CROSSBAR DIAL TELEPHONE SWITCHING SYSTEM 77 



problems which have arisen, but in addition the engineers of the 

 Bell System have continued their search to find new and better means 

 for meeting telephone switching demands. This work has resulted in 

 the adoption of the crossbar type central office switching equipment. 

 Two offices of this type were placed in successful operation during 

 1938 and others are in process of manufacture and installation. 



It will be appreciated that for large metropolitan areas, the develop- 

 ment and economic introduction of a central office switching system 

 which differs materially from the existing systems is a rather large 

 undertaking. The system must fit into the existing plant as a whole 

 without material change. Generally any important changes afifecting 

 the subscribers' use of the telephone or the methods used by switch- 

 board operators should be avoided. Existing numbering plans should 

 not be affected, existing classes of service should be continued, and 

 the addition of others made feasible in case they should be required. 



All of these and many other factors have been taken into account 

 and all requirements have been met by the crossbar system which 

 offers important improvements in telephone switching, both in opera- 

 tion and maintenance. Its introduction does not make any of the 

 existing equipments obsolete in the sense that these equipments will 

 be less serviceable nor will it cause their replacement. Central offices 

 of the crossbar type can be installed in the same building with existing 

 penel central offices without loss in operating economies in either type 

 of office. Certain equipment, such as the existing and additional 

 outgoing trunks to other central offices, manually operated switchboard 

 positions, operating room and maintenance desks, power plant and 

 alarms, can be used in common by the two types of offices in the 

 same building. 



General 



Before describing the crossbar system it is desirable first to give a 

 brief outline of the principal functions of a dial central office equipment. 

 Such an office is capable of serving 10,000 subscriber line numbers, 

 and is provided with a sufficient number of connecting switches, 

 trunks and associated circuits so that under usual peak loads of traffic, 

 calls will be completed promptly. 



The central office circuits, in response to the lifting of the receiver 

 by the calling subscriber, connect the subscriber line to the switching 

 equipment. This equipment then extends the calling line, "link by 

 link," through several switching stages to the called line as determined 

 by the called office code and line number dialed by the calling sub- 

 scriber. When the connection has been established to the called line, 

 the subscriber bell is rung and, when the subscriber answers, the 



