TRANSMISSION DESIGN OF INTERTOLL TELEPHONE TRUNKS 1035 



the amount of traffic will not support the cost of complete automatic 

 alternate routing features. This will cause some additional complication, 

 for each such point introduces another source of echo due to the fact that 

 the capacitance and resistance of the office cabling reduces the balance 

 obtainable when two intertoll trunks are switched together. The effect 

 of such switching on VNL's can be cared for by adding appropriate 

 loss increments which will be small if a careful job of impedance matching 

 is done and the distances from the toll terminal equipment to the switches 

 is held within bounds. 



No increment is added if the return loss for about 84 per cent of the cir- 

 cuits in the group is 24 db or more. These increments increase to about 0.2 

 db for a return loss of 20 db, 0.4 db for 18 db and so on. They are added 

 to VNL of circuits between a two-wire switching point and a four-wire 

 switching point and of circuits between two two-wire switching points. 

 Impedance matching is usually accomplished by adding capacitance 

 across the compromise network and in some cases across the shorter 

 cable runs in an office. 



All circuit losses referred to in this paper are 1000-cycle values, i.e., 

 no allowance is made for the effects of noise and frequency distortion. 

 Careful design, layout, and coordination of individual transmission 

 systems are depended on to keep noise within proper bounds; and all 

 new carrier systems going into the plant have transmitted bands wide 

 enough to require no assignment of distortion transmission impairment 

 (DTI). Circuits having excessive noise and those circuits with large 

 DTI's are earmarked for improvement by any means that may come 

 along. But beyond this, frequency distortion does not enter into VNI 

 calculations since it can not be offset by reducing circuit losses without 

 encountering trouble from the echo or other standpoint. 



While we have considered only circuit design in this paper, it is evi- 

 dent that the success of the whole plan also depends on how closely circuit 

 losses are maintained. This is important from two aspects. 



1. The expected variations determine the allowance which must be 

 made in the assigned loss. As indicated previously, it is expected that an 

 allowance of 0.4 db per link will be adequate for the near future and it is 

 hoped that as time goes on this figure can be reduced. 



2. A more important factor is that unless circuit losses are maintained 

 fairly precisely, large positive or negative excess losses can be accumu- 

 lated on multi-switched connections. Avoidance of such large excesses 

 is particularly important with dial operation since detection and avoid- 

 ance of unsatisfactory transmission conditions by operators will be much 

 less effective. 



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