With CSMA/CA, the CA stands for Collision Avoidance, which is a different concept than Collision Detection in that the stations are trying to avoid any kind of collision instead of simply detecting it. This is commonly used on wireless networks because a wireless device cannot hear whether communication is occurring between other stations at a specific moment (thus, it cannot use CD).
In such an environment, the plan is to avoid any collisions before sending data on the network and this is commonly implemented as RTS/CTS (ready to send/clear to send). This is usually managed by the central point of the wireless network – the access point (AP). Before stations can send traffic, the AP must first grant them permission to do so, confirming that there is no other traffic occurring at that time so collisions can be avoided. Because the station must wait for the clear signal, the AP can be sure that only a single station is sending traffic at a specific time. This decision process is depicted in Figure 19.2 below:
Figure 19.2 – CSMA/CA
In Figure 19.2, you can see that after the frame is assembled the station must wait for confirmation that the channel is clear and only then can it transmit the data. If stations do not receive the CTS approval from the AP, they must wait a random amount of time before sending the RTS request again. This technique also solves the problem in which a station can see the AP but cannot see other stations from other sides of the network that are accessible only through the AP (i.e., the AP can reach every station, as it is the central point).