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MERLIN LEGEND Communications System Release 6.0
System Manager’s Guide
555-660-118
Issue 1
February 1998
About Telecommunications
Page B-8Signaling
B
■ 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
public
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 Figure 2–4 on page 2-8
in Chapter 2 of this
guide.
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
more powerful telephone network applications and services. It can operate in PBX
mode, along with two other modes that define how the system works.
Signaling 2
Telephone service involves a vast network of transmission and switching
equipment whose status and operation must somehow be controlled. This is done
by means of various types of signals.
Originally, a caller alerted the operator that he or she wanted service by turning
the crank on the telephone, which caused a lamp to flash for that line on the
switchboard at the exchange office. The operator plugged in on that line, the caller
verbally gave the number of the called party, and the operator visually checked
the lamp of the called party to see whether the person was available. If not, the
operator told the caller that the line was busy. If the line was available, the
operator rang the called party’s telephone and connected the parties. When the
call was over, the operator could observe that both lamps went out.