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<td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>4.1</b></font></td>
<td bgcolor="#336666"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
<td bgcolor="#336666" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">ISDN Concepts</font></strong></td>
<td width="9" bgcolor="#336666"> </td>
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<td bgcolor="#669999" height="25" width="18"> </td>
<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">4.1.4</font></b></td>
<td bgcolor="#669999"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
<td bgcolor="#669999" height="25" width="100%"><b>
<font face="Arial" size="2" color="#FFFFFF">ISDN functions</font></b></td>
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<p><font face="Arial" size="2">Several exchanges must occur for one
router to connect to another using ISDN. To establish an ISDN call,
the D channel is used between the router and the ISDN switch. Signal
System 7 (SS7) signaling is used between the switches within the
service provider network. </font></p>
<p><font face="Arial" size="2">The D channel between the router and
the ISDN switch is always up. Q.921 describes the ISDN data-link
processes of LAPD, which functions like Layer 2 processes in the OSI
reference model. The D channel is used for call control functions such
as call setup, signaling, and termination. These functions are
implemented in the Q.931 protocol. Q.931 specifies OSI reference model
Layer 3 functions. The Q.931 standard recommends a network layer
connection between the terminal endpoint and the local ISDN switch,
but it does not impose an end-to-end recommendation. Because some ISDN
switches were developed before Q.931 was standardized, the various
ISDN providers and switch types can and do use various implementations
of Q.931. Because switch types are not standard, routers must have
commands in their configuration specifying the ISDN switch to which they
are connecting.</font></p>
<p><font face="Arial" size="2">The following sequence of events occurs
during the establishment of a BRI or PRI call:
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></p>
<ol>
<li>The D channel is used to send the
called number to the local ISDN switch.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></li>
<li>The local switch uses the SS7 signaling protocol to set up a path
and pass the called number to the remote ISDN switch.
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"></li>
<li>The remote ISDN switch signals the destination over the D
channel. <img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12"></li>
<li>The destination ISDN NT-1 device sends the remote ISDN switch a
call-connect message. </li>
<li>The remote ISDN switch uses SS7 to send a call-connect message
to the local switch.
<img border="0" src="../../images/5.gif" align="absmiddle" width="12" height="12"></li>
<li>The local ISDN switch connects one B channel end-to-end, leaving
the other B channel available for a new conversation or data
transfer. Both B channels can be used simultaneously.
<img border="0" src="../../images/6.gif" align="absmiddle" width="12" height="12"></font></li>
</ol>
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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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<p><font color="#808080">
<a href="javascript:void(0)" onClick="window.open('../../activities/ch04/activity_4_1_4.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/ch04/activity_4_1_4.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: ISDN Functions</p>
<p>When the student has completed this activity, the student
will be able to correctly identify the ISDN establishment cycle.</p>
</font><font FACE="Arial" SIZE="2" COLOR="#000000">
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<p><font color="#808080">
<img border="0" src="../../images/links_icon.gif" width="25" height="25">
<b>Web Links</b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>Integrated Services Digital Network (ISDN)<p>
<a target="_blank" href="http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/isdn.htm">
http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ito_doc/isdn.htm</a></font></p>
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