We saw how Unsolicited BSR is handled in 802.11ax <Buffer Status Reports – Unsolicited BSR>. A Solicited BSR is a response for a request for Buffer Status from a non-AP Station responding to a Buffer Status Report Poll (BSRP) trigger frame from an Access Point.

The Trigger frame will have the type set to BSRP Trigger Frame. The Trigger frame format is shown below:

The Trigger Dependent Common Info subfield and Trigger Dependent User Info subfield are not present in the BSRP Trigger frame

The non-AP Station that receives a BSRP Trigger frame can include one or more QoS Null frames containing the below (As per 802.11ax)

1. The QoS Control field(s) with Queue Size subfields for each of the TIDs for which the non-AP STA has queue size to report to the AP.
   
2. The BSR Control subfield with the Queue Size All subfield indicating the queue size for the ACs, indicated by the ACI Bitmap subfield, for which the non-AP STA has queue size to report to the AP if the AP has indicated its support in the BSR Support subfield of its HE Capabilities element.

Down Link Multi-User OFDMA in 802.11ax