Trigger

Trigger is a mechanism by which an AP can notify or request various information for station, or most importantly transmit the scheduling information to stations. It is used mostly in OFDMA process. For this purpose, AP transmit a specific MAC frame called Trigger Frame. There are various types of Trigger Frames as listed below.

• Basic Trigger frame
• Beamforming Report Poll (BFRP)
• Multi-user block ack request (MU-BAR)
• Multi-user request to send (MU-RTS)
• Buffer Status Report Poll (BSRP)
• GCR MU-BAR
• Bandwidth Query Report Poll (BQRP)
• NDP Feedback Report Poll (NFRP)

Structure of Trigger Frame

The structure of the most common / important Trigger Frame that are used for OFDMA process is shown below. Depending on the purpose of the triggering (e.g, Downlink OFDMA or Uplink OFDMA), some of the fields would be set as 'reserved' (not used).

Examples of Triggering

There are many different cases where triggers are used and followings are some of the examples showing how the trigger works. I think the most common use cases for the trigger is OFDMA scheduling process.

• Downlink OFDMA
• Uplink OFDMA

References

• The MU-RTS/CTS process in the HE (802.11ax) - standard
• Generating the Trigger Frame

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