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MERLIN LEGEND Communications System Release 6.0
System Manager’s Guide
555-660-118
Issue 1
February 1998
About the System
Page 2-8Background
2
■ Level 3 and Above: Switching among All SOs. To ensure that there is a
communications path from each SO to any other SO, ever-increasing
levels of SOs combine larger and larger geographical areas. This is
referred to as the
toll network
and comprises national and international
service.
Figure 2–4. The Switching Office Hierarchy
Today, a local area within which there is a single, uniform set of charges for
telephone service is called a
local exchange area.
A number of COs may serve a
local exchange area, 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.
Switching Methods 2
For the first few decades of telephone service, human operators manually
switched calls and made the actual connections of circuits. They made the
connections at switchboards by using cords that had plugs at each end.
Approximately 120 lines terminated at answering jacks on an operator’s
switchboard. In turn, each operator had 18 cords for making connections.
When a telephone service subscriber made a call, a lamp lit at his or her jack,
telling the operator that the person on that line desired service. The operator
connected to the subscriber’s jack, and the calling party would then give the name
(and later, the telephone number) of the party he or she was calling. Then the
operator completed the call (that is, completed the circuit) by connecting the cord
to one of perhaps 10,000 subscriber jacks within reach. When the call was over
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Key:
= Level 5
= Level 4
= Level 3
= Level 2
= Level 1 (CO)
= Customer
Telephone
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= MERLIN
LEGEND
Communications
System
Toll
Network
Tandem
Network
Local
Network