Introduction to Dynamic Frequency Selection

Have you ever wondered why certain channels in the 5 GHz band, more specifically in the 5.25-5.35 GHz (UNII-2) and 5.47-5.725 GHz (UNII-2e) frequency bands are (almost) never in use by Wi-Fi devices? Well, it happens that, worldwide, these unlicensed frequency bands are also used by radar systems. So, for a Wi-fi device to operate in one of these bands, it must support Dynamic Frequency Selection (DFS). DFS is a mechanism (not limited to Wi-Fi) that allows a device to dynamically select or change the operating frequency to avoid interfering with other systems. In other words, regulatory bodies require Wi-Fi devices to use DFS to make sure devices will not interfere with your local weather radar system, for example.

Not all Wi-Fi devices support DFS, so they cannot use DFS channels (hence the unused spectrum), but when they do, they must follow certain rules. DFS states, for example, that if a radar signal is detected on a channel, the device cannot use that channel for a non-occupancy period. It's for this reason that most access points do not allow manual selection of DFS channels and use "auto" instead.

When using DFS, a device selects a DFS channel to use and, prior to initiating communications, it monitors the channel for a certain time to verify no radar systems are present. If radar signals are detected during this channel start-up period, the device doesn't use the channel (and avoids it for the non-occupancy period), then selects another channel and starts again.

The same is done while communicating (in-service). The device monitors the channel being used, and if radar signals are present, it vacates the channel within a channel move time. During this time, the device may indicate other systems to stop transmitting on that channel, which won't be used for the non-occupancy period. 

In the context of Wi-Fi, an access point (AP) that supports DFS must be able to detect radar signals during the channel start-up and in-service phases. If radar signals are detected, the AP will stop transmitting and then select a new channel. Once the new channel is selected, the AP informs nearby clients to clear the channel and move to a different frequency by including the Channel Switch Announcement IE in the beacon frame. DFS-compatible Wi-Fi clients, on the other hand, cannot start transmitting until authorized by the AP and cannot perform active scanning (i.e. transmit probe requests) until Wi-Fi activity from nearby APs is detected. They also must be able to clear a channel when commanded by the AP.