40 THE BELL SYSTEM TECHNICAL JOUHXAL, JANUARY 1957 



system (the \q\q\ of the outgoing New York or London or Montreal 

 switchboard), and a standard deviation of 5 db. It is approximately a 

 normal-law distribution (expressed in vu), except that the very infre- 

 quent high volumes are reduced by load limiting in the inland circuits. 



The other data needed for system load computations are the number 

 of channels, and the "circuit activity", i.e., the per cent of time during 

 the busy hour that the circuit is actually carrying voice in a given direc- 

 tion (eastbound or westbound) . The circuit activity value used in design- 

 ing United States long-distance multi-channel circuits is 25 per cent; 

 for the transatlantic system, 30 per cent waS used. 



The peak value of the computed system multi-channel signal is the 

 same as the peak value of a sine wave having an average powder of -t-17.4 

 dbm at the zero transmission level point of the system. 



This ^'alue, together with the measured sine-wave load capacity of the 

 undersea repeater, determines the maximum permissible output trans- 

 mission level of the repeater. The measured sine-wave load capacity is 

 about +1.3.5 dbm at 164 kc. Hence the maximum permissible output 

 transmission level for the 164-kc channel is +13.5 — 17.4 = —3.9 db. 



If the relati\'e output levels of the various submarine repeaters were 

 precisely known, the highest-level repeater could have an output trans- 

 mission level of —3.9 db at 164 kc. An allowance of about 2 db was made 

 for uncertainty in knowledge of repeater levels, giving — 6 db as the de- 

 sign value for the maximum repeater output level at 164 kc* 



The repeater has frequency shaping in the circuit between the grid of 

 its output tube and the cable. The maximum repeater output at lower 

 frequencies is smaller than at 164 kc, but the maximum voltage on the 

 output grid, from an overload standpoint, is approximately constant 

 over the 20 to 164 kc band. 



The transmission level of the maximum level repeater output is thus 

 determined, based on load considerations. Another factor which might 

 limit this level is modulation noise. This was found to be less of a re- 

 striction than the load limitation, however. 



With the output level determined, the random noise and the modula- 

 tion noise for the system can be computed. The random noise computa- 

 tion is made on the assumption that all repeaters are at the same level, 

 and then a correction is made for the estimated difTerences in levels of 

 the various repeaters. The equation is 



No = Nin-^G - TL + 10 log n + dr 



* At the time of writing, the No. 1 cable (eastbound) is setup with a somewhat 

 lower maximum output level than this; the safe increase in level will be deter- 

 mined later. 



