CARRIER SYSTEMS ON TELEPHONE LINES 



591 



At the carrier frequencies, it has been found impracticable to design 

 transposition arrangements providing for systems where the same 

 frequencies are transmitted in opposite directions. It has been found 

 that, while the two-frequency operation may mean fewer two-way 



Figure 15 — Diagram illustrating occurrence of far-end crosstalk only in carrier 

 systems employing different frequencies for opposite directions of transmission 



operating channels within the same frequency range on a single pair 

 of wires than would be the case if the same frequency bands were 

 provided for opposite directional transmission, the net result in the 

 former case is to make it possible to obtain a greater number of 

 channels on a pole line having many pairs of wires. The need for 

 the directional coordination of frequencies has led to the general 

 adoption of rules throughout the Bell System whereby the systems 

 are all installed so that the low-frequency directional group of channels 

 transmits east to west or north to south and the high-frequency 

 directional group in the reverse direction, west to east or south to 

 north. 



Level Equalization. A situation involving an exaggeration of the 

 crosstalk between two paralleling carrier systems may, of course, 

 arise, even in the case of systems involving the transmission of the 

 same frequency in the same direction for the two systems, if the trans- 

 mission levels of the systems are not the same. If, for example, two 

 systems operating between the same terminals are set up to have the 

 same overall talking equivalent, and one system has a transmitting 

 gain 10 TU higher than the other, the second system will have to 

 have 10 TU greater receiving gain in order to provide the same 

 overall equivalent. This would mean that this system would receive 

 from the first system 10 TU higher crosstalk than if the levels of 

 the two systems were alike. Efforts are, therefore, made in "lining 

 up" the paralleling systems on a pole line so that as nearly as possible 

 the same level relations are obtained for all systems, and the crosstalk 

 tendencies are thus minimized. 



