THE L3 SYSTEM DESIGN 783 



system on existing routes, the design of the LI system, the cable, and 

 the cable route layouts presented the L3 system with a definite plant 

 framework. The present day network of LI coaxial systems is shown in 

 Fig. L There are about 8,000 route miles of cable installed of which about 

 70 per cent consists of eight coaxials, the remainder consisting of six 

 and four coaxials. About 70 per cent of this cable uses coaxials with a 

 y^" diameter outer conductor, the present day standard. The remainder 

 uses the older 0.27'' diameter coaxials. All but a few miles of this cable is 

 plowed into the ground or placed in underground conduit. A piece of a 

 typical eight coaxial cable is shown in Fig. 2. Normally, the coaxials 

 are included in a lead sheath with interstitial pairs which are used for 



Fig. 2 — An 8-coaxial cable. 



control purposes. In many cases additional quads are included in the 



cable for other types of transmission systems. 



The broad objectives of the L3 system planning were: 



L The existing cable was to be reused. Thus, the cable loss and its 



variation with temperature, the cable irregularities due to manufacturing 



and splicing, and the power transmission capabilities of the cable became 



basic restrictions on the design of the L3 system. 



2. The LI telephone terminal equipment was to be reused. This 

 equipment involves channel banks, group and super-group equipment 

 and carrier supplies.^ This hmited the system planning to the use of 

 frequency division multiplex on a single sideband carrier suppressed 

 basis. 



3. It w^ould be desirable to reuse existing LI repeater locations and 

 buildings. The LI auxiliary repeaters are spaced at eight mile intervals 

 and housed in 6' x 9' concrete block huts. The LI main repeaters are 

 spaced at 40 to 160-mile intervals largely dictated by geographical and 

 power transmission considerations. 



