Implementing Extension Termination
Each resource on the private side of the PBX is commonly called an extension.
These devices have a direct, one-for-one connection to a port on the PBX.These
connections are typically digital, however, analog extensions for modems and
other services are available, and you will find that the term Foreign Exchange
Station (FXS) is commonly used for analog stations such as fax machines and
modems attached to the PBX (although this is an erroneous term). In addition,
there is a large population of PBXs attached, via analog links, to the extensions,
and while the current connections from many vendors are digital, there is
nothing wrong with the analog connections apart from the limitations of the
transport.Wiring for these connections is voice grade. However, it may include
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Figure 1.1 The Basic Functions of a PBX
Extension
Termination
Trunk
Termination
PSTN
Other PBX
Systems
System Logic /
Call Processing
Switching
8 Chapter 1 • Old World Technologies
Category-3 or Category-5, and two- or four-wire (single pair or two pair) installations
are common.The PBX must also provide these extensions with dial tone
generation, just as the public phone switch provides this service for non-PBX
attached phones.These interfaces also pass the Dual Tone Multi-Frequency
(DTMF) tones to the call processing engine that will be described shortly.