Unsolicited Buffer Status Reports in 802.11ax and Wi-Fi 6

In a previous blog, we focused on the delivery of buffer status reports (BSRs) from Wi-Fi 6 clients using an explicit process that requires polling from the AP.

To summarize the solicited UL-OFDMA frame exchange we discussed, the following three trigger frames might be used:

  • Trigger 1: BSRP to solicit buffer status reports from the clients
  • Trigger 2: MU-RTS to allocate RUs and set every client’s NAV
  • Trigger 3: Basic trigger to signal the clients to begin their parallel uplink transmissions

It should be understood that all three trigger frames may or may not be needed for uplink transmission. For example, the MU-RTS trigger frame is only needed for protection mechanism purposes for legacy clients. To minimize overhead, an access point can include a BSRP Trigger frame together with other Control, Data and Management frames in one A-MPDU sent to a client that supports the capability.

Additionally, the buffer status report poll (BSRP) trigger frame is not needed when the Wi-Fi 6 clients send unsolicited buffer status report information. Clients can send unsolicited BSRs in frames using either of these subfields:

QoS Control field: Wi-Fi 6 client station can report buffer status information for any given QoS class of traffic in the Queue Size subfield of the QoS Control field in QoS Data or QoS Null frames it transmits. Additionally, an 802.11ax client can report the buffer status of multiple QoS access categories using A-MPDU frame aggregation of QoS Data or QoS Null frames. Traffic identifiers (TIDs) are used to indicate the QoS access category of the buffered client traffic. The Queue Size subfield indicates the amount of buffered traffic for any given traffic class.

BSR Control subfield: Clients can also send BSR information in the BSR Control subfield of any frame it transmits if an 802.11ax AP has indicated support for the BSR Control subfield in an 802.11ax frame. As shown in Figure 1, a Wi-Fi 6 client can report its preferred QoS access category (AC) indicated by the ACI High subfield and how much traffic is buffered of that preferred AC in the Queue Size High subfield. The client station determines which QoS queue deserves higher priority with respect other queues. How this determination is made is not defined in the 802.11ax standard leaving flexibility for different types of Wi-Fi 6 clients.

Figure 1- BSR Control subfield

A Wi-Fi 6 client can also report the buffer status for multiple QoS access categories, indicated by the ACI Bitmap subfield together with the Delta TID subfield. The amount of buffered traffic for multiple QoS access categories is indicated in Queue Size All subfield. As shown in Table 1, the Scaling Factor subfield indicates the unit SF, in octets, of the Queue Size High and Queue Size All subfields. The amount of buffered traffic for either a single AC (Queue Size High) or multiple ACs (Queue Size All) is indicated in units of scaling factor (SF)octets.

It should be noted that an 802.11ax client can optionally carry the buffer status report information in both the QoS Control field and the BSR Control subfield with matching values within the same frame.

Portions of this blog have been excerpted from the 5thedition of Sybex Publishing’s Certified Wireless Network Administrator (CWNA) Study Guide:  http://a.co/bXX3i9F



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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