Over the years, the shape, style, and price of the Plain Old Telephone System
(or POTS) would change many, many times. It is important to understand that the result of
this invention was the intercontinental mesh of telephone wires. These old ???POTS lines??? are
the basis for why we have digital communication today.
If Bell??™s invention provided for the foundation of modern-day communication, the
invention of the Private Branch Exchange (PBX; Figure 1.2) is the ground fl oor. Although
the acronym ???PBX??? has been around since the days of the operator-assisted telephone call,
it is generally used when referring to corporate telephone systems. The fi rst readily available
PBX solution was released in the early 1970s and comprised four core components:
?– Switching matrix
?– Control
?– User stations
?– Trunks
Figure 1.1 A Replica of Bell??™s First Telephone
4 Chapter 1 ??? Unifi ed Communications
Basically, when a call was placed from a user station (keep in mind that a station can be
a phone, an analog device such as a modem, a fax, and so on), the PBX is responsible for
completing the call to either another internal station or the outside world via the Public
Switched Telephone Network (PSTN)??”???Ma Bell??? in the early days, and the ???Baby Bells??? in the
1980s and 1990s. We are oversimplifying the call process, as a number of factors play into it,
including Class of Service, or COS (does the caller have the right to place this call); Least
Cost Routing, or LCR (which phone line is the least expensive to make this call); and many
other pieces of the calling puzzle, all determined in milliseconds.
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