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<td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">4</font></b><font face="Arial" size="2" color="#FFFFFF"><b>.1</b></font></td>
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<td bgcolor="#336666" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">ISDN Concepts</font></strong></td>
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<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">4.1.3</font></b></td>
<td bgcolor="#669999"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
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<strong><font face="Arial" size="2" color="#FFFFFF">ISDN 3-layer model and
protocols</font></strong></td>
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<td>
<font face="Arial" size="2" COLOR="#000000"> ISDN utilizes a suite of
ITU-T standards spanning the physical, data link, and network layers
of the OSI reference model:
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></font><ul>
<li>
<font face="Arial" size="2" COLOR="#000000"> The ISDN BRI and PRI physical layer
specifications are defined in ITU-T I.430 and I.431, respectively. </font>
</li>
<li>
<font face="Arial" size="2" COLOR="#000000"> The ISDN data link
specification is based on LAPD and is formally specified in the
following:
<ul>
<li>ITU-T Q.920</li>
<li>ITU-T Q.921</li>
<li>ITU-T Q.922</li>
<li>ITU-T Q.923</li></font>
</li>
</ul></ul>
<font FACE="Arial" SIZE="2" COLOR="#000000">
<font face="Arial" size="2"> The ISDN network layer is defined in ITU-T
Q.930, also known as I.450 and ITU-T Q.931, also known as I.451. These
standards specify user-to-user, circuit-switched, and packet-switched
connections.</font></font></p>
<p>
<font FACE="Arial" SIZE="2" COLOR="#000000">
<font face="Arial" size="2"> BRI service is provided over a local
copper loop that traditionally carries analog phone service. While
there is only one physical path for a BRI, there are three separate
information paths, 2B+D. Information from the three channels is
multiplexed into the one physical path. </font>
</font>
</p>
<p>
<font face="Arial" size="2" COLOR="#000000"> ISDN physical layer, or Layer 1, frame
formats differ depending on whether the frame is outbound or inbound.
If the frame is outbound, it is sent from the terminal to the network.
Outbound frames use the TE frame format. If the frame is inbound, it
is sent from the network to the terminal. Inbound frames use the NT
frame format.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></font></p>
<p>
<font face="Arial" size="2">Each frame contains two sample frames each
containing the following:</font></p>
<ul>
<li>
<font face="Arial" size="2" COLOR="#000000"> 8 bits from the B1
channel </font>
</li>
<li>
<font face="Arial" size="2" COLOR="#000000"> 8 bits from the B2
channel</font></li>
<li>
<font face="Arial" size="2" COLOR="#000000"> 4 bits from the D channel </font>
</li>
<li>
<font face="Arial" size="2" COLOR="#000000"> 6 bits of overhead </font>
</li>
</ul>
<p>
<font face="Arial" size="2">ISDN BRI frames contain 48 bits. Four
thousand of
these frames are transmitted every second. Each B channel, B1and B2,
have a capacity of 8*4000 = 64 kbps, while channel D has a capacity of
4*4000 = 16 kbps. This accounts for 144 kbps of the total ISDN BRI
physical interface bit rate of 192 kbps. The remainder of the data rate
are the overhead bits that are required for transmission.</font></p>
<p>
<font FACE="Arial" SIZE="2" COLOR="#000000">
<font face="Arial" size="2"> The overhead bits of an ISDN physical
layer frame are used as follows: </font>
</font>
</p>
<ul>
<li>
<font face="Arial" size="2" COLOR="#000000"> <b>Framing bit </b>�
Provides synchronization</font></li>
<li>
<font face="Arial" size="2" COLOR="#000000"> <b>Load balancing bit </b>
� Adjusts the average bit value</font></li>
<li>
<font face="Arial" size="2" COLOR="#000000"> <b>Echo of previous D
channel bits</b> � Used for contention resolution when several
terminals on a passive bus contend for a channel</font></li>
<li>
<font face="Arial" size="2" COLOR="#000000"> <b>Activation bit</b> �
Activates devices</font></li>
<li>
<font face="Arial" size="2" COLOR="#000000"> <b>Spare bit</b> �
Unassigned</font></li>
</ul>
<p>
<font face="Arial" size="2">Note that the physical bit rate for the
BRI interface is 48*4000 = 192 kbps. The effective rate is 144 kbps = 64 kbps + 64 kbps + 16 kbps (2B+D).</font></p>
<p>
<font face="Arial" size="2">Layer 2 of the ISDN signaling channel is
LAPD.</font><font face="Arial" size="2" COLOR="#000000"> LAPD is similar to HDLC. LAPD is used across the D channel to
ensure that control and signaling information is received and flows
properly.</font></p>
<p>
<font face="Arial" size="2" COLOR="#000000"> The LAPD flag and control fields are
identical to those of HDLC.
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"><font face="Arial" size="2"> The LAPD address field is 2 bytes long.
The first address field byte contains the service access point
identifier (SAPI), which identifies the portal at which LAPD services
are provided to Layer 3. The command/response bit (C/R), indicates
whether the frame contains a command or a response. The second byte
contains the terminal endpoint identifier (TEI). Each piece of
terminal equipment on the customer premises needs a unique
identifier. The TEI may be statically assigned at installation, or the
switch may dynamically assign it when the equipment is started up. If the TEI is statically assigned during installation, the TEI
is a number ranging from 0 to 63. Dynamically assigned TEIs range from
64 to 126. A TEI of 127, or all 1s, indicates a broadcast.</font></font></p>
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<b>Web Links</b></font></p>
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<p>Integrated Services Digital Netowrk (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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