Introduction to PBXs
The evolution of phone systems starts with the early experiments of Alexander
Graham Bell. In 1875, communication over long distances was handled by the
telegraph, a simple device that would transmit electrical pulses across a wire.
Though there is some dispute regarding Mr. Bell’s status as the inventor of the
basic telephone, he and his estate have successfully defended the original patent
on the invention. For our purposes, the transmission of sound over electricity is
what’s significant.
Early telephones were little more than extensions of this original discovery,
and party lines and local phone companies were quite common.The party line
placed a multipoint drop from the phone company to a number of homes, and
the operator would signal an incoming call by altering the ring frequency to a
custom signal (for example, one long and three short rings).These systems were
prone to the same issues as broadcast media today, especially eavesdropping.
By 1950, a human operator was needed in a more centralized fashion to service
the hundreds of phones that were installed. Human operators manually connected
calls at the physical layer by using huge switchboards, and call setup times
were very long.This system was a solid first-generation effort to link party lines
and private lines into a national network. However, many of today’s advanced
features, including conferencing, alternate billing, and voice mail were inconceivable
then.
In the last half of the twentieth century, phone technology made huge strides,
including analog switching, digital switching, trunking, and the first versions of
the modern PBX.The human operator was replaced with automated switches
that processed calls automatically, and corporations were able to provide privately
administered services that rivaled the phone company.You may recall that the
original public phone systems were virtually always installed and owned by the
government or by single corporations—a far cry from the divergent world of
today in many countries.
While the PBX remains an entrenched fixture in many organizations, like the
mainframe computer, it also gave life to the next generation of successors and
augmenters. In the mainframe world this is the personal computer, and in the
voice and PBX world this is the IP telephone. Many in the AVVID arena consider
the IP telephone a cornerstone, because it is the simplest of devices for the
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end user to operate and because it integrates so well with the additional services
promised by the technology.Voice over IP (VoIP) is the common term used for
these systems. For completeness, and to simplify installation, IP was selected, as it
is the most common protocol in the current networking world.
NOTE
In subsequent chapters, you will likely find that many of the problems
encountered with VoIP systems, including latency, queuing, and routing,
are related to the early decision of using IP as a protocol.