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<td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">2.2</font></b></td>
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<strong><font face="Arial" size="2" color="#FFFFFF">WAN Technologies</font></strong></td>
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<td bgcolor="#669999" height="25"><b><font face="Arial" size="2" color="#FFFFFF">
2.2.4</font></b></td>
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<td bgcolor="#669999" height="25" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">X.25</font></strong></td>
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<font face="Arial" size="2">In response to the expense of leased
lines, telecommunications providers introduced packet-switched
networks using shared lines to reduce costs. The first of these
packet-switched networks was standardized as the X.25 group of
protocols. X.25 provides a low bit rate shared variable capacity that
may be either switched or permanent.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></font><p>
<font face="Arial" size="2">X.25 is a network-layer protocol and
subscribers are provided with a network address. Virtual circuits can
be established through the network with call request packets to the
target address. The resulting SVC is
identified by a channel number. Data packets labeled with the channel
number are delivered to the corresponding address. Multiple channels
can be active on a single connection.</font></p>
<p><font face="Arial" size="2">Subscribers connect to the X.25 network
with either leased lines or dialup connections. X.25 networks can also
have pre-established channels between subscribers that provide a
PVC.</font></p>
<p><font face="Arial" size="2">X.25 can be very cost effective because
tariffs are based on the amount of data delivered rather than
connection time or distance. Data can be delivered at any rate up to
the connection capacity. This provides some flexibility. X.25 networks
are usually low capacity, with a maximum of 48 kbps. In addition, the
data packets are subject to the delays typical of shared networks.</font></p>
<p><font face="Arial" size="2">X.25 technology is no longer widely
available as a WAN technology in the US. Frame Relay has replaced X.25 at many service provider locations.</font></p>
<p><font face="Arial" size="2">Typical X.25 applications are
point-of-sale card readers. These readers use X.25 in dialup mode to
validate transactions on a central computer. Some enterprises also use
X.25 based value-added networks (VAN) to transfer Electronic Data
Interchange (EDI) invoices, bills of lading, and other commercial
documents. For these applications, the low bandwidth and high latency
are not a concern, because the low cost makes the use of X.25
affordable.</font></p>
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<p>Checkbox: WAN Technologies X.25</p>
<p>Upon completing this activity, the student
will be able to identify the characteristics associated with an
X.25 connection.</p>
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<b>Web Links</b></font></p>
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<p>X.25 Protocol<p>
<a target="_blank" href="http://www2.rad.com/networks/1996/x25/x25.htm">
http://www2.rad.com/networks/1996/ x25/x25.htm</a></font></p>
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