TRANSMISSION DESIGN OF INTERTOLL TELEPHONE TRUNKS 1031 



order of 30 or so milliseconds. With S = 1 there is little margin and with 

 S = 4: there is excessive margin. 



There will be very few connections with delays greater than about 

 30 milliseconds — which is roughly 1,500 miles of carrier — without 

 echo suppressors. While the effect of the margin on probability of observ- 

 ing echo is difficult to compute quantitatively, it is estimated that with 

 S = 2 this probability will be very small. Additional margin is also pro- 

 vided by the fact that on many connections the connecting trunk loss 

 at the talker's end is greater than the loss used in the tests for establishing 

 the echo tolerance curve. 



Crosstalk 



Analysis of many situations indicates, again, that with S = 2 the 

 losses are about as low as are practicable in general from the crosstalk 

 standpoint. With S = 1, they would be too low in many cases, and with 

 S = A, they w^ould be unnecessarily high. 



With S = 2j there may be a few cases where specific attention to cross- 

 talk will be needed, particularly in open wire. 



Final Selection 



From the consideration of factors like the foregoing the value of 2 db 

 has been selected for >S on a judgment basis. 



PROVISION OF s 



The loss S can be provided in any of the following ways as appropriate. 

 (See Fig. 4.) 



1. As a switchable loss pad in the intertoU trunks. 



2. As a fixed loss pad in the toll connecting trunk. 



3. As part of the conductor loss of toll connecting trunks. This can 

 be done only if the structural return loss of the connecting trunks against 

 the balancing network is reasonably good. 



4. If there is to be no switching to other intertoll trunks or to con- 

 necting trunks with more than 2 db loss, S may be provided simply by 

 increasing the circuit loss by 2 db. 



VIA NET LOSS FACTORS 



Table IV lists typical VNLF's of Bell System intertoll trunk facilities 

 for the condition S == 2. T3rpical losses at which circuits would be worked 

 with S = 2 and with the via net loss factors tabulated in Table IV 



