Dynamic Frequency Selection in Wireless Networks

References

https://www.cwnp.com/dynamic-frequency-selection/

Note: All figures taken from internet

 Dynamic Frequency Selection

In US and some other countries some of the Unlicensed National Information Infrastructure (U-NII) bands, 5.25-5.35 Ghz ( UNII -2) are used by radar systems. When wifi devices operate in these bands , they should not cause interference to the radar systems.

Wlan devices operating in these bands or frequencies need to check periodically for the presence of any radar signals. If the device finds any radar signal, then it should not cause interference to the radar systems. The entire BSS must switch to a new channel. This process of detecting the radar signals and switching to a different channel is Dynamic frequency selection.

DFS process consists of the following steps

1) Quieting the Channel: In an Infra structure network, the Ap schedules certain periods to be quiet periods where all the stations stop any transmission. This is helpful to make measurements of the presence of any radar systems in the 5GHz spectrum. The AP indicates when the quiet period happens and the duration of quiet period by including the Quiet Information Element in the Beacon and Probe Response frame.

Following is the structure of Quiet IE


Quiet information element 

 

Quiet count: Specifies the no of beacon intervals after which a quiet period begins

Quiet period: If the Quiet period is  scheduled periodically, it specifies the  nuber of beacon intervals between quiet periods.

Quiet Duration: This specifies the duration of quiet period

2)  Making  Measurement: When the signal strength of radar signal rises above a specific value called interference threshold, it is taken as a radar system is detected. Access point may send a measurement request to the all the station in the BSS. This is a request for the stations to check/measure the radio channel. The stations respond with a measurement report Frame. If any radar signal is detected, then it will be indicated in the measurement report and the AP has to move to a new channel.

3) Channel Switch: Once the AP decides to switch to a new channel, it has to inform all the stations associated of the new channel number . The channel switch announcement IE is used by the AP as part of the beacon to indicate the new channel number.

The following is the structure of Channel Switch IE

Channel Switch Announcement information element

 Channel Switch Mode: It can have a value of 1 or 0. When the value is 1 , it indicates that the operating channel is being changed and the stations should stop transmitting. A value of 0 indicates that the stations can transmit .

New Channel number:  This  indicates the new channel to which a station moving

Channel Switch Count: This indicates the number of beacon intervals after which a channel switch is scheduled to happen


 

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