CSMA/CA coordinates wireless devices competing for airtime on a shared, half-duplex medium. Devices sense the channel before transmitting, reducing—but not eliminating—the chance of collisions. section 18.1.1 section 18.5
A simplified CSMA/CA exchange proceeds as follows:
- Prepare a frame and listen to determine whether the channel is free. section 18.1.1
- Wait briefly for a random period when the channel is busy, then check again. section 18.1.1
- Transmit only after sensing a free channel. section 18.1.1
- Retransmit if simultaneous transmissions still produce a collision and an incoherent signal. section 18.1.1
Two wireless devices contend for airtime
Two devices prepare frames while sharing the same channel. Each listens first; a device that finds the channel busy waits randomly, while a device that finds it free transmits. If both transmit simultaneously, their frames can collide and require retransmission. section 18.1.1
Figure 18.3 The CSMA/CA process. To avoid collisions, a device will listen to ensure the channel is free before transmitting a frame.
Follow the process diagram to visualize channel sensing before frame transmission. section 18.1.1
