Dynamic Frequency Selection (DFS)
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Dynamic Frequency Selection (DFS)

• 802.11h - 2003 - DFS & TPC
• Dynamic frequency selection (DFS) originally proposed to satisfy regulatory requirements for operation in the 5 GHz band in Europe and to detect and avoid interference with 5 GHz satellite and radar systems.
• These same regulatory requirements have also been adopted by the FCC in the United States.
• The main purpose of DFS and TPC is to provide services where 5 GHz 802.11 radio transmissions will not cause interference with 5 GHz satellite and radar transmissions.
• The 802.11h amendment also introduced the capability for 802.11 radios to transmit in a new frequency band called U-NII-2 Extended with 11 more channels
• The 802.11h amendment effectively is an extension of the 802.11a amendment. OFDM transmission technology is used in all of the U-NII bands. The radar detection and avoidance technologies of DFS and TPC are defined by the IEEE.
• RF regulatory organizations in each country still define the RF regulations.
• In the United States and Europe, radar detection and avoidance is required in both the U-NII-2 and U-NII-2 Extended bands.
• DFS is used for spectrum management of 5 GHz channels by OFDM radio devices.
• DFS is essentially radar-detection and radar-interference avoidance technology.
• The dynamic frequency selection (DFS) service provides for the following:
  • An AP will allow client stations to associate based on the supported channel of the access point.
  • The term associate means that a station has become a member of the AP’s wireless network.
  • An AP can quiet a channel to test for the presence of radar.
  • An AP may test a channel for the presence of radar before using the channel.
  • An AP can detect radar on the current channel and other channels.
  • An AP can cease operations after radar detection to avoid interference.
  • When interference is detected, the AP may choose a different channel to transmit on and inform all the associated stations.
  • If a radio determines that a radar signal is present, it must either select another channel to avoid interference with radar or go into a “sleep mode” if no other channel is available.

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Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.