(O)BSS Colouring: What is it?

As I work on my Master’s project, I fell into the concept of BSS colouring. As much as I struggled to find the true definitions of it, I thought there might be some other researchers who are also struggling to understand it. Therefore, I collect various sources and compiled the details I presented below. This article assumes you know what you are looking for, and it does not dig into Wi-Fi basics.

OBSS Colouring

The term OBSS stands for Overlapping Basic Service Set (BSS). BSS represents a collection of stations (STAs) that may communicate together within an 802.11 network. The communicating devices that make up a BSS consist of one access point (AP) radio with one or more client station(s). OBSS occurs when many APs and clients hear each other on the same channel. It is also more commonly known as co-channel interference.

OBSS colouring was introduced to address the challenges of contention overhead, which is the amount of time spent by Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA) to ensure that only one radio can transmit on the same channel at a given time. In addition, it aims at increasing the number of concurrent transmissions in a specific area based on a newly defined Overlapping BSS/Preamble-Detection threshold.

The primary goal of channel reuse patterns is to reduce co-channel interference (OBSS). A channel reuse plan reduces airtime consumption caused by OBSS by isolating frequency domains. However, only three channels are used in the 2.4 GHz band. Because only three channels are available in the 2.4 GHz band and because clients cause OBSS, medium contention deferral is pretty much inevitable in the 2.4 GHz band. OBSS is also a problem in the 5 GHz band, especially if any of the 5 GHz channels are unavailable for a 5 GHz channel reuse plan. To increase capacity in dense environments, frequency reuse between basic service sets needs to be increased.

In reality, the BSS colour identifier is not a colour but instead is a numerical identifier. Therefore, Wi-Fi 6 radios can differentiate between BSSs using a BSS colour (numerical identifier) when other radios transmit on the same channel.

Now since you know what BSS colouring means, let us dig into more details…

How is it communicated?

BSS colour 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 colour field. This field can identify as many as 63 BSSs. At the MAC sublayer, BSS colour information is seen in 802.11 management frames. The High Efficiency (HE) operation information element contains a subfield for BSS colour information. Again, six bits can be used to identify as many as 63 different colours (numerical values) and represent 63 different BSSs. The goal is for Wi-Fi 6 radios to differentiate between BSSs using a BSS colour identifier when other radios transmit on the same channel.

HE operation element, Source

BSS colour detects a colour bit in the PHY header of a Wi-Fi 6 radio frame transmission. Unfortunately, this means that legacy 802.11a/b/g/n radios will not interpret the colour bits because they use a different PHY header format.

When a Wi-Fi 6 radio is listening to the medium and hears the PHY header of an 802.11ax frame sent by another Wi-Fi 6 radio, the listening radio will check the BSS colour of the transmitting radio. Channel access is dependent on the colour detected:

1. If the colour is the same, then the frame is considered an intra-BSS transmission and the listening radio will defer. In other words, the transmitting radio belongs to the same BSS as the receiver; therefore, the listening radio will defer.
2. If the colour is different, then the frame is considered an inter-BSS transmission from an OBSS, and deferral may not be necessary for the listening radio.

BSS Colour uses a 6-bit value carried in the High Efficiency — Signal Field (HE-SIG) along with the UL_Flag (1-bit value) that identifies the link direction of a frame (i.e., DL/UL). Each BSS is identified by the BSS Colour value that ranges from 1 to 63, and a value of 0 indicates that BSS Colour is not used. Thus, frame reception follows the legacy procedure. Nodes may abandon reception if colour mismatch (BSS Colour ≠ 0) occurs and initiate a transmission if channel conditions allow that. The BSS colour can either be assigned manually or coordinated through Radio Resource Management (RRM) to individual APs and the channel assignment.

Furthermore, the colour value is distributed to STAs (from APs) during the association stage and may change during operation if a colour collision is detected and reported to the AP.

How does it work?

BSS colouring decision process

1. Station detect RF energy.
2. Clear channel assessment suggests whether the energy threshold is above -82 or below.
3. If RSSI is greater than -82 and station checks whether it can demodulate traffic?
4. If yes, then it will read the frame header to see the colour of the frame.
5. If it's the same colour, then it means the frame is from intra-BSS and will have to go through the normal CSMA/CA process.
6. If colour is not the same as its own BSS, then it’s an inter-BSS frame.
7. At this point, there is another threshold check. It checks if the signal strength of the frame is above -62 or below.
8. If the signal strength is greater than -62, it’s too close, and the medium will be considered busy.
9. If the signal strength is lower than -62, the station will not contend for this transmission and will continue transmitting.

How is Colour change communicated?

Colour collision detection and reporting, Source

An 802.11ax access point has the ability to change colour if it detects an OBSS using the same colour (colour collision). The details about the collision may be sent by the host AP or clients informing their associated AP about the collision. The client station’s autonomous report will include BSS colour information of all OBSSs that the client can detect. For example, as shown in the figure above, the AP-1 could not see the colour of the AP-2, but an associated client to AP-1 can hear the OBSS with the same colour and then send a colour collision report.

An AP can inform all of its associated clients about a BSS colour change in an Action frame called the BSS colour change announcement frame. The colour-change information can also be included in beacons, probe responses, and reassociation response frames.

As shown in Figure below, an information element called the BSS colour change announcement element contains a Colour Switch Countdown field which tells the associated clients how many more beacons the AP will transmit before the BSS colour change occurs. In addition, the New BSS Colour subfield contains the numerical value of the new BSS colour.

Colour change notification, Source

Enabling BSS colour

BSS colour enabling option is programmed dependent on hardware manufactures. For instance, CISCO describes four ways of configuring the BSS colour option to some of their devices. They designed configuration possibilities on GUI, privileged exec mode, configuration mode, and GUI (colour globally). I present the procedures as presented in the CISCO BSS Colouring guide.

Configuring BSS Colour on AP (GUI)
Step 1 Choose Configuration > Wireless > Access Points.
Step 2 Click the 5 GHz Radios section or the 2.4 GHz Radios section. The list of the AP radios in the band is displayed.
Step 3 Click the required AP name. The Edit Radios window is displayed.
Step 4 From the Edit Radios window, select the Configure tab. The general information, Antenna Parameters, RF Channel Assignment, Tx Power Level Assignment, and BSS Colour are displayed.
Step 5 In the BSS Colour area and from the BSS Colour Configuration drop-down list, choose Custom configuration
• Custom: To manually select the BSS colour configuration for the AP radio.
a) Click the BSS Colour Status field to disable or enable the feature.
b) In the Current BSS Colour field, specify a corresponding BSS colour for the AP radio. The valid range is between 1 and 63.
Step 6 Click Update & Apply to Device.

Limitations

It should be noted that the criteria for changing the BSS colour and the method for selecting a new BSS colour are beyond the scope of the current 802.11ax draft amendment. However, most likely, WLAN vendors will incorporate BSS colour changes into adaptive RF protocols.

References

1. https://www.extremenetworks.com/extreme-networks-blog/how-does-bss-coloring-work-in-802-11ax/
2. http://bit.ly/WiFi6forDummies
3. https://www.rfwireless-world.com/Terminology/What-is-BSS-Coloring-in-WLAN-802-11ax.html
4. https://www.commscope.com/blog/2018/wi-fi-6-fundamentals-basic-service-set-coloring-bss-coloring/
5. https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/17-3/config-guide/b_wl_17_3_cg/m_bss_coloring_ewlc.pdf
6. https://lancomwire.com/more-power-for-your-network-with-bss-coloring/
7. What is BSS Color in 802.11ax? (extremenetworks.com)