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Advanced Data Communication Control Procedures
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<p>In <a href="/facts/Telecommunications/vxoj1cxR">telecommunications</a>, Advanced Data Communication Control Procedures (or Protocol) (ADCCP) is a <a href="/facts/Bit-oriented_protocol/cslzbVrg">bit-oriented</a> <a href="/facts/Data_link_layer/K9yPufbt">data link layer</a> <a href="/facts/Communication_protocol/5uady7cB">protocol</a> developed by the <a href="/facts/American_National_Standards_Institute/p3d1TQCb">American National Standards Institute</a>. It is functionally equivalent to the ISO <a href="/facts/High-Level_Data_Link_Control/aTndRUQ3">High-Level Data Link Control</a> (HDLC) protocol. </p><p>Although the ISO and ANSI standards writers coordinated their work, so the differences between the standards are mainly editorial, there is one meaningful difference: ADCCP's definition of the basic subset required to implement balanced asynchronous mode includes the RSET frame, while HDLC makes it optional. </p><p>One major difference between the two is the unnumbered (U) format. When extended (7-bit) sequence numbers are used, I and S frames have two-byte control fields. Like early versions of HDLC, ADCCP specifies a 2-byte control field format with the P/F flag duplicated. Later HDLC specifications, in particular ISO/IEC 13239, changed that to specify that U frames have 1-byte control fields in all cases. </p> ADCCP control fields<table><tbody><tr><th colspan="8">First byte</th><th colspan="8">Second byte</th><th rowspan="2">Description</th></tr><tr><th>0</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th><th>0</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th></tr><tr><td>0</td><td colspan="3">N(S)</td><td>P/F</td><td colspan="3">N(R)</td><td colspan="8" rowspan="3"></td><td>I frame, N(S) is a 3-bit send sequence number</td></tr><tr><td>1</td><td>0</td><td colspan="2">type</td><td>P/F</td><td colspan="3">N(R)</td><td>S frame, N(R) is a 3-bit receive sequence number</td></tr><tr><td>1</td><td>1</td><td colspan="2">type</td><td>P/F</td><td colspan="3">type</td><td>U frame</td></tr><tr><td>0</td><td colspan="7">N(S)</td><td>P/F</td><td colspan="7">N(R)</td><td>Extended I frame, N(S) is a 7-bit sequence number</td></tr><tr><td>1</td><td>0</td><td colspan="2">type</td><td colspan="4">—0—</td><td>P/F</td><td colspan="7">N(R)</td><td>Extended S frame, N(R) is a 7-bit sequence number</td></tr><tr><td>1</td><td>1</td><td colspan="2">type</td><td>P/F</td><td colspan="3">type</td><td>P/F</td><td colspan="7">—0—</td><td>Extended U frame (ADCCP only)</td></tr></tbody></table>