ELECTRONICS IN TELEPHONE SWITCHING SYSTEMS 



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Fig. 7(b) — Frequency division switching. 



ticular this is the address of the output desired. A service request de- 

 tector (SR-D) is provided for each hne or trunk. 



In electromechanical systems these logic and information gathering 

 functions are performed by relays or electromechanical switches. In 

 order to keep up with the flow of information from a large number of 

 customers, a number of register circuits must be provided to perform 

 the same function simultaneously on different calls. Here information 

 is being gathered on a "space division" basis and therefore a control 

 switching network may be visualized as depicted in Fig. 8. The regis- 

 ters designated R-M constitute the memory used to store the input in- 

 formation as it is being received in a sequential manner from lines and 

 trunks. As in the case of the conversation switching network, a space 

 division control switching network has been used in electromechanical 

 systems because the speed of these devices is not adequate to accom- 

 modate the rate at which information flows into the system. It is inter- 

 esting to note in passing that in the step-by-step system the control 

 and conversation switching networks are coincident. In the No. 5 

 crossbar system^" the same network is used for both control and con- 

 \ersation on call originations but when so used the functions are not 

 coincident, that is, the network is used for either control or conversa- 

 tion. In other common control systems, separate control networks 

 known as "register or sender links" are employed. 



When using relays to receive the information pulsed into the office 

 l)y customers or operators a plurality of register circuits are needed. 

 The number of the registers required is determined by the time required 

 to actuate the calling device and for it to pulse in the information. The 



