1110 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



system, however, one intercept trunk is commonly provided per 100 

 numbers and lines whose service is to be intercepted must be cross- 

 connected to these trunks. 



FURTHER ADVANTAGES OF COMMON CONTROL SYSTEMS ACCRUING FROM 

 THEIR ABILITY TO OPERATE WITH TANDEMS 



Some of the economies permitted by common control systems operat- 

 ing with tandems have been previously mentioned. Tandems are also 

 useful because they provide centralized points at which special features 

 can be concentrated with considerable saving. 



For example, tandems are used for pulse conversion and for concen- 

 tration of message charging equipment. Pulse conversion is needed when 

 it becomes necessary to change from one type of pulsing to another, as, 

 for example, on calls from a panel office to a step-by-step office. Panel 

 can send out only revertive and panel call indicator pulses and step-by- 

 step can receive only dial pulses. The two systems are therefore incom- 

 patible without special arrangements. The following are some of the 

 plans which might be used for handling calls to step-by-step. First, all 

 the panel senders could be modified to send out dial pulses. Second, spill 

 senders could be provided at the outgoing trunks in the panel office or 

 at the incoming trunks in the step-by-step office to receive, say, revertive 

 pulses and convert them to dial pulses. Finally, if there is a tandem in 

 the area, the tandem senders could be arranged (as they actually are) 

 to accept revertive or panel call indicator pulses and send out dial pulses. 

 The first two arrangements are usually more expensive than the last. 

 Therefore, when pulse conversion is required it is generally done by 

 routing calls via tandem. 



To complete calls in the reverse direction, that is from step-by-step 

 to panel, there is a requirement that is due to the use of the step-by-step 

 system, namely that in cases where second dial tone is not employed the 

 equipment at the called office or at an intervening tandem must be ready 

 to accept the step-by-step pulses which are being dialed by the customer 

 within a short time after the incoming trunk is seized. To meet this 

 requirement, special high speed and costly link mechanisms are required 

 to attach senders to incoming trunks or the incoming trunks must be 

 arranged to record and store one or two digits. When calls are made be- 

 tween two systems both using senders, however, cheaper and slower link 

 mechanisms can be employed because the calling senders are arranged to 

 wait for a sender attached signal from the called office. 



