Real-Channel Aspects of an Error-Free Data Transmission System for Toll-Grade Telephone Circuits

A feedback communications system has been designed, constructed and tested for data transmission over toll-grade telephone circuits. The coding technique employed promises an ultra-low data error rate capability; however, it is shown that such reliability can only be assured when considerable attention is also given to the problems associated with bit and block synchronization. This paper describes the decisions which have been made in the design of the system in terms of telephone circuit characteristics. These characteristics were drawn from many thousands of hours of actual on-line testing. The effects of impulse and random noise are outlined, the hazards of long-term circuit dropouts are detailed and some solutions to these problems presented. Test results of a preliminary error-detection system and a discussion of the techniques used in the prototype feedback system conclude the paper.