802.11ax and Medium Contention

As many of you might already know, a key efficiency enhancement for 802.11ax will be the use of OFDMA by Wi-Fi radios. Orthogonal frequency-division multiple access (OFDMA) is a multi-user version of the OFDM digital-modulation technology and is used in other wireless technologies such as LTE. OFDMA subdivides a channel into smaller frequency allocations, called resource units (RUs). By subdividing the channel, parallel transmissions of small frames to multiple users happen simultaneously. Think of OFDMA as a technology that partitions a channel into smaller sub-channels so that simultaneous multiple-user transmissions can occur.

As shown in Figure 1, an 802.11ax AP can partition a 20 MHz OFDMA channel into multiple resource units (RUs) for simultaneous downlink communications with multiple 802.11ax clients. Furthermore, an 802.11ax AP can also synchronize 802.11ax clients for simultaneous uplink transmissions. I have heard some people say that with 802.11ax, the AP is in control of the RF medium. Well that is not entirely true. It should be noted that the rules of medium contention still apply. The AP still has to compete against legacy 802.11 stations for a transmission opportunity (TXOP). Once the AP has a TXOP, the AP is then in control of up to nine 802.11ax client stations for either downlink or uplink transmissions on a 20 MHz channel. The number of RUs used can vary on a per TXOP basis. For the uplink (UL-OFDMA), the AP signals to the clients a synchronized schedule and RU allotment using a trigger frame. We will have a more detailed discussion of this process in a later blog.

A question I get asked is, “Can an 802.11ax client station suspend participation for synchronized uplink OFDMA and contend for the medium for an independent uplink transmission?

The 802.11ax draft proposes an operating mode indication (OMI) procedure for this purpose. A client station can signal to the AP, the maximum number of spatial streams and the maximum channel bandwidth that the client can support for either uplink or downlink transmission. As shown in Figure 2, an 802.1ax client uses the OM Control subfield in 802.11 data and management frames to indicate a change of either transmission or receiver mode of operation. For example, a client can switch between single-user or multi-user UL-OFDMA operations. A client can suspend and resume responses to the trigger frames sent by an AP during the UL-OFDMA process.


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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