ARQ-M

ARQ-M (Automatic Repeat reQuest, Multiplex) is a low-speed radio telegraphy protocol developed to reliably send telex messages over shortwave radio links, which are prone to interference. It was designed to match the performance of landline telex systems and allow long-distance message forwarding. The first ARQ-M link was built in the Netherlands and began exchanging messages with a counterpart in New York in 1947.

The protocol is conceptually similar to ARQ-E, but ARQ-E lacks multiplex capability and uses a different 7-bit alphabet. ARQ-M was created to automatically handle errors on radio circuits. The automatic repeat request system was invented by Hendrik van Duuren of the Netherlands in the 1940s, and the seven-unit error-detecting code used became known as the Van Duuren code, or CCITT Telegraph Alphabet No. 3.

ARQ was first used commercially in 1947 on a link from Amsterdam to New York, with the master station in Amsterdam. In May 1950, the TEX (Overseas teleprinter exchange) service began between those two cities. TEX was an early name for TELEX, a system for automatically routing private messages across the network. Before ARQ-M, errors were so frequent that telecommunications staff had to manually check and confirm messages.

With ARQ-M, messages could be sent automatically and reliably over long distances via shortwave, and it was integrated into automatic systems. By 1956, ARQ radio circuits connected New York to Amsterdam, Berne, Brussels, Frankfurt, London, Paramaribo, Paris, Rome, and Vienna, with plans for Copenhagen, Lisbon, Madrid, and Oslo. Amsterdam had links to Berne, Curaçao, Jakarta, Paramaribo, and Rome. Brussels linked to Leopoldville.

Frankfurt was connected to Madrid and Rome, with plans for Buenos Aires, and a link from Madrid to the Canary Islands was planned. By 1959, most telex radio circuits used ARQ. These international shortwave ARQ data links have since been replaced by undersea cables and satellite circuits.

The New Zealand Post Office used a two-channel multiplex ARQ system to communicate on two circuits between Wellington and Vancouver and Sydney over shortwave radio for a few years until 1961. The radio link was available 22 or 23 hours a day and improved the error rate from 1% to 0.001%.