WiFi 6 deployed in dense device environments will support higher service-level agreements to more concurrently connected users and devices with more diverse usage profiles.
This is made possible by a range of technologies that optimise spectral efficiency, increase throughput and reduce power consumption. These include basic service set (BSS) colouring.
WiFi uses radio frequency communication, which is a half-duplex medium - where only one radio can transmit on a frequency domain at any given time. A frequency domain is a fancy technical phrase for a channel. Everyone must take turns because if everyone ‘talks’ at the same time, no data is communicated because no one is ‘listening’.
Carrier sense with multiple access collision avoidance (CSMA/CA) is the method used in WiFi networks to ensure only one radio can transmit on the same channel at any given time. An 802.11 radio will defer transmissions if it hears transmissions of any other 802.11 radio.
Due to the mobile nature of WiFi client devices, co-channel interference isn’t static – it changes as client devices move.
CSMA/CA is necessary to avoid collisions; however, the deferral of transmissions also consumes valuable airtime. This problem is referred to as ‘contention overhead’. 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). OBSS is also more commonly referred to as co-channel interference.
The BSS is the cornerstone topology of a WiFi network. The communicating devices that make up a BSS consist of one AP radio with one or more client stations. OBSS co-channel interference creates medium contention overhead and consumes valuable airtime because you have two basic service sets on the same channel that can hear each other - thus, the term OBSS. In reality, WiFi clients are the primary cause of co-channel interference.
As shown in Figure 1, if a client associated to AP-2 is transmitting on channel 36, it is possible that AP-1 (and any clients associated to AP-1) will hear the PHY (physical) preamble of the client and must defer any transmissions.
Due to the mobile nature of WiFi client devices, co-channel interference isn’t static – it changes as client devices move.
BSS colour, also known as BSS colouring, is a method for addressing medium contention overhead due to co-channel interference. BSS colour is an identifier of the basic service set (BSS). In reality, the BSS colour identifier is not actually a colour, but is instead a numerical identifier.
WiFi 6 radios are able to differentiate between various BSSs using a BSS colour (numerical identifier) when other radios transmit on the same channel.
BSS colour detects a colour bit in the PHY header of a WiFi 6 radio frame transmission. This means that legacy 802.11a/b/g/n radios will not be able to interpret the colour bits because they use a different PHY header format.
When a WiFi 6 radio is listening to the medium and hears the PHY header of an 802.11ax frame sent by another WiFi 6 radio, the listening radio will check the BSS colour of the transmitting radio. Channel access is dependent on the colour detected.
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.
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.
The goal of BSS colour and spatial reuse is to ignore transmissions from co-channel interference and therefore be able to transmit at the same time. In the example shown in Figure 2, any radio on channel 36 that detected the red colour would defer because that would be considered an intra-BSS. However, deferral may not be necessary if an AP detected a green or blue colour from nearby OBSS transmissions also on channel 36. Keep in mind that this figure is a visual illustration and that the colour information is actually a numerical value.
Conclusion
WiFi 6 access points are designed to optimise the efficient reuse of spectrum in dense deployment scenarios using a range of techniques.
BSS colour, together with spatial reuse operation, has the potential to decrease the OBSS channel contention problem that is symptomatic of existing low SD thresholds.
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