The 802.11ax OFDMA channel consists of 256 sub-carriers. the sub-carrriers can be grouped together into smaller sub-channels known as Resource Units (RUs).

Depending on the channel bandwidth and the number of clients connected, the Resource Units can be as small as a 26 sub-carrier resource unit in a 20 MHz channel with 9 Clients or a 996(2x) for a single client 160 MHz/80+80 MHz channel.

The RU allocation for a 20 MHz channel is provided diagrammatically below.

The RU allocation allows AP to transmit to multiple stations in the downlink direction simultaneously by using only the specific RUs to transmit data to a specific Station. The below diagram provides an example

The below table provides the details of the different RU allocations possible for different channel bandwidths and different number of clients.

In the following articles, we will look at how RU allocations can be used for UpLink Multi- User OFDMA (UL MU-OFDMA) and DownLink Multi-User OFDMA (DL MU OFDMA).

Trigger Frames in 802.11ax