORKS
Within a network owned and operated by a single network operator, the
SS7
signalling
system (or
a
variant of it) will make for close control and monitoring of the network
and highly efficient call routing. However, when networks belonging to different
network operators are connected together, neither operator is likely to want the other
to have full control of his network.
In an international network, an operator in one country is unlikely to let the operator
in another control his network management and routing rearrangements.
Competing network operators in a single country may have their networks interc-
onnected but each guards the monitoring and control of his own network fiercely.
Public network operators may allow direct
SS7
signalling from company private
exchanges but they are unlikely to give up control of the network.
For these reasons, a number of options are permitted within the signalling system
specifications. These allow operators by mutual agreement to restrict the capability of
the signalling system when it is used for network interconnection. Thus a period
of
negotiation is necessary prior to the interconnection of networks.

A
mutual testing
period confirms that the options have been selected correctly.