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 (or C) has one character inverted followed by either three or seven erect characters. Channel B (or D) 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%.