ELECTRONICS IN TELEPHONE SWITCHING SYSTEMS 



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tones in the voice frequency range can be used effectively to call the 

 user to the telephone.^ 



As in most electromechanical switching networks, the concepts of 

 connecting successive stages of switching devices (stages to perform 

 the functions of concentration, distribution and expansion) to form 

 the network also apply. Since there is more than one method of inter- 

 connection, the successive stages of a network may employ different 

 switching techniques — electronic, electromechanical, or both. In 

 electromechanical switching, different devices may also be used in 

 different stages. 



In electromechanical space division networks certain types of cross- 

 points are more adapted to common control operation than others. 

 Systems with electromechanical selector switches most generally are 

 set progressively. In systems with relays or relay-like crosspoints all 

 crosspoints involved in a connection may be actuated simultaneously. 

 In either case the switching device, or the circuit in which it is used, 

 has a form of memory. This memory, shown as a square labeled M in 

 Fig. 4, may be the ability of a selector to remain mechanically held in a 

 particular path connecting position or in a locking or holding circuit 

 associated with a crosspoint relay or crossbar switch magnet. 



To minimize the time consumed by the common control elements, 

 simultaneous operation of relay or relay-like crosspoints is most de- 

 sirable. However, this type of network requires a grid of link testing 

 and control leads such as shown in Fig. 5 for a typical stage of a cross- 

 bar switching network. In a network of this type the calling rate ca- 

 pacity is limited by the slow actuating speed of the electromechanical 

 relay or switch. Efficient network configurations can be devised for 



Fig. 3 — Tone ringer telephone set. 



