TRANSMISSION DESIGN OF INTERTOLL TELEPHONE TRUNKS 1025 



Terminal Return Loss 



As shown in Fig. 2, with four-wire switching and four-wire type cir- 

 cuits the only source of echo is lack of perfect balance between the 

 balancing network of the four-wire terminating set (hybrid coil or its 

 equivalent) and the trunk, loop, and subscriber station connected at the 

 customer side of the set at the distant terminal. The ratio of the amount 

 of power reflected back into the hybrid coil to the amount which goes 

 on toward the listener can be expressed as a loss in db. This * 'terminal 

 return loss" in the echo range (approximately 500 to 2,500 cycles) has 

 been found by tests and computations to have an average value of 11 db 

 and a standard deviation, !)< = 3. 



Round-Trip Circuit Loss 



The round-trip circuit loss plus the terminal return loss is the total 

 loss in the echo path. The round-trip circuit loss, i.e., the over-all loss 

 which the intertoll trunk (or trunks) inserts in the echo path, is the sum 

 of the losses in the east-to-west and west-to-east directions. If the circuit 

 regulation were perfect, this loss would simply be twice the nominal 

 one-way loss of the trunks — which is the thing we are looking for. 



However, regulation is not perfect, and in order to determine what 

 the nominal loss should be we must take into account the deviations 

 from it which are certain to occur in practice. A considerable amount of 

 experience indicates that these de\dations can be treated statistically 

 and considering some improvement in maintenance methods and pro- 

 cedures and a wider use of carrier systems with improved regulation, a 

 standard deviation of D„ = 2 db for round-trip losses seems a not un- 

 reasonable assumption for the next few years. 



Relationship Between Working Echo Net Loss and Round-Trip Delay 



We now have all of the data we need to solve our problem — which 

 as stated before is to find what VNL to assign to a circuit of given length 

 and on a given type of facility. 



Our first step mathematically is to combine the three statistical dis- 

 tributions we have been talking about — i.e., tolerance to echo, terminal 

 return loss and the variations in round-trip circuit loss. 



The combined standard deviation (Dc) of the three sets of distribu- 

 tions is the square root of the sum of the squares of the standard devia- 

 tions of the individual distributions. The first two distributions are 

 independent of the number of links, N, (assuming four-wire switching) 

 but the distribution of circuit loss variations is a function of the number 



