An Automatic Computer-to-Computer Digital Communications System

This paper discusses the design of an automatic on-line computer-to-computer intercommunications system with locations at several data processing centers throughout the world. The solution to the problem is developed in logical order starting with the selection of a transmission medium. Leased private wire telephone facilities were found to be the best world-wide common denominator for economically meeting reliability and speed (1200, 2400 and 600 bit/sec) criteria. Pertinent statistical information characterized the transmission environment with short periods of high burst noise and long periods of error-free transmission. A statistically related burst "error model" of 48 successive bits subject to error was derived. Several available error detecting and correcting codes were considered, and the limitations of each explored. A geometric block code combined with a Hamming cyclical code was found to possess excellent burst error detection capability (343 consecutive bits subject to error or an error correction capacity of one "error model" of 48 bits). When this forward acting code is combined with decision feedback the probability of undetected error entering the system is reduced to less than that in computer systems. Implementation of the code structure is uniquely suitable to time sharing and linear-feedback shift-register techniques.