Specifications

© Copyright IBM Corp. 2000 189
Appendix C. Basic Telephony Terminology
This Appendix contains detailed information on commonly used telephony
systems both in the public network and in traditional enterprise voice networks.
C.1 Central Office Switch
The phone company or PTT central office (CO) is where the local switch is
housed. All the lines to homes and businesses within a geographic area
terminate at their local central office. This is referred to as the local exchange.
The switch provides a dial tone to all the telephones and fax machines connected
to it. Every telephone or fax machine connected to the local exchange can reach
each other directly through the switch. For example, if your telephone number is
453-6790, then you can call anybody else with a 453 prefix. However, if you want
to call 734-3847, then your call will be routed through another central office switch
to the 734 exchange.
Of course, the above example assumes a local exchange of less than 10,000
users. Central office switches vary in size depending upon the area to be served.
The largest central office switches can accommodate as many as 100,000 lines.
The local exchanges are interconnected via inter-office trunk lines to other
switches forming a hierarchy of switches that make up the Public Switched
Telephone Network (PSTN).
C.2 Private Branch Exchange
The Private Branch Exchange (PBX) is a very vital office automation system. It
allows extensions in an establishment to call one another without using the
PSTN. More importantly, it allows an enterprise to share the costly resources of
PSTN trunks. Without the PBX, every telephone in the establishment would need
a separate line to the PSTN.
A PBX is a digital switch that operates very much like those used in the PSTN
itself. It contains line cards that convert analog phone lines into PCM signals and
a TDM switching fabric that is capable of providing a circuit-switched path from a
line port (a telephone) to another line port or to a trunk connection that goes
outside to the PSTN. They work on the principle of stored program control where
the entire database of the switch (for example, telephone numbers, memory
matrix, time slots, addresses, signaling, CODECs and features) are stored on a
hard disk or flash memory. A processor is used to perform call setup and to
control the switching.
Most PBXs employ a high-speed backplane for communications between trunk
cards and line cards. They also employ distributed processing to reduce the load
on the CPU and eliminate single points of failure. This also allows the system to
share resources optimally. These resources include PSTN trunks, tie line trunks,
ISDN lines, and in some geographies even satellite links.
PBXs are rated by their size (number of ports) and traffic handling capabilities. A
PBX that can handle up to 100 telephones generally is considered to be a small
PBX,while a medium-sized PBX can handle up to 1000 telephones. The largest