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994 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



direct control type, such as the step-by-step system, these parts are 

 inexorably integrated. But in the modern systems they have largely 

 been separated. For purposes of the following discussion this type of 

 sj^stem, viz., common control, will be assumed. The bulk nature of thei 

 electronic memory devices makes them more readily adaptable to sys- 

 tems of the common control type, where the control functions con- 

 sisting of the receipt, interpretation, and processing of input signals and 

 the effecting of output signals may be concentrated. 



INTERCONNECTING NETWORK 



In electromechanical switching systems the interconnecting network 

 is composed of crossbar switches or other electromechanical devices. 

 Each connection through the network is physically separated in space 

 from the others and hence the type of network can be called generically 

 a "Space Division" type of network. Such networks are subdivided 

 functionally. First there is the concentration stage where active lines 

 are separated from those not being called or served at a particular time. 

 Next there is distribution stage where interconnection of active lines 

 and trunks is accomplished. Finally there may be an expansion stage 

 where active call paths are connected to selected destinations. 



In electronic switching systems three classes of switching networks 

 have been described. 2 These are: 



a. "Space Division" similar to the space division for electromechani- 

 cal apparatus except that electronic devices such as gas tubes are 

 employed in place of mechanical contacts as the crosspoint element.'^ 



b. "Time Division" where calls are sampled in time, each one being 

 given a "time slot" on a single channel.^ -^ 



c. "Frequency Division" such as employed in carrier systems where 

 each call is modulated to a different frequency level on a single trans- 

 mission medium. ^'^ 



Thus in electronic switching the interconnecting networks derive 

 their basic characteristics from the known methods of telephone trans- 

 mission. Since transmission techniques are used it is generally not 

 feasible to pass direct current signals through such networks. Also 

 certain ac signals such as 20-cycle current now used for ringing are of 

 such a high power level that they would overload the electronic switch- 

 ing devices employed. For this reason it appears that to accomplish 

 switching with an electronic interconnecting network a change is re- 

 quired in the customer's apparatus to make it capable of responding to 

 a lower level ac for the call signal. Telephone sets with transistor ampli- 

 fiers and an acoustical horn are being developed. (See Fig. 3.) Interrupted 



