Lucent Technologies MERLIN LEGEND System Manager's Manual page 391

MERLIN LEGEND Communications System Release 5.0
System Manager's Guide 555-650-118
B About Telecommunications
Switching Equipment
Party-Line Service. Several telephones were connected to one line so a
number of people could communicate in the same conversation. But there
was no way to reach a telephone on any other line.
Station Switching. All telephones were connected to all other telephones.
The telephone itself performed the switching and made the connection.
This was workable for a small number of telephones but quickly became
impractical as hundreds of telephones were installed.
As the number of telephones grew, centralized switching evolved, that is, all the
lines from all the telephones came to a common place, called a central office (CO)
or exchange , where the electrical cross connections could be made between the
telephones. The actual connections were made manually by human operators.
As geographical areas enlarged, it was impractical to bring all the lines into one
CO, so more COs were created to serve the nearby surrounding areas. Eventually
a hierarchy of special switching offices (SOs) was created to connect the COs
locally and then connect cities and countries for long-distance (toll) switching:
Trunking between COs. A CO was interconnected to another CO by a
dedicated line called a trunk , so a call from a party served by one CO could
be made to a party served by another CO. This is referred to as the local
network and is the first level in the switching hierarchy.
Tandem Switching between COs. As growth continued, special SOs,
called tandem offices , were developed to function as intermediaries and
handle the switching of calls over trunks between COs. This is referred to
as the tandem network and is the second level in the hierarchy.
Toll Switching between Cities. As even more growth occurred, extended
switching systems, called toll offices , were then developed to handle
long-distance switching between cities. This is referred to as the toll
network and comprises the third and higher levels in the hierarchy. The toll
network involves national and international service.
The SO hierarchy is illustrated in
Today, the hierarchy of the local exchange of the CO through tandem offices and
toll offices is still in use. An area within which there is a single uniform set of
charges for telephone service is called an exchange area . An exchange area may
be served by a number of COs, and a call between any two points within an
exchange area is a local call . A toll call is a call made to a point outside the local
exchange area, and includes service through the switching office hierarchy.
In addition to the telephone company switches and switching hierarchy, private
switching systems (PBXs) were developed. In a PBX, the switch is located on the
company's premises. The telephone company's Centrex service enables a
business to have the services of a PBX, but supplied from the CO.
As noted in the previous section, the MERLIN LEGEND Communications System
is a private switch, located on a company's premises, that offers access to even
Figure 2–4 on page -8
Issue 1
June 1997
Page B-7
in Chapter 2 of this guide.