218 BELL SYSTEM TECHNICAL JOURNAL 



tolerable crosstalk per repeater section may be estimated by probability 

 methods. The total of the irregularity crosstalk and the type un- 

 balance crosstalk in a repeater section is a matter of chance and may 

 be estimated from probability theory. Conversely, the part of the 

 tolerable crosstalk which may be assigned to type unbalance crosstalk 

 may be estimated. As noted above, the allowance for type unbalance 

 crosstalk for adjacent or nearby pairs is usually made so small that 

 irregularity crosstalk controls the total. The maximum permissible 

 carrier frequency is, then, the frequency at which the irregularity 

 crosstalk just reaches the tolerable value. Having determined toler- 

 able values of type unbalance crosstalk for a repeater section for the 

 various pair combinations, tolerable values for the individual trans- 

 position sections must be determined. 



If a repeater section involves a number of different types of trans- 

 position sections it is not likely that there will be a systematic building 

 up of type unbalance crosstalk. Factors are, therefore, worked out 

 to relate the crosstalk in a succession of similar transposition sections 

 to that in one section. Numerous factors are required since they 

 depend upon the transpositions at the junctions of the sections. A 

 study of such factors indicates values which it is reasonable to assign 

 to an individual transposition section in order to avoid excessive type 

 unbalance crosstalk in a complete repeater section. 



In the case of a voice-frequency transposition system, both near-end 

 and far-end type unbalance limits must be set. The far-end limits 

 are usually easily met. In the case of a transposition system for 

 carrier systems, far-end crosstalk is controlling and the far-end type 

 unbalance limits are important. The "reflection crosstalk" previously 

 discussed depends, however, on both the magnitude of the near-end 

 crosstalk and on the impedance mismatches. Information on the 

 degree to which it is practicable to reduce these mismatches must be 

 available in order to set limits on near-end type unbalances at carrier 

 frequencies. 



Pairs used for carrier systems are usually also used for voice- 

 frequency telephone systems and in designing transpositions for these 

 pairs crosstalk limits suitable for both types of systems must be met. 

 In practice, an existing line may have only a part of the pairs retrans- 

 posed for carrier operation and in designing a system of transpositions 

 for such retransposed pairs limits must be set for the crosstalk at 

 voice frequencies between the retransposed pairs and the pairs not 

 retransposed. 



It has been the practice to transmit certain carrier telegraph fre- 

 quencies in the opposite directions used for these frequencies in 



