• Case 1: AP has something to transmit to an ax client so the AP will contend for the channel. When the APs NAV reaches zero it will have its turn. At the beginning of the transmission the AP will have to tell how long it is going to use the channel. How does it know? It knows how much it is going to transmit, but will it also start giving out resource units to clients? The AP can't reserve the channel indefinitely or it would starve ac/n clients waiting outside of OFDMA.

    While the AP is going to sent a DL - OFDMA transmission to multiple clients, AP will know the length of the DATA transmission to each clients and will add the NAV to the MU-RTS frame, which is a trigger frame which will contain RU's allocated for each clients. 

    When multiple STA's are going to send UL -OFDMA transmission, this is scheduled by the AP as well, and AP should know how much is buffered data that each client is going to send, this should be informed by the clients using Trigger-BSR(Buffer status report) to the AP, so I think AP will come to know on how much DATA the clients are going to send,AP will calculate & pick the  largest NAV from the clients 

    Whether this Trigger-BSR frame is MU-OFDMA or legacy format frame, will be interesting to know. 

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