COMMON' COVrUOL SW 1 !'( 11 1 .\<} SYSTEMS ] 10") 



Alst), as brought out later, destiuatiou codes arc re(iuire(l iu order to 

 realize the importaut tiunkiug economies of automatic alternate routing. 



CODE CONVERSION 



In passing, another feature of some eoniniou control S3'slems, namely 

 code conversion, can be brought out here because the illustration, I'ig. (>, 

 fits. (\dls originating in a common control system can use office name 

 codes (such as MA 2 for calls to the ]\IAin 2 oflice) to n^ach destinations 

 \ia step-by-step switching equipment where route codes (such as 157) 

 ai-e widely used. The translating e(|uii)meiit at tiie common control ofhce 

 can be arranged to substitute arbitrary digits for the office name code 

 digits or in some cases to prefix arbitrarj^ digits ah(\i(l of the called 

 number. The arbitrary digits substituted or prefixed conform to the 

 re(iuirements of the office using route codes. In Fig. 6, office C when 

 equipped with common controls could be arranged to convert MA 2 to 

 157, and therefore codes conforming to the nationwide numbering plan 

 could be used for area 217 even though the calls were routed through 

 step-l)y-step equipment. 



RKLATION BETWEEN TYPE OF SYSTEM AND TRUNKING ECONOMIES 



The pro^'ision of a system w^hich makes the most economical use of 

 the trunk plant is important in any network but it is not as important 

 in a small network as in a large one. Small networks can derive only 

 small economies from arrangements whicfi permit saving trunks. P^or 

 example, in a single office network the trunks consist of wires ruiniing 

 from originating to terminating equipment in the same building plus 

 relativel}^ cheap associated relay circuits. However, in a large toll net- 

 work the trunks may include expensive repeaters, signaling equipments, 

 carrier equipment and perhaps echo suppressors, as well as transmission 

 channels running up to hundreds of miles in length and expensive toll 

 relay circuits. For the larger networks there is therefore considerable 

 urge to save as many trunks as possible. It is important therefore to 

 operate these networks with switching plant that makes the most effi- 

 cient use of the trunk plant by providing full access to groups, and to 

 use an arrangement that permits the trunking economies of I'outes via 

 tandems and of automatic alternate routing. These are features provided 

 by common control systems and hel|) explain why these systems are 

 more attractive in the larger networks, l)oth toll and local. 



The cost of rearrangements for growth, new routes, load l)alancing 

 and for restoring ser\ic(> under cincrgency conditions \-ai-y with the type 



