How does CSMA/CA work?

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is a network protocol for carrier transmission that operates at the MAC.

Compared to wired networks, wireless systems cannot be monitored as securely. The node beyond the first station's range may cause collisions since it cannot detect the other's attempts to send data packets over the network. CSMA/CA helps lower the probability of such collisions. Each station in a decentralized network must abide by a set of rules and manage communication among themselves.

LBT principle

The "Listen Before Talk" (LBT) principle is the foundation of CSMA/CA. Before the station can begin transmitting, the line must check to verify if it is free. However, this is only the first action. Collisions are effectively prevented with the help of additional functions that are part of the procedure.

Working

The following illustration shows the workflow of CSMA/CA in detail.

The steps above are followed by wireless networks that adopt CSMA/CA. These are explained in more detail below.

1. First, the stations sense the transmission medium. This implies that the carrier sense keeps a close eye on the radio channel and determines whether other stations are transmitting at the same time.

2. If the transmission medium is already in use, a random backoff is initiated, during which the station waits until a new check begins for a determined period. The same happens at all other stations that are not actively broadcasting or receiving. It only occurs if the station is unaware that the medium is already occupied (which is set by the wireless listening stations to tell a station how long it must wait before accessing the wireless medium).

3. If the network is available, the station initiates, which regulates how long a station waits before commencing a media transmission. Before $t$, which is the amount of time a station should wait before sending its request frame, the channel is examined thoroughly. If it remains free, a random backoff begins, and the optional exchange starts, which minimizes frame conflicts caused by the hidden node problem. If the request to send has been received successfully at the receiver side and there hasn't been a collision, the frame grants the sender permission to use the transmission medium.

4. All other stations are alerted that the network is in use. As a result, they raise their network allocation vector again and delay making another attempt to verify if the channel is free.

5. The station then begins the transmission. The receiver confirms that the data packet has been successfully received after waiting for the $t$, which is the time required to process a data package, and then sends an ACK (acknowledgement) frame to indicate to the sender that the data frame has been received. It also sets the network allocation vector to 0, indicating that the network is now open for a new transmission.

Example

Host A reserves the transmission channel to deliver a data frame to host B. Host C pauses its backoff timer when it senses that the channel is busy and only resumes it when the transmission media gets free.