34 HliU. SYSTEM TF.CHSICAL JOVRSAL 



lenancc stan(l|xiint this means that certain additional testing methods 

 must Ik- emplo\e<l whicli will insure the proper generation and trans- 

 mission of the carrier currents and that the nuxiulation and de- 

 mmlulation of the voice fre(|uency currents is accomplished without 

 distortion or excess loss in overall transmission. 



To give a general picture of the more important features in\olved 

 in the transmission maintenance of carrier systems, Fig. G shows a 

 schematic diagram of a carrier layout ha\ing one carrier repeater. 

 The particular arrangement shown is for the l>pe B system described 

 by Messrs. Colpitis and Blackwell.* although the same general main- 

 tenance considerations ai)ply to any of the present systems. It will 

 be noted that three si-ries of tests are rec|uired, one for the carrier 

 repeaters, one for the carrier terminals and one for the system as a 

 whole. The nature of these \arious tests and the approximate points 

 in the carrier ssstem where the\- are ajiplied will be e\ iilent from the 

 names and numln-rs used in the figure. 



For both telephone repeaters and carrier systems, provision is 

 matie in the regular testing e(|uipment so that the tests can be ver\- 

 quickly ajiplied lM>th as a routine proposition and also when rec|uired 

 for trouiili- ioc.itiiii). 



I k \N^Mi>si<>\ li.Ms ON lv\( n.\N(;ic Ari;.\ C'ikclits 



The transmission conditions in the exchange area plant are im- 

 portant not only from the standpoint of insuring good local .service 

 but also to insure gcMid tcdl service, since the local plant forms the 

 terminals of toll connections. The exchange or local plant offers a 

 somewhat different transmission maintenance problem than the toll 

 plant, particularly with res|H-ct to the routine testing procedures 

 whiih must be followed to insure s^itisfactory transmission. This 

 will Ik- evident when it is considered that in each city and town a 

 com[)lete telephone system is in oper.ttion which involves the use of 

 a large numl)er of circuits of various types. There are also in use 

 three general types of iele|)hone switching efjuipmcnts; manual, 

 panel machine switching, and step-by-step machine switching, and 

 in cert.iin cities cimbinations of these eiiuipments. It is estimated 

 that at the present time in the Mell System there are in the neighbor- 

 hood of two and one-half million exchange area circuits, exclusive of 

 subM-riU-rs' lines, in\ol\ing eiiuipmenl other than contacts and 

 wiring which ma\ directly affect the transmission of speech. 



The general clas.ses nf exchange area circuits in both manual and 

 ni.ii liiMi xuit, liiiu. .,Mi,.-s. important from ;i tr.msmission maintenance 



