• When a number of stations (users) use a common link of communication system we have to use a multiple access protocol in order to co-ordinate the access to the common link.

• The three techniques used to deal with the multiple access problem are as follows:

• Random access
• Controlled access
• Channelization

• This is the CSMA protocol with collision avoidance.

• Figure shows the flow chart explaining the principle of CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance).

• The station ready to transmit senses the line by using one of the persistent strategies.

• As soon as it finds the line to idle, the station waits for an IFG (Inter-frame gap) amount of time.

• It then waits for some random time and sends the frame.

• After sending the frame, it sets a timer and waits for an acknowledgment from the receiver.

• If the acknowledgement is received before expiry of the timer then the transmission is successful.

• But if the transmitting station does not receive the expected acknowledgment before the timer expiry then it increments the back off parameter, waits for the back off time and re-senses the line.