Lucent Technologies 5 manual Switching Methods, Switching Office Hierarchy

MERLIN LEGEND Communications System Release 5.0	Issue 1
System Manager’s Guide 555-650-118	June 1997

2About the System

Background Page 2-8

■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.

Toll

Network

Tandem

Network

Local

Network

☎ ☎ ☎ ☎

Key:

= Level 5

= Level 4

= Level 3

= Level 2

= Level 1 (CO)

= MERLIN

LEGEND

Communications

System

☎= Customer

Telephone

☎ ☎ ☎

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