TRANSMISSION DESIGN OF INTERTOLL TELEPHONE TRUNKS 1027 



The problem then is to find the best method of determining "VNL" 

 and "S". As indicated earlier, this problem has a wide variety of solu- 

 tions among which the best can be selected on a judgment basis. The 

 process is as follows: 



(a) In Fig. 3 a solid curve is shown giving the relation between working 

 loss and round-trip delay for a single link, the information being taken 

 from Table III. 



(b) With the plant as it will be in the reasonably near future the round- 

 trip delay on any connection without an echo suppressor will not exceed 

 about 45 ms. This figure is based on a survey of geographical lengths, 

 with some adjustment for the expected more extensive use of carrier and 

 taking into account the ''rules" (discussed later) for the use of echo 

 suppressors. 



Table III 



* With the switching arrangements which will be used, not more than 6 inter- 

 toll trunks will be used in tandem without an echo suppressor. 



(c) Then starting at any arbitrarily selected value of S, a straight 

 line can be drawn from 2S (since there is S at each end) plus 0.4 (re- 

 quired to be added per link for variations) and intersecting the curve at 

 45 ms. 



(d) In Fig. 3, three such straight lines are drawn, ior S = I, S = 2 

 and *S = 4, which have slopes (in db per millisecond) about as follows: 



S Slope (db/ms) 



0.15 

 0.10 

 0.016 



(e) From these data, equations for VNL and TNL for four-wire 

 circuits can be worked out in terms of round-trip delay, *'d," thus: 



VNL 



0.15d + 0.4 



O.lOd + 0.4 



0.016d + 0.4 



TNL = VNL + 25 



0.15d + 2.4 

 O.lOd + 4.4 

 0.016d + 8.4 



