L5 Coaxial-Carrier Transmission System, Foreword

December 1974 Telephone and Telegraph Company, Printed in Number 10 U.S.A. L5 Coaxial-Carrier Transmission System Foreword Coaxial-carrier transmission systems constitute a significant portion of the Bell System long-haul transmission facilities. These systems have been developed over several decades to provide the basis of a high-quality, high-capacity, long-distance communications network. After extensive exploratory work on wideband amplifiers and coaxial cable, the feasibility of a coaxial-carrier system was demonstrated in 1936 between New York and Philadelphia. The success of this trial was followed by development of the first Bell System coaxial-carrier transmission system, the LI. Placed in service in 1941, the LI system was initially capable of carrying 480 four-kHz two-way message channels per pair of 0.270-inch-diameter coaxial cables, with a repeater spacing of 5.5 miles. Soon, 0.375-inch-diameter cable became standard and, with system improvements, the vacuum-tube-operated LI system was capable of carrying 600 circuits per coaxial pair with 8-mile repeater spacing. Its capacity was later increased to 720 circuits. The major expense of the coaxial system has been in the outside plant area: cable, cable placement, right-of-way, and buildings, including aboveground or underground structures for housing repeaters. Once this portion of the system is established, development of electronic equipment to provide maximum utilization of the cable becomes economically attractive.