MEMORY REQUIREMENTS IN A TELEPHONE EXCHANGE 



347 



s 



The approximation in replacing (5) by (6), of the order of — log e, is equiva- 

 lent to the memory required to allow connections to be set up from a sub- 

 scriber to himself. With .V = 10,000, 6" = 1,000, we obtain M = 26,600 



S INTERCONNECTING ELEMENTS 

 Fig. 3 — Minimum separate memory exchange. 



N = 2M- 



-2M = N 



Fig. 4 — Interconnecting network for Fig. 3. 



from (6). The considerable discrepancy between this minimum required 

 memory and the amount actually used in standard exchanges is due in part 

 to the many control and supervision functions which we have ignored, and 

 in part to statistical margins provided because of the limited access property. 

 The lower bound given by (6) is essentially realized with the schematic 

 exchange of Fig. 3. Each box contains a memory 2 log ;V and a contact 

 network capable of interconnecting any pair of inputs, an ordered pair being 

 associated with each possible state of the memory. Figure 4 shows such an 

 interconnection network. By proper excitation of the memory relays 1, 2, 

 • • • , M, the point p can be connected to any of the ;Y = 2'" subscribers on 

 the left. The relays 1', 2', ■ ■ -, M' connect p to the called subscriber on 



