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221 lines
7.0 KiB
HTML
221 lines
7.0 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<!-- CSMA.html (C) K. J. Turner 18/12/14 -->
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<title>CSMA/CD ("Ethernet")</title>
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<link rev="made" href="http://www.cs.stir.ac.uk/~kjt/"/>
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<script type="text/javascript" language="JavaScript">
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<!--
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var simulator;
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var serviceCount;
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var serviceMode;
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function validInteger(name, value, min, max) {
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if (!value.match("\\d+") || value < min || value > max) {
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alert("Value for " + name + " must be in range " + min + " to " + max);
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return false;
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}
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else
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return true;
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}
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function set() {
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retryLimit = document.settings.retryLimit.value;
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if (validInteger("Retry Limit", retryLimit, 0, 5)) {
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simulator.setParameter("retryLimit", retryLimit);
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simulator.restart();
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}
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}
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//-->
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</script>
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</head>
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<body background="simulator.jpeg"
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onload="simulator=document.ProtocolSimulator; set();">
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<div style="text-align: center">
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<h1>CSMA/CD (Ethernet)</h1>
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<img src="simulator.gif" alt="Simulator Logo"/>
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</div>
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<h2>Protocol Description</h2>
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<p>
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CSMA/CD (Carrier Sense, Multiple Access, Collision Detection) illustrates
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how multiple systems can share a common communications medium. CSMA/CD is
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best known in the form of Ethernet - a kind of LANs (Local Area Network).
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LANs have a MAC layer (Medium Access Control) that operates the data link
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protocol.
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</p>
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<p>
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Many systems (known as stations) can be attached to an Ethernet - the
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Multiple Access aspect. A station will not transmit if it finds that another
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station is already using the Ethernet - the Carrier Sense aspect. However,
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it is still possible for two stations to transmit at almost the same time.
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While a station is transmitting, it may find that another station has
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already begun its own transmission - the Collision Detection aspect. In such
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a case, each station backs off and tries again - hopefully avoiding a
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collision on the second or later attempt.
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</p>
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<p>
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An Ethernet is a broadcast medium: whatever one station sends is received by
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all others. Each message carries the address of its intended destination;
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only the station with this address will act on the message. A transmission
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takes a very short (but not zero) time to arrive at its destination. Since
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stations will be at different distances from each other, the transmission
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time between them varies. The maximum transmission time between any two
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stations is called the slot time.
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</p>
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<p>
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Ethernet does not provide any guarantee of correct delivery, so its protocol
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can be simple. Instead, a higher-level protocol such as TCP (Transmission
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Control Protocol) is responsible for detecting and correcting errors.
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</p>
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<p>
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Transmission proceeds in one of the following ways:
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</p>
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<dl>
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<dt><b>Normal:</b></dt>
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<dd>
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The station transmits its message from start to finish. When the message
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has been fully received at the intended destination, that station
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processes the message.
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</p>
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</dd>
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<dt><b>Deferral:</b></dt>
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<dd>
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<p>
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In fact, a station listens for a transmission from another station
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before beginning its own transmission. If it hears such a transmission,
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it defers to the other station. Once that transmission has finished, the
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station begins its own transmission.
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</p>
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</dd>
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<dt><b>Collision:</b></dt>
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<dd>
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<p>
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However, this does not avoid interference. For example, a distant
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station may have started transmission but its signal has not yet
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arrived. The local station may therefore think the network is not in use
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and begin its own transmission. Both transmissions will then collide in
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the network. This can be detected by the transmitting stations, but this
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is not always fully reliable. The stations therefore ensure that the
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collision is properly detected by sending a jam signal.
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</p>
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<p>
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Following this, each station backs off a random delay (a number of slot
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times) before retrying its transmission. Ideally, one station should
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choose a smaller delay and so start retransmitting before the other one.
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</p>
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<p>
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For the first retransmission, the random delay is 0 or 1 slot times.
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Since two stations may choose the same delay, there is a 50% chance that
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they retry at the same point (and therefore collide again). On further
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collisions, the maximum delay is doubled (0 to 2, 0 to 4, etc.). A limit
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is set on the maximum number of retries.
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</p>
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</dd>
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</dl>
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<p>
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For this simulation, only two stations are involved; addresses are therefore
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not required. For simplicity, the protocol simulation below assumes that the
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LAN works perfectly (no message corruption, loss or misordering). However,
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interference among transmissions is still a source of potential error that
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the protocol must cope with.
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</p>
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<h2>Protocol Parameters</h2>
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<p>
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By default, the simulation has a retry limit of 0. Since this means that
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retries are not allowed, the simulation internally ensures that collisions
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cannot occur. Retry limits from 1 to 5 can be used, but the possibility of
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collisions complicates the behaviour. Click Change Settings after altering
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the retry limit. This will restart the whole simulation.
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</p>
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<form name="settings">
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<center>
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<table cellspacing="5">
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<tr>
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<td align="right">Retry Limit (0 = no collisions, 5 maximum):</td>
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<td><input name="retryLimit" size="4" value="0"></td>
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</tr>
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<tr>
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<td align="center" colspan="2">
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<input type="button" name="btnSet" value="Change Settings"
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onclick="set()"/>
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</td>
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</tr>
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</table>
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</center>
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</form>
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<h2>Protocol Simulation</h2>
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<p>
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The protocol simulation shows a time-sequence diagram with two stations, the
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MAC entities that support them, and a LAN that carries messages between
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them. Stations transmit messages in the format <var>DATA(Dn)</var>. Data
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messages are numbered <var>D0</var>, <var>D1</var>, etc.; no explicit data
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content is given. If the retry limit is reached, this is reported to a
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station with the message <var>FAIL(Retry Limit)</var>. Protocol messages
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have the form <var>START(Dn)</var> to start a transmission,
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<var>FINISH(Dn)</var> to finish the transmission, and <var>JAM</var> to jam
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transmission.
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</p>
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<center>
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<applet code="simulator.ProtocolSimulator.class"
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archive="ProtocolSimulator.jar" width="750" height="700"
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name="ProtocolSimulator">
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<param name="protocol" value="CSMA"/>
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</applet>
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</center>
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<hr/>
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<p>
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<a href="index.html"><img src="uparrow.gif" alt="Up Arrow"/></a>
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Up one level to <a href="index.html">Protocol Simulators</a>
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</p>
|
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</body>
|
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</html>
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