1102 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



Direct dial control systems cannot operate economically with a uni- 

 versal numbering plan in a network requiring any given call to have the 

 possibility of completion over a variable number of links. The need for 

 operating in this fashion arises Avhen calls may be completed directly to 

 the called office or via one or more tandem or toll systems. Numbering 

 difficulties of a plan which attempts to use tandems with direct dial 

 control systems can be illustrated by reference to Fig. 5. Assume that 

 A, B, C represent three direct dial control type offices in a 6-digit number- 

 ing plan area and that these are connected b}'' direct trunks between 

 offices. Office B is designated ACademy (22 on the dial) and office C is 

 designated BLue Hills (25 on the dial). Analysis of the trunk layout in 

 this network indicates, let us say, that trunking economies can be made 

 by establishing a tandem and that the direct route from A to C is no 

 longer economical as compared to the route via the proposed tandem. 

 The digits 25 must now select a route via tandem. However, if we use 

 both digits for selecting the route to tandem we have none left for select- 

 ing the route to office C at the tandem office. Since this plan mil not 

 work, let us see what results if we assume that the tandem trunks are 

 selected by means of the first digit. Now all calls starting with the code 

 digit 2 at ofiice A must be routed via tandem and even though economies 

 call for a direct route to the ACademy office from A we are forced to use 

 the uneconomical route through tandem for this office. Actually we must 

 consider the economy of routing the traffic for all offices whose codes 

 begin with a given digit via tandem, or routing it over direct trunks, or 

 we must change the designation of one of the offices. We could, of course, 

 adopt the undesirable expedient of using non-universal numbering, i.e., 

 numbering that varied by points of origin, as, for example, by introduc- 

 ing extra digits on calls through tandem from A to C and omitting them 

 on calls from B to C, 



y' >, LOCAL OFFICE 



^^ ° /ACADEMY (22) 



LOCAL 

 OFFICE 



^ A LOCAL OFFICE 

 ^ J BLUE HILLS (25) 



Fig. 5 — Trunking scheme with a tandem office. 



