The dilemma of Wi-Fi DFS Channels
Activate ? Do not activate? Learn more about DFS channels to make the right choice for your network
In order to better understand this mechanism, we will go through its history, its advantages and disadvantages so that we can better choose when configuring a Wi-Fi network for professional or domestic use.
The regulation used as a basis for this article will be that of the European ETSI region.
The limits of the 2.4 GHz band
The 2.4 GHz band used with the popular 802.11b/g standards in the early 2000s quickly became limited, partly due to the limited number of channels, and partly due to the arrival of Bluetooth and consumer wireless devices such as home automation and many others.
On the other hand, the bandwidth on which the channels used for Wi-Fi transmission are based is low: only 80 MHz. For this reason, only 14 channels are available in 2.4 GHz (each one spaced at 22 MHz as shown in the diagram below), but only 3 do not overlap:
With the development of wireless devices on this band, the channels started to get very busy, and this is still often the case in dense environments.
A new 5 GHz band, new rules
An additional band has emerged as necessary to cope with the increasing number of connected devices: the 5 GHz band.
In France, it was authorized in May 2008 by a decision of ARCEP, the national regulator. Small problem, it is already used in France (and many other countries) for other purposes: weather or military radars, telecommunications satellites.
So how do you make Wi-Fi equipment and important applications in other areas coexist? The IEEE found the solution in 2003, by introducing the 802.11h amendment to the 802.11 (Wi-Fi) standard: it is called Dynamic Frequency Selection (DFS).
When this mechanism is activated, the Wi-Fi access point permanently listens to the 5 GHz band and if its current channel is disturbed by a radar, two cases are possible :
- The access point is in automatic channel mode : the AP changes channels automatically to select a free one instead. In this case, the Wi-Fi service is maintained, but does not interfere with the radar.
- The access point is in manually set channel mode : the DFS deactivates the AP.
In both cases, they are often logged in event logs as “DFS Events” .
In order to limit the risk of interference, timed parameters are also defined by standards :
DFS Parameters
CAC — Channel Availability Check : Minimum time on which the Wi-Fi AP will listen to a channel to monitor the presence of speed cameras before selecting it if it is free (60 sec).
NOP — Non occupancy period : Period over which the channel will be unusable if radar interference is detected (30 min).
Channel Move Time : Maximum time for the Wi-Fi AP to change channels once interference is detected (10 sec).
Channel Closing Transmission Time : The maximum time the Wi-Fi AP can take to complete transmissions once interference is detected (1 sec).DFS parameters standardized by the ETSI EN 301 893 standard, applicable to Europe
These parameters have some practical consequences: an AP will only be able to transmit after 60 seconds after its initialization (due to the Channel Availability Check), and during a channel change following an interference, a transition time may be felt by the customers, especially during real-time use (VoIP, video streaming…).
The competent jurisdictions, depending on the use of speed cameras in their territory, set channels on which DFS is mandatory: in Europe, these are channels 52 to 140.
What can be done?
Know your environment and its use
A dense environment (e.g. building, store) will benefit from additional channels (with DFS) to reduce interference on adjacent channels by better distributing the channels allocated to the access points. The same applies if a bandwidth greater than 20 MHz is chosen, in order to increase throughput.
Many 5 GHz Wi-Fi devices do not use DFS by default, and in this type of environment generate high usage on non-DFS channels (36/40/44/48), which creates interference at the same time.
Disadvantages appear in particular during radar interference (due to the proximity to one of them or sometimes even the too high sensitivity of the AP), which leads to a channel change with a possible, although limited, dropout of the connected client.
In an environment that is isolated from other Wi-Fi access points, selecting the non-DFS channels reduces the risk of instability if a radar is nearby.
Monitor DFS Events
Radars are not only located at airports or military zones, hence the interest in monitoring the frequency of “DFS Events” to judge the stability or not of the network at a given location. Unfortunately, it is often difficult to predict in advance whether an area will be subject to radar interference.
Many manufacturers of access points store these DFS Events in the form of logs. This is particularly the case of Meraki, which even allows to retrieve them via the Dashboard REST API.
Personal opinion
Activating DFS channels has far more advantages than disadvantages, particularly in terms of the large increase in the number of channels available.
In a professional environment, on fleets of several thousand APs, I activate these channels by default, and then deactivate them if necessary, when the DFS Events monitoring indicates a lot of channel changes, or when they are noticeable by users.
For example, as a network responsible for Meraki equippied sites, I used the power of Dashboard API to develop a script that counts DFS events on an entire organisation or single network :
python getMerakiEventCount.py -p wireless -t dfs_event -o 549236
Using default timespan : 1 day
Timespan : between 2020-11-14 16:37:07.722869 and 2020-11-15 16:37:07.722869
Network : DevNet Sandbox ALWAYS ON , Number of dfs_event : 17
Network : DNSMB3 , Number of dfs_event : 0
Network : DNSMB2 , Number of dfs_event : 1References
To go further on the subject, I recommend the following sources of information:
ETSI IN 301 893
List of WLAN channels
DFS — Meraki Documentation
