With the ability to transmit/receive to or from multiple non-AP Stations using Downlink/Uplink Multi-User operation, 802.11ax had to devise a set of operations to indicate to the no-AP stations which Resource Units to use.

The “Trigger frame” was introduced in 802.11ax to indicate to a non-AP Station which RUs to utilise for its receive/transmit operation. The Trigger Frame could also be used to elicit data response from the non-AP station.

The Trigger Frame format as depicted in the 802.11ax standard is shown below

The Common Information field is depicted below

The User info sub-field is provided below

To understand the various parameters in the Common Information field/User info sub-field and what they signify, kindly refer section “9.3.1.22 Trigger Frame Format” in the 802.11ax standard.

There are several types of trigger frames defined in the 802.11ax standard. They are as below

In the following articles, we will look at a couple specific trigger frames and how they may be employed in Uplink and Downlink Multi-User transmissions.

Basic Trigger Frame in 802.11ax