LONG DISTANCE TELEPHONE CIRCUITS 



531 



These curves include allowance for the unavoidable variations in 

 the net loss which occur from time to time due to repeater battery 

 variations, residual variations left over after pilot wire regulators 

 have removed major transmission variations due to temperature 

 changes in the cables, humidity effects in office wiring, etc. 



Performance Characteristics of Four-Wire Circuits 



Fig. 4 shows, for four-wire circuits used exclusively for terminating 

 business, the minimum working net losses for various distances as 

 limited separately by crosstalk and echoes. The possibility of singing 

 does not enter as a limitation on these circuits. In these curves 

 suitable allowance has also been made for the effect of the unavoidable 

 transmission variations. 



KILOMETERS 

 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 



io 8 

 5 



2000 

 MILES 



3600 4000 



Fig. 4 — Minimum working net loss of 19-ga. H-4:4-25 four-wire facilities versus 

 circuit length for terminating business. 



It will be observed that crosstalk is controlling only for the shorter 

 distances up to a little over 400 miles (640 kilometers). Up to this 

 length meeting proper crosstalk limits requires that the net loss be 

 kept above about 6 db. 



Echo constitutes the important limitation to the net loss of four-wire 

 circuits. As is well known, echo suppressors go a long way toward 

 eliminating echo effects but do not remove these effects, which remain 

 the most important limiting factor on the longer circuits. 



Transmission Requirements 



The transmission requirements established for toll circuits are based 



on the provision of adequate transmission for the complete connection 



between any two points in the United States and the southern part 



of Canada, and also between any point of the country and the terminals 



