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CSMA/CD
Last modified: Monday, February 28, 2005
Short for Carrier Sense Multiple Access /
Collision Detection, a set of rules determining how
networkdevices respond when
two devices attempt to use a
data channel
simultaneously (called a collision). Standard
Ethernet networks use
CSMA/CD to physically monitor the traffic on the line at participating stations.
If no transmission is taking place at the time, the particular station can
transmit. If two stations attempt to transmit simultaneously, this causes a
collision, which is detected by all participating stations. After a random time
interval, the stations that collided attempt to transmit again. If another
collision occurs, the time intervals from which the random waiting time is
selected are increased step by step. This is known as exponential back off.
CSMA/CD is a type of
contention protocol. Networks using the CSMA/CD procedure are simple
to implement but do not have deterministic transmission characteristics. The
CSMA/CD method is internationally standardized in
IEEE 802.3 and ISO
8802.3.
Ethernet and CSMA/CD This article describes how and why CSMA/CD works on an Ethernet network, and how it can fail.
