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<td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">5</font></b><font face="Arial" size="2" color="#FFFFFF"><b>.1</b></font></td>
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<td bgcolor="#336666" width="100%"><b style="mso-bidi-font-weight:normal"><span style="font-family:Arial;mso-bidi-font-family:"Times New Roman""><o:p>
<font size="2" color="#FFFFFF">Frame Relay Concepts</font>
</o:p>
</span></b></td>
<td width="9" bgcolor="#336666"> </td>
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<tr>
<td bgcolor="#669999" height="25" width="18"> </td>
<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">5.1.6</font></b></td>
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<td bgcolor="#669999" height="25" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">Frame Relay LMI</font></strong></td>
<td bgcolor="#669999" height="25" width="9"> </td>
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<font FACE="Arial" SIZE="2">
Frame Relay was designed to provide packet-switched data transfer with
minimal end-to-end delays. Anything that might contribute to delays
was omitted. When vendors came to implement Frame Relay as a separate
technology rather than as one component of ISDN, they decided that
there was a need for DTEs to dynamically acquire information about the
status of the network. This feature was omitted in the original
design. The extensions for this status transfer are called the Local
Management Interface (LMI).</font><p>
<font FACE="Arial" SIZE="2">
The 10-bit DLCI field allows VC identifiers 0 through 1023. The LMI
extensions reserve some of these identifiers. This reduces the number of
permitted VCs. LMI messages are exchanged between the DTE and DCE
using these reserved DLCIs.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></font></p>
<p>
<font FACE="Arial" SIZE="2">
The LMI extensions include the following:</font></p>
<ul>
<li>
<font FACE="Arial" SIZE="2">
The heartbeat mechanism, which verifies that a VC is operational
</font>
</li>
<li>
<font FACE="Arial" SIZE="2">
The multicast mechanism</font></li>
<li>
<font FACE="Arial" SIZE="2">
The flow control</font></li>
<li>
<font FACE="Arial" SIZE="2">
The ability to give DLCIs global significance</font></li>
<li>
<font FACE="Arial" SIZE="2">
The VC status mechanism</font></li>
</ul>
<p>
<font FACE="Arial" SIZE="2">
There are several LMI types, each of which is incompatible with the
others. The LMI type
configured on the router must match the type used by the service
provider. Three types of LMIs are supported by Cisco
routers:
</font>
</p>
<ul>
<li>
<font FACE="Arial" SIZE="2">
<b>Cisco</b> � The original LMI extensions
</font>
</li>
<li>
<font FACE="Arial" SIZE="2">
<b>Ansi</b> � Corresponding to the ANSI standard T1.617 Annex D</font></li>
<li>
<font FACE="Arial" SIZE="2">
<b>q933a</b> � Corresponding to the ITU standard Q933 Annex A</font></li>
</ul>
<p>
<font FACE="Arial" SIZE="2">
LMI messages are carried in a variant of LAPF frames. This variant
includes four extra fields in the header so that they will be
compatible with the LAPD frames used in ISDN. The address field
carries one of the reserved DLCIs. Following this are the control,
protocol discriminator, and call reference fields that do not
change. The fourth field indicates the LMI message type.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></font></p>
<p>
<font FACE="Arial" SIZE="2">
There are one or more information elements (IE) that follow the header.
Each IE consists of the following:</font></p>
<ul>
<li>
<font FACE="Arial" SIZE="2">
A one byte IE identifier
</font>
</li>
<li>
<font FACE="Arial" SIZE="2">
An IE length field
</font>
</li>
<li>
<font FACE="Arial" SIZE="2">
One or more bytes containing actual data that typically includes
the status of a DLCI</font></li>
</ul>
<p>
<font FACE="Arial" SIZE="2">
Status messages help verify the integrity of logical and physical
links. This information is critical in a routing environment because
routing protocols make decisions based on link integrity.</font></p>
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<TD width="5">
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<TD><IMG alt="" height="1" width="3" src="../../images/s.gif"></TD><TD align="right" valign="top">
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<TR>
<TD bgcolor="#ffffff" width="15"> </TD>
<TD bgcolor="#ffffff"><font FACE="Arial" SIZE="2" COLOR="#000000">
<p><font color="#808080">
<a href="javascript:void(0)" onClick="window.open('../../activities/ch05/activity_5_1_6.html','Interactivity','width=740,height=390,toolbar=0,resizable=yes,scrollbars=0,screenX=26,screenY=38,left=26,top=38');">
<img border="0" src="../../images/media_icon.gif" width="25" height="25"></a> <b>
<a onClick="window.open('../../activities/ch05/activity_5_1_6.html','Interactivity','width=740,height=390,toolbar=0,resizable=yes,scrollbars=0,screenX=26,screenY=38,left=26,top=38');" href="javascript:void(0)">Interactive Media Activity</a></b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>Drag and Drop: LMI Message Format</p>
<p>When the student has completed this activity, the student
will be able to correctly order the fields in a LMI message
frame.</p>
</font><font FACE="Arial" SIZE="2" COLOR="#000000">
<p><IMG alt="" height="2" width="1" src="../../images/s.gif"></p></TD>
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<p><font color="#808080">
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<b>Web Links</b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>Frame Relay<p>
<a target="_blank" href="http://www.tele.sunyit.edu/internetworking/55149.htm">
http://www.tele.sunyit.edu/
internetworking/ 55149.htm</a></font></p>
<p>
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