4 November 2014

ARQ-E (ARQ-1000, ARQ-N)

The ARQ-E (also known as ARQ-1000D and ARQ-N) is a single channel duplex ARQ system made by Siemens and mainly used by French Military Forces. Many signals idle for long periods of time in order to maintain links up and send very little traffic.

ARQ-E uses FSK modulation and operates at symbol rate of 46.2, 48, 50, 64, 72, 86, 96, 144, 184.6, 192, and 288 Baud with different shift values: the system can be adjusted with different gear variants to support user requirements.

ARQ-E/184-6/850                                                                                      ARQ-E/184.6/388

ARQ-E/184-6/850                                                                                      ARQ-E/184.6/388

 ARQ-E synchronization may be started by the selection of a baud rate. An AUTO program
start causes the automatic determination of the frequency shift and baud rate to be executed first. The signal polarity (USB or LSB sidebands) is automatically detected.
After synchronization to an ARQ-E system has been achieved, the detected repetition rate is displayed (4 or 8 cycles). This parameter may give clues to the identity of the transmission.
If a continuously repeated character (often FFFF) is decoded whilst working in the ARQ-E mode, it is most likely an ARQ-E3 system being monitored. ARQ-E employs the ARQ-1A alphabet with parity checking which allows the detection of transmission errors.
Full-duplex systems transmit a RQ character after having detected an erroneous character or in the presence of excessive signal distortions. The remote station subsequently repeats the last three, four or seven characters preceded by the RQ character.

To maintain synchronization between the two stations both transmitters operate continuously and send the idle bit pattern if no traffic is transmitted.


ARQ-E alphabet
Using the 7 bit ITA 2-P alphabet with 4 or 8 character repetition cycle, inverting every 4th or 8th character to facilitate block synchronisation.
Each 5 unit character arriving from the landline connection is converted very easily into a 7 unit code. The first bit of the 7 unit code is used as a "character identifier element". It identifies the character as either being a traffic character or a supervisory or control character. If it is of start polarity if the character is a traffic character, and of stop polarity if the character is a control signal, such as a idle Alpha, Beta or an RQ.
There then follows, for bits 2 to 6, the normal 5 unit telegraph ITA-2 code used on the landline connection. The 7th bit is a polarity bit. It is chosen so that the total number of stop polarity bits will be an odd number. If an even number of stop polarity bits results from the first 6 bits of the 7 unit code, a stop polarity element will be added as the 7th bit. If on the other hand, the number of the stop polarity elements is found to be an odd number, a start polarity element will be added.

Although only a 4 or 8 character cycle has been mentioned above, there is also a special 5 character block length. When using on-line crypto, it is extremely important not to lost any characters in the continual stream of random traffic characters. To ensure that a RQ block is not missed for what it is, a double RQ is transmitted. The sending of a double RQ character thus ensures that the sending station is fully aware that a repetition was requested.
There is also a SELCAL facility. When a station is called with its station number, it automatically brings up the transmitter and returns its selcal number back and enters the idle Beta signal state. When traffic is finished, either station can send a special cleardown signal which is also acknowledged but turning off both transmitters.

ARQ-N is identical to ARQ-E except that all characters are Erect, i.e. there are no character inversions every 4 or 8 character cycle.

RFFA French Mil, F - ARQ-E/184.6/850 (Cycle 8) in idle status
An ARQ-E short recording may be heard here

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