Experimental data transmission system for switched telephone lines

Revision of "Systemtechnik der Datenuumlbertragung auf Fernsprechleitungen", in Nachrichtentech. Fachber. (NTF) (Germany), Vol. 19, 63-69 (1961). After stating the objectives of a high-speed data transmission system on ordinary telephone circuits the authors approach the chosen solution step by step. For transmission the duplex method without echo suppressors and the serial system are selected. Reasons for favouring frequency modulation follow. An extensive section is devoted to the decision of the method for coding and error correction, discussing two-motion and crossbar switch noise contributions, code redundancies and error rates. Finally the experimental system is described, operating with a telegraph speed of 800 Bauds and frequencies of 1500 and 2100 c/s, whilst the return channel, carrying repeat or proceed signals, operates on 150 Bauds at frequencies of 540 and 660 c/s. With 63-element blocks, of which 21 are redundant check elements, the calculated character error rate of between 10-7 and 10-8, depending on the signal level at the final selector, has been achieved.