Talk:SITOR

Unfortunately when SITOR was designed, it was named Simplex [Telex, Teleprinting, Teletype] Over Radio. It was an attempt to add error-checking to the previously used 5-bit Baudot system, which has none, and while they were at it they increased the baud rate to 100 and generally tightened up the timing, allowing stations to exchange rapid-fire bursts on the same frequency.

Actually a very closely related variant called AMTOR - Amateur Teleprinting Over Radio - is pretty much as a rule used simplex in Mode A, the chirp-chirp mode. You can hear the two stations alternating chirps - weird sound.

People who are into digital communications will recognize this as a simple ack/nak protocol. Hence the name ARQ, Automatic Repeat ReQuest, for mode A. The one shown in the sample spectrogram, which btw I made, though I certainly have no problem with its use, is actually a broadcast in continuous mode B, FEC, Forward Error Correction. The signal shown right next to it is the type of ARQ marker described, which indeed is 3 or 4 1-second alternations of mark and space at 100 baud, followed by the mark tone being keyed on and off in Morse code with the station's callsign. This is not the only marker in use, but it allows easy phasing by the other station and it is the most common. The receiving station has to phase to the sender before making a call, regardless of whether simplex or duplex, due to the tight timing mentioned before.

Technically SITOR-A is an assymetrical protocol, where stations take turns being the ISS, Information Sending Station, and IRS, Information Receiving Station. The receive burst is shorter, as it is a simple ack/nak. The ISS burst is something like 3 characters long. It lasts half a second. A nak'd or not ack'd burst is repeated until confirmed, making SITOR get veeeeeerrrrrrrryyyyyyyyyyyy sssssssssslllllllllooooooooowwwwwwwwwwww, like this, when conditions are bad.