What Is BSS Coloring In 802.11ax?
Last week, we looked at the new features that 802.11ax brings to the 802.11 standard. Let’s take a closer look at BSS Coloring this week.
BSS Coloring was a mechanism originally introduced in 802.11ah to assign a different “color” per BSS. Why? To increase capacity in dense environments, we need to increase frequency reuse between BSS’s. However, with existing medium access rules, devices from one BSS will defer to another co-channel BSS, with no increase in network capacity.
With 802.11ax, BSS Coloring is a method for addressing this medium contention overhead due to overlapping basic service set (OBSS) and spatial reuse. 802.11ax radios can differentiate between BSS’s by adding a number (color) to the PHY header and new channel access behavior will be assigned based on the color detected. The same color bit indicates an intra-BSS while different color bits indicate inter-BSS. Inter-BSS detection means that a listening radio treats the medium as busy and must defer. However, adaptive CCA implementation could raise the signal detect (SD) threshold for inter-BSS frames while maintaining a lower threshold for intra-BSS traffic. BSS Coloring could potentially decrease the channel contention problem that is a result of existing 4 dB signal detect (SD) thresholds.
The goal of BSS Coloring is to increase reuse, while not causing a significant reduction in selected MCS due to interference. The bottom line here is that the medium is only busy when my color is detected, which ultimately helps mitigate OBSS. As with many of the MU-MIMO features, BSS coloring may not be available in any first generation products. Next week we will discuss Target Wake Time.
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