TRANSMISSION DESIGN OF INTERTOLL TELEPHONE TRUNKS 1029 



For two-wire circuits the local echo paths at the repeaters make it 

 impracticable to establish a straightforward relationship between over-all 

 delay and echo performance. The via net loss factors for such circuits 

 are approximations based on judgment and experience. 



SELECTION OF VIA NET LOSS FACTORS AND S 



From the foregoing, it is evident that the values of S and VNLF are 

 interrelated and that there is a wide variety of possible relationships. 



Fig. 3 shows that, up to fairly long delays, the lower the value of S, 

 the lower the over-all losses at which the circuits can be worked from the 

 echo standpoint. 



Since the lower delay calls are much more numerous than longer delay 

 calls, it is desirable to use as low an S as is practicable. However, in 

 selecting a value, the other factors which have been neglected to this 

 point — crosstalk, singing and noise — must now be taken into account 

 and we must be sure that echo margin is now added. Each of these factors 

 is discussed separately in the following. 



Singing 



The more extensive use of carrier reduces the importance of singing 

 because voice frequency circuits are becoming shorter, thus eliminating 

 the difficult singing problems associated with multi-repeater-section 

 two-wire circuits. On the other hand, some of the conditions at circuit 

 terminals may become more severe from the singing standpoint. 



Studies indicate that over-all losses obtained with S = 2 are adequate 

 to care for singing under most conditions but that ii S = 1 were adopted, 

 singing would be more important. With S = 2 the necessity for increasing 

 circuit losses to avoid excessive danger of singing will probably be con- 

 fined to a few open wire circuits having large discrete irregularities. 



Noise 



Noise is usually not a factor in the assignment of circuit losses. Carrier 

 systems are designed so that under normal conditions the noise is low 

 enough so that any desired loss can be used. If, in a specific case, noise 

 in either carrier or voice frequency circuits is too high, the approach is 

 to get rid of it by one or more of the means available. 



Echo Margin 



Reference to Fig. 3 will indicate that ior S = 2 there is 2 db or more 

 round-trip echo margin in all cases with round-trip delays less than the 



