The Trigger frame format and the need for the Trigger Frame was discussed briefly in the article <Trigger Frames in 802.11ax>. The Basic Trigger frame has the same frame format as defined in the previous article. It is depicted below:

One major exception is that the “Trigger Dependent Common Info Field” in the Common Information portion of the trigger frame is not present.

In the User information field <Refer Trigger Frames in 802.11ax>, the AID12 determines which AID the RUs are allocated for. The “RU allocation” field in the User information alongwith UL BW in the common info field determine the size and location of the RU.

The trigger dependent per user information field is shown below:

The parameters are as follows

• MPDU MU spacing factor – value by which the minimum MPDU start spacing is mltiplied
• TID aggregation Limit – indicates the MPDUs allowed in an A-MPDU carried in the HE TB PPDU and the maximum number of TIDs that can be aggregated by the STA in the A-MPDU
• Preferred AC – indicates the lowest AC that is recommended for aggregation of MPDUs in the A-MPDU contained in the HE TB PPDU sent as a response to the Trigger frame

The basic Trigger frame is used to allocate RU resources to different stations

MU-RTS Frame in 802.11ax