Dial Telephone Systems

When the dial telephone handset (or the telephone receiver) is removed from position, prior to dialing, the calling subscriber line is immediately connected to the central-office switching equipment. When the calling party dials the desired number, the connection is extended through various switching stages to the desired subscriber line.

If the called line is busy, busy tone is sent to the calling party. If the called line is idle, the bell of the called party is rung. When the telephone is answered, the central-office equipment completes the talking circuit and holds connection during the talking period. When both parties replace their receivers, the central-office equipment is released immediately for use on other calls.

Dial systems appear complicated and difficult to understand; however, the basic principles are relatively simple if a system is analyzed. Nevertheless, it is recognized that those entirely unfamiliar with dial systems may have to do considerable reading elsewhere to understand, in particular, the details of the panel and crossbar systems.

Five different types of dial systems are used in the United States: (1) the step-by-step or Strowger system, (2) the all-relay system, (3) the rotary dial system, (4) the panel system, and (5) the crossbar system. Each of these systems is controlled by electric impulses transmitted from the dial of the calling party to the central office over the subscriber line, or loop. A brief description of the operation of the dial will now be considered before studying the different types of central-office equipment.⁽¹⁾

(1)	Most of the material on dial systems in this book was prepared by Mr. Dwight L. Jones. (See Preface)