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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">
2.1.6</font></b></td>
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<td bgcolor="#669999" height="25" width="100%"><b>
<font face="Arial" size="2" color="#FFFFFF">WAN link options</font></b></td>
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Figure <img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12">
provides an overview of WAN link options.
<p>Circuit switching establishes a dedicated physical connection for voice
or data between a sender and receiver. Before communication can start, it
is necessary to establish the connection by setting the switches. This
is done by the telephone system, using the dialed number. ISDN is used
on digital lines as well as on voice-grade lines. If the local loop is
not directly connected to the telephone system, a digital subscriber
line (DSL) may be available.</p>
<p>To avoid the delays associated with setting up a connection,
telephone service providers also offer permanent circuits. These
dedicated or leased lines offer higher bandwidth than is available
with a switched circuit. Examples of circuit-switched connections
include:</p>
<ul>
<li>Plain Old Telephone System (POTS)</li>
<li>ISDN Basic Rate Interface (BRI)</li>
<li>ISDN Primary Rate Interface (PRI)</li>
</ul>
<p>Many WAN users do not make efficient use of the fixed bandwidth
that is available with dedicated, switched, or permanent circuits, because the data flow
fluctuates. Communications providers have data networks available to
more appropriately service these users. In these networks, the data is transmitted in labeled
cells, frames, or packets through a packet-switched network. Because
the internal links between the switches are shared between many users,
the costs of packet switching are lower than those of circuit
switching. Delays (latency) and variability of delay (jitter) are
greater in packet-switched than in circuit-switched networks. This is
because the links are shared and packets must be entirely received at
one switch before moving to the next. Despite the latency and jitter
inherent in shared networks, modern technology allows satisfactory
transport of voice and even video communications on these networks.</p>
<p>Packet-switched networks may establish routes through the switches
for particular end-to-end connections. Routes established when the
switches are started are PVCs. Routes established on demand are SVCs. If the routing is not
pre-established and is worked out by each switch for each packet, the
network is called connectionless.</p>
<p>To connect to a packet-switched network, a subscriber needs a local
loop to the nearest location where the provider makes the service
available. This is called the point-of-presence (POP) of the service.
Normally this will be a dedicated leased line. This line will be much
shorter than a leased line directly connected to
the subscriber locations, and often carries several VCs.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"> Since it
is likely that not all the VCs will require maximum demand
simultaneously, the capacity of the leased line can be smaller than
the sum of the individual VCs. Examples of packet or cell switched
connections include:</p>
<ul>
<li>Frame Relay</li>
<li>X.25</li>
<li>ATM</li>
</ul>
<p>
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<p><font color="#808080">
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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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