COMMON CONTROL SWITCHING SYSTEMS 



1097 



CALLING 

 TELEPHONE 



TRANSMISSION PATH 



PRE- 

 SELECTOR 



BACKWARD 

 SELECTOR 



LINE 

 FINDER 



CALLED 

 TELEPHONE 



Fig. 1 — Curves developed by E. C. Molina for trunk engineering. 



be cluiugccl for any item of traffic; and (4) a still furtiier variation where 

 the function of hiuiting for an idle path is removed from the selectors 

 and placed in new circuits called markers. Each variation resulted in 

 improvements over preceding methods of operation. 



The first plan is the simplest but also the least flexible. An advantage 

 of this arrangement as well as of the other plans which also store the 

 digits over step-by-step is that the interdigital time does not control 

 the group size. By-path sA^stems are examples of this method of operation. 

 A system of this type is shown in Fig. 2. By-path systems use an auxiliary 

 switch train that is under direct control of the dialed pulses to set up a 

 connection. The talking circuit is then established over a parallel system 

 of switches. The auxihary train releases after the talking connection is 

 set up and is available for use in setting up other connections. The Lori- 

 mer system avoided the penalties resulting from hunting during the 

 interdigital interval by storing the digits at the station. 



A further step in the direction of flexibility, but with added compli- 

 cation, can be taken by a fixed translation from a decimal to a non- 

 decimal basis, i.e., a form of translation wherein a given decimal digit 

 or a set of decimal digits is always changed into the same predetermined 

 non-decimal equivalent. This permits the use of switches with less than 

 ten groups of outlets thereby providing economies by permitting larger 

 groups of outlets with a given size of switch. 



A third variation with still greater flexibility than the first two, but 

 also with greater complication, is a system with changeable translation. 

 Changeable translation is achieved by providing some means such as 

 cross-connections for readily changing the output pattern of the trans- 

 lators generally for sets of digits as, for example, for the called office 



