FUNDAMEXTAL PLAN'S FOR TOLL TELEPHONE PLANT 835 



the piiiiciples of this comproheiisivo plan for han(llin<j; telephone toll 

 traffic in the United States and Eastern Canada. It involved two classes 

 of major switching centers - Regional Centers and Primary Outlets - and 

 some classes of less imi)ortant centers. It also s(M np methods of designing 

 toll triniks to give adecinate transmission efficienc}- on all possible toll 

 connections. In use for the last two decades this basic plan has been of 

 great vii\\w in accommodating the tnnneiidons gi-owth of telephone* toll 

 bnsiness during this j)eriod. 



SWITCHING PLAN FOR NATIONWIDE TOLL DIALING 



The earlier general switching plan w'as based on manual switching 

 and on a toll plant made up for the most part, of voice fre(|uency cir- 

 cuits. The probal)ility of operating irregularities and delays increases 

 with the numl)er of manual switches in tandem. Likewise, the trans- 

 mission problem of operating many voice frequency trunks in tandem 

 was so formidable that the number of intertoll trunks in tandem had 

 to be limited to fi\e. In practice, switching was avoided where practicable 

 and economical. 



Impact of Mechanization and Improved Transmission Facilities 



On the other hand, mechanical switching is very fast and is designed 

 to be practically free of operating irregularities. Delays can be minimized 

 by fast switching to alternate routes. Also, in the last tw-o decades the 

 use of carrier has grown from a relatively minor place in the toll plant to 

 the point w^here it is now commonplace. Carrier provides superior trans- 

 mission performance. Limitations on switching are thus greatly reduced 

 and economies are achieved under many conditions. 



In addition, mechanization of local switching systems has proceeded 

 rapidly. With mechanized toll switching, it is becoming possible to 

 establish many toll connections with only a single toll operator and in 

 some cases by customer dialing, without the assistance of any opera- 



. 3, 4 



tor. 



Along with these developments has come tremendous growth in traffic. 

 Since 1930 toll messages in the Bell Operating Companies and the Bell 

 Telephone Company of Canada have more than trebled, growing from 

 an annual volume of about G.IO million to about 2 billion. Intertoll trunks 

 over 25 miles in length have increased in mmil)er from about 28,000 to 

 about 100,000. This continuing growth in traffic volume has required a 

 large scale development of plant facilities and has permitted a more 



