802.11ax HD WLAN

Sorting Out BSS Color, Spatial Reuse, and Dual NAV

This post first appeared on

We usually only hear about BSS Coloring in the marketing of Wi-Fi 6, but Spatial Reuse and Dual NAV are related important features of 802.11ax. Let’s sort them out, but first some background.

All 802.11 stations (AP’s and clients) must make sure that the channel they are operating on is free before transmitting. This prevents collisions with other stations operating on the same channel. 802.11 stations accomplish this through two methods: physical carrier sense at layer 1 and virtual carrier sense and layer 2. Physical carrier sense listens for 802.11 preambles that are transmitted at the beginning of every frame. This is the clear channel assessment signal detect (CCA-SD), sometimes called preamble detect. Physical carrier sense also checks for any RF energy on the channel. This is the clear channel assessment energy detect (CCA-ED). Virtual carrier sense operates at layer 2 using a frame’s MAC header Duration/ID field to determine how long an ongoing frame exchange will last. It sets the station’s NAV timer (network allocation vector), which prevents the station from transmitting until it counts down to zero, even if physical carrier sense determines the channel to be idle. Both carrier sense methods must determine that the channel is available before the station can transmit.

Because modern 802.11 radios are very sensitive, CCA-SD causes a station to defer transmitting even if it detects a very low RSSI signal from a distant BSS operating on the same channel. Co-channel interference, referred to as overlapping BSS (OBSS) in the standard, is a problem then even at very low RSSI, as most 802.11 radios will trip their CCA-SD check when they detect a transmission just 4 dB above the noise floor, and defer transmission. If instead, such a station transmitted despite the low RSSI OBSS transmission, it is likely that the receiving station would hear it successfully, which would increase overall spectral efficiency and limit the negative effects of CCI.

802.11ax introduces enhancements to both physical and virtual carrier sense to help address this issue. Spatial Reuse works at the physical carrier sense level and enhances CCA-SD, and Dual NAV works at the virtual carrier sense level and enhances the NAV timer. Both features cause stations to act on the BSS color field, although a BSS color is not required in all cases for them to work. When used in combination, these features can increase the spectral efficiency of 802.11ax.

BSS Coloring

BSS Coloring is simply the ability for an AP to advertise a BSS color, which is actually a number, in its beacon and probe response frames, as well as include the same BSS color field in the HE preamble of the 802.11ax frames that it transmits. Clients that support BSS Coloring also add a BSS color field to the HE preamble of the 802.11ax frames that they transmit. The AP and all its clients in the BSS use the same color value. Overlapping BSS’s on the same channel use a different color to indicate that their frames are OBSS, and therefore they may be treated differently using one or both of the techniques below. The presence of BSS coloring on its own doesn’t change station behavior, it must be acted on using the following techniques to provide any benefit.

Note that some AP vendors and the Wi-Fi Alliance talk very generally about BSS Coloring and I suspect that they really mean BSS Coloring with Spatial Reuse operation.

Spatial Reuse

Spatial Reuse introduces the concept of an OBSS-PD threshold (overlapping BSS packet detect) to CCA-SD. In the OBSS scenario, each BSS will use a unique BSS color. Spatial Reuse allows the stations in each BSS to use a less sensitive preamble detection threshold for OBSS frames during their normal CCA-SD check. That way, even though there may be an OBSS frame making the channel busy, if it is not very loud and there is still significant SNR, an 802.11ax station that supports Spatial Reuse can transmit anyway. To account for the temporarily lower SNR, it may use a lower, more robust MCS. One limitation of Spatial Reuse is that the OBSS transmitting station can’t make the same adjustment to its MCS because it has no knowledge of the other station’s future intention to transmit. 802.11be may solve this problem with new multi-AP coordination features.

Spatial Reuse support is indicated in beacon and probe responses by the Spatial Reuse Parameter Set IE. This is also where the specific thresholds are defined along with which spatial reuse method is to be used. The two methods are OBSS-PD-based operation and parameterized spatial reuse-based operation (PSR), the details of which are beyond the scope of this blog.

