Dueling NAVs in 802.11ax

Wi-Fi radios use two methods of carrier sense to determine if the RF medium is busy and possibly defer transmissions. The clear channel assessment (CCA) is a physical carrier sense mechanism which works together with MAC layer virtual carrier sense. Virtual carrier sense uses a timer mechanism known as the network allocation vector (NAV).

In previous blogs, we discussed how 802.11ax BSS color information can be used in modes of Spatial Reuse Operation to apply adaptive clear channel assessment (CCA) thresholds for detected OBSS frame transmissions. The goal of spatial reuse is to ignore transmissions from an OBSS and therefore be able to transmit at the same time.

The adaptive CCA capabilities of BSS color and spatial reuse operation will also work together with virtual carrier sense, with a new requirement for two NAV timers. The 802.11ax draft amendment defines the use of two network allocation vector (NAV) timers for 802.11ax client stations: an intra-BSS NAV timer and a basic NAV timer. The intra-BSS NAV timer of an 802.11ax station can only be reset by the Duration/ID value from frame transmissions of stations that belong to the same BSS. The basic NAV timer of an 802.11ax station is reset by the Duration/ID value from frame transmissions of stations that belong to a different BSS (inter-BSS). If either NAV has a non-zero value, the medium is considered to be busy.

Maintaining two NAVs is beneficial in dense deployment scenarios in which an 802.11ax station requires protection from frames transmitted by other stations within its BSS (intra-BSS), and avoid interference from frames transmitted by stations in a neighboring BSS, (inter-BSS).

As shown in Figure 1, AP-1 sends an RTS frame with a duration value of 200 μs to protect the remaining frame exchange with client STA
#1. Because STA #1 is associated to AP-1, the client resets its intra-BSS NAV for 200
μs. However, during the frame exchange, the initial client that belongs to BSS #1 may also hear an RTS frame from a nearby client station that belongs to a different BSS. The duration value of 125 μs in the RTS frame sent by client STA #2 will reset the basic NAV timer of STA #1. The intra-BSS NAV timer will expire first, however, the basic NAV timer of STA #1 will continue to decrement, and STA #1 cannot transmit until both timers have expired. This ensures that the frame exchange in nearby BSS #2 is protected.

But what if we do not want to protect a frame transmission from an overlapping basic service set? The whole point of spatial reuse operation is to possibly ignore transmissions of an 802.11ax radio that belong to an OBSS. As previously stated, spatial reuse operation will apply adaptive clear channel assessment (CCA) thresholds for detected inter-BSS frame transmissions. Additionally, spatial reuse operation will not update the basic NAV timer during inter-BSS transmissions.

For more information about 802.11ax, please download the free 802.11ax for Dummies eBook.


David Coleman is a wireless mobility consultant, public speaker, and trainer. For the last twenty years, David has instructed IT professionals from around the globe in enterprise WLAN design, WLAN security, WLAN administration and WLAN troubleshooting. In his spare time, David writes white papers, blogs, and books about enterprise Wi-Fi networking. David is the co-author of Sybex Publishing’s Certified Wireless Network Administrator (CWNA) Study Guide and numerous other books about Wi-Fi. David is the Senior Product Evangelist for Aerohive Networks and is CWNE #4.

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