Common-Battery System - Multiple Switchboard

As explained, in this system the various lines are connected to the board only through an individual line jack. It was mentioned that this type was limited to small cities or to private branch exchange installations. One operator can effectively handle only about 150 to 200 lines, depending on the make of the board and the type of service given. Thus, when the number of incoming lines approaches this amount, a second position must be provided, or if required, a third position added. By reaching in front of each other, the operators can complete each incoming call. If more than three positions are required, however, this method would be unsatisfactory, and transfer circuits (or interposition trunks) would be necessary between the different positions.

The multiple switchboard was early designed^ref.3 to eliminate this trunking or transfer problem. With this type of board, each operator receives certain calls and completes all calls for parties connected to that central office. Each operator must therefore be able to reach calling jacks connected to each line in that central office area. Accordingly, in a central office serving 10,000 users, each operator must be able to reach 10,000 calling jacks. An operator can, by reaching in front of the two adjacent operators, cover three positions, and thus each line must terminate in multiple (or parallel) calling jacks at every third position down the board. As an illustration, if the party connected to line 250 wishes to talk with the party on line 5050, the first party will be answered by the operator at whose position the answering jack for line 250 appears. This operator then completes the call by selecting from the multiple line 5050 and connecting line 250 to it.

Since one operator can reach conveniently only about 10,000 multiple jacks, a city having more than this number of telephone users must have more than one central office. When a customer connected to one central office wishes to talk with one connected to another office, the calls must be trunked to the second office for completion. Each operator must accordingly be supplied with trunks to each of the other offices in an exchange. Thus, in addition to the multiple calling jacks and answering jacks, trunk jacks appear before the operator somewhat as shown in Fig. 15.

Figure 15. Typical arrangement of jacks in a common-battery multiple switchboard.