Buffer status reports are non-AP stations sending a report to the AP on “Amount of buffered data at station“. This helps the Access Point to determine the RU allocation for specific station.

The non-AP station can send buffered data implicitly via QoS control field or BSR control sub-field of any frame transmitted to the Access Point. This method of implicitly transmitting the buffer status reports is termed as “unsolicited BSR“.

The Access point can also request non-AP stations to respond with BSRs in any frame sent to the Access Point in response to a BSRP Trigger frame. This method of explicit request for BSR is termed as “Solicited BSR“.

Unsolicited BSR

Buffer status report in the Queue subfield which is part of QoS Data packet –> QoS control field defines a queue size value for a particular Traffic Identifier (TID). The bit 4 is set in QoS control field in QoS data frames sent by a non-AP Station or in QoS Null Data frames sent in non-AP HE Station. Refer section 9.2.4.5.6 in the 802.11ax standard.

The Queue Size subfield consists of a Scaling Factor subfield in B14–B15 of the QoS Control subfield and an unscaled value (UV) in B8–B13 of the QoS Control subfield. The scaling factor sub-field provides a Scaling Factor (SF). The scaling factor alongwith the queue size determine the amount of octets present at the station for transmit. The 802.11ax standard provides the below.

A High Efficiency (HE) Station can also send the BSR in the BSR control subfield of frames it transmits to the Access Point. For this to happen, the Access Point should also indicate support for BSR in the HE capabilities element. if the BSR support is not indicated, the non-AP Station will not send the buffer status in the BSR control field. The below HE capability element snippet shows the BSR Support field:

The BSR operation using the BSR support field and the BSR control field as stated in the standard is below

Buffer Status Reports – Solicited BSR