FUNDAMENTAL I'LAXS FOK TOLL TKLKIMIONE PLANT 



845 



iiig rhaniioliiifz; (equipment at the tormiiiuls of these systems similar to 

 that used for the present coaxial system, eaeh pair of radio channels may 

 provide up to (lOO telephone channels. Several pairs of such radio channels 

 may be operated throuf>h the same antennas (Fi^- 10). 



Radio systems are also u.seful in some cases where the number of toil 

 triniks re(iuire(l is moderate, where diversity is desired or where water 

 or other natural l)arri(M's make tiie proxision of wire circuits difficult oi' 

 impracticable. 



The type of facility to be used on a particular route is sometimes 

 affected by re(iuirements for other services such as teletypewriter, tele- 

 vision network facilities, program facilities, private lines and other 

 factors. 



Trend to Carrier Type Facilities and Advantages to Toll Sivi telling Plan 



About 70 per cent of the long haul toll message mileage in Bell Operat- 

 ing Companies is provided on carrier type facilities as contrasted with 

 7 per cent in 1930 (Fig. 11). 



From the transmission standpoint, carrier facilities offer marked ad- 



Fig. 11 — Growth in Bell Sy.stem iiitertoll trunk mileage showing trend toward 

 more extensive use of carrier type facilities. 



