Lucent Technologies 5 manual B-7

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

BAbout Telecommunications

Switching Equipment Page B-7

■Party-Line Service. Several telephones were connected to one line so a number of people could communicate in the same conversation. But there was no way to reach a telephone on any other line.

■Station Switching. All telephones were connected to all other telephones. The telephone itself performed the switching and made the connection. This was workable for a small number of telephones but quickly became impractical as hundreds of telephones were installed.

As the number of telephones grew, centralized switching evolved, that is, all the lines from all the telephones came to a common place, called a central office (CO) or exchange, where the electrical cross connections could be made between the telephones. The actual connections were made manually by human operators.

As geographical areas enlarged, it was impractical to bring all the lines into one CO, so more COs were created to serve the nearby surrounding areas. Eventually a hierarchy of special switching offices (SOs) was created to connect the COs locally and then connect cities and countries for long-distance (toll) switching:

■Trunking between COs. A CO was interconnected to another CO by a dedicated line called a trunk, so a call from a party served by one CO could be made to a party served by another CO. This is referred to as the local network and is the first level in the switching hierarchy.

■Tandem Switching between COs. As growth continued, special SOs, called tandem offices, were developed to function as intermediaries and handle the switching of calls over trunks between COs. This is referred to as the tandem network and is the second level in the hierarchy.

■Toll Switching between Cities. As even more growth occurred, extended switching systems, called toll offices, were then developed to handle long-distance switching between cities. This is referred to as the toll network and comprises the third and higher levels in the hierarchy. The toll network involves national and international service.

The SO hierarchy is illustrated in Figure 2–4 on page -8 in Chapter 2 of this guide.

Today, the hierarchy of the local exchange of the CO through tandem offices and toll offices is still in use. An area within which there is a single uniform set of charges for telephone service is called an exchange area. An exchange area may be served by a number of COs, 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.

In addition to the telephone company switches and switching hierarchy, private switching systems (PBXs) were developed. In a PBX, the switch is located on the company’s premises. The telephone company’s Centrex service enables a business to have the services of a PBX, but supplied from the CO.

As noted in the previous section, the MERLIN LEGEND Communications System is a private switch, located on a company’s premises, that offers access to even