ARQ-M

Lore & Background

ARQ-M was developed to automatically correct errors in telex messages sent over radio, which suffered from noise and signal problems. The automatic repeat request system was invented by Hendrik van Duuren of the Netherlands in the 1940s, using a seven-unit code known as the Van Duuren code. Before ARQ-M, staff had to manually check and confirm messages; after its introduction, messages could be automatically and reliably sent over long distances via shortwave. By 1956, ARQ radio circuits connected New York to multiple European cities, and Amsterdam had links to Berne, Curaçao, Jakarta, Paramaribo, and Rome. By 1959, most telex radio circuits used ARQ.

The protocol uses a 7-bit error-detecting code (CCITT Telegraph Alphabet No. 3) and time-division multiplexing for two or four channels (ARQ-M2 and ARQ-M4). Synchronization is achieved by sending characters erect or inverted. Transmission cycles vary with latency: four characters for normal latency, eight for long latency. Standard baud rates include 96/192 baud for compatibility with 50-baud telex landlines, and 100/200 baud for point-to-point links. The alphabet is based on Baudot (CCITT No. 2) but remapped to a seven-bit system with three bits on and four off, allowing detection of any single-bit error.

Reader's Guide

ARQ-M's significance lies in its role as a reliable bridge between landline telex networks and long-distance shortwave radio, enabling automated international telegraphy decades before undersea cables and satellites became dominant. The New Zealand Post Office used a two-channel multiplex ARQ system between Wellington, Vancouver, and Sydney, achieving 22–23 hours of daily availability and reducing error rates from 1% to 0.001%. In 1961, that radio link became a backup for the COMPAC undersea cable. The Australian Department of Transport ran multiplexed ARQ links to remote airports for aeronautical and meteorological data before AUSSAT. Radio Corporation of America, the first U.S. user, called their equipment 'automatic error reduction and correction equipment.' By 1990, use had declined, but ARQ-M was still employed by French Forces, the British military, and various national networks, including links from Paris to N'Djamena, Djibouti, and Papeete, and from Moscow to Kabul, Havana, and Hanoi. Canadian armed forces used shortwave ARQ where satellite links into the Arctic were difficult. International shortwave ARQ data links were eventually replaced by undersea cables and satellite circuits.

Did You Know?

• The first ARQ-M link began exchanging messages between Amsterdam and New York in 1947.
• The Van Duuren code used in ARQ-M is a seven-bit error-detecting code with exactly three bits on and four bits off.
• In 1961, a New Zealand ARQ radio link became a backup for the COMPAC undersea cable.
• The European patent office rejected an NEC patent for a multiplexing system in 2005, citing prior art including the Siemens Elmux 1000.