Lucent Technologies 5 manual Switching Equipment, Evolution of Switches

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

2About the System

Background Page 2-6

■Digital Signal. Information transmitted in a coded form (from a computer) represented by discrete signal elements; for example, off and on or zero and one.

Switching Equipment 2

As described earlier, the telephone network is composed of a number of centralized switching locations, callcentral offices (COs), where a telephone circuit is connected, or switched, to another circuit. That is, the caller’s line is connected to the called party’s line so the two can hold a conversation.

Telephone operators, who supplied the first manual switching, were slow and costly but afforded some special functionality: calls could be forwarded, messages taken, and calls interrupted. Electromechanical switching automated that manual labor and made telephone service universally affordable, but the technology was inflexible and did little more than switch calls. Now, with electronic, computer-controlled switches, both flexibility and functionality are affordable for everyone.

The Evolution of Switches 2

The method, type, capabilities, and capacities of switches have evolved as geographic areas expanded and technological advances became available. The following list describes each of these progressive innovations. Figure 2–3 illustrates this evolution of switching equipment.

■Private-Line Service. In the first telephone installations, communication was directly from one telephone to another, as in Bell’s demonstration. Thus, one telephone could communicate with only one other telephone.

■Party-Line Service. Several telephones were connected to one line so that 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.

■Centralized Switching. As the number of telephones grew, all the lines from all the telephones came to a common central office (CO) or exchange, so that the lines could be electrically cross-connected. Human operators made the connections.