1104 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



extreme example iu Fig. G. This iiguie shows a multi-switch route 

 through four automatic intertoll switching systems, A, B, C, D, to a 

 customer whose listed number is 2345 in the centi-al office, MAin 2, 

 MAin 2 is in numbering plan area 217, a different area from that of the 

 calling office. Typical digit combinations are shown at each place for 

 reaching the next place with direct dial control systems. On a call from 

 the A toll center area to the nimrber MA 2-2345, the originating toll op- 

 erator must dial 16 digits, such as 059 076 097 157 2345. Calls starting at 

 intermediate points or in other networks use different numbers depending 

 on the route. (Note that the route codes start with or 1 to distinguish 

 them from local codes.) It is rather obvious that dialing such combina- 

 tions is cumbersome and requires elaborate routing information at each 

 toll center. Intertoll calls through direct dial control systems are there- 

 fore generally limited to being smtched at one place along the route, 

 with infreciuent use of two switching points. 



However, with common control systems the situation is quite different. 

 The originating point need dial only the ten digits of the destination 

 217 MA 2-2345. At each point except the one preceding the called area 

 the full complement of digits is sent ahead. At that point the area code 

 is dropped. At the last point, D, which is assumed to have direct circuits 

 to the called office, MA 2 is skipped and 2345 is sent ahead. If calling 

 and called points had been in the same numbering plan area, only seven 

 digits would have been required. Note that since destination codes are 

 used all points outside the numbering plan area dial the same 10 digits 

 to reach a given line and all points within dial the same seven digits. 



While only a small proportion of toll calls require multi-switch con- 

 nections of the type just described, connections such as these are never- 

 theless required for an economically feasible nationwide network in 

 which all calls are dialed to completion, and this objective cannot be 

 attained practically "wdthout systems operating with destination codes. 



(059) (076) (097) 



CALLED NUMBER 

 2345 IN MAIN 2 

 CENTRAL OFFICE 



TYPICAL DIGITS \ \ 



\ \ 



\ \ 



DIRECT DIAL 0590760971572345 0760971572345 0971572345 1572345 T 23.45 . 



COMMON CONTROL 217 Ma'z 2345 217 MA2 2345 217 MA2 2345 MA2 2345 |; 2345 : 



Fig. 6 — Numbering with direct dial control and common control systems. 



