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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 2-7Background
2
Figure 2–3. The Evolution of Switches
Eventually, as more and more COs were created, a hierarchy of special
switching
offices
(SOs) connected the COs locally and then between cities and countries for
long-distance (toll)
switching.
The dedicated lines between COs were called
trunks.
The following list provides an overview of the hierarchy of switching offices. Figure
2–4 illustrates the hierarchy.
■ Level 1: Trunking between COs. The first level in the hierarchy consists
of local COs with direct trunk connections between them. This is referred to
as the
local CO network.
The customer premises served by each CO can
be a residence with a single phone line or a business with a customer
premises switch such as the MERLIN LEGEND Communications System.
■ Level 2: Switching between Tandem (Intermediary) SOs. When the
traffic between two COs exceeds the amount that direct trunking can
efficiently and cost-effectively serve, the COs are each connected to a third
switching office (SOs) that functions as an Intermediary. This is referred to
as the
public tandem network.
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Private-Line
Service
Telephone
Station
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Party-Line
Service
Station Switching
Telephone
Station
Switch
Central Office
Centralized Switching
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