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<td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">3</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">Serial Point-to-Point Links</font></strong></td>
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<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">3.1.5</font></b></td>
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<font face="Arial" size="2" color="#FFFFFF">HDLC encapsulation</font></strong></td>
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<font face="Arial" size="2" COLOR="#000000"> Initially, serial
communications were based on character-oriented protocols.
Bit-oriented protocols were more efficient but they were also
proprietary. In 1979, the ISO agreed on HDLC
as a standard bit-oriented data link layer protocol that encapsulates
data on synchronous serial data links. This
standardization led to other committees adopting it and extending the
protocol. Since 1981, ITU-T has developed a series of HDLC derivative
protocols. The following examples of derivative protocols are called
link access protocols:</font><ul>
<li>
<font face="Arial" size="2" COLOR="#000000"> Link Access Procedure,
Balanced (LAPB) for X.25
</font>
</li>
<li>
<font face="Arial" size="2" COLOR="#000000"> Link Access Procedure on
the D channel (LAPD) for ISDN
</font>
</li>
<li>
<font face="Arial" size="2" COLOR="#000000"> Link Access Procedure for
Modems (LAPM) and PPP for modems
</font>
</li>
<li>
<font face="Arial" size="2" COLOR="#000000"> Link Access Procedure for
Frame Relay (LAPF) for Frame Relay</font></li>
</ul>
<p>
<font face="Arial" size="2" COLOR="#000000"> HDLC uses synchronous
serial transmission providing error-free communication between two
points. HDLC defines a Layer 2 framing structure that allows for flow
control and error control using acknowledgments and a windowing
scheme. </font><font face="Arial" size="2">Each frame has the same
format, whether it is a data frame or a control frame.</font></p>
<p>
<font face="Arial" size="2" COLOR="#000000"> Standard HDLC does not
inherently support multiple protocols on a single link, as it does not
have a way to indicate which protocol is being carried. Cisco offers a
proprietary version of HDLC. The Cisco HDLC frame uses a proprietary
�type� field that acts as a protocol field. This field enables
multiple network layer protocols to share the same serial link. HDLC
is the default Layer 2 protocol for Cisco router serial interfaces.</font></p>
<p>
<font face="Arial" size="2" COLOR="#000000"> HDLC defines the
following three types of frame, each with a different control field
format: <img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></font></p>
<ul>
<li>
<font face="Arial" size="2" COLOR="#000000"> <b>Information frames
(I-frames)</b> � Carry the data to be transmitted for the station.
Additional flow and error control - data may be piggybacked on an
information frame.</font></li>
<li>
<font face="Arial" size="2" COLOR="#000000"> <b>Supervisory frames
(S-frames)</b> � Provide request/response mechanisms when
piggybacking is not used.</font></li>
<li>
<font face="Arial" size="2" COLOR="#000000"> <b>Unnumbered frames
(U-frames)</b> � Provide supplemental link control functions, such
as connection setup. The code field identifies the U-frame type.</font></li>
</ul>
<p>
<font face="Arial" size="2" COLOR="#000000"> The first one or two bits
of the control field serve to identify the frame type. In the control
field of an Information (I) frame, the send-sequence number refers to
the number of the frame to be sent next. The receive-sequence number
provides the number of the frame to be received next. Both sender and
receiver maintain send and receive sequence numbers.
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<p>Configuring Synchronous Serial Ports<p>
<a target="_blank" href="http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_configuration_guide_chapter09186a0080087283.html">
http://www.cisco.com/en/US/products/sw/
iosswrel/ps1828/ products_configuration_
guide_ chapter09186a0080087283.html</a></font></p>
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