The 802.11ax Trigger Frame

In 802.11ax communications, the trigger frame is used for multiple purposes. One of them is to allocate ressources for a specific multi-user OFDMA transmission.

In this article, we are going to take a deeper look at some of the interesting fields we will find in this trigger frame. For more details, you can look at section "9.3.1.22 Trigger frame format" of the 802.11ax-D4 IEEE draft.
​

The Frame Format

​Here is the format of the frame:

The trigger frame is a broadcast frame and you can use this filter to fidn it in Wireshark: wlan.fc == 0x2400.

It has the following characteristics:

• Type: Control (wlan.fc.type == 1)
• Sub-Type: 2 (wlan.fc.subtype == 2)

​
Here is what you see in the packet capture:

Two information fields are very interesting:

• The Common Info
• The User Info

Let's take a deeper look at what they contain.
​

The Common Info Field

Here is the structure of the common info field:

Here are some of the interesting sub-fields:

• UL Length: it indicates the length of the expected response frame (filter = wlan.trigger.he.ul_length)
• UL BW (Up Link Bandwidth): it indicates the bandwidth of the transmission (filter = wlan.trigger.he.ul_bw)
  • ​0 means 20MHz
  • 1 means 40MHz
  • 2 means 80MHz
  • 3 means 80+80MHz or 160MHz
• GI and LTF Type: it indicates which guard interval and long training field will be used for the transmission (filter = wlan.trigger.he.gi_and_ltf_type)
  • 0 means 1x HE-LFT + 1.6us GI will be used
  • 1 means 2x HE-LFT + 1.6us GI will be used
  • 3 means 4x HE-LFT + 3.2us GI will be used
• AP TX Power: when the AP sends the trigger frame, it will provide the Tx Power used to transmit the frame (filter = wlan.trigger.he.ap_tx_power)

​
​Here is an example of a common field:

In this example we can see that we will be using a 20MHz wide channel, 2x LFT + 1.6us GI for the multi-user communication. The AP used a combined transmit power of 21dBm to send the trigger frame.
​

The User Info Field

The User Info field provides details on each client devices participating in the same upcoming OFDMA transmission.

Here are some of the interesting sub-fields:​

• Association ID: indicates the association ID of the addressed STA (filter = 

wlan.trigger.he.user_info.aid12)

• RU Allocation: indicates the size and location of the ressource unit allocated for the addressed STA (filter = wlan.trigger.he.ru_allocation)
• UL MCS: indicates which MCS is expected the STA to use (filter = wlan.trigger.he.mcs)
• SS Allocation: indicates the number of spatial streams to be used by the addressed STA. You will have to minus 1 to the number. (filter = wlan.trigger.he.ru_number_of_spatial_stream)
• Target RSSI: indicates the the expected RSSI of the PPDU to be sent on the RU (filter = wlan.trigger.he.target_rssi)

​Here is an example of a common field:

In this example, we can see that 2x STA will be sharing the 20MHz channel in 2x 106-tones Ressource Units.
Both client devices are expected to be using MCS 11, 1 spatial streams. And their PPDUs is expected to be received with a RSSI of -30dBm.

Some Additional Resources

Here are a couple of interesting articles you can read to learn more about it:

• UL OFDMA Basic Trigger Frame and Multi-STA-BlockACK From Gjermund Raeen: https://gjermundraaen.com/2019/08/26/ul-ofdma-basic-trigger-frame-and-multi-sta-blockack/​
• LENGTH - The Underestimated Parameter in 802.11 by Gjermund Raeen: https://gjermundraaen.com/2019/11/19/length-the-underestimated-parameter-in-802-11/
• 802.11ax remote packet captures: https://www.semfionetworks.com/blog/80211ax-remote-packet-captures-using-the-jetson-nano
• Clear To Send Podcast Series on 802.11ax: http://cleartosend.net/ax
• Identifying 802.11ax support using Wireshark by Rowell Dionicio: https://rowelldionicio.com/identifying-802-11ax-support-wireshark/​

​​
Thank you for reading!

written by François Vergès