How Does DL-OFDMA Work in 802.11ax?

You should always remember that multi-user OFDMA communication is only for 802.11 data frame exchanges between an 802.11ax AP and multiple 802.11ax clients. For backward compatibility, management and control frames will still be sent using single-user OFDM. In a previous blog, we mentioned that 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.

Let us take a look at how trigger frames can be used for multi-user DL-OFDMA communications between an 802.11ax AP and 802.11ax clients. An 802.11ax AP will first need to contend for the medium and win a transmit opportunity (TXOP) for the entire DL-OFDMA frame exchange. As shown in Figure 1, once an 802.11ax AP has won a TXOP, the AP might send a multi-user request-to-send (MU-RTS) frame. The MU-RTS frame has two purposes:

  • Reserve the medium: The MU-RTS frame is transmitted using OFDM (not OFDMA) across the entire 20 MHz channel so that legacy clients can also understand the MU-RTS. The duration value of the MU-RTS frame is needed to reserve the medium and reset the NAV timers of all legacy clients for the remainder of the DL-OFDMA frame exchange. The legacy clients must remain idle while the multi-user OFDMA data frames are transmitted between the 802.11ax AP and 802.11ax clients.
  • RU allocation: The MU-RTS frame is also an extended trigger frame from the AP used to synchronize uplink clear-to-send (CTS) client responses for 802.11ax clients. The AP uses the MU-RTS as a trigger frame to allocate resource units (RUs). The 802.11ax clients will send CTS responses in parallel using their assigned RUs.

Figure 1

After the parallel CTS response from the clients, the AP will begin multi-user DL-PPDU transmissions from the AP to the OFDMA-capable clients. Keep in mind that the AP determined how to partition the 20 MHz channel into multiple RUs. Once the 802.11ax clients receive their data via their assigned RUs, they will need to send a Block ACK to the AP. The AP will send a Block ACK request (BAR) frame followed by the clients replying with Block ACKs in parallel. Optionally, an automatic Block ACK can be sent by the clients in parallel.

Once the frame exchange is over, the AP or clients that win the next TXOP will then be able to transmit on the medium. For example, if an 802.11n/ac client wins the next TXOP, the 802.11n/ac will send a data frame uplink to the AP using OFDM.

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


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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