TRANSMISSION DESIGN OF INTERTOLL TELEPHONE TRUNKS 1021 



circuit groups and in (b) there are 26 circuit groups. More specific ideas 

 regarding the effects of these differences can be obtained by considering 

 how calls between specific centers (for example, Al to Bl) would be 

 routed in the two plans. 



From the transmission standpoint the principal impact of the new 

 plan is that the situation will be changed from one in which as much of 

 the traflftc as practicable was handled over direct circuits with a minimum 

 of switched traffic (circuits in tandem) to one in which two or more (up 

 to a maximum of eight) circuits will be used in tandem on many calls 

 and in which different numbers and make-ups of circuits may be en- 

 countered on successive calls between the same two telephones, as a 

 result of the alternate routings employed with machine switching. This 

 means that the losses of circuits must be low in order to provide adequate 

 transmission on all calls and to avoid large differences in transmission on 

 successive calls between the same two places. 



The ideal method in such a situation would be to operate all circuits 

 at zero loss since this would make the results independent of the number 

 of circuits in tandem. However, the distances involved in the Bell System 

 are so great that the propagation times, which affect echo, and the cross- 

 talk between circuits require that even carrier circuits be operated at 

 finite losses. Also, the plan must accommodate many voice frequency 

 circuits on which the noise and singing conditions, as well as echo and 

 crosstalk, may be more severe than on carrier circuits. The practical 

 plan, therefore, is to: 



1. Operate every circuit at the lowest loss practicable considering its 

 length and the type of facilities used. 



2. Assign circuits with different transmission capabilities in accordance 

 with the parts they have to play in the operation of the over-all network. 



The principal problem is to determine how low circuit losses can be 

 made without getting into trouble due to one or more of the limitations 

 mentioned above. This problem is complicated by the fact that the effects 

 of these limitations are not directly proportional to circuit length or to 

 the number of circuits in tandem. For example, if circuit (a) can be oper- 

 ated by itself at a loss of X db and circuit (b) can be operated by itself 

 at a loss of Y db, the loss permissible when circuits (a) and (b) are 

 .switched together is less than X + F. Ideally, therefore, each circuit 

 should have a different loss in each different connection in which it is 

 used. However, this is not practic-able and a compromise must be 

 adopted. This compromise provides that in some connections a particular 

 circuit will operate at its lowest practical loss while in other connections 

 higher losses will be employed to give over-all figures that will be ade- 



