OP EN- WIRE CROSSTALK 215 



pairs in other repeater sections in order to minimize the overall cross- 

 talk between these two circuits. This crosstalk will usually be largely 

 due to those parts of the parallel where the circuits are on adjacent 

 or nearby pairs, since the pole line seldom has enough pairs to make it 

 practicable to keep any two circuits far apart for a large proportion 

 of the total parallel. It is important, therefore, to strive for the lowest 

 possible crosstalk between adjacent or nearby pairs even though this 

 requires permitting higher crosstalk between widely separated pairs 

 than would otherwise be necessary. 



For the adjacent or nearby pairs with naturally high crosstalk, 

 limits on the type unbalance crosstalk are set which make this type of 

 crosstalk small compared with that due to irregularities. Since the 

 type unbalance crosstalk varies with frequency and, in general, 

 increases with frequency, these limits are imposed only for the range 

 of frequency which the line will be required to transmit. It is not 

 advisable to go beyond this, since more severe limits require closer 

 spacing of transpositions and the increased number of transpositions 

 would make the "pole spacing" irregularity crosstalk larger. For 

 the well-separated pairs with naturally lower crosstalk, the type 

 unbalance crosstalk rather than the irregularity crosstalk may be 

 allowed to control with the same idea in mind of requiring a minimum 

 number of transposition points. 



Fig. 28 indicates the method used generally in the Bell System for 

 arranging transpositions with 32 transposition poles. The arrange- 

 ments shown are called fundamental types. They are iterative, i.e., 

 if the first two-interval length is transposed at the center, each fol- 

 lowing two-interval length is likewise transposed, etc. Various other 

 arrangements called hybrid types are possible but in the long run 

 there appears to be no advantage from their use except in the case of 

 side circuits of phantoms. In this case the transposition pattern may 

 change when the side circuit changes pin positions at a phantom 

 transposition. 



The fundamental types may be extended to involve 64, 128, 256, 

 etc., transposition poles. Types involving 128 transposition poles are 

 often used. 



A long line, say 100 miles, is divided into short lengths called 

 transposition sections. With the latest transposition designs, sections 

 having 128, 64, 32, 16 and 8 transposition poles are provided. The 

 nominal lengths of these sections vary from 6.4 to .25 mile. The 

 purpose of these sections is to provide an approximate balance against 

 crosstalk (and induction from power circuits) in short lengths and 

 thus to allow for unavoidable discontinuities in the exposure between 



