AT&T MERLIN LEGEND Release 3.1 System Manager's Manual page 371

About Telecommunications
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 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
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.
If the call was between switching offices, the two operators completed the steps
of the process.
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System Manager's Guide