SITOR

Key takeaways

SITOR belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect SITOR to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of SITOR from memory before moving on to harder problems.

Reference excerpt

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 170 Hz 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 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 2×26 = 52 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 single-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.

SITOR control characters The number of valid binary code values 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: ( 7 3 ) = ( 7 4 ) = 35 . {\displaystyle \ \textstyle {\binom {7}{3}}={\binom {7}{4}}=35\ .} 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.

SITOR-A Transmission in synchronous frames of 450 ms. Three characters are transmitted by the Information Sending Station (ISS), which takes 210 ms. The ISS then waits 240 ms 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).

Worked examples

Example 1 — a first encounter with SITOR

Start with the simplest possible case. Write down what SITOR claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to SITOR before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about SITOR ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of SITOR

In research: SITOR appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses SITOR in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: SITOR is common in secondary-school and first-year university syllabi. It links to neighbouring topics Quantized radio modulation modes, so understanding it makes those chapters shorter.
In everyday life: Look for SITOR outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study SITOR in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what SITOR means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain SITOR out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is SITOR in simple terms?

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).

Why does SITOR matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study SITOR?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on SITOR.