EHT Trigger Frame

The EHT (Extremely High Throughput) Trigger Frame is a Wi-Fi 7 (802.11be) management frame which co-ordinates multi-user uplink data transmission which enables simultaneous data transmission for multiple client devices via OFDMA or MU-MIMO.

The EHT Trigger frame is broadcasted by the AP to solicit frames from the client devices participating in simultaneous data transmission. Below is an example of decoding EHT trigger frame and the fields in it.

Test Scenario- Four Wi-Fi 7 capable client devices trying to send UL-OFDMA traffic

Filter for EHT trigger frames in wireshark using the below filter (ensure the frame is a broadcast frame)

wlan.fc.type_subtype == 0x0012

Verifying Common Info field in the trigger frame-

Expand IEEE 802.11 Trigger > Common Info > EHT Trigger Common Info

• Trigger Type- Basic (0)
• UL Length>0
• UL BW- 0 (20 MHz)
• GI-LTF- 1 or 2
• AP Tx Power>0
• Number of HE/EHT-LTF symbols<5 (5-7 are reserved)
• Special User Info Field flag- 0 (EHT)

Verifying User Info field in trigger frame-

Expand IEEE 802.11 Trigger > User Info

• Special User Info for the first user- AID12- 2007
• Phy version Identifier- 0 (EHT)
• UL BW extension- 0

Special user info will be listed for each of the stations participating in simultaneous data transfer and can be verified based on the AID in each User Info. (User 2 onwards)

• AID12- 172 (AID of the client)
• RU Allocation: 53 (RU size:106) (RU allocation for the client device)
• UL FEC coding type-0 (BCC)
• UL EHT MCS- 0x9 (MCS support negotiated between client and AP)
• Starting Spatial Stream-1
• Number of Spatial Streams- 1 (NSS supported)