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+
+<head>
+<meta http-equiv="Content-Language" content="en-us">
+<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
+
+
+
+<title>Content</title>
+<base target="_self">
+</head>
+
+<body background="../../images/bg.gif" topmargin="0" leftmargin="0" marginheight="0" marginwidth="0" onLoad="window.focus();" link="#808080" vlink="#808080" alink="#808080">
+
+<table border="0" cellpadding="0" cellspacing="0" width="100%">
+ <tr>
+ <td bgcolor="#336666" width="18" valign="top">
+ <img border="0" src="../../images/content_lines.gif" width="16" height="25">
+ <img border="0" src="../../images/transdot.gif" width="2" height="1"></td>
+ <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>
+ <td bgcolor="#336666"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
+ <td bgcolor="#336666" width="100%"><strong>
+ <font face="Arial" size="2" color="#FFFFFF">WAN Design</font></strong></td>
+ <td width="9" bgcolor="#336666"> </td>
+ </tr>
+ <tr>
+ <td bgcolor="#669999" height="25" width="18"> </td>
+ <td bgcolor="#669999" height="25"><b><font face="Arial" size="2" color="#FFFFFF">
+ 2.3.4</font></b></td>
+ <td bgcolor="#669999"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
+ <td bgcolor="#669999" height="25" width="100%"><strong>
+ <font face="Arial" size="2" color="#FFFFFF">Three-layer design model</font></strong></td>
+ <td bgcolor="#669999" height="25" width="9"> </td>
+ </tr></table>
+
+<table border="0" cellpadding="0" cellspacing="0" width="95%" bordercolor="#111111">
+ <tr>
+ <td width="15"></td>
+ <td><font face="Arial" size="2">
+ A systematic approach is needed when many locations
+ must be joined. A hierarchical solution with three layers offers many
+ advantages.
+ <img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12">
+ <p>Imagine an enterprise that is operational in every country of the
+ European Union and has a branch in every town with a population over
+ 10,000. Each branch has a LAN, and it has been decided to interconnect
+ the branches. A mesh network is clearly not feasible because nearly
+ 500,000 links would be needed for the 900 centers. A simple
+ star will be very difficult to implement because it needs a router
+ with 900 interfaces at the hub or a single interface that carries 900
+ virtual circuits to a packet-switched network.</p>
+ <p>Instead, consider a hierarchical design model. A group of LANs in an
+ area are interconnected, several areas are interconnected to form a
+ region, and the various regions are interconnected to form the core of
+ the WAN. </p>
+ <p>The area could be based on the number of locations to be connected
+ with an upper limit of between 30 and 50. The area would have a star
+ topology,
+ <img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12">
+ with the hubs of the stars linked to form the region.
+ <img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12">
+ <img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12">
+ Regions could be geographic, connecting between three and ten areas,
+ and the hub of each region could be linked point-to-point.
+ <img border="0" src="../../images/5.gif" align="absmiddle" width="12" height="12"></p>
+ <p>This three-layer model follows the hierarchical design used in telephone
+ systems. The links connecting the various sites in an area that
+ provide access to the enterprise network are called the access links
+ or access layer of the WAN. Traffic between areas is distributed by
+ the distribution links, and is moved onto the core links for transfer
+ to other regions, when necessary.</p>
+ <p>This hierarchy is often useful when the network traffic mirrors the
+ enterprise branch structure and is divided into regions, areas, and
+ branches. It is also useful when there is a central service to which
+ all branches must have access, but traffic levels are insufficient to
+ justify direct connection of a branch to the service.</p>
+ <p>The LAN at the center of the area may have servers providing
+ area-based as well as local service. Depending on the traffic volumes
+ and types, the access connections may be dial up, leased, or Frame
+ Relay. Frame Relay facilitates some meshing for redundancy without
+ requiring additional physical connections. Distribution links could be
+ Frame Relay or ATM, and the network core could be ATM or leased line.</p>
+ <p>
+
+ </font></p>
+ </td>
+ </tr>
+ </table>
+
+</body>
+
+</html>
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