Sunday, April 27, 2008

Digital Mode of the Week: SITOR

SITOR stands for Simplex Telex Over Radio or Simplex Teleprinting Over Radio. It uses the same type of frequency-shift keying (FSK) as RTTY. Mark and space are used. The shift is always 170 Hz, and speed is always 100 baud.

SITOR was developed in the 1960s for use in the radiotelex and maritime narrowband direct printing services, as an improvement on RTTY. It adds error checking, reducing garble over noisy and fading HF circuits. The trade-off is that timing is far more important than in RTTY. Much tighter technical standards are needed for acceptable communication.

SITOR has two modes, A and B. SITOR-A is a fully synchronous two-way mode for traffic handling. Two stations alternate half-second transmit intervals with half-seconds for listening, exchanging short bursts which give the system its distinctive chirp-chirp-chirp sound. You can't mistake this one.

Mode A uses an error checking protocol named ARQ, for Automatic Repeat reQuest. Messages are broken up into 3-character blocks. The station sending these is the Information Sending Station (ISS). The other station is the Information Receiving Station (IRS). The IRS replies with a signal that the received block passed an error check (ACK for acknowledgement), or that it did not (NAK for negative acknowledgement).

Bad blocks are resent as many times as needed, within reason. The effect is that SITOR-A handles degraded circuits not by garbling the message but by slowing down, to a maddening snail's pace if necessary.

To copy SITOR-A, you need to be tuned to the ISS. Its bursts are a little longer than those of the IRS. With some experience, you can tell the difference. It is tricky for the casual listener to properly phase with the ISS. When you do, most decoders will simply print repeated blocks over and over again.

That incredible screech you hear on maritime telex channels is the SITOR-A tuning marker sent by the coast station. It consists of pulses at the baud rate sent in several bursts a couple of seconds long, usually followed by the station callsign in Morse code keyed with the mark tone.

SITOR mode B is a continuous broadcast system using Forward Error Correction (FEC). It sounds like a sped-up, less chattery version of standard RTTY. Being a broadcast, the only station transmitting is the sender. Everyone else listens. Characters are sent in a stream, but with a built-in redundancy in which each character is sent again three characters later. Such combined repetition sequences are called interleave in the jargon.

The concepts of ARQ, FEC, and interleave come up again and again in different digital modes. Many systems use different interleaves. In this one, ABCDE [end of message] would be sent ABCADBEC D E [end of message]. Characters not received twice are dropped, with the idea being that a missing character is better than a wrong one. Therefore, SITOR-B does not slow down on degraded circuits. If everything works right (a big if), it just stops printing.

SITOR-B is easier to receive than A, but it is still necessary to achieve sync for the error check. The special characters ALPHA and BETA are provided for phasing. This is especially evident in the NAVTEX service, which sends these phasing pairs between each of its short messages.

SITOR also uses a different transmission alphabet, called CCIR 476. The scheme is called 4/7. Each character is 7 bits long, but as an additional error check, there are always a total of 4 ones and 3 zeroes in the character. While a larger number of bits would usually mean a larger character set, this requirement means that most bit patterns are in fact error characters, and the character set is actually smaller. Again, there are LTRS and FIGS cases, selected by the appropriate shift characters. Again, optional USOS (UnShift On Space) is usually provided to partially deal with missed shift-outs.

A ham radio version of SITOR is called AMTOR. The differences between the two are slight, and for us, AMTOR can be considered the same thing. The same software usually works for both. ARRL headquarters station W1AW in CT transmits daily bulletins in AMTOR mode B on the same frequencies as RTTY. AMTOR mode A imposes timing and transmit/receive switching demands that amateur gear is rarely designed for, and it is not widely used.