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<td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">2</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">WAN Technologies Overview</font></strong></td>
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<td bgcolor="#669999" height="25"><b><font face="Arial" size="2" color="#FFFFFF">
2.1.4</font></b></td>
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<b><font face="Arial" size="2" color="#FFFFFF">WAN encapsulation</font></b></td>
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Data from the network layer is passed to the data link layer for
delivery on a physical link, which is normally point-to-point on a WAN
connection. The data link layer builds a
frame around the network layer data so the necessary checks and
controls can be applied. Each WAN connection type uses a Layer 2
protocol to encapsulate traffic while it is crossing the WAN link. To
ensure that the correct encapsulation protocol is used, the Layer 2
encapsulation type used for each router serial interface must be
configured. The choice of encapsulation protocols depends on the WAN
technology and the equipment. Most framing is based on the HDLC
standard.<p>HDLC framing gives reliable delivery of data over
unreliable lines and includes signal mechanisms for flow and error
control.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"> The frame
always starts and ends with an 8-bit flag field, the bit pattern
01111110. Because there is a likelihood that this
pattern will occur in the actual data, the sending HDLC system always
inserts a 0 bit after every five 1s in the data field, so in practice
the flag sequence can only occur at the frame ends. The receiving
system strips out the inserted bits. When frames are transmitted
consecutively the end flag of the first frame is used as the start
flag of the next frame. </p>
<p>The address field is not needed for WAN links, which are almost
always point-to-point. The address field is still present and
may be one or two bytes long. The control field indicates the frame
type, which may be information, supervisory, or unnumbered:</p>
<ul>
<li>Unnumbered frames carry line setup messages.</li>
<li>Information frames carry network layer data.</li>
<li>Supervisory frames control the flow of information frames and
request data retransmission in the event of an error.</li>
</ul>
<p>The control field is normally one byte, but will be two bytes for
extended sliding windows systems. Together the address and control
fields are called the frame header. The encapsulated data follows the
control field. Then a frame check sequence (FCS) uses the
cyclic redundancy check (CRC) mechanism to establish a two or four
byte field.</p>
<p>Several data link protocols are used, including sub-sets and
proprietary versions of HDLC.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"> Both PPP
and the Cisco version of HDLC have an extra field in the header to identify the network layer protocol of the encapsulated
data.
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"></p>
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<b>Web Links</b></font></p>
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<p>WAN<p>
<a target="_blank" href="http://www.cisco.com/en/US/tech/tk713/tech_topology_and_network_serv_and_protocol_suite_home.html">
http://www.cisco.com/en/US/tech/tk713/
tech_topology_and_ network_serv_and_
protocol_suite_ home.html</a></font></p>
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