Instruction manual

About Telecommunications
The No1. ESS, developed by AT&T and installed in 1965, served from 10,000 to
65,000 lines at a maximum of 25,000 calls in the busy hour. With ever-
increasing innovations in technology, the AT&T 5ESS digital switching system in
1983 could handle 100,000 lines and 650,000 telephone calls per hour.
Because most trunks are digital, the newer digital switching systems interface
easily with digital trunks.
The Evolution of Switching Offices
In the early days of the telephone network, there was little or no switching
involved in telephone service (see Figure 2−3 in Chapter 2, “
About the
System”):
■
Private-Line Service. In the initial telephone installations, telephone
communication was from one telephone directly to another, as in Bell’s
demonstration that went from an instrument in one room to another
instrument a few rooms away. Thus, one telephone could communicate
with only one other telephone.
■
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.
System Manager's Guide
B–
7