ARQ Sincrono Duplex. Utilizzato da due stazioni su frequenze differenti. Utilizza 7 bit, l'alfabeto ITA 2-P con 4 o 8 caratteri per ciclo di ripetizione. Inverte ogni 4° o 8° carattere. Nel modo ARQ-N tutti i caratteri sono Eretti.
ARQ1000 DUPLEX (aka ARQ-E and ARQ-N)
This system was invented by Siemens of Germany.It is a Synchronous Duplex ARQ system with each station using a different frequency. 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 (see code table). 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.
