90 BELL SYSTEM TECHNICAL JOURNAL 



VII. Stability 



As has been pointed out, the magnitude of the line transmission 

 loss in a repeatered circuit is of comparatively small importance in 

 determining its possible transmission equivalent, whether the cir- 

 cuit be worked on a four-wire or two-wire basis. However, it is of 

 extreme importance to be sure that the repeater gains are kept adjusted 

 so as to compensate exactly for a large part of the transmission loss 

 in the circuit, so that the difference between the total loss in the cir- 

 cuit and the total gain, which represents the net equivalent of the 

 circuit, will be kept constant. 



On certain of the long circuits this difference is very small as com- 

 pared to the quantities which are subtracted. For example, in the 

 case of a 1000-mile four-wire circuit using X.L.L. 19-gauge conductors, 

 the total line transmission loss is about 500 miles. Not counting 

 the gain required to make up for losses in apparatus and office cabling, 

 the total gain is about 488 miles, the difference, 12 miles, represent- 

 ing the net equivalent. Evidently only a very small percentage 

 change in either the transmission losses or the gains will have a large 

 effect on the net equivalent. This represents about the most severe 

 condition. Some examples of less severe conditions are — 



2-Wire 19-gauge M.H.L. circuit 200 miles long (320 kilometers). 

 Line equivalent 58 miles. Repeater gain exclusive of gain 

 required to make up for loss in apparatus and office cabling 46 

 miles. Net equivalent 12 miles. 



4-Wire 19-gauge M.H.L. circuit 500 miles long (800 kilometers). 

 Line equivalent 145 miles. Repeater gain exclusive of gain 

 required to make up for loss in apparatus and office cabling 133 

 miles. Net equivalent 12 miles. 



In order to maintain the necessary constancy of the overall or net 

 transmission equivalent of long repeatered circuits in cable, it is 

 necessary first of all to maintain the gains of the individual repeaters 

 within close limits. In addition, periodic transmission measurements 

 are required over the complete circuits, supplemented by suitable 

 adjustment of certain of the individual repeaters whenever the overall 

 equivalent falls outside of the prescribed limits. Also, on the very 

 long small gauge circuits, the changes in attenuation, due to the 

 resistance changes caused by temperature variations, become so large 

 that it is practically essential to provide automatic means for over- 

 coming these effects. 



The methods employed in maintaining the gains of the individual 

 repeaters and of the overall transmission equivalents within proper 



