written by Jim Vajda, CWNE #183
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 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 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 (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.
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.
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.
Jim Vajda is the Director of Global Wi-Fi Solutions at 7SIGNAL. He is CWNE #183 and hold several networking vendor certifications as well. His professional background includes wireless network engineering in healthcare, higher education, MSP, K12, non-profit, and more. You can follow him on Twitter @jimvajda.