Dual NAV

Dual NAV (referred to as “two NAV operation” in the standard) works at layer 2 using the duration field of a frame’s MAC header, and it also takes advantage of the new TXOP field present in the HE preamble. It requires 802.11ax clients to establish two NAV timers, an intra-BSS NAV for all frames within the BSS, and a basic NAV for OBSS frames (often called inter-BSS frames).

The intra-BSS NAV timer is set by frames that match the station’s BSS color or frames with a BSSID field that matches the station’s associated AP. The basic NAV timer is set by OBSS frames with a different BSS color, frames with no BSS color in the case of legacy frames, or frames with a BSSID field that doesn’t match the station’s associated AP.

This helps 802.11ax stations overcome several problems. A legacy station with a single NAV can have its NAV incorrectly shortened by an OBSS frame declaring a shorter duration than its current NAV value. This scenario is particularly troublesome during the long TXOP’s an AP holds for OFDMA frame exchanges. Dual NAV prevents OBSS frames from resetting the intra-BSS NAV.

On the other hand, the basic NAV can also protect OBSS frames during OFDMA if an AP has set the carrier sense required field with its preceding trigger frame. If a client in that scenario has a non-zero basic NAV, it will not respond to the trigger frame in order to avoid a collision with the OBSS transmission. Therefore, the state of the CS required field in the trigger frame determines if a client will respect the basic NAV or transmit anyway, but this only applies to OFDMA operation.

In all other scenarios, both NAV’s must equal zero in order for an 802.11ax station to transmit.

A key difference in 802.11ax is that there is a new TXOP field present in the HE preamble which sets the NAV timer. This allows the NAV to be set at the PHY level, removing the need for RTS/CTS TXOP protection when legacy PHY’s are not present. It blurs the layer 1/layer 2 distinction between the preamble and NAV. It also allows the NAV to be set at lower SNR and at greater distance than previous generations of 802.11, which only set the NAV via a frame’s duration field or RTS/CTS protection. Dual NAV became necessary to prevent the OBSS NAV reset issue from becoming much worse in 802.11ax with the NAV now set by the robustly modulated HE preamble.

Dual NAV can operate using the BSSID field present in non-HE frames to distinguish OBSS frames, like in a mixed environment with 802.11ac and 802.11n stations present. It also operates when BSS coloring is disabled on an AP.

Putting it All Together

Spatial Reuse can make a station less sensitive to OBSS transmissions and increase the likelihood of successful simultaneous transmissions, increasing the spectral efficiency of 802.11ax. Dual NAV on its own will probably only have a marginal impact on spectral efficiency. Its value lies in ensuring virtual carrier sense is accurate and reducing collisions. However, when these features are used in combination, Spatial Reuse will prevent OBSS frames below the OBSS-PD threshold from setting the basic NAV, increasing spectral efficiency by desensitizing both physical and virtual carrier sense to OBSS frames.

Now it is helpful to understand how these features are implemented. 802.11ax has different requirements for AP’s and clients as to what mix of them is mandatory.

Station TypeBSS ColoringSpatial ReuseDual NAV

Most 802.11ax AP’s come with BSS Coloring enabled by default, although the standard allows it to be disabled. Unfortunately, Spatial Reuse is optional for all stations, however Cisco has announced AP support for OBSS-PD-based Spatial Reuse in recent code versions. It seems unlikely that client vendors will implement it if it is not required. Dual NAV is optional for AP’s and mandatory for clients. The standard doesn’t explain this, but perhaps this is because the AP owns the TXOP during both upload and download OFDMA, so it will not reset its NAV due to OBSS frames during that period. However, it may also be due to the mobile nature of clients who can be anywhere within an AP’s coverage and are more likely to encounter and create OBSS conditions than their associated AP.

In practice, 802.11ax stations that only support Dual NAV without Spatial Reuse won’t see a significant improvement to spectral efficiency under OBSS conditions, perhaps only benefiting during OFDMA operation. Combining BSS Coloring with Dual NAV and Spatial Reuse is the key to significantly improving spectral efficiency through reducing physical and virtual carrier sense sensitivity to OBSS transmissions.

