SITOR
Encyclopedia
SITOR is a system for transmitting text messages. Although it uses the same frequency-shift keying
(FSK) modulation used by regular radioteletype
(RTTY), SITOR uses error detection, redundancy, and/or retransmission to improve reliability.
There are two SITOR modes:
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.
SITOR-B is used for narrow-band direct printing (NBDP).
The United States Coast Guard
uses SITOR-B for NAVTEX
. NAVTEX 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 HTML is layered on top of TCP.)
Amateur radio
uses SITOR but calls it AMTOR
, AMateur Teleleprinting 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.
) character code. ITA2 is a five bit code with 32 possible code points. Four code points are used for null (BLANK), space (SPACE), carriage return (CR), and line feed (LF). Two code points are used for a letter shift (LTRS) or a figure shift (FIGS). The remaining 26 code points are used for characters in the letters and figures sets. Consequently, ITA2 can represent 52 (2*26) additional characters..
SITOR recasts ITA2 into a 7 bit code called CCIR 476. Each 7-bit character in CCIR 476 has 4 marks (ones) and 3 spaces (zeros). Each valid character code has a Hamming distance
of at least 2 from every other character. A one bit error will disrupt the balance of marks and spaces; a second bit error may (or may not) bring the count back to 4 marks and 3 spaces. Consequently, the CCIR 476 alphabet is guaranteed to detect all single bit errors within a character.
The number of valid characters in CCIR 476 is the number of ways to choose 4 marks for 7 bit positions, and the number can be calculated using the binomial coefficient
:
. Thus CCIR 476 has 3 additional 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. (CCIR Rec. 625 Service Information Signals.)
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
(FSK) modulation used by regular radioteletype
Radioteletype
Radioteletype is a telecommunications system consisting originally of two or more electromechanical teleprinters in different locations, later superseded by personal computers running software to emulate teleprinters, connected by radio rather than a wired link.The term radioteletype is used to...
(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 requestARQAutomatic Repeat reQuest , also known as Automatic Repeat Query, is an error-control method for data transmission that uses acknowledgements and timeouts to achieve reliable data...
(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 is said to use forward error correctionForward error correctionIn telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
(FEC).
SITOR sends 7-bit characters as a bit stream at 100 baud
Baud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...
(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).
The United States Coast Guard
United States Coast Guard
The United States Coast Guard is a branch of the United States Armed Forces and one of the seven U.S. uniformed services. The Coast Guard is a maritime, military, multi-mission service unique among the military branches for having a maritime law enforcement mission and a federal regulatory agency...
uses SITOR-B for NAVTEX
Navtex
NAVTEX is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships...
. NAVTEX 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 HTML is layered on top of TCP.)
Amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
uses SITOR but calls it AMTOR
AMTOR
AMTOR is a type of telecommunications system that consists of two or more electromechanical teleprinters in different locations that send and receive messages to one another. AMTOR is a specialized form of RTTY protocol...
, AMateur Teleleprinting 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.
Character set
RTTY uses the ITA2 (Baudot codeBaudot code
The Baudot code, invented by Émile Baudot, is a character set predating EBCDIC and ASCII. It was the predecessor to the International Telegraph Alphabet No 2 , the teleprinter code in use until the advent of ASCII. Each character in the alphabet is represented by a series of bits, sent over a...
) character code. ITA2 is a five bit code with 32 possible code points. Four code points are used for null (BLANK), space (SPACE), carriage return (CR), and line feed (LF). Two code points are used for a letter shift (LTRS) or a figure shift (FIGS). The remaining 26 code points are used for characters in the letters and figures sets. Consequently, ITA2 can represent 52 (2*26) additional characters..
SITOR recasts ITA2 into a 7 bit code called CCIR 476. Each 7-bit character in CCIR 476 has 4 marks (ones) and 3 spaces (zeros). Each valid character code has a Hamming distance
Hamming distance
In information theory, the Hamming distance between two strings of equal length is the number of positions at which the corresponding symbols are different...
of at least 2 from every other character. A one bit error will disrupt the balance of marks and spaces; a second bit error may (or may not) bring the count back to 4 marks and 3 spaces. Consequently, the CCIR 476 alphabet is guaranteed to detect all single bit errors within a character.
The number of valid characters in CCIR 476 is the number of ways to choose 4 marks for 7 bit positions, and the number can be calculated using the binomial coefficient
Binomial coefficient
In mathematics, binomial coefficients are a family of positive integers that occur as coefficients in the binomial theorem. They are indexed by two nonnegative integers; the binomial coefficient indexed by n and k is usually written \tbinom nk , and it is the coefficient of the x k term in...
:
. Thus CCIR 476 has 3 additional 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. (CCIR Rec. 625 Service Information Signals.)| Code (bits 6543210) | Mode A (ARQ) | Mode B (FEC) |
|---|---|---|
| 1100101 | Control signal 1 (CS1) | |
| 1101010 | Control signal 2 (CS2) | |
| 1011001 | Control signal 3 (CS3) | |
| 0110101 | Control signal 4 (CS4) | |
| 1101001 | Control signal 5 (CS5) | |
| 0110011 | Idle signal β | Idle signal β |
| 0001111 | Idle signal α | Phasing signal 1 Idle signal α |
| 1100110 | Signal repetition (RQ) | Phasing signal 2 |