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Wi-Fi 6 – Basic Service Set Coloring

Written by Jim Rousseau
May 19, 2022

802.11ax feature – BSS Coloring 

We have discussed 5G and Wi-Fi 6e for use in the DoD, but now we want to focus on the 802.11ax (Wi-Fi 6) feature set. There are several features to Wi-Fi 6 that improve efficiency with lower latency and simultaneous deliver over that of 802.11n (Wi-Fi 5). 

  • 802.11ax standard covers both 2.4 and 5 GHz bands whereas 802.11ac is 5 GHz only
  • QAM modulation is increased to 1024. More data is getting pushed onto the sine wave
  • OFDMA use of sub-channels for improved Multi-user MIMO
  • Target Wake Time feature improves the battery life of a client. Huge for IoT implementations
  • Basic Service Set (BSS) Coloring to improve airtime utilization and interference reduction

Let’s first define a BSS. A basic service set is a local network made up of one access point with associated clients. Picture your conference room. On the ceiling or wall, there is an access point. Devices in that room such as a TV, printer, cell phone, and tablet connect to that access point. That environment is a BSS. Now picture a number of cubicles outside the conference room. That area is being supported by another access point with devices connected to it as well. This is a separate BSS adjacent to the conference room.  

If both these access points are on the same channel, say channel 6 at 2.4 GHz. This creates a situation known as co-channel interference, also known as Overlapping Basic Service Set or OBSS. The 802.11 standards are written in a manner that if adjacent BSSs find themselves on the same channel, they must share airtime with each other and all the clients associated with them. The clear channel assessment mechanism built into the 802.11 makes it so that each conversation with each client from their respective access points must happen one at a time. Everyone connected to each access point, and the access points themselves, need to take turns sending information over the air. This not only takes up a whole bunch of nanoseconds, but it also adds to airtime utilization. In short, it is inefficient which is why proper channel design is always promoted in engineered deployments. But there are only so many channels, in the case of 2.4 GHz only 3 are non-overlapping. There are times when channel separation may not be possible. 

Wi-Fi 6 introduces BSS coloring. This is another way of identifying the BSS as being unique. So, our conference room access point sends specific data to the client letting it know it is in the conference room, Blue, while the cubical access point tells its clients where they are by identifying them as Red. In reality, the access points are setting a value in a 6-bit frame in the PHY header understood by each access point and client. 

Now what happens is the conference room recognizes the cubical BSS is on the same channel but is operating as Red; the clients in each BSS recognize the same thing. For as long as the signal is less than a pre-determined threshold, the clear channel assessment is not triggered between the colors, only within the same color, despite being on the same channel. The conference room access point ignores the neighboring cubical AP on the same channel and concentrates on serving the conference room clients. Likewise, the clients ignore the cubical BSS as well. This greatly improves efficiency in airtime utilization. 

One caveat to this feature, both access point and client need to be 802.11ax (Wi-Fi 6) capable. Wi-Fi 5 (802.11ac) clients are not capable of taking advantage of BSS coloring. In some cases, this feature will need to be turned on. There are additional features within the Wi-Fi 6 standard: Spatial Reuse and two NAV operation. When combined with BSS coloring there is a significant improvement in spectral efficiency. 

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