Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

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CSMA/CD is a type of media access control method for placing signals on baseband transmission networks.

What is CSMA/CD (Carrier Sense Multiple Access with Collision Detection)?

CSMA/CD is a type of media access control method for placing signals on baseband transmission networks. Since baseband networks can carry only one data signal at a time, there must be some way of controlling which station has access to the media at any given time. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is one such control method.

How it works

In networking technologies that use CSMA/CD as their access method, a station first «listens» to the network media to make sure there is no signal already present from another station before it tries to place its own signal on the media. If a carrier signal is detected on the media, which indicates that a station is currently transmitting a signal, no other station can initiate a transmission until the carrier stops. If no carrier is detected, any station can transmit a signal.

If two stations listen to the wire and detect no carrier signal, they may both decide to send signals simultaneously. If this happens, a collision occurs between the two signals generated. Next, both stations detect the collision and stop transmitting their signals immediately, sending out a jamming signal that informs all other stations on the network that a collision has occurred and that they should not transmit. Meanwhile, the two stations whose signals created the collision cease transmitting and wait random intervals of time (usually a few milliseconds) before attempting to retransmit.

CSMA/CD Process
CSMA/CD Process

CSMA/CD is known as a contention method because computers contend for the chance to transmit data onto the network media. CSMA/CD is the standard access method for Ethernet networks. This method has two main drawbacks:

  • Only a relatively small number of computers can exist within any one collision domain. More computers will produce more collisions and slow overall network traffic.
  • CSMA/CD is not reliable beyond a distance of 1.5 miles (2500 meters) because of signal attenuation.


The designation CSMA/CD derives from the following:

  • CS means that stations first s ense a c arrier present on the media before transmitting their own signals.
  • MA means that m ultiple stations can a ccess the network media.
  • CD means that if a c ollision is d etected because of multiple simultaneous transmission of signals, the stations that are transmitting signals stop, and then retransmit a short time later.

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