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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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<td bgcolor="#336666" width="100%"><strong>
<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">
2.3.3</font></b></td>
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<font size="2" color="#FFFFFF"><b><span style="font-family: Arial">How to
identify and select networking capabilities</span></b></font></td>
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<td><font face="Arial" size="2">Designing a WAN essentially consists of
the following:</font><ul>
<li><font face="Arial" size="2">Selecting an interconnection pattern
or layout for the links between the various locations</font></li>
<li><font face="Arial" size="2">Selecting the technologies for those
links to meet the enterprise requirements at an acceptable cost</font></li>
</ul>
<p><font face="Arial" size="2">Many WANs use a star topology. As the
enterprise grows and new branches are added, the branches are connected
back to the head office, producing a traditional star topology.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"> Star
end-points are sometimes cross-connected, creating a mesh or partial
mesh topology.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"> This
provides for many possible combinations for interconnections. When
designing, re-evaluating, or modifying a WAN, a
topology that meets the design requirements must be selected.
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"></font></p>
<p><font face="Arial" size="2">In selecting a layout, there are several
factors to consider. More links will increase the cost of the network
services, and having multiple paths between destinations increases
reliability. Adding more network devices to the data path will increase
latency and decrease reliability. Generally, each packet must be completely received at one
node before it can be passed to the next. A range of dedicated
technologies with different features is available for the data links.
<img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12"></font></p>
<p><font face="Arial" size="2">Technologies that require the establishment of a connection before data
can be transmitted, such as basic telephone, ISDN, or X.25, are not
suitable for WANs that require rapid response time or low latency. Once
established, ISDN and other dialup services are low latency, low jitter
circuits. ISDN is often the application of choice for connecting a small
office or home office (SOHO) network to the enterprise network,
providing reliable connectivity and adaptable bandwidth. Unlike cable
and DSL, ISDN is an option wherever modern telephone service is
available. ISDN is also useful as a backup link for primary connections
and for providing bandwidth-on-demand connections in parallel with a
primary connection. A feature of these technologies is that the
enterprise is only charged a fee when the circuit is in use.</font></p>
<p><font face="Arial" size="2">The different parts of the enterprise may
be directly connected with leased lines, or they may be connected with
an access link to the nearest point-of-presence (POP) of a shared
network. X.25, Frame Relay, and ATM are examples of shared networks.
Leased lines will generally be much longer and therefore more expensive
than access links, but are available at virtually any bandwidth. They
provide very low latency and jitter.</font></p>
<p><font face="Arial" size="2">ATM, Frame Relay, and X.25 networks carry
traffic from several customers over the same internal links. The
enterprise has no control over the number of links or hops that data
must traverse in the shared network. It cannot control the time data
must wait at each node before moving to the next link. This uncertainty
in latency and jitter makes these technologies unsuitable for some types
of network traffic. However, the disadvantages of a shared network may
often be outweighed by the reduced cost. Because several customers are
sharing the link, the cost to each will generally be less than the cost
of a direct link of the same capacity.</font></p>
<p><font face="Arial" size="2">Although ATM is a shared network, it has
been designed to produce minimal latency and jitter through the use of
high-speed internal links sending easily manageable units of data,
called cells. ATM cells have a fixed length of 53 bytes, 48 for data and
5 for the header. ATM is widely used
for carrying delay-sensitive traffic. Frame Relay may also be used for
delay-sensitive traffic, often using QoS mechanisms to give priority to
the more sensitive data.</font></p>
<p><font face="Arial" size="2">A typical WAN uses a combination of technologies that are usually chosen
based on traffic type and volume. ISDN, DSL, Frame Relay, or
leased lines are used to connect individual branches into an area. Frame Relay, ATM, or
leased lines are used to connect external areas back to the backbone.
ATM or leased lines form the WAN backbone.</font></p>
<p><font face="Arial" size="2"> </font>
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