C H A P T E R 24 Customer-Premise Switching System Features

The main purpose of both TDM and IP customer-premise switching systems is
identical. They connect voice users in the office to one another and to external
users over a pool of shared trunks. The differences lie in the way the two types of
switch accomplish their objectives. Telephone functions have evolved over more
than a century and users are accustomed to how they work. Each new generation
of switches has brought features that its predecessor could not support, but it also
supported the older features as well. The result is a suite of features that most
customer-premise systems support regardless of technology.
If trunk pooling was the only purpose of CPE switches, they could be
quite simple devices, but PBXs, and to some degree key systems, have a more
important role of unifying the office. CPE switches do for voice what the LAN
does for data; they bind the office with shared applications. It is logical, therefore,
to consider unifying voice and data into a single system that supports both. To
some degree this is possible, but the requirements of voice and data are much
different and most users care little or nothing about how the mission is accomplished.
They do care about features, however. Users expect similar features
whether the platform is TDM or IP. As we will discuss in Chapter 26, the strength
of IP lies in its ability to provide features that TDM switches find difficult or
impossible.
The office switching system is a strategic investment for most companies.
A PBX ties your company with your customers, suppliers, the public, and
other parts of the organization. The features it has and the way it is set up
and administered have a significant effect on how those on the outside view
your company. This chapter discusses the principal features of CPE switching
systems.

KEY SYSTEM FEATURES
Most manufacturers offer separate key system and PBX product lines, although
the functions may be nearly identical, at least in larger line sizes. Most TDM manufacturers
also offer a cross between a PBX and a key system known as a hybrid.
Table 24-1 lists the principal differences between these three types of system, but
these distinctions are not absolute. The feature differences between product lines
are many and varied, and some applications such as voice mail are universally
available on all systems. This section discusses features that are common to key
systems, which can be defined as CPE switches having the central office lines terminated
on telephone set buttons. In such systems, users answer incoming calls
by pressing a line button. If the call is for someone else, it is announced over an
intercom. To place an outgoing call, the user selects an idle line by pressing a button.
The physical size of the telephone set eventually places upper size limits on a
key system. When the system terminates more than a dozen or so lines it is easier
for users to dial 9 to access a line than to cope with all the buttons on the phone.
Key systems and hybrids often use the same hardware and software platform.
The distinction between the two is a matter of how the system is set up.