CSMA with Collision Avoidance (CSMA/CA)

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is a network protocol for carrier transmission that operates in the Medium Access Control (MAC) layer. In contrast to CSMA/CD (Carrier Sense Multiple Access/Collision Detection) that deals with collisions after their occurrence, CSMA/CA prevents collisions prior to their occurrence.

Algorithm

The algorithm of CSMA/CA is:

• When a frame is ready, the transmitting station checks whether the channel is idle or busy.

• If the channel is busy, the station waits until the channel becomes idle.

• If the channel is idle, the station waits for an Inter-frame gap (IFG) amount of time and then sends the frame.

• After sending the frame, it sets a timer.

• The station then waits for acknowledgement from the receiver. If it receives the acknowledgement before expiry of timer, it marks a successful transmission.

• Otherwise, it waits for a back-off time period and restarts the algorithm.

The following flowchart summarizes the algorithms:

Advantages of CMSA/CD

• CMSA/CA prevents collision.

• Due to acknowledgements, data is not lost unnecessarily.

• It avoids wasteful transmission.

• It is very much suited for wireless transmissions.

Disadvantages of CSMA/CD

• The algorithm calls for long waiting times.

• It has high power consumption.