Uplink Orthogonal Division Multiple Access (UL-OFDMA) in 802.11ax

As we discussed in a previous blog, an 802.11ax AP must contend for the medium and win a transmit opportunity (TXOPbefore multi-user (MU) communications can occur. Once the 802.11ax AP wins a TXOP, it can then coordinate uplink transmissions from 802.11ax clients that support UL-OFDMA.

UL-OFDMA is more complex than DL-OFDMA and may require the use of as many as three trigger frames. Each trigger frame is used to solicit a specific type of response from the Wi-Fi 6 clients. UL-OFDMA also requires the use of buffer status report (BSR) frames from the clients. Clients use BSR frames to inform the AP about the client’s buffered data and about the QoS category of data. The information contained in BSR frames assists the AP in allocating RUs for synchronized uplink transmissions. The AP will use the information gathered from the clients to build uplink window times, client RU allocation, and client power settings for each RU. Buffer status reports (BSRs) can be unsolicited or solicited. If solicited, the AP will poll the clients for BSRs.

As shown in Figure 1, once an 802.11ax AP has won a TXOP, the AP will send the first trigger frame. A buffer status report poll (BSRP) frame is used to solicit information from the 802.11ax clients about their need to send uplink data. The clients will then respond with buffer status reports (BSRs). The whole purpose of the buffer status report (BSR) information is so the Wi-Fi 6 clients can assist an 802.11ax AP to allocate uplink multi-user resources. The AP will use this information to decide how to best allocate RUs to the clients for synchronized uplink transmissions.

Figure 1- Uplink OFDMA

If legacy clients exist, the AP may send a multi-user request-to-send (MU-RTS) frame, which functions as a second type of trigger frame. 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 UL-OFDMA frame exchange. The AP uses the MU-RTS as a trigger frame to allocate resource units (RUs). The 802.11ax clients will send clear-to-send (CTS) responses in parallel using their assigned RUs.

A third basic trigger frame is needed to signal the 802.11ax clients to begin uplink transmission of their data with their assigned RUs. The basic trigger frame also dictates the length of the uplink window. The uplink client devices must all start and stop at the same time. The basic trigger frame also contains power control information so that individual clients can increase or decrease their transmit power. This will help equalize the received power to the AP from all uplink clients and improve reception. Once the uplink data is received from the clients, the AP will send a single multi-user Block ACK to the clients. The AP also has the option of sending separate Block ACKs to each individual client.

In our next 802.11ax blog, we will discuss how Wi-Fi 6 clients can send unsolicited buffer status reports to an AP for coordinated uplink communications.

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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