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SCCP provides services similar to those of the OSI network layer to the SS7 user parts. It serves as a means of using higher layer OSI protocols to communicate
with SS7. Five classes of SCCP service are identified: 0. Basic connectionless class.
1. Sequenced connectionless class. 2. Basic connection-oriented class.
3. Flow control connection-oriented class. 4. Error recovery and flow control connection-oriented class.
Class 1 denotes the MTP service described earlier. The connection-oriented service is defined in terms of the same set of service primitives as presented for the
OSI network layer see Chapter 4 and will not be further discussed here. SCCP messages appear as the SIF field in the signal unit. Figure 10.Error Bookmark not
defined.
illustrates the general structure of an SCCP message.
Figure 10.14 SCCP message structure. Field
Description
Routing Label The routing label in SIF as explained earlier.
Message Type Denotes the type of SCCP message.
Mandatory Fixed Part Denotes the mandatory message parameters.
Mandatory Variable Part Denotes additional parameters for variable messages.
Optional Part Denotes optional parameters.
10.3.5. User Parts
User part messages appear as the SIF field in signal units. Also important, is the SIO field which, as mentioned earlier, guides message distribution. A subfield in SIO
determines the message category as one of:
•
SCCP message
•
Telephone user part message
•
ISDN user part message
•
Data user part message
•
Signaling network management message
•
Signaling network testing and maintenance message
The Telephone User Part TUP defines the SS7 telephone signaling messages
and functions for national and international calls. It directly uses the services of the
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MTP. The structure of a TUP message is dependent on the message type and, because of its complexity, is beyond the scope of our discussion. The TUP is defines
by CCITT recommendations Q.721-Q.725.
The ISDN User Part ISUP defines the SS7 signaling functions and messages
for supporting ISDN applications. ISUP provides a variety of services: control of circuit-switched connections between subscribers, calling line identification, malicious
call identification, call waiting, call redirection, as well as others. ISUP either uses the SCCP for end-to-end signaling transport between local exchanges or uses its own
internal service for this purpose. The structure of ISUP messages is very similar to SCCP messages. The ISUP is defined by CCITT recommendations Q.761-Q.766.
10.3.6. Operations and Maintenance Applications Part
This part provides network management functions and messages for the Operation, Administration, and Maintenance OAM of the network. It is specified by CCITT
recommendation Q.795.
10.4. Private Telephone Networks
Just as WANs and LANs address different, but often complementary, data communication requirements, private telephone networks coexist with the public
telephone network to address the specific telecommunication needs of independent organizations.
Private telephone networks come in different configurations and sizes, with varying degrees of connectivity or integration with public telephone networks and
data networks. Most private networks revolve around a private exchange, which we will look at first.
10.4.1. PBX Networks
A Private Branch Exchange PBX; also called a Private Automatic Branch Exchange, or PABX is a small to medium scale telephone switch typically used in
office environments. One can think of a PBX as a local exchange, with the difference that it is owned and operated by a private organization other than a phone company.
It is connected to a central office exchange via trunk lines and provides a number of local lines or extensions ranging from tens to thousands for connecting office
telephones. All calls between the extensions are locally handled by the PBX. Outside calls are routed by the PBX as normal public network calls via the trunk lines.
Modern PBX systems are digital and can be configured to provide integrated voice and data services. Consequently, the PBX can be used in a LAN capacity to
interconnect personal computers, mainframes, and other computing devices though typical PBX data rates are much lower than LAN data rates. Furthermore, it can be
