802.11ax feature – BSS Coloring 

Understanding the technical components of 802.11ax (Wi-Fi 6) can help IT organizations decide how and where to adopt this technology. There are several features to Wi-Fi 6 that improve efficiency and lower latency while providing simultaneous delivery over 802.11n (Wi-Fi 5).

• 11ax standard covers both 2.4 and 5 GHz bands whereas 802.11ac is 5 GHz only.
• QAM modulation increases to 1024 allowing more data to be pushed over the wireless network.
• OFDMA use of sub-channels for improved Multi-user MIMO
• Target Wake Time features improve battery life of clients to ease IoT implementations.
• Basic Service Set (BSS) Coloring improves airtime utilization and reduces interference.

Let’s first define a BSS. A basic service set is a local network made up of one access point with associated clients connected to it.

When more than 1 access point/basic service sets are on the same channel (i.e., channel 6 at 2.4 GHz). this creates a situation known as co-channel interference, also known as an 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 their associated clients. The clear channel assessment mechanism built into the 802.11 forces each conversation to each client from their respective access points to 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 increases latency and adds to airtime utilization. This inefficiency can be mitigated through proper network design. Network design is still fundamentally limited under WiFi 5 by issues like the 2.4GhZ band only having 3 non-overlapping bands creating times where channel separation is not possible.

Wi-Fi 6 introduces BSS coloring. This is another way of identifying a BSS as being unique. Technically, the access points are setting a value in a 6-bit frame in the PHY header understood by each access point and client.

Access Points and their BSS can now be on the same channel, but the added network intelligence allows for better load sharing and traffic management.

To use the BSS feature, the access points 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. There are additional features within the Wi-Fi 6 standard that can be combined with BSS coloring to further improve spectral efficiency.  These include Spatial Reuse and two NAV operation.  We will explore these features in future articles.