The Wi-Fi Professor: Perry Correll

Dynamic Frequency Selection (DFS)

Dynamic Frequency Selection (DFS) When additional channels were added to the available 5GHz spectrum there was much concern on how Wi-Fi’s use of these channels may impact devices already operating in these ranges. To address this the 802.11h spec, commonly referred to as Dynamic Frequency Selection (DFS) was created to define a set of procedures to detect and avoid interference with Radar systems operating in the 5 GHz range (UNII channels – 52-64 & 100-140). Note that many people believe that DFS is only required for the extended UNII band, when in fact it also includes the upper 4 channels in UNII (52-64). There are several parts of the specification, however the one that is most visible to users is the ability of an AP to detect and move from a channel that interferes with radar systems. Basically AP’s supporting the standard will designate a ‘Quiet’ period using information in the Beacon frame. This information will tell the stations to set their Network Allocation Vector (NAV) to allow for a quiet period when the AP can listen for transmitting Radar. If radar is detected the AP must alter the channel it is operating on and most have the ability to tell associated stations what channel they will be moving to. This allows stations to re-associate with minimum interruption. APs that do not support DFS are not allowed to operate on the channels where interference occurs; this significantly limits the number of channels available in the 5GHZ spectrum.

Thanks for visiting, The Prof

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