MACHINE SWITCHING TELEPHONE SYSTEM 



85 



In order to eliminate the inefficiency that such an arrangement 

 would entail, it has been the practise in manual operation to handle 

 the traffic from one part of the area to another part of the area over 

 main trunk routes. The collecting and distributing points on these 

 trunk routes are known as "tandem centers," and the plan of oper- 

 ation is known as "tandem operation." 



Fig. 26 — Typical Tandem Trunking Plan 



Fig. 26 shows an arrangement of offices in a typical tandem trunk- 

 ing plan. Offices marked M are local offices, either manual or ma- 

 chine switching. The office marked T is a tandem office. If a call 

 is originated by a subscriber in office M-l for a subscriber in offices 

 M-4, M-5, or M-6, to which no direct trunks are provided, the call is 

 routed at office M-l to trunks terminating at tandem office T. At 

 this point they are connected to trunks leading to the proper office, 

 where the connection is completed to the desired subscriber in the 

 usual manner. Likewise, calls from offices M-2 and M-3 are com- 

 pleted over the same groups of trunks from the tandem office T to 

 offices M-4, M-5, or M-6. 



The plan described above is typical of that followed in the New 

 York Metropolitan area for many years, the completion of the call 

 being controlled at the tandem office by operators. 



The machine switching system is not only adapted to fit into the 

 existing tandem plan, either when used in the local central office or 

 at the tandem office, but also makes available possibilities for con- 

 siderably extending the field of usefulness of the tandem system, due 

 to certain advantages in handling calls at tandem points by the use 

 of machinery. 



The use of a sender at the machine switching office which is cap- 

 able of routing a call in any way desired permits locating the selectors 

 which have access to the interoffice trunks at any convenient point 

 either at the originating office or at some distant point. In other 



