What are the “DFS” Channels and how are they dealing with RADAR systems?

5Ghz band is huge spectrum. However, there is a one big issue with this channel plan: DFS Channels.

DFS stands for “Dynamic Frequency Selection” and if your want to use some channels in the 5Ghz band, you should obey DFS rules.

DFS channels are used by some military and civilian RADAR systems and WiFi should be shut itself off, immediately after it detects the presence of RADAR pulses. So, if an AP has no RADAR detection capabilities, cannot operate in DFS channels.

Why are we dealing with this? Are there any non-DFS frequencies? Yes, there are.

For example, the below chart shows the band plan (5Ghz, ETSI) including DFS channels.



Let’s remove the DFS channels, and make them grey-out:




Oh! We lost nearly all our space, right? We only have some indoor channels with limited power levels, no outdoor channels and very limited spectrum to use (only 80Mhz total)

So, this non-DFS area tells us that we MUST use DFS channels and we need to understand DFS rules.

What are the DFS rules? Where are they written?

For ETSI regulatory domain that I’m working in, these details are writting in the document called “ETSI EN 301 893 v1.8.1 (2015-03)” document. (I’m not sure if there are updated version of this doc)

Link to the original document:  http://www.etsi.org/deliver/etsi_en/301800_301899/301893/01.08.01_60/en_301893v010801p.pdf

According to this document, the section 4.7 states the details about the DFS operation, such as:

  • There should be “channel availability check” before attempting the first transmission on the channel. CAC is 60 seconds but 10 minutes (for frequencies between 5600-5650Mhz). So, APs should wait first, then if there is no RADAR transmission heard on the channel for at least 60 seconds or 10 minutes, then start transmitting.
  • The device may conduct off-channel scanning to understand the presence of RADAR signals, these off-channel scannings should last minimum 6 minutes or 1 hour for frequencies between 5600-5650Mhz.
  • If RADAR is seen in a channel, no more Tx there, in the next 30 minutes.
  • If RADAR is seen in a channel, the channel should be emptied in max 10 seconds.
  • During these 10 second channel move time, total Tx duration should not last more than 1 seconds.

Screen Shot 2017-12-07 at 18.21.52

If every WLAN device obeys these rules, then RADAR operators will be happy, but why?

Here is a civilian weather radar output for Istanbul, Turkey. This is to see rain, snow etc. Meteorological Institutions are using these RADARs to detect atmospheric events in an area. So, for the below image, we can see some rain in the south of Istanbul and some other areas as well. This is a regular output for a rainy day(s):


However, what about the below image for Ankara?


What are those straight lines nearly around the top right of the image. Clouds with rain? Looks unnatural, right?

These are some WiFi signals captured by Ankara RADAR, probably coming from in the middle of the city. RADARs are using very very high gain antennas and they can pick up your very weak WiFi transmission. So, your WiFi transmission, if you do not obey DFS rules, literally blinds the RADAR and it cannot see the atmosphere, it cannot work as expected.

This is the critical point about DFS and we should all know DFS and its rules, to be careful about that.



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