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<title>Modified AMI code</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Modified AMI code</span></span>

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<p><b>Modified AMI codes</b> are a digital telecommunications technique to maintain <a href="System" title="System">system</a> <a href="Synchronization" title="Synchronization">synchronization</a>. <a href="Alternate_mark_inversion" class="mw-redirect" title="Alternate mark inversion">Alternate mark inversion</a> (AMI) <a href="Line_code" title="Line code">line codes</a> are modified by deliberate insertion of <a href="Bipolar_violation" title="Bipolar violation">bipolar violations</a>. There are several types of modified AMI codes, used in various <a href="T-carrier" title="T-carrier">T-carrier</a> and <a href="E-carrier" title="E-carrier">E-carrier</a> systems.

</p>

<div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2></div>

<p>The <a href="Clock_rate" title="Clock rate">clock rate</a> of an incoming T-carrier is extracted from its bipolar line code. Each signal transition provides an opportunity for the receiver to see the transmitter's clock. The AMI code guarantees that transitions are always present before and after each mark (1 bit), but are missing between adjacent spaces (0 bits). To prevent loss of <a href="Synchronization" title="Synchronization">synchronization</a> when a long <a href="String_(computer_science)" title="String (computer science)">string</a> of zeros is present in the <a href="Payload_(computing)" title="Payload (computing)">payload</a>, deliberate bipolar violations are inserted into the line code, to create a sufficient number of transitions to maintain synchronization; this is a form of <a href="Run_length_limited" class="mw-redirect" title="Run length limited">run length limited</a> coding. The receive <a href="Terminal_equipment" title="Terminal equipment">terminal equipment</a> recognizes the bipolar violations and removes from the user data the marks attributable to the bipolar violations.

</p><p>T-carrier was originally developed for voice applications. When voice signals are digitized for <a href="Transmission_(telecommunications)" class="mw-redirect" title="Transmission (telecommunications)">transmission</a> via T-carrier, the <a href="Data_stream" title="Data stream">data stream</a> always includes ample 1 bits to maintain synchronization. (To help this, the <a href="%CE%9C-law_algorithm" title="Μ-law algorithm">μ-law algorithm</a> for digitizing voice signals encodes silence as a continuous stream of 1 bits.) However, when used for the transmission of <a href="Digital_data" title="Digital data">digital data</a>, the conventional AMI line code may fail to have sufficient marks to permit recovery of the incoming clock, and synchronization is lost. This happens when there are too many consecutive zeros in the <a href="User_(telecommunications)" title="User (telecommunications)">user</a> <a href="Data" title="Data">data</a> being transported.

</p><p>The exact pattern of bipolar violations that is transmitted in any given case depends on the line rate (<i>i.e.</i>, the level of the line code in the <a href="T-carrier" title="T-carrier">T-carrier</a> hierarchy) and the polarity of the last valid <a href="Mark_frequency" class="mw-redirect" title="Mark frequency">mark</a> in the user data prior to the unacceptably long string of zeros. It would not be useful to have a violation immediately following a mark, as that would not produce a transition. For this reason, all modified AMI codes include a space (0 bit) before each violation mark.

</p><p>In the descriptions below, "<style data-mw-deduplicate="TemplateStyles:r886049734">

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</style><span class="monospaced">B</span>" denotes a balancing mark with the opposite polarity to that of the preceding mark, while "<span class="monospaced">V</span>" denotes a bipolar violation mark, which has the same polarity as the preceding mark. In order to preserve AMI coding's desirable absence of <a href="DC_bias" title="DC bias">DC bias</a>, the number of positive marks must equal the number of negative marks. This happens automatically for balancing (<span class="monospaced">B</span>) marks, but the line code must ensure that positive and negative violation marks balance each other.

</p>

<div class="mw-heading mw-heading2"><h2 id="Zero_length_code_suppression">Zero length code suppression </h2></div>

<p>The first technique used to ensure a minimum density of marks was <b>zero code suppression</b> a form of <a href="Bit_stuffing" title="Bit stuffing">bit stuffing</a>, which set the least significant bit of each 8-bit byte transmitted to a 1. (This bit was already unavailable due to <a href="Robbed-bit_signaling" title="Robbed-bit signaling">robbed-bit signaling</a>.) This avoided the need to modify the AMI code in any way, but limited available data rates to 56,000 bits per second per <a href="Digital_Signal_0" title="Digital Signal 0">DS0</a> voice channel. Also, the low minimum density of ones (12.5%) sometimes led to increased clock <a href="Slip_(telecommunication)" title="Slip (telecommunication)">slippage</a> on the span.

</p><p>Increased demand for bandwidth, and compatibility with the <a href="G.703" title="G.703">G.703</a> and <a href="ISDN" title="ISDN">ISDN</a> <a href="Primary_Rate_Interface" title="Primary Rate Interface">PRI</a> standards which called for 64,000 bits per second, led to this system being superseded by B8ZS.

