COMMON" CONTROL SWITCHING SYSTEMS 1103 



It is a situation such as has been described which has led to the prac- 

 tice, in some cases, of putting offices whose designations begin with the 

 same first digit in the same buihhiig in step-b3^-step areas. This, of 

 course, leads to restrictions. 



Another alternative is to use selector repeaters. With these devices a 

 "mitlaufer" action takes place in the local and tandem office selectors, 

 i.e., both the local office selectors and the tandem office selectors follow 

 the dial pulses until sufficient information is received to determine the 

 route, whereupon the unneeded etjuipment is released. This equipment 

 makes possible both the direct route to office B and the route via tandem 

 to office C without an office designation change. However, selector re- 

 peaters are expensive and the cost of introducing them may be con- 

 siderable. They also waste some trunk and equipment capacity because 

 selector repeaters operate by seizing both local selectors and tandem 

 trunks on every call. More often than not, perhaps, it would be cheaper 

 to forego the trunk economy than to introduce the selector repeaters. 



Now take the same network and assume common control equipment 

 at all points. Prior to the introduction of the tandem the local offices 

 translate the first two digits into information for selecting an outgoing 

 trunk and then outpulse only the last four numerical digits directly to 

 the called office. When the tandem is introduced, the translation at 

 office A is changed to select a trunk to tandem on calls to BLue Hills 

 and to tell the sender at A to spill ahead the code digits or equivalent 

 information as well as the line number for these calls. For calls to ACad- 

 emy the existing arrangement is retained. There is no special problem at 

 tandem since the code for the called office, BLue Hills, is made available 

 there. The translator at the tandem office tells the tandem sender to 

 omit the office code digits in outpulsing to BLue Hills. 



There is an essential difference in the coding between direct dial con- 

 trol and common control which is obscured by the use of the same codes 

 in the examples. In the direct dial control case the codes are route codes 

 (sometimes called group codes) ; that is, the digits directly correspond to 

 the route through the switches and are expended in the switching oper- 

 ations. In the common control case they are destination codes and it is 

 not necessary to have them conform to the route nor are they used up 

 in the switching process. Only common control systems can operate with 

 destination codes. Therefore common control systems are required where 

 it is necessary to route calls to some offices by direct trunks and calls to 

 other offices via tandems without numbering restrictions. 



Another example of a numbering difficulty with direct dial control 

 systems tracing back to the use of route codes, is illustrated by an 



