Operating your Wi-Fi network on the 5GHz band? Make sure you deploy DFS first!

Allion Labs/ Flash Liu

The under-capacity, over-crowdedness and poor efficiency of the 2.4GHz band have long been common knowledge. The number of channels on the 5GHz is greater than that one the 2.4GHz. However, a large portion of these channels is DFS channels. The advantage of the 5GHz is reduced by more than half if the DFS channels are not in use.
What exactly is DFS? Why is it so influential?

What is DFS?
DFS stands for dynamic frequency selection. You have to ensure that your Wi-Fi network supports for DFS when using the 5GHz band, which is a channel also frequently used by military radars and meteorological radar applications.

Wi-Fi products that utilize these overlapping channels must pass the DFS certification to ensure security. When we talk about DFS channels, we refer to the 5GHz band shared by both Wi-Fi networks and radars.
DFS certification requires Wi-Fi networks on the 5GHz band to automatically switch to other channels in the detection of any radar signals on the same frequency. Different governments adopt different regulatory requirements on Wi-Fi networks operating on the 5GHz band.

Some countries with advanced competence in anti-interference technology would free up more channels in the 5GHz band, while some countries open up fewer channels.
For instance, in the U.S. and certain countries, radar systems use unlicensed bands (i.e. U-NII, Unlicensed National Information Infrastructure). The Wi-Fi networks operating in these channels are required to have radar detection and interference detection/avoidance. These functions can be achieved by supporting DFS for the shared channels.

Table 1: 5GHz Spectrum Policies in Different Countries

Why do Wi-Fi networks adopt DFS Channels?
The 802.11n standard is currently under development to respond to wider adoption of wireless networks, and the ever-increasing demand for less-congested bandwidth. In the 2003 World Radiocommunication Conference (WRC), the global wireless communication conference responsible for the standardization of radiocommunication protocols stipulated that 5250-5350 MHz (100 MHz bandwidth) and 5470-5725 MHz (255 MHz bandwidth) are available for users around the world.

When did DFS come into being?
Back in 2007, the U.S. government started to resolve security issues for shared wireless bandwidths. To avoid any inference to the meteorological radar or military applications, the Federal Communications Commission (FCC) requires the use of dynamic frequency selection and transmit power control on the 5.250 GHz – 5.350 GHz and 5.470 GHz – 5.725 GHz bands.
In 2010, the FCC released the regulations governing the use of the 5.470 GHz – 5.725 GHz to avoid signal interference to the Doppler radar systems at airports. Most governments around the world have come up with similar spectrum regulations and restrictions on transmit powers.

What functions are required for DFS?
The use of the DFS channels is limited to indoors. No use of these channels in the outdoor environment is allowed. When using this channel, users have to wait and make sure that they do not interfere with any military or meteorological radar. If detected, the Wi-Fi network should automatically switch to other channels to avoid inferencing with the military or meteorological radar functions.
The use of the 802.11a protocol adopting the 5GHz is accepted in the U.S. but is being strongly resisted in Europe. This is because the European military radar systems use this spectrum extensively. To resolve the security concern, the WLAN products sold in Europe must be equipped with both TPS and DFS features.
Our brief introduction of DFS regulations shall pause here. We will resume later by discussing how DFS functions are certified, learning the Wi-Fi Alliance certification process, and introducing some frequently seen issues and solutions concerning DFS certifications.