Filter
Top Products
MERLIN LEGEND Communications System Release 6.0
System Manager’s Guide
555-660-118
Issue 1
February 1998
About Telecommunications
Page B-7Switching Equipment
B
It was, therefore, a natural progression to the idea of using a computer, with its
inherent programmable flexibility, to control the operation of the switching network
that resulted in the new generation of switching technology called an
electronic
switching system
(ESS).
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 Lucent Technologies 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 2
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
CO network
and is the first level in the switching hierarchy.