Two access points on the same channel normally take turns. Wi-Fi 6 puts a small number, the BSS color, at the start of every frame, so an AP can tell at once that a frame comes from a neighbor and, if it is weak enough, talk over it at reduced power. Move the APs, change the colors and thresholds, and see when that helps and who pays for it.
A color in the preamble
The BSS color is a 6-bit number, 1 to 63, in the HE-SIG-A field of every Wi-Fi 6 frame (IEEE 802.11-2024 27.3.11.7) and the U-SIG of every Wi-Fi 7 frame (IEEE 802.11be-2024 36.3.12.7). A station reads it about 32 µs into the frame, long before the MAC addresses.
1 · Two networks on one channel
Neighbor AP B is
Spatial reuse (OBSS PD)
Drag the APs and their clients. The rings show how strong AP B's frames are where they land.
2 · The threshold and the price
AP spatial streams
3 · What AP A does with AP B's frame
The path the frame takes is ticked. Click another branch to change the setup.
4 · Airtime and speed
Speed each client gets (PHY rate × share of airtime)
Without spatial reuseWith these settings
5 · When two neighbors pick the same color
Definitions
↗ opens a term's full entry in the Wi-Fi Glossary, which explains over 400 Wi-Fi terms with their IEEE 802.11 references.
Notes
Sources: classifying a frame as intra-BSS or inter-BSS, IEEE 802.11-2024 26.2.2; ignoring an inter-BSS frame below the OBSS PD level, IEEE 802.11-2024 26.10.2.2; the OBSS PD level and transmit power rule (Equation 26-5), IEEE 802.11-2024 26.10.2.4, with its −82 to −62 dBm range in IEEE 802.11-2024 Table 26-11; the power limit lasting until the end of the transmit opportunity, IEEE 802.11-2024 26.10.2.5; the BSS color (1 to 63), disabling it, the BSS Color Change Announcement and collision reports, IEEE 802.11-2024 26.17.3; CCA at −82 dBm for a Wi-Fi frame and −62 dBm for any signal on the primary 20 MHz channel, IEEE 802.11-2024 27.3.22.6.3; BSS Color in HE-SIG-A bits B8–B13, IEEE 802.11-2024 Table 27-19.
Only 20 MHz: the OBSS PD level applies to a 20 MHz frame on the primary channel. For a wider frame it rises by 10 × log10(width ÷ 20 MHz), 3 dB per doubling. The model uses 20 MHz frames throughout.
Both APs use the same settings and always have data to send (downlink only). In the reuse case they take turns being the one that wins the channel normally, at full power, while the other reuses it at reduced power; so each AP spends half its time at each power. The power limit applies only to the transmit opportunity that the ignored frame made possible. The reusing AP's frames also land on the BlockAcks the other client sends back; that is not modeled.
Simplified radio model: signal = transmit power + 3 dBi AP antenna − path loss, received with a 0 dBi antenna (APs too). Path loss is free-space loss over the first metre at 5180 MHz (channel 36), then rises with an indoor exponent of 3.5, a rule of thumb for an office with walls. Noise is −94 dBm (thermal noise over 20 MHz plus a 7 dB noise figure). The MCS is the highest whose minimum SNR is met (4 dB for MCS 0 up to 34 dB for MCS 11, from the standard's minimum sensitivities), with rates for 802.11ax, 20 MHz, 2 spatial streams, 0.8 µs guard interval. Real throughput is lower: protocol overhead, retries and contention are left out.
Frames that are never ignored: even from a neighbor, a non-HT Ack or BlockAck; a non-HT CTS, unless it answers a neighbor's RTS that was itself ignored; a non-HE frame addressed to the station itself, or carrying a Public Action, NDP Announcement or FTM frame; and sounding NDPs. A frame whose Spatial Reuse field says PSR_AND_NON_SRG_OBSS_PD_PROHIBITED, or an AP that sets Non-SRG OBSS PD SR Disallowed, also turns reuse off (IEEE 802.11-2024 26.10.2.2). APs in the same network can also list each other as a spatial reuse group (SRG) with its own, usually higher, threshold (IEEE 802.11-2024 26.10.2.3); that is not modeled here.
Wi-Fi 5 neighbors: their frames carry no BSS color, and the neighbor's rates in this model are 802.11ac (up to MCS 8 at 20 MHz, 2 streams). The standard lets a station tell they are from another network by other means, such as the MAC addresses once the header is decoded (IEEE 802.11-2024 26.2.2), but that comes much later in the frame. This model, like most early decisions, relies on the color and waits. The same goes for a color collision: once the MAC header is decoded, the addresses win over the color, but by then the chance to ignore the frame early has passed.
The color does more than spatial reuse: a client can keep a separate NAV for its own network and for neighbors (IEEE 802.11-2024 26.2.4), and can doze through a frame with another color (IEEE 802.11-2024 26.14.1). A color collision spoils both.