F.XaiXF.l-.RIXC COST SIC PI IS 7 



\i). ■Jl-^aiim" i'.il)li-, it is, of course, iiri-i'ss.ir\- tli.il .ill silliscriinTs 

 lia\'ii)^ loops lon^rr th.in tho aiuount of No. 2I-Kaiij;c cable rcpri'scnti-d 

 !)>• this resistance sh.ill hv put in No. 22-^augc or No. 19-Raune calile 

 as ni.iy he recpiireil. 



Tlie transmission losses, iuitli ir.uisinit ling an<l receiving, ari- 

 then coinputeil for the assunieil loops. The transmission losses in 

 central otVue apparatus are constant and known. Subtracting ifie 

 losses in the otVices and in the substation loops for each assumed 

 grade of loop from the transmission standards, leaves the amount 

 ()f transmission loss which can he allowed in the interoffice trunks 

 correspomling to each limiting grade of suhscril)cr's loop. On the 

 h.isis of this allowable transmission loss in the trunks and knowing 

 the distances between central offices, we are enabled to fix the size 

 of conductor reijuired in the trunks. 



Knowing the grade of loops and trunks reciuired for each of the 

 above assumptions, we can then compute the total annual charge 

 of giving .service according to that assuminion. If the assumptions 

 h.ive lieen wisely chosen it will usually work out that the first 

 assumption, that is, a very high grade of subscriber's loop, will not 

 be as economical as some others, due to the relatixcly high cost of the 

 subscribers' loops taken as a whole. Neither will the last assumption, 

 that is, a very low grade of subscriber's loop, be the most economical, 

 on account of the relatively high cost of the triniks. Somewhere 

 between, however, there will be sf)me assumption wiiicli will show 

 the smallest total annual charge. 



To find more precisely the most economical arrangement, the vari- 

 ous values are plotted with the assumptions as to subscribers' loops 

 forming one set of ordinates and the total annual cost forming the 

 other. The point on the curve representing the lowest annual cost 

 then indicates the proper grade of subscribers' loops to employ. In 

 the case of the longer interoffice triniks, loading is, of course, em- 

 ployed. In the design of toll lines antl toll switching trunks generally 

 similar cost balancing methods are employed. 



In many cases, the problem can be solved b\- the determination 

 of what we term "the warranted annual charge" of transmission 

 which may be defined as the annual cost of improving the talking 

 efficiency of the circuit in the cheapest way by a definite small amount. 

 By means of studies of this kind, we obtain a plant closely approxi- 

 mating a balanced cost condition. That is, in such a plant, a dollar 

 can be spent in improving transmission efficiency, ncj more effectivelv 

 in one part than in another. 



