Some Results of Data Transmission Tests Over Leased Telephone Circuits

Data transmission tests were performed on leased telephone circuits using two different sets of terminal equipments. The first applied phase-reversal modulation (PM), the second, frequency-shift keying (FSK). The transmission speeds were 750 or 1625 baud in the former and 750 or 1500 baud in the latter case. Nine different circuits, having an average length of 980 km, were tested. Only erroneous blocks were recorded, the blocklength being in the PM case one quarter and in the FSK case one half of the 250 bits provisionally recommended by the CCITT. A block error occurred, on the average, in every 1000-10,000 correct blocks. On shorter circuits or under special conditions average error rates of one in 100,000 seem possible. On the average, 250-millions bits were transmitted per test. The test procedure is illustrated in one selected example, which is also particularly interesting because of the unusually high outage time of the circuit during the test period.