ARQ-E is a radio transmission method used to send data over short wave radio. ARQ-E is a full duplex synchronous data communications system that requests repeats if data is not received correctly. It uses an alphabet that can detect errors. Another name for this is ARQ-1000 duplex or ARQ-1000D. Alphabet The alphabet used in the ARQ-E protocol is an extension of 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. 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. In asynchronous transmissions a steady stop signal indicates that there is nothing to send. The ARQ-E characters are extended with an identification bit or signal element at the start to indicate whether it is a normal character or a function signal. This would add another possible 32 combinations to the code. But of the 32 only three are used. An extra symbol labelled α shows a start polarity, and another symbol labelled β indicates a steady stop polarity. So a stream of β characters will be sent if there is nothing else to send. The α and β symbols are called idle signals. The third symbol used is the RQ signal used to request a retransmit. A seventh bit is added to the character to indicate parity. Odd parity is used so that the number of stop polarity elements (1) is always odd. The parity bit is checked by the receiver to tell if an error has occurred in the transmission of the character. 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 cells with blank entries are undefined for international communications, but may have meaning within one country. Marking After the five bit characters are extended to seven bit, the polarity may be inverted to form a marking pattern, Either every fourth or every eighth character has its 0s and 1s (space and mark elements) transposed. There is also a variation with a cycle length of five characters, to be used when encrypters are in-line.