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What is the process an AP must follow to choose a DFS channel and what happens if radar is detected

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TitleWhat is the process an AP must follow to choose a DFS channel and what happens if radar is detected
Question
What is the process an AP must follow to choose a DFS channel and what happens if radar is detected
Environment
  • ExtremeWireless
  • DFS (Dynamic Frequency Selection)
  • 802.11h
Answer
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)
  1. 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.
  2. Assuming the channel is clear of radar, transmissions may begin
  3. 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
  4. AP does CAC on newly selected channel as per 1 and starts to transmit as per 2.
  5. After non-occupancy time expires, the channel on which the AP previously detected radar is removed from the NOL and made available for reuse.
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