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Arrangement of Central-Office Equipment

As indicated in Fig. 21, the various subscriber lines entering a central office are in cables that terminate on the line side of the main distributing frame, abbreviated M.D.F. At this point, protection in the form of lightning arresters and heat coils is inserted in the circuits (page 568). The cable terminations are marked with the number of the cable and with the cable-pair number for identification.

On the opposite side of the M.D.F. the terminations are marked with the multiple numbers, or with what might be termed the telephone number designations. These numbers correspond with the numbers on the calling multiple on the "B" board. These M.D.F. terminations are permanently connected with the intermediate distributing frame or I.D.F. terminations as shown in Fig. 21. Wires also lead from I.D.F terminations to the multiple on the "B" board. From the opposite side of the I.D.F., wires go to the answering jacks on the-"A" board. These are marked with switchboard panel and jack designations.

Figure 21. Typical arrangement of apparatus for the "A"-"B" board operation with 100 per cent trunking. The "A" operator's telephone is connected to the call circuit by a key arrangement. One circuit only is shown. Switchboard cord circuits are not shown. This shows the use of a call circuit between the two operators, but often the straightforward method is used (page 367).

One purpose of the M.D.F. and the I.D.F. is to provide a flexible method for terminating the telephone sets on the switchboards, and thus to provide a means for distributing the switchboard load. When a customer requests telephone service a vacant pair in the cable near his residence is selected. A vacant multiple jack on the "B" board is also chosen, and the "A" board panel and jack determined. These latter must be at a position where the "A" operator is not already fully loaded during the busy hour. As can be seen from Fig. 21, after these selections are made, twisted wires called jumpers are used to connect the line to the corresponding points on the M.D.F. and I.D.F. That is, a jumper is placed on the M.D.F. between the cable-pair termination and the multiple termination on the opposite side of the frame. This completes the connection to the multiple on the "B" board. The multiple side of the I.D.F. is then connected by a jumper to the proper panel and jack terminations on the opposite side of the I.D.F. In this manner the connections are made to the "A" board.



Last Update: 2011-05-30