ARQ-M
ARQ-M, short for Automatic Repeat reQuest, Multiplex, is a radio telegraphy protocol used to reliably forward telex messages over partially reliable radio links. It is a low-speed system designed to match the performance of landline telex systems and allow those messages to be forwarded over long distances using shortwave radios. The first ARQ-M link was built in the Netherlands, and began exchanging messages with a counterpart in New York in 1947.
ARQ-M is similar in concept to ARQ-E, but ARQ-E has no multiplex capability and uses a different 7-bit alphabet.
History
The telex system, which developed out of the telegraph system, is based on defined electric current levels that are interpreted as a mark or space signal. These are normally sent over well-defined networks with considerable infrastructure that keeps error rates very low. In contrast, radio communications are subject to a wide variety of noises and other signal problems that leads to losses. To forward telex messages successfully over radio links, some form of error correction should be applied.ARQ-M was developed to automatically deal with errors. The automatic repeat request system was invented by Hendrik van Duuren of the Netherlands in the 1940s and so it became known as the Van Duuren automatic error correction system. The seven unit code used was called the Van Duuren code.
ARQ was first used commercially in 1947 with a link from Amsterdam to New York with the master station in Amsterdam. In May 1950 the TEX service between New York and Amsterdam started. TEX was an early name for TELEX which is a system to allow automatic routing of private messages from a sender to a receiver on the TELEX network. Prior to the introduction or ARQ-M there were so many errors in the messages received that staff from the telecommunications companies had to manually check messages and confirm correct receipt. Once ARQ-M was introduced, messages could be automatically and reliably sent over long distances via shortwave radio, and so was introduced into automatic systems. By 1956 ARQ radio circuits were running from New York to Amsterdam, Berne, Brussels, Frankfurt, London, Paramaribo, Paris, Rome, and Wien, with plans for Copenhagen, Lisbon, Madrid, and Oslo. Amsterdam had radio links to Berne, Curaçao, Jakarta, Paramaribo and Rome. Brussels linked to Leopoldville. Frankfurt was connected to Madrid and Rome with plans for Buenos Aires. A link from Madrid to Canary Islands was planned. By 1959 most radio circuits for Telex were using ARQ. International shortwave ARQ data links have 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 till 1961. The radio link was available for 22 or 23 hours a day, and could improve the error rate from 1% to 0.001%. In 1961 the radio system became a backup for the primary Commonwealth Pacific Cable System undersea cable, with telegraph operations conducted using voice frequency telegraphy over telephone channels instead.
The Department of Transport in Australia used to run several multiplexed ARQ data links to remote airports prior to the use of AUSSAT. These were calls the aeronautical fixed teletype network. It was used to communicate aeronautical and meteorological data. A station at Perth, VZPH and VKP communicated with Cocos Island VZCC, Port Hedland VZPD, Darwin VZPM, and Sydney VZSY. The stations in Sydney communicated with Alice Springs, VZAS, Mount Isa, VZMA, Norfolk Island VZNF, and Lord Howe Island, VZLH. International radioteletype links connected Sydney to Singapore on 9ME and Jakarta 8BB.
Radio Corporation of America, who were the first in USA to use ARQ-M, named their ARQ equipment "automatic error reduction and correction equipment".
By 1990 use of ARQ-M had reduced. However it was still used by French Forces to communicate between Paris, and N'Djamena, Djibouti, Port de France, Papeete, Dakar, Port-Bouët, Le Port, Reunion. One link ran between Companhia Portuguesa Radio Marconi in Lisbon and the Vatican. The British Royal Army, Navy and Air force still used it from England and Gibraltar. Moscow had a link with Kabul, Havana and Hanoi. ASECNA had a network of west African airports. Moroni in the Comoros has a link to Paris, Stockholm had a link to Ho Chi Minh City, and Brussels had a link to Kinshasa. Canadian armed forces used shortwave radio as satellite links into the Arctic are difficult. Martin-de-Viviès, and Port-aux-Français connected back to France for Direction des Telecommunications des Reseaux Exterieurs.
