Legacy 802.11a/b/g/n/ac Wi-Fi clients must contend for the medium and win their own TXOP if they want to transmit uplink. However, the uplink transmissions of 802.11ax clients are synchronized and controlled by the access point. A question that I am often asked is, “Can an 802.11ax client station suspend participation for synchronized uplink OFDMA and contend for the medium for an independent uplink transmission?”
802.11ax defines an operating mode indication (OMI) procedure for this purpose. As shown in Figure 1, the Wi-Fi 6 client that transmits a frame with an OM Control subfield is defined as the OMI initiator and the AP is the OMI responder. An 802.11ax 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.
A client can switch between single-user or multi-user UL-OFDMA operations with the change in transmit operating mode (TOM). Therefore, a Wi-Fi 6 client can both suspend and resume responses to the trigger frames sent by an AP during the UL-OFDMA process.
Additionally, a Wi-Fi client station can signal a change in receive operating mode (ROM) to the AP. The client indicates to the AP, the maximum number of spatial streams and the maximum channel bandwidth that the client can support for downlink transmission. As shown in Figure 2, the client can indicate a change in channel size and number of supported spatial steams.
Most likely, WLAN vendors will be required to support OMI in the 802.11ax AP whereas the OMI capability will be optional for Wi-Fi 6 clients.