CROSSBAR DIAL TELEPHONE SWITCHING SYSTEM 79 



new ofiices and changes in the character or extent of existing central 

 office areas. 



In the panel system, senders are provided which record and store 

 the dial pulses as they are dialed and then independently control the 

 operation of the switching units. The large panel type switches 

 provide access to large groups of trunks and to a large number of 

 groups, and at the same time permit considerable variation in the sizes 

 of the groups. The necessary flexibility in the size and location of the 

 trunk groups is obtained by flexibly wired routing equipment provided 

 in decoder circuits which are associated with the senders. These 

 facilities permit trunk group locations on the switches as dictated by 

 traffic regardless of the office codes listed in the directories and dialed 

 by the subscribers. The panel system also readily provides for the 

 routing of calls through intermediate or tandem offices where the 

 traffic between offices can be more economically handled in this 

 manner. 



The crossbar system also makes use of the sender and decoder 

 method of operation and provides a still greater flexibility in the 

 trunking arrangements than is obtained by the panel system. 



The Crossbar System 



The two outstanding features of the crossbar system are the "cross- 

 bar switch" which is used for all major switching operations, and the 

 "marker" system of control which is used in the establishment of all 

 connections throughout the crossbar office. 



The crossbar system is essentially a relay system employing simple 

 forms of relays and relay type structures for all switching operations. 

 The apparatus consists almost wholly of crossbar switches, multi- 

 contact relays and the usual small relays similar to those generally 

 employed in all telephone systems. The switching circuits are wired 

 to the contacting springs of the switches, and the connections through 

 the switches are made by pressing contacts together by means of 

 simple electromagnetic structures instead of the moving brushes and 

 associated fixed bank terminals of other systems. 



The use of rela}^ type apparatus with its small, pressure type 

 contact surfaces economically permits the use of twin or double 

 contacts with thin layers of precious metal for all contact points. 

 Obviously, double precious metal contacts make for reliable operation, 

 especially with the low speech and signaling currents inherent to a 

 telephone system. 



The short mechanical movements and the inherently small operating 

 time intervals of the "relay-like" crossbar switch permit the use of 



