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Wikipedia / SITOR

SITOR (SImplex Teletype Over Radio) is a system for transmitting text messages. It was developed in the 1960s by Koninklijke TNT Post as an improvement over radioteletype (RTTY). Although it uses the same frequency-shift keying (FSK) modulation used by regular RTTY, SITOR uses error detection, redundancy, and/or retransmission to improve reliability.

There are two SITOR modes:

  • SITOR-A is used for point to point links. SITOR-A uses automatic repeat request (ARQ) to gain reliability. If the receiver detects an error, it requests a retransmission.

  • SITOR-B is used for broadcast links. SITOR-B transmits each character in a message twice to gain reliability. If the receiver detects an error in the first character, it uses the copy. If both characters are garbled, the receiver won't know what was sent.

  • SITOR-B by definition uses forward error correction (FEC), versus ARQ for SITOR-A.

SITOR sends 7-bit characters as a bit stream at 100 baud (which, in this case, is 100 bits per second, 10 milliseconds per bit, or 70 milliseconds per character).

The bitstream is FSK modulated with a 170Hz frequency shift. The high frequency is a mark, and the low frequency is a space.

Applications

SITOR is used commonly on shortwave bands, where it is used to transmit maritime-related information such as weather forecasts and storm warnings.

SITOR-B is used for narrow-band direct printing (NBDP).

NAVTEX marine weather and safety messages are broadcast using SITOR-B. The NAVTEX messages have a specific format that is interpreted by NAVTEX receivers. (NAVTEX is layered on top of SITOR-B just as HTTP is layered on top of TCP.)

Amateur radio uses SITOR but calls it AMTOR, AMateur Teleprinting Over Radio. AMTOR-A is SITOR-A. AMTOR-B (also called AMTOR-FEC) is SITOR-B. In 1991, an AMTOR extension was described that includes lower case and other printable ASCII characters.

Technical Details

Character set

RTTY uses the ITA2 (Baudot code) character code. ITA2 is a five bit code with 32possible code points. Four code points are used for null (), space (), carriage return (), and line feed (). Two code points are used for a letter shift () or a figure shift (). The remaining 26code points are used for characters in the letters and figures sets. Consequently, ITA2 can represent 2×26= 52additional characters.

SITOR recasts ITA2 into a 7bit code called CCIR 476. Each 7bit character in CCIR476 has 4marks (ones) and 3spaces (zeros). Each valid character code has a Hamming distance of at least2 from every other character. A single-bit error will disrupt the balance of marks and spaces; a second bit error may (or may not) bring the count back to 4marks and 3spaces. Consequently, the CCIR476 alphabet is guaranteed to detect all single bit errors within a character.

SITOR control characters

The number of valid binary code values in CCIR476 is the number of ways to choose 4marks for 7bit positions, and the number can be calculated using the binomial coefficient Thus CCIR476 has 3additional code points over ITA2. SITOR uses the additional code points for idle, phasing, and repeat requests. In addition, some of the ordinary characters are reused as control signals.

  1. {| class="wikitable"

|- ! Code(bits6543210) ! modeA(ARQ) ! modeB(FEC) |- | 1100101 ["L"] | Control signal1 (CS1) | |- | 1101010 [] | Control signal2 (CS2) | |- | 1011001 ["N"] | Control signal3 (CS3) | |- | 0110101 ["G"] | Control signal4 (CS4) | |- | 1101001 ["H"] | Control signal5 (CS5) | |- | 0110011 | Idle signal | Idle signal |- | 0001111 | Idle signal | Phasing signal1 Idle signal |- | 1100110 | Signal repetition (RQ) | Phasing signal2 |}

SITOR-A

Transmission in synchronous frames of 450ms. Three characters are transmitted by the Information Sending Station (ISS), which takes 210ms. The ISS then waits 240ms for a response. The Information Receiving Station (IRS) receives the three characters and checks that they each have four marks and three spaces. If they do, then the IRS transmits an acknowledgement. If they don't, then the IRS requests retransmission. At the beginning of the next frame, the ISS either retransmits the last three characters or transmits the next three characters.

SITOR-B

SITOR has an aurally easy to identify idling pattern. Synchronization bursts are transmitted every second or so and last for slightly more than one second. Every few sync bursts, a Morse identifier of three letters is transmitted (for example NMO in Honolulu).

See also

References