TRANSMISSION DESIGN OF INTERTOLL TELEPHONE TRUNKS 1033 



dem giving a total length of 4,000 miles, an echo suppressor would be 

 imperative, if over-all loss is not to be excessive. 



It is not practicable to take care of this problem merely by reducing 

 the delay time at which an echo suppressor is applied, since if this were 

 done it is conceivable that eight circuits each with an echo suppressor 

 might be connected in tandem. It has been necessary, therefore, to 

 establish more or less arbitrary rules to insure at least one echo suppressor 

 on long connections and to make it very improbable that more than two 

 will be encountered. In general, these rules specify that echo suppressors 

 will be placed on: 



a. All RC-NC circuits. 



b. All RC-RC circuits. 



c. On high-usage group circuits when the desired losses can not be 

 met without them. 



Our ideas as to when suppressors of Item c will be required may change 

 with the trend from voice-frequency towards high-velocity carrier cir- 

 cuits. Experience will be a valuable guide, for it is not likely that an 

 intolerable situation will build up overnight and without casting some 

 shadow of coming echo ; and the echo suppressor, being a discrete equip- 

 ment unit, can be installed after it is found to be needed without appre- 

 ciable lost motion or additional cost. 



ALLOCATION OF FACILITIES 



If the intertoll plant were homogeneous the over-all problem would be 

 solved at this point — each circuit would be designed in accordance 

 with the preceding and that would be that. 



But the plant is not homogeneous — it consists of everything from 

 loaded voice frequency circuits to circuits on microwave radio with 

 VNLF's ranging from 0.03 to 0.0014. It is, therefore, necessary to allo- 

 cate these facilities among different circuit groups in such a way that 

 as far as practicable the higher performance facilities are used in the 

 more demanding parts of the network. 



As an aid to allocating facilities, charts like Fig. 5 are used. This 

 chart shows ranges of losses within which circuits in different parts of 

 the network are expected to fall. The losses shown there are exclusive 

 of S which must be added, as indicated before, at both ends of each 

 connection. It should be emphasized that these losses are not ''limits" 

 in the usual sense, neither are they attempts to divide up over-all' losses 

 among circuits. They simply help in allocating facilities in the non- 

 homogeneous plant among different circuit groups. As the use of carrier 



