An Experimental 224 Mb/s Digital Repeatered Line

Digital transmission of information is becoming increasingly attractive in the telephone plant because it competes favorably both in performance and in cost for telephone service and it allows all kinds of other services such as data and television to share the transmission media with virtually no interaction among the various signals. The digital transmission process1 is based on (i) pulse code modulation of the analog signals to be transmitted, (ii) time interleaving of the resultant pulse streams to form a composite pulse stream, and (in) regeneration 993 994 T H E BELL SYSTEM TECHNICAL J O U R N A L , S E P T E M B E R 1 9 6 ( 5 in the repeatered line to nullify the effects of noise and distortion encountered in transmission. The T1 carrier system,2 3 extremely successful since its introduction in the Bell System in 1962, provided the first wide use of the digital transmission concept. In T l , 24 exchange area voice signals are pulse code modulated, time division multiplexed, and transmitted over cable pairs at a rate of 1.544 megabauds. Tl repeatered lines are now also in use for short-haul data services,4 since these lines are virtually insensitive to the source of the pulse streams transmitted. There has also been interest in making use of the features of digital transmission for long-haul transmission. Recently, at Bell Telephone Laboratories, a 224 megabits/second (Mb/s) experimental system was constructed to demonstrate the feasibility of a coast-to-coast system. The PCM terminals and time division multiplex portions of this system were described by J.