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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
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interconnected to other public and private networks, such as a public X.25 network or a corporate LAN. Figure 10.Error Bookmark not defined. illustrates a sample
PBX system connected to a variety of other systems.
Figure 10.15 Sample PBX configuration.
Extensions Voice
Services Data
Services LAN
Services Admin.
Services
PBX
LAN WAN
Cetral Office Exchange
Trunks
Telephone companies have also attempted to gain a share of the PBX market by offering PBX-equivalent services on their public networks. The Centrex service in
North America is an example. It provides the same features as a typical PBX system but uses the public network for handling calls. Given that most PBX installations
represent significant investments with continued upgrading and reconfiguration costs, the Centrex approach has obvious economic attractions.
10.4.2. Corporate Networks
Many large corporations operate their own distributed private telephone network, which consists of a number of PBX systems at different sites, interconnected through
leased trunk lines. Each user is provided with a network-wide number which can be used to contact them, independent of their geographic location. The network also
provides an interface to the public network for contacting people outside the corporation. By concentrating traffic on leased trunk lines, significant reductions in
running costs can be achieved. Figure 10.Error Bookmark not defined. illustrates the structure of a simple corporate network.
