518 BELL SYSTEM TECHNICAL JOURNAL 



only for calls between the two cities at which it terminates) or for 

 through business (i.e., the circuit may be connected at one or both 

 ends to circuits to other cities). Evidently in the case of circuits 

 used for this second purpose consideration must be given to various 

 combinations of circuits which may be connected together, as dealt 

 with in the paper entitled "General Switching Plan for Telephone 

 Toll Service" by H. S. Osborne {B. S. T. J., Vol. IX, p. 429, July, 

 1930). Also, the working out of such a plan involves various com- 

 promises. While in working out a general transmission plan, consid- 

 eration must be given to the fact that a given through circuit some- 

 times appears in one connection and sometimes in another, there is 

 little difference between the computation of the minimum working 

 net loss of a single link connection and the computation for some 

 particular assumed combination of through circuits into a multi-link 

 connection. The discussion which follows is written as if applying 

 particularly to terminal circuits. However, the reader may take the 

 methods as practically applying to a long built-up connection. 



The method of determining the echo limitation is to determine 

 the minimum echo net loss ^ and then to add an allowance for varia- 

 tions to determine the minimum working echo net loss. In the case 

 of singing and crosstalk, however, the minimum working net losses 

 are determined directly, allowance for variations being made, re- 

 spectively, in the singing margin required under average conditions 

 and in the average amount of crosstalk considered allowable. After 

 the minimum working echo, singing and crosstalk net losses have been 

 computed separately, the largest one of the three values is taken as 

 the minimum working net loss of the circuit. 



The echo, singing and crosstalk limitations and the normal varia- 

 tions are considered in detail in what follows: 



Echoes 



In the telephone art, the term "echo" ^ is applied to more or less 

 faithful repetition of the conversation to which the talker or listener 

 is a party, which reaches him through some path other than the side- 

 tone path or the main channel of communication. If the delay of 

 the echo is sufficient, a distinct repetition of the sound is heard which 

 produces a sensation similar to the one usually associated with the 

 word echo in common parlance. If the delay is very small the echo 

 tends to merge with the sidetone or direct transmission. 



2 The minimum echo (singing, crosstalk) net loss is the smallest net loss at which 

 a circuit, free of variations, is satisfactory with respect to echoes (singing, crosstalk). 



^ See "Telephone Transmission Over Long Cable Circuits," by A. B. Clark. 

 (Jour. A. I. E. E., January, 1923, and Bell Sys. Tech. Jour., January, 1923.) 



