Updated: Dec 10, 2020

5 GHz DFS channels highlighted in red TDWR channels in yellow


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.

DFS is all about radar detection and avoidance. Radar stands for "Radio detection and ranging." In the past, the radars used to operate in frequency ranges where they were the only type of device operating there. Now that regulatory agencies are opening those frequencies for other uses (like wireless LAN), they need those devices to operate alongside incumbent radar units.

The general behavior of a device complying with the DFS protocol is to detect when a radar is occupying the channel, then stop using that occupied channel, monitor another channel, and jump on it if it is clear. (i.e., no radar there as well).

A radio's process to detect a radar is a complicated task that is not part of the standard. Hence, false radar detections can occur and is an art that combines the Wi-Fi vendor algorithm with the Wi-Fi chip capabilities. However, the detection itself is mandatory by the regulatory agency and defined clearly. Therefore scanning parameters are not configurable.

DFS has been required early on for European Telecommunication Standard Institute (ETSI) devices working in the European Union (and countries following ETSI regulations) in the ETSI 5ghz band. It is not necessarily mandatory in other parts of the world and also depends on the frequency range. The American Federal Communication Commission (FCC) has now made it compulsory for UNII-2 and UNII-2 extended frequency range like ETSI.

DFS operations use different ways of exchanging information between stations. Data can be put in specific elements in the beacon or probe response, but an action frame can also report information.


DFS detects radar interference and moves the wireless network to another frequency with no interference. It maintains a list of channels where radar has been detected in the NOL (non-occupancy list). The AP avoids using these channels for at least 30 minutes after detecting radar on them.

It is important to note that the responsibility for scanning the spectrum for existing radar falls to the Access Point not the client (unless ad-hoc network in use).

When DFS is enabled, the AP:

  • Looks for radar detection before securing a frequency channel.

  • Scans continuously for radar signal patterns during normal operation.

Before transmitting on a DFS channel, 802.11 stations must validate (by first listening for 60 seconds in the US) that there is no radar activity. If an 802.11 radio should detect radar while using the DFS channel, it must vacate that channel quickly. Thus, if a radio should detect radar in its serving channel, then switch to another DFS channel, this will impose (at least) a one-minute outage.

If a radar signal is detected while an access point (AP) is on a DFS channel, the AP will behave as follows:

  • Stop transmission of data frames on that channel

  • Broadcasts an 802.11h channel-switch announcement

  • Disassociates clients

  • Selects a different channel from the DCA (Dynamic Channel Assignment) list

  • If the selected channel is not DFS, then AP enables beacons and accepts client associations

  • If the AP selects a DFS-required channel, it scans the new channel for radar signals for 60 seconds. If there are no radar signals on the new channel, the AP enables beacons and accepts client associations. If a radar signal is detected, the AP selects a different channel

TPC - Transmit Power Control

TPC is a feature of 802.11h along with DFS by which the access point can define local rules for maximum transmit power. There are many reasons why this would be used. One could be that the administrator wants to set another set of rules than the regulatory domain maximum because of more specific local rules or environment.

Another could be that the administrator knows it is a very dense Wi-Fi deployment with intense coverage. Therefore, APs set themselves to a lower transmit power (thanks to the RRM algorithm).

TPC is a dynamic way to force clients to lower their power and reduce their coverage so that they do not disturb neighbor clients/AP's on the same channel.

Terminal Doppler Weather Radar (TDWR)

The Terminal Doppler Weather Radar (TDWR) is an advanced technology weather radar deployed near 45 of the US's larger airports. The radars were developed and deployed by the Federal Aviation Administration (FAA) beginning in 1994 to respond to several disastrous aircraft crashes in the 1970s and 1980s caused by strong thunderstorm winds.

These crashes occurred because of wind shear, a sudden change in wind speed and direction. Wind shear is common in thunderstorms due to a downward rush of air called a microburst or downburst. The TDWRs can detect such dangerous wind shear conditions and have been instrumental in enhancing aviation safety in the US over the past 15 years.

The TDWRs also measure the same quantities as our familiar network of 148 NEXRAD WSR-88D Doppler radars--precipitation intensity, winds, rainfall rate, echo tops, etc. However, the newer Terminal Doppler Weather Radars are higher resolution and can "see" in much more sufficient detail close to the radar. This high-resolution data has generally not been available to the public until now.

Since thunderstorms are uncommon along the West Coast and Northwest US, there are no TDWRs in California, Oregon, Washington, Montana, or Idaho.

TDWR Radar Coverage Areas

TDWR Site Exact Locations and Frequencies


Wireless Lan Professionals

Cisco - Overview on 802.11h, Transmit Power Control (TPC) and Dynamic Frequency Selection

Wi-Fi Alliance - What is dynamic frequency selection (DFS)?

WiFi Insider - DFS

Cisco - Radar Detection in Dynamic Frequency Selection (DFS Channels)

Weather Underground - Understanding Weather Radar


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