TYPE-O CARRIER SYSTEM 691 



K cables to Fort Dodge and Waterloo. Eventually a second cable route 

 will extend across the state through Waterloo and Fort Dodge, as shown 

 by the dashed lines. A second coaxial route cuts across the southwestern 

 corner on its waj^ to Kansas City and a third coaxial route coiniects 

 Sioux City with Omaha. The rest of the state is served by open-wire 

 lines. 



Fig. 2 shows the distribution with length of the Bell System open-wire 

 short-haul toll and tributar}' circuits in Iowa in 1950, including both 

 voice-freciuency aiul carrier facilities. It will ])e noted that 95 per cent 

 of the circuits are less than 100 miles in length, while 90 per cent are 

 less than 70 miles, the point where type C systems just become economi- 

 cal. For tributary circuits 98 per cent are less than 30 miles in length. 

 There arc a total of some 2700 toll and tributary circuits in Iowa. In 

 addition, there is also a sizable connecting company development. Fig. 

 3 is a distribution of the number of circuits per group, where a group is 

 composed of the circuits used for via or terminating business between 

 two towns. There are about four circuits per group for short-haul toll 

 and two circuits per group for tributary service in the median case. As 

 the dial conversion program proceeds the average number of circuits per 

 tributary group is expected to increase. 



Because of the short distances involved, and the small number of 

 circuits per group, carrier development in Iowa has been restricted, to a 

 large extent, to single-channel systems, and type M. Only four or five M 

 channels can be operated on a given open-wire line, and while these 

 systems have some transmission disadvantages, they have been employed 

 to a large extent. However, further M development is blocked because of 

 the expense of isolating M systems from adjacent lines. There are a few 

 C systems on such routes as Sioux City-Spencer-Mason City, Waterloo- 

 Dubuque, Muscatine-Keokuk, and Atlantic-Spencer. 



The type-0 system, therefore, is being made available to provide 

 short-haul toll and tributary circuits on open- wire lines in the range 

 from 15 to 150 miles. WTien completely developed it will provide four 

 four-channel systems in the frequency range from 2 to 156 kc, as shown 

 on Fig. 4. The use for separate channels of both sidebands of a single 

 carrier, called a "twin-channel," results in economical use of the fre- 

 ciuency space. The four channel systems are designated OA, OB, OC, and 

 OD respectively, and cover substantially the same frequency range as 

 the C and J systems. 



Considerable attention has been given to keeping the line rearrange- 

 ments as simple as possible. The use of non-loaded entrance cable is 

 proposed, and simple arrangements are available for adding groups 



