What is dynamic frequency selection (DFS)?

In many countries, regulatory requirements may limit the number of 5 GHz channels available or place additional restrictions on their use because the spectrum is shared with other technologies and services. For instance, in the US and other countries, some of the Unlicensed National Information Infrastructure (U-NII) bands are used by radar systems. Wi-Fi networks operating in those bands are required to employ a radar detection and avoidance capability. The IEEE 802.11h standard addresses this requirement by adding support for DFS and transmit power control (TPC) on every DFS channel.