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