AT&T MERLIN LEGEND Release 3.1 System Manager's Manual page 43

Background
Background
Alexander Graham Bell and his assistant, Thomas A. Watson, demonstrated the
first working model of a telephone on March 10, 1876. Bell made the call from a
transmitter in one room to a receiver a few rooms away.
The first telephone installations were set up like that first call, as direct
connections between one telephone and another. When more and more
telephones were installed, it quickly became impractical to have every phone
connected directly to every other phone. Thus, the concept of switching
developed, that is, all telephones connected physically to all other telephones,
but each telephone could make the electrical cross-connection between itself
and another phone so that the caller was connected to the called party.
Again, as more and more telephones and lines were installed, it became
impractical to have each telephone perform this switching function, so all lines
from all phones were brought into a common place, called a central office (CO)
or exchange (see Figure 2–1) where human operators switched calls at
switchboards. This two-way connection between the telephone and the CO was
(and still is) called the local loop . Eventually, more and more COs were created
and interconnected, until the current global telephone network evolved (see
Figure 2–2).
As geographic areas expanded and the global telephone network evolved, and
as technological advances became available, switches also evolved and are
now fully automatic and controlled by computers.
There are now also private switches that, rather than being located at the
telephone company's CO, are located on a company's premises. These
systems, called private branch exchanges (PBXs), made sense because most
of a business' calls are between telephones on-site within the company.
The MERLIN LEGEND Communications System includes such a switch, located
on a company's premises, that offers access to even more powerful telephone
network applications and services. It can operate as a PBX ( Hybrid/PBX mode )
or can be set up to operate in one of two other modes that define how the
system works. The system can also use state-of-the-art telephone equipment.
The next sections briefly describe the evolution of telephone equipment and
switching. For more information, see Appendix B, "About Telecommunications."
2–2
About the System