The current article tries to elaborate on how the Access Point notifies client stations on the presence of radar and the steps it employs to detect radar

• An access Point will periodically quiet the entire BSS to scan for radar. This is achieved by transmitting the quiet element in the beacon
• The quiet element can be also incorporated into a beacon when the AP detects a signal which seems like radar. The quiet element and its description is provided below

FIG COURTESY – 802.11-2012^TM standard

•  The parameters of the quiet element are defined below
  • Element ID – set to 40 indicating quiet element
  • Length is set to 6 bytes
  • Quiet count — the count value indicates the number of TBTTs before the start of next quiet period. A value of ‘1’ indicates that the quiet interval starts at the beacon interval of the next TBTT
  • Quiet Period — the quiet period indicates the number of beacon intervals between regularly scheduled quiet periods. A value of ‘0’ indicates that there is no scheduled periodic quiet period setup
  • Quiet duration — it indicates the duration of the quiet interval, time is expressed in TUs.
  • Quiet offset — set to the offset of the start of the quiet interval from the TBTT specified by the Quiet Count Field, expressed in TUs. The value of the quiet offset should be lesser than one beacon interval

An example Quiet element in a packet capture is shown below

• After sending the quiet element and start of quiet period – the AP will scan the medium for the detection of Radar
• If the Access Point detects a radar repetitive in-band signal, the Access point will send out a channel switch announcement element to all associated stations. The channel switch announcement element is provided below

FIG COURTESY – 802.11-2012^TM standard

• The Parameters are defined below
  • Element ID is 37 — indicating channel switch announcement element
  • Length — is set to 3 octets
  • Channel Switch Mode — it can take 2 values – 0 or 1
  • When set to 1 — The Station in the BSS will not transfer any frames till the channel switch operation is complete
  • When set to 0  — The access point does not set any restrictions on the station
  • New Channel Number — The new channel that the Access Point is moving to
  • Channel Switch Count — indicates the number of TBTTs till the Access Point can move to the new channel. A value of ‘0’ indicates that the switch can occur any time after the channel switch announcement element is transmitted

An example channel switch announcement element in a packet capture is shown below

• The Access point after the transmission of the Channel switch announcement element will cease all transmissions and move to the new channel within “Channel Move Time”. The channel move time is specified as “10 seconds”.
• The channel on which the Access point detected the radar becomes unavailable for a “non-occupancy period” – which is specified as “30 minutes”
• The Access point needs to complete all its transmission after detecting radar within – “Channel Closing Transmission Time” – which is specified as “1 second”
• On Moving to the new channel – if the new channel selected is a DFS channel, then the Access point needs to monitor the new channel for a “Channel Availability Check Time” – which is specified as “60 seconds”
• Once the above processes are completed – the Access point starts beaconing on the new channel
• The Channel switch Announcement element can also be transmitted in a channel switch announcement action frame by the Access Point

Extended Channel switch announcement and its usage