802.11 stations follow Carrier sense-Multiple Access and Collision Avoidance (CSMA/CA) and the signal detect is around 4 dB above noise floor. Since, the Signal is detected at such a low level, an 802.11 station can latch on to a very far away signal from another BSS (termed Overlapping BSS) and backoff from performing its own transmissions.

The above situation can begin to cause issues in highly congested networks wherein most 802.11 stations will start backing off as soon as a signal is detected which in actuality would not affect the Station transmission. This effect would severely degrade the overall throughput of the network.

Another, not so evident issue is if the signal from a far off BSS is detected and the NAV value in the OBSS frame is much smaller than the current BSS last received frame, then, the 802.11 Station might reset the NAV to the smaller value and this might lead to collisions later on.

To address this issue of co-channel interference from overlapping BSS, the 802.11ax standard brought in the concept of BSS coloring. In actuality, it is not a color, but a unique identifier which identifies a specific BSS.

The 802.11ax standard defined steps to not back-off for certain situations if the BSS-color of the received frame was different from the BSS color of the current BSS to which the 802.11ax station is associated to.

The 802.11ax standard also defined different signal thresholds to be configured for intra-BSS and inter-BSS packet receptions. The below diagram shows how BSS color can be used.

In the above image, if a station in BSS color blue containing AP1/AP2 receives a packet from another BSS which is in channel 36 but has the color purple, it can choose to ignore the packet reception and not reset its NAV and continue its own transmission based on the thresholds it has configured for intra-BSS and inter-BSS frame receptions.

Inter-BSS frames are frames that do not belong to the BSS to which the station is associated to. Intra-BSS frames are frames that belong to the BSS to which the station is associated to. The 802.11ax description of the rules on what constitutes a Inter-BSS or Intra-BSS frame is provided in the next article

Inter-BSS and Intra-BSS PPDU