Electrical Communication is a free textbook on the basics of communication technology. See the editorial for more information....


Multioffice Dial Exchanges

Before considering the details of making a call in a multioffice exchange, the following additional pieces of equipment will be explained briefly.

Outgoing Trunk Switches. Outgoing trunk switches sometimes are called outgoing secondary line switches and are used as selecting switches between the selector banks and outgoing impulse repeaters. Impulse Repeaters. Impulse repeaters are used in multioffice systems and are associated with the interoffice trunks. There are three reasons for using these impulse repeaters on interoffice trunks; first, they eliminate the need for the third wire (control circuit); second, the repeater reduces the distance over which direct current must be supplied; third, the repeater provides capacitors to insulate the local-office battery from the interoffice trunk.

In a multioffice step-by-step exchange having the arrangement shown in Fig. 28, the "G" office is shown in detail; however, other offices are similar, except for the manual office "S." The toll, information, and other services requiring the help of operators are concentrated at "M" office.

When a call is initiated in the "G" office, the line finder finds the calling line, connects it to an idle local first selector, and dial tone is received by the calling party. The first digit dialed will operate the first selector to choose the office to which the call is being made. If this digit is "G," which corresponds to the digit 4, the called line must be terminated in the "G" office, and the call will be directed to the local second selector in the "G" office. The second digit will operate the second selector to the level corresponding to the dialed digit, thus selecting the desired group of local third selectors. Likewise, the third digit will step the third selector to a level leading to the desired group of connectors. The last two digits operate the chosen connector switch to extend the connection to the called line.

Figure 28. Arrangement of equipment in a typical dial step-by-step multioffice exchange.

Calls to other offices proceed in a similar manner. The first digit dialed selects the chosen office, and the outgoing trunk switch selects an idle impulse repeater and associated trunk to that office. The succeeding digits operate switches in the distant office to give the desired connection.

If it is desired to make a call to the manual "S" office, the first digit must be a seven, corresponding to the "S" on the dial. This steps the local first selector to the seventh level and selects an idle outgoing trunk switch, which in turn selects an idle trunk and associated impulse repeater to the manual office. These trunks from the dial offices usually terminate at a "B" operator's position in the manual office. The calling party may, in some systems, dial only the first digit and then speak the desired number to the operator. In other systems the complete number is dialed and the last four digits stored in call indicator equipment and then displayed visually in front of an idle "B" operator, who reads the number and plugs the calling line in on the proper calling multiple, thus completing the connection to the line desired.



Last Update: 2011-05-30