DFS Channels and RADARs

Overview:

Few of the 5Ghz channels is shared with Radars Systems. The channels that are used for Radar Systems depend on the Regulatory rules of that country and may differ from country to country and these channels are called DFS channels (Dynamic Frequency Selection). When a 5Ghz channel is used by a Radar system, any wlan device cannot operate on that 5Ghz channel. This is becuase we do not want to interfere the operation of a miltary Radar System.

So who takes care of not operating on the DFS channel? Well its usually the AP who has the capability of detecting Radar Signals and switching away from that channel and also, announcing to the clients that there is a Radar Signal detected and the AP would be switching to a new channel.

How is it informed to the clients? Answer is a new IE, called 'Channel Switch Announcement IE'.

Length : 

Set to 3 bytes

Channel Switch Mode :

Indicates any restrictions on transmission until a channel switch. An AP in a

BSS or a STA in an IBSS sets the Channel Switch Mode field to either 0 or 1 on transmission. In an MBSS,the Channel Switch Mode Field is reserved

New Channel Number: 

Set to the number of the channel to which the STA is moving.

Channel Switch Count:

For non-mesh STAs, this  field either is set to the number of TBTTs until the STA sending the Channel Switch Announcement element switches to the new channel or is set to 0. A value of 1 indicates that the switch occurs immediately before the next TBTT. A value of 0 indicates that the switch occurs at any time after the frame containing the element is transmitted.

This Channel Switch Announcement element present in beacons & probe responses(So that Clients which are trying to associate are aware that this AP will be switching its channel anytime). Channel Switch Announcement element also associated with an action frame (spectrum management type or category type =0) that can be sent by the AP between beacons to announce the channel switch.

So let us now understand the flow:

-- Only an AP is allowed to transmit a Channel Switch announcement IE in a BSS once it detects a radar signal. AP sends CSA IE in 10 beacons (usually) before moving it to a new channel, in this way, it gives a fair chance to all Stations to move to new channel.

-- The AP may force STAs in the BSS to stop transmissions until the channel switch takes place by setting the Channel Switch Mode field in the Channel Switch Announcement element to 1.

-- The STA can move to the new channel or even choose to move to a new BSS. 

-- Once an AP detects a Radar signal on a particular channel it will not use that channel for next 30 minutes.

-- If an AP is configured to use a DFS channel, it will first scan the channel for 1 minute to check if there is a Radar signal on that channel and only if it finds there is no Radar signal it will start using that channel.