MEMORY REQUIREMENTS IN A TELEPHONE EXCHANGE 



345 



If the exchange were constructed using only relays it must contain at least 

 log2 X^ .Yl/2"'ml{X — 2m) I relays. If 10 X 10 point crossbars are used in 



the normal fashion it must contain at least — - logio ^ Nl/2'"m\(N 



2m) 



of these, etc. If fewer are used there are not enough stable configurations of 

 connections available to distinguish all the possible desired interconnections. 

 With N = 10,000, and a peak load of say 1000 simultaneous conversations 

 M = 16,637 bits, and at least this many relays or 502 10 X 10 crossbars 

 would be necessary. Incidentally, for numbers N and S of this magnitude 

 only the term m = S is significant in (3). 



The memory computed above is that required only for the basic function 

 of remembering who is talking to whom until the conversation is completed. 

 Supervision and control functions have been ignored. One particular super- 

 visory function is easily taken into account. The call should be charged to 



MEMORY RELAYS 



SWITCHING 

 NETWORK 



u 



u 



R2 



5 



A 



CONTROL CIRCUIT 



Fig. 2 — Minimum memory exchange. 



the calling party and under his control (i.e. the connection is broken when 

 the calling party hangs up). Thus the exchange must distinguish between 

 a calling b and b calling a. Rather than count the number of pairs possible 

 we should count the number of ordered pairs. The effect of this is merely 

 to eliminate the 2"' in the above formulas. 



The question arises as to whether these limits are the best possible — could 

 we design an exchange using only this minimal number of relays, for ex- 

 ample? The answer is that such a design is possible in principle, but for 

 various reasons quite impractical with ordinary types of relays or switching 

 elements. Figure 2 indicates schematically such an exchange. There are M 

 memory relays numbered 1, 2, . . ., M. Each possible configuration of calls 

 is given a binary number from to 2^' and associated with the corresponding 

 configuration of the relay positions. We have just enough such positions to 

 accommodate all desired interconnections of subscribers. 



The switching network is a network of contacts on the memory relays 

 such that when they are in a particular position the correct lines are con- 

 nected together according to the correspondence decided upon. The control 

 circuit is essentially merely a function table and requires, therefore, no 

 memory. When a call is completed or a new call originated the desired con- 



