Instruction manual

Background
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 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 like 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
tandem network .
■
Level 3 and Above: Switching Between 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.
☎ ☎
☎ ☎
☎ ☎
☎
Key:
= Level 5
= Level 4
= Level 3
= Level 2
= Level 1 (CO)
= Customer
Telephone
☎
= MERLIN
LEGEND
Communications 
System
Toll
Network
Tandem
Network
Local
Network
Figure 2–
4. The Switching Office Hierarchy
2–8 About the System