Dot11 Guru

802.11, 802.11h

802.11h DFS

DFS Began in Europe with ETSI, and later FCC.  The IEEE proposed the DFS solution along with TPC.

The certification process checks that the AP successfully detects 6 different types of DFS waveforms.  The AP must meet a particular % of success to receive certification.

Europe certification for DFS is very fast, and often clients are released with all channels available immediately.

America certification is much slower, usually a 6 month backlog, so devices are sometimes released without supporting these channels, and often firmware is not updated or channels are never available for these devices

This is why we must profile our devices for channel capability

802.11n devices also have an issue because the DFS certification was suspended for 2 years, so devices manufactured during these years often don’t support DFS either

There are two types of devices when considering DFS:

Master Device   AP/Device capable of doing radar detection (certification required here)
Client Device Can, but is not required to do DFS detection (unless it’s doing ad-hoc mode)

Channel Availability Check (CAC) monitors if a radar waveform exists on a specific channel

if a waveform exists and is louder than -64dbm, an AP must move.  When the AP boots, it must perform the CAC for 60 seconds before it can begin beaconing.  In Europe for channels 120,124,128, an AP must listen for 10 minutes before beaconing (TWDR)

If an AP hears a pulse after the initial CAC, it has 10 seconds to move channel (often moving to channel 36).  To move channels, the AP will send Channel Switch Announcement (CSA) frames.

A CSA can be within a beacon, probe response, or action frame.  The AP should send multiple frames over a 10 second period.

Image result for channel switch announcement frame format

If the channel switch mode is set to 1, no client can transmit until the channel change has happened.

After a channel change, the client will need to re-associate to the new channel.

The non-occupancy time of a DFS channel is 30 minutes, and the AP has to monitor the channel for 60 seconds before moving back.

there is a feature called Zero wait DFS, which monitors the channel with an additional radio instead of impacting clients (Broadcom)


 Terminal Doppler Weather Radar (TDWR) is a Doppler weather radar system with a three-dimensional “pencil beam” used primarily for the detection of hazardous wind shear conditions, precipitation, and winds aloft on and near major airports situated in climates with great exposure to thunderstorms

CAC (Channel Availability Check): The period of time in which AP will monitor for presence of Radar signals.

NOP (Non occupancy period) : The period of time in which radar detected channel will become unusable channel (or) un-available channel .

ISM : It is an process which will continuously monitor the operating channel for   detecting the presence of radar signals.

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