What is BSS Color in 802.11ax?
As we discussed in the last few blogs, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) dictates half-duplex communications, and it says only one radio can transmit on the same channel at any given time. An 802.11 radio will defer transmissions if it hears the PHY preamble transmissions of any other 802.11 radio. Unnecessary medium contention overhead that occurs when too many APs and clients hear each other on the same channel is called an overlapping basic service set (OBSS). The more commonly used terminology for OBSS is co-channel interference (CCI). For this discussion, we will use the term overlapping basic service set (OBSS).
The main reason for channel reuse patterns is to minimize airtime consumption and the degradation of performance due to OBSS. The 802.11ax amendment defines a method to further deal with OBSS conditions using a procedure called spatial reuse operation. Spatial reuse operations can use adaptive clear channel assessment (CCA) thresholds for detected OBSS frame transmissions.
BSS color, also known as BSS coloring, is a method for identifying overlapping basic service sets (OBSSs). BSS coloring was first defined in the 802.11ah-2016 amendment and is now also defined in the 802.11ax draft amendment. The BSS color is a numerical identifier of the BSS. 802.11ax radios are able to differentiate between BSSs using BSS color identifier when other radios transmit on the same channel. If the color is the same, this is considered to be an intra-BSS frame transmission. In other words, the transmitting radio belongs to the same BSS as the receiver. If the detected frame has a different BSS color from its own, then the STA considers that frame as an inter-BSS frame from an overlapping BSS.
BSS color information is communicated at both the PHY layer and the MAC sublayer. In the preamble of an 802.11ax PHY header, the SIG-A field contains a 6-bit BSS color field. This field can identify as many as 63 BSSs.
As shown in Figure 1, the BSS color information is also seen in 802.11 management frames. The HE operation information element contains a subfield of BSS color information. Six bits can be used to identify as many as 63 different colors and represent 63 different BSSs.
Channel access is dependent on the BSS color detected. Spatial reuse operation can use the BSS color information to apply adaptive clear channel assessment (CCA) thresholds for detected OBSS frame transmissions. The goal of spatial reuse is to ignore transmissions from an OBSS and therefore be able to transmit at the same time. The 802.11ax amendment defines two independent spatial reuse modes, one called OBSS PD-based spatial reuse and the other called SRP-based spatial reuse. We will discuss these modes of spatial reuse operation in future 802.11ax blogs.
In the next blog, we will discuss how an 802.11ax AP has the ability to change its BSS color if it detects a BSS color collision. An AP might decide to change its BSS color if it hears a frame from an OBSS AP or OBSS client with the same color. Associated clients might also autonomously report a BSS collision to their AP if the clients detect frames from an OBSS station of the same color.