Dynamic Frequency Selection (DFS)

Dynamic Frequency Selection is a technique of detecting the RADAR signals which must be protected against interference from 5GHz radios and upon detection switching to 5G operating frequency which does not interferes with the RADAR.

So using DFS, WLAN 802.11ac 5G devices are able to use 5G channels which are also used by RADAR systems. Thirteen out of twenty two 20Mhz 5G channels are DFS channels. WLAN devices operating in DFS channels require the capability of RADAR detection and avoidance capability. IEEE 802.11h addresses this requirement by adding DFS and Transmit Power Control(TPC) to all DFS channels. Here is list of 5G channels which require DFS :

Channel
- 52
- 56
- 60
- 64
- 100
- 104
- 108
- 112
- 116
- 120
- 124
- 128
- 132
- 136
- 140

When a 5G radio is operating in DFS channel it continuously monitors that 5G channel for RADAR signals.And if RADAR is detected it:

• Stops the transmission on that channel.
• Flushes power save client queues.
• Disassociates the clients.
• Broadcasts the 802.11h channel change announcement.
• Randomly selects another channel : 
1. If the selected channel is non DFS channel, it starts the beaconing and accepts client associations on that channel.
2. If the selected channel is DFS channel, then it waits for 60 sec to see if RADAR is detected and if RADAR is not detected, It starts the beaconing and accepts the client associations on this channel. If a RADAR is detected, It searches for another 5G channel.

WLAN Scanning

Ever wondered how does your mobile phone or laptop scan for WiFi networks around?

All Wireless radios scan for wlan transmitters continuously. 802.11 b/g/n radios scan in 2.4G frequency band and 802.11 a/ac radios scan in 5G frequency band. WLAN stations listen for beacons,probe responses  and finds out what Access Points (APs) are available in range to connect.

WLAN scanning is of two types:

•  Passive Scanning
•  Active scanning

Passive Scanning

All APs select one RF channel for their operation(Like for 2.4G band it could be channel number between 1 to 11  and for 5G band it could be channel 36,40,44,48,52,56,60,64,100, 104,108,112,116,120,124,128,132,136,140,144,149,153,153,157,161 or channel 165) and  send out beacon every 100ms (or more based on beacon interval configured).While doing a passive scan 2.4G WLAN station listens to  beacons  in  channels  allowed for 2.4G band and 5G station will  listen to channels allowed for 5G  frequency band and jolts down the results in the form of SSIDs of the different APs in proximity.

Passive scans are always enabled and can never be disabled because it's a bare minimum requirement for a client to connect to wireless AP.

Passive scan is a time consuming process because a WLAN station has to go to each channel and listen for approximately 200ms to listen to beacons. Once it is done scanning each channel, it will have list of APs in range.

Active Scanning

Active scans can be enabled and disabled based on requirement of the device in radio profile. During active scan,  WLAN station sends a broadcast probe request and listen to the probe responses from other WLAN devices.Active scans are more thorough than passive scans. Probe response received in response to active scan contains SSID and BSSID of an AP.

Active scan is little faster then passive scan and hence the preferred method for operating systems to connect to an AP.In active scan station goes to each channel and sends out either a broadcast probe request(asking any body out there?) or a directed probe request (asking some specific AP say : Hey, Neelam, are you out there ?)  and wait for <= 100ms for probe response and move to next channel.