Carrier Sense Multiple Access/with Collision Avoidance

What Does Carrier Sense Multiple Access/with Collision Avoidance Mean?

Carrier Sense Multiple Access/with Collision Avoidance (CSMA/CA) is a network contention protocol used for carrier transmission in networks using the 802.11 standard. In contrast to the Carrier Sense Multiple Access/Collision Detect (CSMA/CD) protocol, which handles transmissions only after a collision has taken place, CSMA/CA works to avoid collisions prior to their occurrence.


CSMA/CA increases network traffic as it requires sending out a signal to the network even before transmitting any real data. This is to listen for any collision scenarios in the network and to inform other devices not to transmit.

Techopedia Explains Carrier Sense Multiple Access/with Collision Avoidance

In CSMA/CA, the moment a node receives a packet intended for sending, the first thing it does is to listen to the broadcast channel for a pre-specified time frame to determine if another node is broadcasting on the channel inside the wireless range. If the broadcast channel is detected as "idle," the node can then start transmitting the data packet.

If the broadcast channel is detected as "busy," the node holds the transmission, waits for a random time frame and then checks all over again to find out whether the channel is free. This time frame is referred to as the backoff factor. The backoff factor is counted down using a backoff counter.

If the channel is free when the backoff counter gets to zero, the node sends the data packet. If the channel is not clear even when the backoff counter reaches zero, the backoff factor is scheduled yet again, and the entire scenario is repeated. This is repeated until the channel becomes available. As soon as the channel becomes available, the data packet is transmitted. Once the data is received by the receiving node, it sends back an acknowledgment packet (ACK) after a short while. If the ACK is not received, it is assumed that the packet is lost and then a retransmission is set up.

The additional signaling makes CSMA/CA a slower access technique when compared to the CSMA/CD technique used in Ethernet networking.


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Margaret Rouse is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical, business audience. Over the past twenty years her explanations have appeared on TechTarget websites and she's been cited as an authority in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine and Discovery Magazine.Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages. If you have a suggestion for a new definition or how to improve a technical explanation, please email Margaret or contact her…