</p>

<div class="mw-heading mw-heading2"><h2 id="B8ZS_(North_American_T1)">B8ZS (North American T1) </h2></div>

<p>Commonly used in the North American T1 (<a href="Digital_Signal_1" title="Digital Signal 1">Digital Signal 1</a>) 1.544 Mbit/s line code, <b>bipolar with eight-zero substitution (B8ZS)</b> replaces each string of 8 consecutive zeros with the special pattern "<span class="monospaced">000VB0VB</span>". Depending on the polarity of the preceding mark, that could be <span class="monospaced">000+−0−+</span> or <span class="monospaced">000−+0+−</span>.

</p>

<div class="mw-heading mw-heading2"><h2 id="B6ZS_(North_American_T2)">B6ZS (North American T2) </h2></div>

<p>At the North American T2 rate (6.312 Mbit/s), bipolar violations are inserted if 6 or more consecutive zeros occur. This line code is called <b>bipolar with six-zero substitution (B6ZS)</b>, and replaces 6 consecutive zeros with the pattern "<span class="monospaced">0VB0VB</span>". Depending on the polarity of the preceding mark, that could be <span class="monospaced">0+−0−+</span> or <span class="monospaced">0−+0+−</span>.

</p>

<div class="mw-heading mw-heading2"><h2 id="HDB3_(European_E-carrier)">HDB3 (European E-carrier) </h2></div>

<p>Used in all levels of the European <a href="E-carrier" title="E-carrier">E-carrier</a> system, the <b>high density bipolar of order 3 (HDB3)</b> code replaces any instance of 4 consecutive 0 bits with one of the patterns "<span class="monospaced">000V</span>" or "<span class="monospaced">B00V</span>". The choice is made to ensure that consecutive violations are of differing polarity; i.e., separated by an odd number of normal <span class="monospaced">+ or −</span> marks.

</p>

<caption>HDB 3 coding of 0000<sub>2</sub>

</caption>

<th>Parity of +/− bits<br>since previous V</th>

<th>Pattern</th>

<th>Previous pulse</th>

<th>Coded

</th></tr>

<tr>

<td rowspan="2">B00V

</td>

<td>−00−

</td></tr>

<tr>

<td>+00+

</td></tr>

<tr>

<td rowspan="2">000V

</td>

<td>000+

</td></tr>

<tr>

<td>000-

</td></tr></tbody></table>

<p>These rules are applied on the code as it is being built from the original string. Every time there are 4 consecutive zeros in the code they will be replaced by either 000−, 000+, +00+ or −00−. To determine which pattern to use, one must count the number of pluses (+) and the number of minuses (−) since the last violation bit V, then subtract one from the other. If the result is an odd number then 000− or 000+ is used. If the result is an even number then +00+ or −00− is used. To determine which polarity to use, one must look at the pulse preceding the four zeros. If 000V form must be used then V simply copies the polarity of last pulse, if B00V form must be used then B and V chosen will have the opposite polarity of the last pulse.

</p>

<div class="mw-heading mw-heading3"><h3 id="Examples">Examples</h3></div>

<p>Here are some examples of bit streams codes with AMI and HDB3. All assume the same starting conditions: the previous 1 bit was −, and the previous violation was an even number of 1 bits ago. (E.g. the preceding bits could have been ++−.)

</p>

<table>

<td><b>Input</b></td>

</td></tr>

<tr>

</td></tr>

<tr>

</td></tr>

<tr>

</td></tr></tbody></table>

<table>

<td><b>Input</b></td>

</td></tr>

<tr>

</td></tr>

<tr>

</td></tr>

<tr>

</td></tr></tbody></table>

<table>

<td><b>Input</b></td>

</td></tr>

<tr>

</td></tr>

<tr>

</td></tr>

<tr>

</td></tr></tbody></table>

<table>

<td><b>Input</b></td>

</td></tr>

<tr>

</td></tr>

<tr>

</td></tr>

<tr>

</td></tr></tbody></table>

<div class="mw-heading mw-heading2"><h2 id="B3ZS_(North_American_T3)">B3ZS (North American T3) </h2></div>

<p>At the <a href="DS-3" class="mw-redirect" title="DS-3">North American T3</a> rate (44.736 Mbit/s), bipolar violations are inserted if 3 or more consecutive zeros occur. This line code is called <b>bipolar with three-zero substitution (B3ZS)</b>, and is very similar to HDB3. Each run of 3 consecutive zeros is replaced by "<span class="monospaced">00V</span>" or "<span class="monospaced">B0V</span>". The choice is made to ensure that consecutive violations are of differing polarity, i.e. separated by an odd number of normal <span class="monospaced">B</span> marks.