802.11ax WLAN

What’s Different About 802.11ax in 6 GHz

There has been some consternation that 802.11ax should have a greenfield mode in 6 GHz, leaving behind all the protocol overhead used for backwards compatibility in the 2.4 and 5 GHz bands. This mythical mode could also have fantastic new capabilities that would now be possible without legacy PHY requirements. 6 GHz is an opportunity to so radically overhaul 802.11 that we could increment the 802.11 version bit in all 802.11 6 GHz frames (It’s been 0 for the entire history of Wi-Fi). Of course, it probably isn’t reasonable to expect the same amendment that must provide backwards compatibility in the legacy bands to also do something radically new in 6 GHz. It may also be unrealistic to expect 802.11ax client chipsets that operate in the legacy bands in legacy modes to do something radically different using the same radio in 6 GHz. Still, there are real protocol differences in 6 GHz 802.11ax operation. 802.11ax is not ratified, so it is still possible for some things to change, but I thought I’d run down what’s different and what opportunities I think were missed with 802.11ax in 6 GHz.

  • Security Upgrade – 802.11ax will make SAE and OWE mandatory replacements for PSK and open auth respectively in 6 GHz. MFP will be required. I still don’t understand how this will work with SSID’s that span the legacy bands to support legacy WPA2 clients as well as WPA3-only clients in 6 GHz. If the answer is “Just add another SSID for Wi-Fi 6E clients-only,” then I will be disappointed.
  • OFDM and HE-Only – There is no HT or VHT operation allowed in 6 GHz. Unique HE beacon IE’s indicate support for features inherited from HT and VHT. There are no HT/VHT Capabilities IE’s in a 6 GHz beacon. No HT or VHT MCS will be used in 6 GHz. OFDM is there because its shorter preamble consumes less airtime than the new HE preamble, so it will be used for those frames that don’t require the bigger HE preamble.
  • Basic HE-MCS and NSS (number of spatial streams) Set – We aren’t using legacy rates outside of the legacy preamble, RTS/CTS, and legacy ACK frames. Most frames can be modulated with HE-MCS encoding, including beacon, multi-STA blockACK, and trigger frames, some of which can be transmitted with multiple spatial streams.
  • Spatial Reuse Can Work – 6 GHz STA’s can take full advantage of BSS Coloring and OBSS CCA-PD. Without legacy STA’s to conflict with, we can design for these features to significantly desensitize all intra-BSS STA’s from OBSS frames, and allow for increased spectral efficiency by reusing the channel more aggressively. If implemented, those robustly-modulated preambles are a smaller problem. However, Spatial Reuse (the OBSS CCA-PD) is optional in 802.11ax. Dual NAV is required for clients, and optional for AP’s. Confused yet?
  • EDCA Optimization – This might be the trick to getting OFDMA operation to take place more often. All 6 GHz STA’s will support OFDMA, so why not increase the contention window for the SU EDCA access categories advertised in beacons (there is a separate MU EDCA table for OFDMA)? That would reduce the likelihood of clients winning access to the channel for SU operation, and increase the likelihood that the AP will win the channel for OFDMA operation to take place.
  • Less RTS/CTS Overhead? – Because all 6 GHz STA’s can interpret the HE preamble, which includes the duration of the TxOP, normal RTS/CTS protection is redundant. The 802.11ax draft allows for several ways to establish a TxOP, including the legacy duplicate RTS/CTS method, so we will have to wait and see what the vendors choose to use in 6 GHz. In ideal circumstances, 802.11ax in 6 GHz will look like this: AP wins arbitration, trigger frame, MU-PPDU, BlockACK, repeat, repeat, repeat…
  • Reason Code 71 – 6 GHz AP’s can deny an association request from a client with poor RSSI using status “DENIED_POOR_CHANNEL_CONDITIONS” or disassociate a low RSSI client with a new reason code 71,”POOR_RSSI_CONDITIONS.” A 6 GHz client must respond to this sensibly (e.g. not blacklisting the BSSID/SSID as clients sometimes do in the legacy bands). Although vendors have had features that accomplished this for a long time, client behavior in response has always been a mixed bag.
  • 6 GHz AP Discovery and Association – A 6 GHz STA can discover, and in some cases associate to, a 6 GHz radio while operating in the 2.4 or 5 GHz bands. An AP’s beacon, probe response, and neighbor report frames in those bands can indicate the channel and channel width of their matching 6 GHz radio. Additionally, for 6 GHz-only operation, a specific subset of channels will be identified as preferred scanning channels (PSC) where the primary channel of a wide channel BSS should reside, limiting the channels a client needs to scan to discover a 6 GHz-only AP. PSC’s are spaced 80 MHz apart, so a client would only need to scan 14 channels in the US. Active probing in the 6 GHz band in the US is only allowed after a client has heard an AP transmission on the channel, which includes a beacon frame, an unsolicited probe response sent to the broadcast address, or a FILS discovery frame. However, one of these frames can be transmitted at least every 20 TU’s, which allows for less required dwell time on the channel for passive scanning. Less required dwell time and a limited set of PSC’s will make passive AP discovery faster in 6 GHz than 5 GHz.
  • 80 MHz AP Channel Width Minimum in 6 GHz – They weren’t lying when they said “80 is the new 20.” Maybe things will change before 802.11ax is ratified, but I’ve learned that a 6 GHz AP will have to indicate support for at least 80 MHz channel width. This aligns well with the PSC’s the standard will define. I don’t know why the IEEE would require this, as it is extremely undesirable in the LPV WLAN’s where 802.11ax in 6 GHz would otherwise provide the most benefit. It’s an even larger problem in countries with unlicensed access to a smaller portion of the band. Clients may still use smaller channel widths, including a 20 MHz-only operating mode.
  • How Much of the Wide Channel Can Be Used? – The use of wide channels followed wasteful logic in 802.11ac with dynamic bandwidth operation (DBO): If the primary 20 MHz channel of a 160 MHz BSS is busy nothing can be transmitted. If the secondary 20 MHz channel that would be used for a 40 MHz channel-width is busy then the STA can only use 20 MHz, the rest of the 120 MHz is unused. If those first two 20 MHz channels are clear, another 40 MHz is checked to see if 80 MHz of the channel is available, etc. And this pattern of checking for the next wider channel width is done serially through the CCA process which also introduces more overhead on its own. Remember, OFDMA only happens within the TxOP gained from a wide-channel arbitration process, which can be subject to the logic I just described. DBO was optional in 802.11ac and not widely implemented, and it appears to be optional in 802.11ax as well. This is important because…
  • Preamble Puncturing is Optional – What improves spectral efficiency in the scenarios above is a new 802.11ax feature called preamble puncturing, which allows a HE STA to transmit across the full 160 MHz of spectrum, but not within the specific 20 MHz subchannels that are busy. One busy subchannel doesn’t prevent the use of others, but the primary channel must still always be free for anything to be transmitted. However, preamble puncturing is an optional feature in 802.11ax. So even in the best case scenarios, OFDMA can only subdivide the channel within the 20 MHz subchannels determined to be available via CCA and (maybe) RTS/CTS. In the worst case scenario (no DBO or preamble puncturing), no data frame, OFDMA or otherwise, can be transmitted unless the entire wide channel is available (minimum 80 MHz in 6 GHz!).