On 17 November 2005 the European patent office rejected an appeal about overturned patent number 0309763 from NEC which attempted to patent a multiplexing system, after they agreed that it did not add anything new beyond European patent 0099101 A and the Siemens Elmux 1000.
Description
ARQ-M is a duplex radio communications protocol used to ensure that data is transmitted correctly over a partially unreliable radio path.Data is encoded using 7 unit binary codes sent using a seven bit error detecting code called Van Duuren code or CCITT Telegraph Alphabet No. 3. The data is sent in two channels labelled A and B, or four channels labeled A, B, C and D. Data from the different channels are interleaved using a time division multiplex system. The two channel system is called ARQ-M2, the four channel system is called ARQ-M4.
In order to synchronise to the synchronous data stream the encoded characters can be sent erect or inverted. Inverted means that the 0 and 1 are swapped, whereas erect means that the character is sent without modification. There are two possible channel arrangements depending on the latency of the circuit. For normal latency the cycle consists of four characters, and if the channel has extra long latency it has eight characters. Channel A has one character inverted followed by either three or seven erect characters. Channel B has one character erect followed by either three or seven inverted characters.
Transmission rates
There are several standard transmission rates. The preferred standard rates are 96 baud for the two channel system and 192 baud for the four channel system. These rates allowed running at the same speed as 50 baud Telex landline systems. The transmission cycle was ms long. Another standard rate allowed interoperation with 45 baud networks, with a repetition cycle lasting ms and baud rates of and for the two and four channel systems. The fastest standard speed used 100 and 200 baud, with a transmission cycle of 140 ms and was incompatible with land based networks, but more commonly used for point to point links.Performance
Two performance measures give quality characteristics of an ARQ-M link. These are error rate and throughput. Residual errors can be due to transpositions of the symbol elements or double errors. The chances that this happens is about 100 to 1000 times less than for a working unprotected link. A log graph of residual error rate against raw error rate shows a steeper line with slope 2 intercepting at 100% errors. If the unprotected 5 unit code had an error rate of 1%, the ARQ-M protected code error rate is 0.0025%.Throughput is reduced by errors. If the raw error rate increases to 3% the link will be blocked all the time with retransmissions. Each transmitted cycle of 28 bits has a likely chance of an error in these conditions. An eight character repeat cycle has twice the impact. At 1% raw errors the throughput for the 4 character cycle system is about 80%. On real equipment, an efficiency meter may indicate how good the throughput is.
Alphabet
The alphabet used in the ARQ-M protocol is based on the CCITT Telegraph Alphabet No. 2 more commonly known as Baudot. This alphabet has five bits, and therefore has 25 or 32 different possible symbols. It uses letters shift and figures shifts to select different sets of characters similar to a typewriter. The ARQ-M alphabet being synchronous always has to send data and does not have gaps between characters. It does not include start and stop bits that would be used in asynchronous transmissions and so saves on those two bits per character. In asynchronous transmissions a steady stop signal indicates that there is nothing to send. A prolonged start signal can be used for signalling, for example to select a destination for a telex connection.The ARQ-M characters are remapped to a seven bit system so that there are always three bits on and four bits off. The ARQ code is a Moore code, which can detect the modification of any single bit. This alphabet is CCITT Telegraph Alphabet No. 3 aka TOR No. 3. This differs from the alphabet used in the ARQ-E single channel system. Three additional control signals are added to the 32 from the Baudot set. Out of 128 seven bit characters there are 35 that have three one bits set, so allowing for the 32 Baudot and three control codes to just be accommodated. The extra control characters are RQ, meaning request repeat, α meaning constant space condition, which may indicate the end of a connection or be used to signal information, such as a number to connect to, and the β character, meaning constant mark condition, used when there is nothing to send. The RQ signal is also called Signal Roman one and may be designated "I".
The number of one bits is checked by the receiver to tell if an error has occurred in the transmission of the character. There should be three ones in each character received. The return channel will include the RQ signal if an error is detected.
- ltrs is the symbol to activate the letters shift.
- figs is the symbol to activate figures shift.
- Space is equivalent to the space bar
- cr is carriage return
- lf is line feed
- Blank entries have undefined meaning in international communications, but may be defined within a country.