368 BELL SYSTEM TECHNICAL JOURNAL 



circuit noise measured at the end of a repeater section usually decreased 

 as the attenuation increased. This is important because otherwise the 

 extra increase in the repeater gain to take care of the higher attenuation 

 at such times would make the noise excessive. The study of ice 

 conditions throughout the country which has been carried on and is 

 still continuing will be useful in laying out repeater stations along some 

 of the routes which eventually will be candidates for the application of 

 type J systems. 



Open-Wire Crosstalk ^ 



The crosstalk problem on open-wire lines is one of the most im- 

 portant. Crosstalk is controlled by transpositions which are intro- 

 duced into the various pairs in accordance with a predetermined design. 

 The creation of the necessary designs requires consideration both of the 

 complex theory of transpositions and measurements on lines con- 

 structed by practical methods. 



However, the design of transposition systems is considerably 

 simplified by the use of different frequencies for the two directions of 

 transmission. The only crosstalk between systems which is directly 

 important is that known as far-end crosstalk, which is that between a 

 talker at one end of one circuit and a listener at the opposite or far end 

 of another. Near-end crosstalk, which is that between a talker and a 

 listener at the same or near ends of two circuits, becomes a source of 

 interference between circuits only when portions of it appear as far-end 

 crosstalk because of reflections at points of impedance irregularity in 

 the circuits. 



Because of the high cost of a transposition design to keep both near- 

 end and far-end crosstalk down to small values, only small reflections 

 are permitted where open-wire and cable meet, or where circuits are 

 terminated in equipment. A number of the difficulties which had to be 

 overcome to attain small reflections are discussed later in the paper. 

 With this control the transposition designer can concentrate most of his 

 attention on far-end crosstalk, the near-end crosstalk requirements are 

 relaxed, and a cheaper transposition arrangement can be used. 



What can happen when reflection occurs may be seen by comparison 

 of the near-end and far-end crosstalk curves in Fig. 4. The similarity 

 in the shapes of the two curves, and particularly the fact that the peaks 

 occur at the same frequencies, show that what appears to be far-end 

 crosstalk is in this case mostly reflected near-end crosstalk. It is for 

 pair combinations such as this one, where the near-end crosstalk is 

 much larger than the far-end, that the closest control of reflection 

 effects is required. With the values of reflection realized in the J 

 system, reflected crosstalk will ordinarily be unimportant. 



