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Bit stuffing
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<h2 id="zero-bit-insertion">Zero-bit insertion</h2> <p>The <a href="/facts/NRZI/8f3cfm9r">NRZI</a> coding scheme transmits a 0 bit as a signal transition, and a 1 bit as no change. In this case, bit stuffing is most easily described as the insertion of a 0 bit after a long run of 1 bits. </p><p>It was popularized by IBM's <a href="/facts/Synchronous_Data_Link_Control/KoVGwb3j">SDLC</a> (later renamed <a href="/facts/HDLC/aTndRUQ3">HDLC</a>), and is also used in Low- and full-speed <a href="/facts/USB/XcIKWLyg">USB</a>. </p><p>After a long sequence of 1 bits there would be no transitions in the transmitted data, and it would be possible for the transmitter and receiver clocks to lose synchronisation. By inserting a 0 after five (SDLC) or six (USB) consecutive 1 bits the transmitter guarantees a maximum of six (SDLC) or seven (USB) bit times between transitions. The receiver can synchronise its clock against the transitions to ensure proper data recovery. </p><p>In SDLC the transmitted bit sequence "01111110" containing six adjacent 1 bits is the <a href="/facts/High-Level_Data_Link_Control/aTndRUQ3">Flag byte</a>. Bit stuffing ensures that this pattern can never occur in normal data, so it can be used as a marker for the beginning and end of the frame without any possibility of being confused with normal data.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> </p><p>The main disadvantage of bit-stuffing is that the <a href="/facts/Code_rate/xPrLLTwt">code rate</a> is unpredictable; it depends on the data being transmitted. </p><p><i>Source: from <a href="/facts/Federal_Standard_1037C/CA4jqghV">Federal Standard 1037C</a> in support of <a href="/facts/MIL-STD-188/D5rsvdsW">MIL-STD-188</a></i> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Consistent_Overhead_Byte_Stuffing/54CB0JLL">Consistent Overhead Byte Stuffing</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Kevin R. Fall and W. Richard Stevens, TCP/IP Illustrated Volume 1: The Protocols, Second Edition, Addison-Wesley, 2012, Kindle Edition loc 3505 <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> </ol>