222 BELL SYSTEM TECHNICAL JOURNAL 



In dealing with the problems of crosstalk coupling between open- 

 wire circuits, consideration must be given not only to the direct effect 

 of one circuit on another but also to the indirect effect of the other 

 circuits on the line. What happens is that the disturbing circuit 

 crosstalks not only directly into the circuit under consideration but 

 also into the group of other circuits and thence into the disturbed 

 circuit. The name "tertiary" circuit has been given to this group of 

 circuits although it is not in reality one circuit but rather any or all 

 of the possible circuits which may be formed of the different wires. 

 The system of transpositions must, therefore, not only substantially 

 balance out the direct couplings between disturbing and disturbed 

 circuits but must also substantially balance the couplings from the 

 disturbing circuit into the "tertiary" circuit and from this "tertiary" 

 circuit into the disturbed circuit. 



Reflections of the electrical waves also add interest and complexity 

 to the problem. Such reflections tend to increase crosstalk because 

 the electrical waves which are changed in direction as a result of 

 reflections crosstalk differently, and in many cases more severely, 

 into neighboring circuits than do the waves traveling in the normal 

 direction. The most important reflections occur at junctions between 

 lines and office apparatus. The possibility of other reflections must 

 also be considered, however, at intermediate points in the line which 

 might be caused by inserted lengths of cable, change in spacing of 

 wires, etc. 



In working out the transposition designs, the fact that crosstalk 

 between two paralleling circuits tends to manifest itself at both ends 

 is of great importance. At the "near end" crosstalk coming from 

 the disturbed circuit in a direction opposite to the transmission in the 

 disturbing circuit must be considered. At the "far end" crosstalk 

 coming in the same direction as the transmission in the disturbing 

 circuit must be considered. 



For telephone circuits which use the same path for transmission in 

 both directions, the "near-end" crosstalk is considerably more severe 

 than the "far-end" for two reasons: (1) The crosstalk per unit length 

 of the paralleling circuits is greater; (2) the gains of the repeaters 

 especially augment the "near-end" crosstalk. Voice-frequency open- 

 wire telephone circuits have always been worked on this "one-path" 

 basis and are good examples of circuits in which "near-end" crosstalk 

 is controlling and must be given principal consideration in working 

 out transposition designs. 



In the case of carrier circuits, it was found early in the development 

 that if these circuits were worked on a one-path basis, the crosstalk 



