What are OFDMA Trigger Frames?

In previous blogs, you have learned that an 802.11ax access point uses OFDMA technology to partition a channel into smaller sub-channels called resource units (RUs) so that simultaneous multiple-user transmissions can occur. The AP mandates the RU allocation of a 20 MHz for multiple clients for both downlink and uplink OFDMA. When referencing downlink and uplink OFDMA transmissions, the acronyms of DL-OFDMA and UL-OFDMA are often used.

A series of frame exchanges are used for both DL-OFDMA and UL-OFDMA. In both cases, trigger frames are needed to bring about the necessary frame exchanges for multi-user communications. Trigger frames are also used for MU-MIMO frame exchanges. Multiple types of 802.11 control frames can function as trigger frames, as shown in the table below.

As you learned in an earlier blog, trigger frames contain information about RU allocation. RU allocation information is communicated to clients at both the PHY and MAC layers. RU allocation information can be found in the HE-SIG-B field of the PHY header of an 802.11 trigger frame. Additionally, RU allocation information is delivered in the user information field in the body of a trigger frame.

An UL-OFDMA trigger frame exchange is depicted in Figure 1. For UL-OFDMA, a trigger frame sent by the AP is also used to tell the clients how many spatial streams and which modulation and coding scheme (MCS) to use when transmitting uplink on their assigned RUs. This information can be found in the SS Allocation and UL MCS subfields of the user information field within the body of a trigger frame.

OFDMA trigger frames

Trigger frames can also be used by an AP to tell clients to adjust their power settings for synchronized uplink transmissions. Within a trigger frame, the UL Target RSSI subfield indicates, in a dBm value, the expected receive power at the AP across all antennas for the assigned resource unit (RU) transmissions from the uplink 802.11ax clients. The UL Target RSSI subfield uses values of 0 to 90 which are directly mapped from –110 dBm to –20 dBm. A value of 127 indicates to the client station to transmit at its maximum power for the assigned MCS. Based on this information provided by the trigger frame, the transmit power could be adjusted by the uplink clients. Please note that a client station might be unable to satisfy the target RSSI due to its hardware or regulatory limitations.

Portions of this blog have been excerpted from the 5thedition of Sybex Publishing’s Certified Wireless Network Administrator (CWNA) Study Guide:  http://a.co/bXX3i9F


David Coleman is a wireless mobility consultant, public speaker, and trainer. For the last twenty years, David has instructed IT professionals from around the globe in enterprise WLAN design, WLAN security, WLAN administration and WLAN troubleshooting. In his spare time, David writes white papers, blogs, and books about enterprise Wi-Fi networking. David is the co-author of Sybex Publishing’s Certified Wireless Network Administrator (CWNA) Study Guide and numerous other books about Wi-Fi. David is the Senior Product Evangelist for Aerohive Networks and is CWNE #4.

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