1114 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



Even fast dials are inefficient as compared to the push button keysets 

 used bj^ operators for key pulsing and it is obvious that subscriber sets 

 with push buttons would be faster and more convenient than dials. Such 

 sets were used at Media, Pa., on an experimental basis and have func- 

 tioned in a highly satisfactory manner. Their introduction merely re- 

 quired the design and installation of registers to receive the pulses they 

 generate. This was done with little difficulty or expense at the central 

 office end. However, with ordinary step-by-step systems such devices are 

 impractical because of the short interdigital interval they allow and be- 

 cause of the cost of adding the pulse receiving equipment in every 

 selector and of providing translation to change the key pulses into a 

 form to drive the switch. 



CLASSES OF SERVICE 



Differences in the handling of calls from non-coin, coin and PBX lines 

 and differences in rate treatments require the recognition of classes of 

 customers at the central office. In step-by-step separate groups of line 

 finders are provided to permit segregation in classes and where routings 

 for different classes vary, separate selector multiples are required for 

 these routings. Class distinctions within a line finder group can be made 

 by normal post springs and by marking a fourth conductor in the line 

 circuit. 



Common control systems permit the economical handling of many 

 classes of service. The No. 5 crossbar, for example, is most flexible in 

 this respect. As many as thirty classes of service can be handled in a 

 single line link frame, including coin and non-coin. Special handling, 

 reroutes and restrictions are mostly functions of the common controls 

 and inefficiencies due to segregation of traffic in small groups of switching 

 equipment are largely avoided. 



DOUBLE CONNECTIONS 



In systems such as panel and step-by-step in which selectors do the 

 hunting, several selectors may be hunting over the same terminals simul- 

 taneously, and since there is an unguarded interval just after an idle 

 terminal has been found before it is made busy by the release of the busy 

 testing relay, double connections occur. Considerable effort and expense 

 have been expended to reduce the probability of double connections in 

 these systems. In systems which employ markers, on the other hand, 

 the trunk testing schemes do not normally permit double connections to 



