56 BELL SYSTEM TECHNICAL JOURNAL 



transposition arrangements which may exist in a single transposition 

 section. Since the attenuation in a transposition section is not great, 

 these calculations need not take into account differences in the attenu- 

 ation constants of the various tertiary circuits. A long line has a 

 series of transposition sections of various types and the total far-end 

 crosstalk for any two circuits is a summation of the crosstalk values 

 obtained from the type unbalances for the various sections plus 

 interaction crosstalk between the various combinations of sections. 

 With practical methods of transposition design, the transposition 

 arrangements are so chosen that the interaction crosstalk between two 

 sections is usually small compared with the far-end crosstalk in one 

 section. A long line for the most part consists of a succession of 

 similar sections with occasional sections of other types. Inter- 

 action crosstalk between dissimilar sections does not ordinarily 

 contribute appreciably to the total far-end crosstalk. For the im- 

 portant case of a succession of similar sections interaction crosstalk 

 between sections must be carefully considered since it may build up 

 systematically and the total may be large compared with the summa- 

 tion for the far-end crosstalk values for the individual sections. 



Serious interaction crosstalk between similar sections is guarded 

 against by computing factors relating the far-end type unbalance in 

 one section to that in various numbers of successive sections with 

 various transposition arrangements at the junctions of sections. The 

 factors actually computed are somewhat in error since they involve 

 long distances and assume the same attenuation constants for all 

 circuits. The errors are not sufficient, however, to prevent the 

 factors from being a proper guide in avoiding systematic building up 

 of interaction crosstalk between sections. 



The above discussion assumes that the tertiary circuits are indef- 

 initely extended or terminated to simulate their characteristic im- 

 pedance. The tertiary circuits may not be terminated at the ends of 

 a line since many of them are not used for transmission of speech or 

 signals. Complete reflections of the crosstalk current in the tertiary 

 circuits will, therefore, occur at their ends and these reflections some- 

 what modify the crosstalk currents in other circuits. This effect is 

 important in a very short line since the reflected wave is again reflected 

 at the distant end and at particular frequencies large changes in the 

 tertiary crosstalk currents may occur due to multiple reflections. 

 In a long line such multiple reflections are damped out and, in general, 

 tertiary circuit reflection effects are not important. 



If all the pairs on a line are transposed for the same maximum 

 useful frequency, the transposed pairs will usually be relatively 



