OPEN-WIRE CROSSTALK 217 



Evolution of Transposition Designs 



In designing transposition systems it must be kept in mind that 

 much of the crosstalk is due to irregularities and is a matter of chance. 

 Theoretically the crosstalk elements due to all of the various irregu- 

 larities might chance to add directly. This is highly improbable and 

 if the design were based on making this limiting condition satisfactory, 

 the expense would be very great. Practically, therefore, the designs 

 are based on exceeding a tolerable value a small percentage of the 

 time. If, in practice, the tolerable value happens to be exceeded and 

 this is not found to be due to an error in construction, the unfortunate 

 adding up of crosstalk elements can be broken up by a different 

 connection of circuits at the offices. 



The tolerable values commonly chosen are 1000 crosstalk units 

 (60 db) for open-wire carrier circuits and 1500 units (56 db) for voice- 

 frequency open-wire circuits, which tend to have more line noise than 

 cable or carrier circuits. These limits apply to the crosstalk between 

 terminating test boards with the circuits worked at net losses of about 

 9db. 



Before proceeding with the design of the individual transposition 

 sections which are but a few miles long, it is evidently necessary to 

 determine what part of the overall limit can properly be assigned to an 

 individual section. Assumptions must first be made as to typical 

 and limiting lengths in which circuits are on the same pole line and 

 in which adjacent or nearby circuits continue in this relation. A 

 representative repeater layout must then be chosen. The repeater 

 layout is very important, since the crosstalk in each repeater section 

 is propagated to the circuit terminal and amplified or attenuated, 

 depending on the arrangement of the repeaters. As a matter of fact, 

 the layout of repeaters must be governed to a considerable extent by 

 crosstalk considerations. 



On the assumption that the relative magnitudes and phase relations 

 of the crosstalk couplings in the various repeater sections are a matter 

 of chance the tolerable crosstalk in a single repeater section can be 

 estimated by the use of probability laws. Similarly the tolerable 

 value for any part of the repeater section can be estimated. These 

 probability methods apply very well to crosstalk due to irregularities. 

 Type unbalance crosstalk is systematic, however, and in assigning 

 tolerable values of type unbalance crosstalk in a transposition section, 

 it is necessary to consider how the crosstalk values for various trans- 

 position sections may add up. 



It is not likely that there will be systematic building up of type 

 unbalance crosstalk in successive repeater sections and, therefore, the 