</p>

<caption>B3ZS coding of "000"

</caption>

<th>Number of B bits<br>since last V</th>

<th>Pattern</th>

<th>Polarity<br>of last B</th>

<th>Coded

</th></tr>

<tr>

<td rowspan="2">00V

</td>

<td>00+

</td></tr>

<tr>

<td>00−

</td></tr>

<tr>

<td rowspan="2">B0V

</td>

<td>−0−

</td></tr>

<tr>

<td>+0+

</td></tr></tbody></table>

<p>Other line codes that have 3 states:

</p>

<ul><li><a href="Bipolar_encoding" title="Bipolar encoding">Bipolar encoding</a> or alternate mark inversion</li>

<li><a href="Hybrid_ternary_code" title="Hybrid ternary code">Hybrid ternary code</a></li>

<li><a href="MLT-3_encoding" title="MLT-3 encoding">MLT-3 encoding</a></li>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><div id="Line_coding_(digital_baseband_transmission)443" style="font-size:114%;margin:0 4em"><a href="Line_coding" class="mw-redirect" title="Line coding">Line coding</a> (digital baseband transmission)</div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Main articles</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Unipolar_encoding" title="Unipolar encoding">Unipolar encoding</a></li>

<li><a href="Bipolar_encoding" title="Bipolar encoding">Bipolar encoding</a></li>

<li><a href="On%E2%80%93off_keying" title="On–off keying">On–off keying</a></li>

<li><a href="Mark_and_space" title="Mark and space">Mark and space</a></li></ul>

</div></td><td class="noviewer navbox-image" rowspan="4" style="width:1px;padding:0 0 0 2px"><div><span typeof="mw:File"></span></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Basic <a href="Line_code" title="Line code">line codes</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Return-to-zero" title="Return-to-zero">Return to zero (RZ)</a></li>

<li><a href="Non-return-to-zero#Non-return-to-zero_level" title="Non-return-to-zero">Non-return-to-zero, level (NRZ/NRZ-L)</a></li>

<li><a href="Non-return-to-zero#Non-return-to-zero_inverted" title="Non-return-to-zero">Non-return-to-zero, inverted (NRZ-I)</a></li>

<li><a href="Non-return-to-zero#Non-return-to-zero_space" title="Non-return-to-zero">Non-return-to-zero, space (NRZ-S)</a></li>

<li><a href="Manchester_code" title="Manchester code">Manchester</a></li>

<li><a href="Differential_Manchester_encoding" title="Differential Manchester encoding">Differential Manchester/biphase (Bi-φ)</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Extended line codes</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Conditioned_diphase" class="mw-redirect" title="Conditioned diphase">Conditioned diphase</a></li>

<li><a href="Bipolar_encoding#Alternate_mark_inversion" title="Bipolar encoding">Alternate mark inversion</a></li>

<li><a href="Coded_mark_inversion" title="Coded mark inversion">Coded mark inversion</a></li>

<li><a href="MLT-3_encoding" title="MLT-3 encoding">MLT-3 encoding</a></li>

<li><a href="Hybrid_ternary_code" title="Hybrid ternary code">Hybrid ternary code</a></li>

<li><a href="6b/8b_encoding" title="6b/8b encoding">6b/8b encoding</a></li>

<li><a href="8b/10b_encoding" title="8b/10b encoding">8b/10b encoding</a></li>

<li><a href="64b/66b_encoding" title="64b/66b encoding">64b/66b encoding</a></li>

<li><a href="Eight-to-fourteen_modulation" title="Eight-to-fourteen modulation">Eight-to-fourteen modulation</a></li>

<li><a href="Delay_encoding" class="mw-redirect" title="Delay encoding">Delay/Miller encoding</a></li>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Optical line codes</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Carrier-suppressed_return-to-zero" class="mw-redirect" title="Carrier-suppressed return-to-zero">Carrier-suppressed return-to-zero</a></li>

<li><a href="Alternate-phase_return-to-zero" class="mw-redirect" title="Alternate-phase return-to-zero">Alternate-phase return-to-zero</a></li></ul>

</div></td></tr><tr><td class="navbox-abovebelow hlist" colspan="3"><div>

<ul><li><i>See also:</i> <a href="Baseband" title="Baseband">Baseband</a></li>

<li><a href="Digital_signal" title="Digital signal">Digital signal</a></li>

<li><a href="Digital_transmission" class="mw-redirect" title="Digital transmission">Digital transmission</a></li>

<li><a href="Ethernet_physical_layer" title="Ethernet physical layer">Ethernet physical layer</a></li>

<li><a href="Modulation" class="mw-redirect" title="Modulation">Pulse modulation methods</a></li>

<li><a href="Pulse-amplitude_modulation" title="Pulse-amplitude modulation">Pulse-amplitude modulation</a> (PAM)</li>

<li><a href="Pulse-code_modulation" title="Pulse-code modulation">Pulse-code modulation</a> (PCM)</li>

<li><a href="Serial_communication" title="Serial communication">Serial communication</a></li>

<li>Category:Line codes</li></ul>

</div></td></tr></tbody></table></div></div>

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</style><span class="citation 1037C"><span class="noviewer" typeof="mw:File"><span></span></span> This article incorporates <a href="Copyright_status_of_works_by_the_federal_government_of_the_United_States" title="Copyright status of works by the federal government of the United States">public domain material</a> from <style data-mw-deduplicate="TemplateStyles:r1238218222">

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</style><cite class="citation cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20220122224547/https://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm"><i>Federal Standard 1037C</i></a>. <a href="General_Services_Administration" title="General Services Administration">General Services Administration</a>. Archived from <a rel="nofollow" class="external text" href="https://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm">the original</a> on 2022-01-22.</cite></span>

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