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<td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>2.</b></font><b><font face="Arial" size="2" color="#FFFFFF">3</font></b></td>
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<font face="Arial" size="2" color="#FFFFFF">WAN Design</font></strong></td>
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<td bgcolor="#669999" height="25"><b><font face="Arial" size="2" color="#FFFFFF">
2.3.1</font></b></td>
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<font face="Arial" size="2" color="#FFFFFF">WAN communication</font></strong></td>
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<td><font size="2" face="Arial">WANS are considered to be a set of data
links connecting routers on LANs. User end stations and servers on LANs
exchange data. Routers pass data between networks across the
data links.</font><p><font size="2" face="Arial">Because of cost and
legal reasons, a communications provider or a common carrier normally
owns the data links that make up a WAN. The links are made available to
subscribers for a fee and are used to interconnect LANs or connect to
remote networks. WAN data transfer speed (bandwidth) is considerably
slower than the 100 Mbps that is common on a LAN. The charges for link provision are
the major cost element of a WAN and the design must aim to provide
maximum bandwidth at acceptable cost. With user pressure to provide more
service access at higher speeds and management pressure to contain cost,
determining the optimal WAN configuration is not an easy task.</font></p>
<p><font face="Arial" size="2">WANs carry a variety of traffic types
such as data, voice, and video. The design selected must provide
adequate capacity and transit times to meet the requirements of the
enterprise. Among other specifications, the design must consider the
topology of the connections between the various sites, the nature of
those connections, and bandwidth capacity. </font>
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<p><font size="2" face="Arial">Older WANs often consisted of data links
directly connecting remote mainframe computers.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"> Today�s
WANs, though, connect geographically separated LANs.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"> End-user
stations, servers, and routers communicate across LANs, and the WAN data
links terminate at local routers. By exchanging Layer 3 address
information about directly connected LANs, routers determine the most
appropriate path through the network for the required data streams.
Routers can also provide quality of service (QoS) management, which
allots priorities to the different traffic streams.
</font>
</p>
<p><font size="2" face="Arial">Because the WAN is merely a set of
interconnections between LAN based routers, there are no services on the
WAN. WAN technologies function at the lower three layers of the OSI
reference model.
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"> Routers
determine the destination of the data from the network layer headers and
transfer the packets to the appropriate data link connection for
delivery on the physical connection.</font></p>
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<b>Web Links</b></font></p>
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<p>Introduction to WAN Technologies<p>
<a target="_blank" href="http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introwan.htm">
http://www.cisco.com/univercd/cc/</a><br>
<a target="_blank" href="http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introwan.htm">
td/doc/cisintwk/ito_doc/introwan.htm</a></font></p>
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