CSMA/CA — Wireless Medium Access Control Protocol
Carrier Sense Multiple Access with Collision Avoidance
The medium access control (MAC) is a sublayer of the data link layer of the open system interconnections (OSI) reference model for data transmission. It controls the transmission of data packets via remotely shared channels. It sends data over the network interface card.
CSMA/CA is a medium access control for IEEE 802.11 wireless local area network.
Carrier sense means that a node on the network can detect what is going on in the transmission medium. Carrier means shared medium. Sense means a node can listen and detect. Multiple access means every node has an equal right to access the transmission channel but must take turns.
All put together means that a node can listen or check if the transmission medium is free or busy so that when the medium is not busy the node can transmit the data or its signal to the destination via the medium.
Unlike CSMA/CD the nodes can’t detect collision because of the medium being wireless instead of cables, so CSMA/CA tries to avoid the collision in the first place. This whole process is called IEEE 802.11 RTS/CTS Exchange.
Here is how it works between a node and a wireless Access Point (WAP):
The laptop wants to send some data over the medium, the first thing it will do is check the medium for ongoing activity if it detects the activity it waits for some time and tries again, if the medium is not busy it sends a Request to Send (RTS) to the WAP to ask for permission to transmit. If the WAP is busy it rejects the RTS and the laptop waits for some time and after some time the laptop tries again and if the WAP is idle it confirms the RTS and states that it issues a Clear to Send (CTS) receipt. The WAP suspends communication with all the other nodes and waits for the laptop to complete the transmission.