RU allocation information is communicated to clients at both the PHY and MAC layers. At the Physical layer, RU allocation information can be found in the HE-SIG-B field of the PHY header of an 802.11 trigger frame. The HE-SIG-B field is used to communicate RU assignments to clients. As shown in Figure 1 the HE-SIG-B field consists of two sub-fields: the common field and user-specific field. A sub-field of the common field is used to indicate how a channel is partitioned into various RUs. For example, a 20 MHz channel might be subdivided into one 106-tone RU and four 26-tone RUs. The user-specific field comprises multiple user fields that are used to communicate which users are assigned to each individual RU.

_{Figure 1- RU Allocation at the PHY layer}

What about how RU allocation information is communicated at the MAC layer? RU allocation information is delivered in the user information field in the body of a trigger frame. Figure 2 displays a table of how RU allocation information is communicated at the MAC layer. The table highlights all the possible RUs within a 20 MHz channel and the subcarrier range for each RU. Each specific RU is defined by a unique combination of 7 bits within the user information field of the trigger frame, known as the RU allocation bits.

_{Figure 2- RU allocation at the MAC layer}

In the example, in Figure 3, the trigger frame allocates specific RUs to three client stations for simultaneous uplink transmission within a 20 MHZ OFDMA channel. Clients STA-1 and STA-2 are each assigned to a 52-tone RU, whereas client STA-3 is assigned to a 106-tone RU.

_{Figure 3- RU Allocation with Trigger frames}

802.11ax technology is the foundation of a generational paradigm shift occurring with Wi-Fi technology…. Wi-Fi 6.  If you want to learn more about 802.11ax, please download the FREE eBook I authored: ‘Wi-Fi 6 for Dummies’, a complete guide for everything you need to know to get ahead of this shift and future-proof your network.  To download your free copy, scan the QR code or follow this link:

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