AUTOMATIC SWITCHING SYSTEM 445 



at the Bell Telephone Laboratories, suggested that a program be started 

 to explore the possibilities of a new system whicli would ixMiuiic only a 

 single control circuit of each type. This would reciuiio that each group of 

 functions assigned to a common contiol ciicuit \)e pciformod on a one- 

 call-at-a-time basis. It might be accomplished in a fi'csh approach to 

 system design employing recent developments in high speed components. 

 High speed in the common control units alone would not be sufficient. 

 1 1 would also be necessary to have fast switches since the operating time 

 of a switching network is part of the holding time of the control circuit 

 which operates the network. Similarly, since the signaling time is part 

 of the holding time of the control circuit which receives and registers 

 the signals, some form of liigh speed signaling would also be required. 

 Further, the subscribers should have no direct control of the holding 

 time of any common control unit. It was hoped that a great reduction 

 in the number of common control circuits and connectors would result 

 in a reduction in the size and cost of a central office even if the individual 

 control circuits were somewhat more expensive. Furthermore, a speed 

 permitting one-at-a-time operation would result automatically in faster 

 service for the subscriber. 



Consideration of the various factors of one-at-a-time operation was 

 undertaken by the members of the Switching Research Department 

 and possible system components evolved. Primary elements of inherently 

 high speed, such as cold cathode gas tubes, thermionic electron tubes, 

 dry-reed relays and mercury relays, were immediately adopted for the 

 system. A network of high-speed switches with its high-speed control 

 circuits was designed. A pre-set dialing device in the subscriber telephone 

 set ^vith transmission of high-speed dialing signals to the central office 

 under control of common equipment in the office was selected as a means 

 of eliminating the direct influence of subscribers on control circuit holding 

 time. A code of high-speed signals, suitable for transmission over all 

 existing types of local telephone facilities, with means for the pre-selec- 

 tion and controlled generation of telephone numbers w^as designed into 

 the subset. Such a subset is necessarily complex since it becomes a form 

 of manually operated register wath all digits of a number stored before 

 transmission to the central office. Circuits to control the generation of 

 subset signals from the central office and receiver circuits to decode 

 and register the signals were constructed. 



These parts were then combined in the design of the Electronicallj'' 

 Controlled Automatic Switching System, ECASS. A skeletonized labo- 

 ratory version was built and tested to investigate the feasibility of com- 

 bining the circuit elements and techniques, and to prove the operability 



