326 BELL SYSTEM TECHNICAL JOURNAL 



cable were in some cases found to be inadequate, and in answer to the 

 demand a cable has been developed containing 1,818 pairs of 26 A.W.G. 

 wires within a sheath having an inside diameter of 2^ in. 



These wires are insulated with paper 732 in. wide and 1^ mils thick 

 by the use of specially designed insulating heads and, instead of being 

 stranded in reverse layers as is the case with older types of cables, 

 they are first stranded in groups of 101 pairs, 18 of these groups being 

 then cabled together to form a compact core. 



This method of cabling, called the "unit" type to distinguish it from 

 the layer type, has several advantages, particularly in splicing in the 

 field. Development work on this 1,818-pair cable is not yet complete 

 but there is no reason to doubt that, if there is a demand for a 2,400- 

 pair cable, the demand will be met. 



For convenient reference Table 1 has been shown giving the specified 

 limiting characteristics of some of the standard types of non-quadded 

 cables. From the table it will be seen that the larger gauge cables are 

 used mostly for trunk work and the smaller gauges for connections to 

 subscribers. While the electrical characteristics of these non-quadded 

 cables are of prime importance, they do not demand quite the extreme 

 refinement in manufacturing processes required for quadded cables. 



The discussion so far has been confined mainly to cable intended for 

 local service, that is, cable providing conductors to connect subscribers 

 directly with the central office and different offices with one another. 

 Gradually, the network of long lines connecting different exchange areas 

 or cities grew and while the early lines were mostly open-wire, it was 

 necessary to provide cable' in and near the larger cities to bring these 

 lines into the central offices. Most of the long lines were operated on 

 the phantom principle where four wires are combined to provide two 

 ordinary pair circuits and a third or phantom circuit which uses the 

 four wires simultaneously. It was, therefore, necessary to provide 

 cable for these toll entrances which could also be operated on the same 

 phantom principle. More recently many long toll lines have been 

 placed for their entire length in cables of this type. 



One of the greatest difficulties in providing this type of cable was that 

 of building it with sufficiently good electrical balance to avoid serious 

 interference or "crosstalk" between the various circuits in the same 

 four-wire group or "quad," such crosstalk being especially liable to 

 occur because practically all of these lines are loaded. For a given 

 degree of imperfection in capacitance balance, crosstalk is much more 

 serious if the line is loaded than otherwise. A very considerable 

 amount of work was necessary to determine the principles of design 

 and manufacture which have the most influence in bringing about the 

 best balance reasonably attainable. 



