To explain this process which is mandated by the regulatory authorities (FCC/ETSI) some terminology must be defined
Channel Availability Check Time - This is the time the system must monitor a channel from radar prior to any data being transmitted over the channel (known as CAC)
Channel move time - The time taken by the system to vacate a channel, it's measured from the end of the radar burst to the end of the data transmission on that channel. Must be 10 seconds or less.
Non-Occupancy time - The time during which a channel must not be used after radar has been detected (must be a minimum of 30 minutes)
Non-Occupancy list - A list of channels the AP has seen radar on (NOL)
Master device - A device that can detect radar typically an AP (and referred to as AP from now on) but could be a client device operating in ad-hoc mode
Client device - A device that cannot detect radar, instead it relies on seeing valid data on a channel from a master device (eg an 802.11 Beacon frame)
- System starts up and does CAC check to monitor for radar, this takes about 60-90 seconds, no transmission will occur, the AP is only listening. If radar is detected during this time another channel will be selected and the CAC process restarted.
- Assuming the channel is clear of radar, transmissions may begin
- In the event that radar is detected on the channel the AP:
- AP informs clients that it is servicing of its intention to vacate the channel (802.11h channel switch announcement)
- AP initiates move to another channel
- The above is the channel move time.
- AP adds vacated channel to NOL
- AP does CAC on newly selected channel as per 1 and starts to transmit as per 2.
- After non-occupancy time expires, the channel on which the AP previously detected radar is removed from the NOL and made available for reuse.