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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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<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">
2.1.5</font></b></td>
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<font face="Arial" size="2" color="#FFFFFF">Packet and circuit switching
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Packet-switched networks were developed to overcome the expense of
public circuit-switched networks and to provide a more cost-effective
WAN technology.<p>When a subscriber makes a telephone call, the dialed
number is used to set switches in the exchanges along the route of the
call so that there is a continuous circuit from the originating caller
to that of the called party. Because of the switching operation
used to establish the circuit, the telephone system is called a
circuit-switched network. If the telephones are replaced with modems,
then the switched circuit is able to carry computer data.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"></p>
<p>The internal path taken by the circuit between exchanges is shared
by a number of conversations. Time division multiplexing (TDM) is
used to give each conversation a share of the connection in turn. TDM
assures that a fixed capacity connection is made available to the
subscriber. </p>
<p>If the circuit carries computer data, the usage of this
fixed capacity may not be efficient. For example, if the circuit is
used to access the Internet, there will be a burst of activity on the
circuit while a web page is transferred. This could be followed by no
activity while the user reads the page and then another burst of
activity while the next page is transferred. This variation in usage
between none and maximum is typical of computer network traffic.
Because the subscriber has sole use of the fixed capacity allocation,
switched circuits are generally an expensive way of moving data.</p>
<p>An alternative is to allocate the capacity to the traffic only when
it is needed, and share the available capacity between many users.
With a circuit-switched connection, the data bits put on the circuit
are automatically delivered to the far end because the circuit is
already established. If the circuit is to be shared, there must be
some mechanism to label the bits so that the system knows where to
deliver them. It is difficult to label individual bits, therefore they
are gathered into groups called cells, frames, or packets.
The packet passes from exchange to exchange for delivery through the
provider network. Networks that implement
this system are called packet-switched networks.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></p>
<p>The links that connect the switches in the provider network belong
to an individual subscriber during data transfer, therefore many
subscribers can share the link. Costs can be significantly lower
than a dedicated circuit-switched connection. Data on packet-switched
networks are subject to unpredictable delays when individual packets
wait for other subscriber packets to be transmitted by a switch.</p>
<p>The switches in a packet-switched network determine, from
addressing information in each packet, which link the packet must be
sent on next. There are two approaches to this link
determination, connectionless or connection-oriented. Connectionless
systems, such as the Internet, carry full addressing information in
each packet. Each switch must evaluate the address to
determine where to send the packet. Connection-oriented systems
predetermine the route for a packet, and each packet need only carry
an identifier. In the case of Frame Relay, these are called Data Link
Control Identifiers (DLCI). The switch determines the onward route by
looking up the identifier in tables held in memory. The set of entries
in the tables identifies a particular route or circuit through the
system. If this circuit is only physically in existence while a packet
is traveling through it, it is called a Virtual Circuit (VC).</p>
<p>The table entries that constitute a VC can be established by
sending a connection request through the network. In this case the
resulting circuit is called a Switched Virtual Circuit (SVC). Data
that is to travel on SVCs must wait until the table entries have been
set up. Once established, the SVC may be in operation for hours, days
or weeks. Where a circuit is required to be always available, a
Permanent Virtual Circuit (PVC) will be established. Table entries are
loaded by the switches at boot time so the PVC is always available.</p>
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<p>Packet vs Circuit Switching<p>
<a target="_blank" href="http://www.cina.org/ac051599/sld011.htm">
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