ELECTRONICS IN TELEPHONE SWITCHING SYSTEMS 



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point element will be actuated if the link to which it connects is idle. 

 h]ventually all available paths between input and output will be 

 marked. Means must be provided for sustaining only one of the possible 

 idle paths. Here the memory property of the crosspoint device takes over 

 to hold the path until it is released by release marks or removal of the 

 sustaining voltages. So it may be seen that in space division networks 

 the memory requirements must be satisfied the same as in electrome- 

 chanical networks. 



Multiplexing and carrier transmission systems^ employ time and fre- 

 quency division but the physical terminals at both ends of a channel 

 for which the facilities are derived have a one-to-one correspondence 

 Avhich can only be changed manually. In a switching system means 

 must be provided to change automatically the input-output relations 

 as required for each call. Here the need arises for a changeable memory 

 for associating a given time or fref[uency slot to a particular call at any 

 given time. At some other time these points in time or frequency must 

 be capable of being assigned automatically to different inputs and out- 

 puts. For the period that they are assigned, some form of memory must 

 record this assignment and this memory is consulted continuously or 

 periodically for the duration of the call. 



With time division switching this new concept in the use of memory 

 in a switching network appears most clearly, see Fig. 7(a). To associate 

 an input with an output during a time slot the memory must be con- 

 sulted which associates the particular input with the particular output. 

 To effect the connection during a time slot the input and output must 

 be selected A memory is consulted to operate simultaneously high speed 



GAS TUBE 

 SWITCHING NETWORK 



Fig. 6 — Typical "End Marking" control of a gas tube switching network. 



