Designing with Legacy Systems in Mind
Before you tackle the converged world of Cisco’s AVVID—even if you configure
PBX systems daily—it may be a good idea to read this chapter to renew your
understanding of what a PBX is and how it works.
NOTE
Please note that this chapter is written from the Cisco AVVID perspective
as it relates to PBX systems and telephony, and, as such, some definitions
and concepts will differ from the phone company or PBX system origins.
These are not errors, but rather, are simplifications of these terms and
ideas to a common, related reference point. For example, FXS and
FXO in carrier terms can refer to other companies and their respective
connections.
However, before we enter the world of the PBX, there is a legacy system that
needs introduction.This is the key system.A key system is a multiline phone historically
found in offices with up to ten users. It is best thought of as those old,
clicking phones with the large, lit buttons.
It is possible to find such systems servicing up to 100 users, however, modern
economics and the lack of advanced features makes these installations less
common, and well-suited for replacement.
As contrasted with the PBX, these systems function by placing a single line
on more than one physical phone and, typically, a one-for-one relationship is
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Old World Technologies • Chapter 1 5
maintained between the number of phones and the number of outside lines. As
such, unlike the PBX, these systems do not scale to hundreds of users, nor do
they save circuit charges.
So, why do we introduce the key system before the PBX? Well, the key
system is to the PBX what, presumably, the PBX is to VoIP and AVVID.The services
provided by the key system were invaluable to companies of the mid-twentieth
century, as calls needed to be routed from one resource to another. In
addition, many PBXs today emulate the key system’s multiline presence, and this
service is available with the current offering of AVVID. As you read about the
internal functions of the PBX, consider the legacy of phone and key systems previously
described, and consider those services in the VoIP environment.