14 April 2015


SITOR-ARQ is a simplex/duplex FSK 100Bd/170 system where both stations transmit alternatel. The CCIR recommendation 476-3 defines a cycle of 450 ms and the two stations are designated as follows:

ISS: information sending station
IRS: information receiving station

The SITOR 7 bit alphabet employs a 3:4 mark-space ratio. The ISS transmits blocks of 210 ms duration each containing 21 bits. In turn the IRS transmits acknowledgement bursts with a duration of 70 ms. If the ISS does not receive an acknowledgement, a block containing the RQ character is sent. In the case of blocks being received in error by the IRS, the last data block sent is repeated.

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