The Switching Office Hierarchy - AT&T MERLIN LEGEND Release 3.1 System Manager's 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.
Toll
Network
Tandem
Network
Local
Network
Figure 2–4. The Switching Office Hierarchy
2–8
About the System
Key:
= Level 5
= Level 4
= Level 3
= Level 2
= Level 1 (CO)
= MERLIN
LEGEND
Communications
System
= Customer
Telephone