534 BELL SYSTEM TECHNICAL JOURNAL 



TABLE \l— {Continued) 



TRANSMISSION VARIATIONS (AVERAGE OF THE TWO DIRECTIONS) 



Bell System C. C. I. 



250-niile (400-kilometer) circuits ±1.5 No limits, 

 db, 1000-mile (1600-kiiometer) circuits 

 ± 3.0 db (these limits are exceeded 

 approximately 5 per cent of the time). 



ECHOES 



Echoes are limiting in accordance with Echoes are limiting in accordance with 

 curves representing the results of ex- curves representing the results of ex- 



perience as to permissible loudness of perience as to permissible loudness of 



echo without interference for different echo without interference for different 



times of propagation. See page 129 times of propagation. See page 129 



of the "Red Book" of the C. C. I. of the "Red Book" of the C. C. I. 



On four-wire circuits echo suppressors 

 are employed for all circuits used for 

 switched business in excess of 270 

 miles (430 kilometers) of H-44-25 

 facilities. Four-wire circuits used for 

 terminal business only are equipped , 



with echo suppressors when they ex- 

 ceed approximately 650 miles (1000 

 kilometers) of H-44-25 facilities. 

 Echoes on circuits so equipped are 

 limiting as indicated in Fig. 4 of this 

 paper. 



SINGING MARGIN (TERMINAL CONDITION) 



10 db for two and four-wire circuits. Two-wire 6.8 db. 



Four-wire 9.0 db. 



CROSSTALK 



For quiet circuits: 54 db at least. 



Average — 70 db. 

 Maximum — 60 db. 



(Based on 1 per cent chance of e.xceed- ' 



ing 60 db.) 



MINIMUM WORKING NET LOSS* 



4 db. No limits established. 



TIME OF PROPAGATION 



For continental communication — 250 For continental communication — 250 

 milliseconds. milliseconds. 



Continental circuits in an interconti- Continental circuits in an interconti- 

 nental connection — 100 milliseconds. nental connection — 100 milliseconds. 



For continental communication: For continental communication: 

 Delay between echo suppressors — 100 Delay between echo suppressors — 100 



milliseconds. milliseconds. 



NOISE (including BABBLE) 

 TERMINAL CONDITION MEASURED AT THE TOLL SWITCHBOARD 



-f 26 db (200 noise units) referred to 2 Millivolts (Approximately 160 Noise 

 reference noise (approximately 10 noise Units) 



units). For noise values greater than (Equivalent intensity of 800-cycle tone 

 above limit N. T. I.'s are applied. of the limiting voltage across a receiver 



whose impedance is adjusted to 600 



ohms.) 



* For more detailed discussion refer to text. 



I 



