998 THE RET.L SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



selectors for both the input and output. Each selector receives informa- 

 tion from a memory which actuates crosspoints to associate the input 

 or output with the common transmission medium. The information 

 from the memory which controls the selection process is known as an 

 "address". The crosspoint is non-locking since it must open when the 

 selector receives its next address. The individual memory of crosspoints 

 for space division networks has thus been changed by time division to 

 changeable memory, usually in the form of a coded address associated 

 with each time slot. Furthermore since the successive addresses actuate 

 the same selectors and hence may be held in a common high speed 

 device, electronic bulk memory is ideally suited for this task. The 

 memory must be changeable to allow for different associations of input 

 to output at different times. 



In frequency division the control characteristics of the interconnect- 

 ing network require a modulation frequency to be assigned each simul- 

 taneous conversation to be applied within the bandwidth of the com- 

 mon medium. As shown in Fig. 7(b) the application of the modulation 

 frequencies requires a separate selector for each input and output. These 



I o- 

 20- 



1 



-O A 

 -OB 



COMMON 

 MEDIUM 



Fig. 7(a) — Time division switching. 



selectors are nothing more than space division switching networks and 

 therefore require memory in the switching devices whether they are 

 electromechanical or electronic. 



In addition to memory for associations within the switching network, 

 selecting means are also needed to activate a terminal to be chosen in 

 space division (e.g.. Fig. 6), to place address information in the proper 

 time slot in time division switching or to set the frequency applying 

 switching network in frequency division. 



CONTEOL 



The control of the switching system provides the facilities for receiv- 

 ing, interpreting and acting on the information placed into it. In par- 