The primary channel bottleneck is particularly troublesome because future generations of 802.11 in 6 GHz will have to account for 802.11ax operation to maintain backwards compatibility, although a lot of traffic that used to be primary channel-only can be included in OFDMA transmissions now (ACK, null-data frames). I wish we could have left behind the legacy channel arbitration process as well, or at least made preamble puncturing mandatory. Other problems may be improved in future amendments. The potential for reduced overhead and higher likelihood of the AP winning channel access are significant improvements when coupled with all 802.11ax clients in 6 GHz.

To Be Determined

  • The Wi-Fi Alliance – The WFA could decide that features that are optional in 802.11ax are mandatory for Wi-Fi 6E certification. Preamble puncturing and spatial reuse are very beneficial features that should be mandatory.
  • 802.11md – This is the maintenance work being done to roll-up 802.11 into 802.11-2020. It will include 802.11ax-2020 and leaves open the possibility of additional changes to 6 GHz operation occurring that are not part of the 802.11ax drafts, but happen within the roll-up process separately. An example of this happening in the past is 802.11 Fine Timing Measurement, which was added to the standard through the 802.11mc roll-up to 802.11-2016. That feature does not have its own 802.11xx amendment. Hopefully this will all be sorted out in December. Stay tuned.
  • 802.11ax, TBH – It’s not finalized yet, so perhaps some of this will change. I’ll update this blog if that happens.