All Things Digital

Terms

• Simplex Telex Over Radio – SITOR• Radio Teletype - RTTY– Narrow Band Direct Printing - NBDB– Frequency Shift Keying – FSK

• Error Correction– SITOR A - Automatic Repeat Request (point to

point)– SITOR B – Forward Error Correction (broadcast)

Terms

• The CharactersSITOR 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.Wikipedia.org

ASCII Code

American Standard Code for Information InterchangeWikipedia.org

The Code• RTTY uses 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.

Wikipedia.org

The Code• CCIR 476 - Extends ITA2 from 5 to 7 bits, using the extra 2 bits as

check digits. • 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.• 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:

• 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.

Wikipedia.org

Error Correction

• Error CorrectionSITOR A - Automatic Repeat Request (point to point)– Transmission in synchronous frames of 450ms. – Three characters are transmitted by Information Sending Station

(ISS). – Takes 210 milliseconds. – The ISS then waits 240 ms for a response. – Information Receiving Station (IRS) receives the three characters and

checks that they each have 4 marks (1) and 3 spaces (0). – 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

3 characters or the next three characters.

Error Correction

• Error CorrectionSITOR B - Broadcast– Time Diversity Error Correction• Each character is sent twice DX and the RX.• The first transmission (DX) of a specific character is

followed by the transmission of four other characters, after which the retransmission (RX) of the first character takes place, allowing for time-diversity reception of 280 ms.

Digital Selective Calling

Digital Selective Calling

• The system is a synchronous system using characters composed from a ten-bit error-detecting code

Digital Selective Calling

• Apart from the phasing characters, each character is transmitted twice (time diversity aka FEC) in a time-spread mode; the first transmission (DX) of a specific character is followed by the transmission of four other characters before the re-transmission (RX) of that specific character takes place, allowing for a time-diversity reception interval of:– 400 ms for HF and MF channels– 331/3 ms for VHF radio-telephone channels

• The classes of emission, frequency shifts and modulation rates are as follows:– F1B or J2B 170 Hz and 100 Bd for use on HF and MF channels– G2B for use on VHF at 1200 Bd

Digital Selective Calling• Dot Pattern and Phasing

– The phasing sequence provides information to the receiver to permit correct bit phasing and unambiguous determination of the positions of the characters within a call sequence.

– To provide appropriate conditions for earlier bit synchronization and to allow for scanning methods to monitor several HF and MF frequencies by ship stations, the phasing sequence should be preceded by a dot pattern (i.e. alternating B-Y or Y-B sequence bit synchronization signals) with duration of:• 200 bits

– At HF and MF, for "distress", "distress acknowledgement", "distress relay" and "distress relay acknowledgement" calls and for all calling sequences to ship stations.

• 20 bits– At HF and MF, for all acknowledgement sequences (except distress

acknowledgements and distress relay acknowledgements – see § 3.4.1 and Note 1) and for all calling sequences to coast stations (except distress relay calls – see § 3.4.1). At VHF for all calls.

NAVTEX