CSMA/CD


CSMA/CD stands for "Carrier Sense Multiple Access with Collision Detect”. CSMA/CD
is a random control access method.

The CSMA/CD access method is used as the access control method in Ethernet and is defined in a standard from IEEE. The CSMA/CD algorithm is quite simple and the efficiency for an ordinary Ethernet is about 65%. This means that the effective bandwidth for a 10 Mbps Ethernet is about 6.5 Mbps. The rest is lost, mainly due to collisions.


Before one host will transmit it must ”listen” on the medium whether or not another host is
transmitting. If the medium is ”quiet” the host can send its data. The term "Carrier Sense" indicates that a host listens before it transmits.

"Multiple Access" means that many hosts can be connected to the network and all hosts
have the same right to transmit.

With CSMA/CD, it occasionally happens that two hosts send their packets at the same
time. This will make a collision on the network. The information about the collision is
detected by all the other hosts on the network. This is called "Collision Detect". If a host
detects a collision it will wait a random period of time before it tries to transmit again.

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