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ITA-5 → ← ITA-3 ← ITA-2
Simplex Teletype-Over-Radio · CCIR 476-3
SITOR, short for Simplex Teletype-Over-Radio, also known as AMTOR,
is a system for transmitting digital text-based messages
(telex, teletype) over narrowband radio channels,
developed in the 1960s by the Dutch Post Office (PTT).
It is an improvement over standard radioteletype (RTTY) in that it has
provisions for error detection and correction, known as
Automatic Repeat Request (ARQ).
It uses a 7-bit remapped telegraph alphabet which is similar
(but not identical) to ITA-3.
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- Different mapping of the ITA-2 teletype characters
- Three additional control signals in the return path (CS1, CS2, CS3)
- 4:3 MARK/SPACE ratio (instead of 3:4)
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- SITOR-A
Basic form of SITOR, also kown as SITOR-ARQ,
using detection based on ARQ
[1].
In the CCIR Recommendation 476-3 it is defined as mode A [2].
It uses its own specific alphabet which is described below.
Each character is defined by 7 bits, consisting of four 1s and three 0s.
This means it has a MARK/SPACE ratio of 4:3. 1
Furthermore, the positions of
the 1s are chosen in such a way that a minimum of 2 bits are different between
any two adjacent characters. This means that the codes have a Hamming Distance
of ≥ 2.
- SITOR-B
This is the broadcast mode of SITOR, in which redundant data is added to the
data stream to allow error detection and correction. This is also known as
Forward Error Correction (FEC).
In the CCIR Recommendation 476-3 it is defined as mode B [2].
It uses the same alphabet as SITOR-A.
SITOR B is also known as SIFEC or SITOR-FEC.
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The table below shows the assignment of the characters of the SITOR alphabet
to 7-bit values. Of the 128 characters that can be made with 7-bits (27),
only 38 are used. 32 of these characters are the same as the 32 characters of
the ITA-2 alphabet (Baudot), remapped to 7-bit space.
The remaining six are special ARQ/SITOR characters
(RQ, α, β, CS1, CS2, CS3), shown below in green.
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| Hex | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | A | B | C | D | E | F |
| 0x | | | | | | | | | | | | | | | | α |
| 1x | | | | | | | | J | | | | F | | C | K | |
| 2x | | | | | | | | W | | | | Y | | P | Q | |
| 3x | | | | β | | G | FIG | | | M | X | | V | | | |
| 4x | | | | | | | | A | | | | S | | I | U | |
| 5x | | | | D | | R | E | | | N | LTR | | SP | | | |
| 6x | | | | Z | | L | RQ | | | H | NUL | | LF | | | |
| 7x | | O | B | | T | | | | CR | | | | | | | |
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Control character
Printable character
ARQ/SITOR character
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SITOR-A, commonly known as SITOR or SITOR-ARQ, is the basic version
of the SITOR format. The sending station transmits short bursts of data
and then waits for an acknowledgement.
In the description of the data flow, the following expressions are used:
ISS TX Information Sending Station
IRS RX Information Receiving Station
Master MSTR Station determining the time frame
Slave SLAV Station adapting itself to the Master time frame
The baudrate is always 100 baud, which means that each bit has a duration of
10 ms. The ISS sends pulses of 210 ms, which is equal to 21 bits or 3
characters. The IRS sends 70 ms acknowledge pulses, by means of
one of the three special control characters (α), (β) or (RQ).
Alternating CS1 and CS2 characters notify the ISS of correct reception.
If the ISS receives the same acknowlege character on two successive
frame (i.e. 2 x CS1 or 2 x CS2), it repeats the last three characters.
Framing is different for Master and Slave, depending on which of the two
is the ISS. In the examples below, the upper line (red) is the Master,
whilst the lower line (blue) is the Slave. In example A the ISS is the Master.
In example B, the IRS is the Master.
Note the position of the
acknowledge character at the switch-over point of a station from ISS to
IRS. The Master places its acknowledge at the end of a time slot, whereas
the Slave puts the acknowledge at the beginning of the time slot.
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SITOR-B, also known as SITOR-FEC,
is described in CCIR Recommendation 476-3 as mode B.
It is the broadcast mode, in which the sending station transmits
continuously wihtout interruption, with no feedback from the receiver.
Data is sent as 7-bit characters, using the same alphabet as
SITOR-A, at a speed of 100 baud with an FSK shift of 170 Hz.
It has a collective mode, in which many recipients are addressed, and a
selective mode, in which a single recipient is addressed.
Forward Error Correction (FEC) is achieved by sending each chracter
twice at a 35-bit distance (i.e. 350 ms), as illustrated in the diagram above.
Every second character is the repetition of a previously sent character.
A transmission starts with the so-called phasing signals CS1 and CS2,
followed by the actual data. In selective mode, the data is inverted.
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The table below shows the 7-bit SITOR telegraphy alphabet (orange) in
comparison to the regular 5-bit ITA-2 alphabet and the
7-bit ITA-3 ARQ alphabet. SITOR is very similar to ITA-3
but uses a different mapping of the telegraph characters and has
a 4:3 MARK/SPACE radio of the seven bits.
In the table below,
all binary values are shown with the least significant bit (lsb) at the
right. This is the regular notation in computer software. The dot (·) shows the
position of the sprocket hole in punched paper tape. Note that
5-channel paper tape (ITA-2)
has the same layout as the bit order shown here (lsb at the right).
With 7-channel paper tape (ITA-3, SITOR), the lsb is at the left.
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Name SITOR-A Method Synchronous, burst, half-duplex (simplex-ARQ) Inventor PTT (Netherlands) Users Radio Amateurs, Marine service, Government, Diplomatic services Modulation FSK2 Speed 100 baud M/S ratio 4:3 Frequency HF FSK shift 170 Hz (amateurs, government)
300 Hz, 400 Hz, 850 Hz (Air Force, Navy, Ministry) Standard ITU-R Recommendation M.476-5
ITU-R Recommendation M.625-3
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- SITOR-A
- SITOR Mode A
- SITOR-ARQ
- ARQ
- ARQ 625
- TOR
- AMTOR
- AMTOR-A
- AMTOR-ARQ
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Name SITOR-B Method Synchronous, continuous, simplex broadcast Inventor PTT (Netherlands) Users Radio Amateurs, Marine service, Government, Diplomatic services Modulation FSK2 Speed 100 baud M/S ratio 4:4 Frequency HF FSK shift 170 Hz Standard ITU-R Recommendation M.476-5
ITU-R Recommendation M.625-3
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- SITOR-B
- SITOR Mode B
- SITOR Broadcast
- SITOR-FEC
- SIFEC
- AMTOR-B
- AMTOR-FEC
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© Crypto Museum. Created: Monday 10 February 2025. Last changed: Tuesday, 18 February 2025 - 10:09 CET.
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