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SITOR (SImplex Teletype Over Radio) 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-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).〔check this〕 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 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.〔 claims groups of 5 characters are sent, but it is probably in error.〕 In 1991, an AMTOR extension was described that includes lower case and other printable ASCII characters. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「SITOR」の詳細全文を読む スポンサード リンク
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