OFDMA RU Allocator
802.11ax splits a channel into Resource Units (RUs) of 26, 52, 106, 242, 484, or 996 tones. The AP allocates RUs to stations through HE-SIG-B. Click any cell below to allocate it. Larger RUs override overlapping smaller ones — exactly the geometry the spec constrains.
— Shankar K. · Source: IEEE 802.11ax Section 27.3.2.2, Figures 27-5/6/7 · Wireshark: wlan.he.ru
200 employees in an open office. Wi-Fi 6 was deployed specifically to fix density problems. Six months later, voice calls still stutter and the helpdesk is still flooded with complaints.
OFDMA was supposed to let the AP pack many voice clients onto small Resource Units and trigger them to transmit simultaneously. Two things silently sabotage that. First, around 40 of the 200 clients are still Wi-Fi 5 devices that can't parse Trigger Frames, so during their transmissions the AP has to schedule them with legacy single-client bursts and OFDMA doesn't run. Second, some AP firmware ships with OFDMA disabled by default pending vendor validation, and many deployments never re-enable it after firmware updates.
Confirm OFDMA is actually active by looking for Trigger Frames (type=Control, subtype=Trigger) from the AP in a capture — their presence means OFDMA is running. Then split the SSID: a Wi-Fi 6-only SSID for voice clients, a mixed SSID for laptops. With the voice-only SSID, the AP can schedule roughly 8-9 voice clients on 26-tone RUs within a 20 MHz channel simultaneously, and jitter typically drops from tens of milliseconds to single digits.
OFDMA benefits small, frequent packets (voice, IoT telemetry, notifications). For bulk data transfer — one laptop downloading a large file — single-user transmission or DL-MU-MIMO is usually faster. Don't expect OFDMA to improve everything; it's specifically a density-and-latency win, not a raw-throughput win.
OFDMA stands for Orthogonal Frequency Division Multiple Access. Before Wi-Fi 6, one client transmitted on the full channel width at a time — if four devices all wanted to talk, they took turns, one at a time. OFDMA changes that. The AP slices the 20 / 40 / 80 MHz channel into smaller frequency chunks called Resource Units (RUs), and multiple clients transmit at the same instant, each on a different RU.
Think of a four-lane highway. Before OFDMA, every car had to drive in the whole four-lane width. With OFDMA, four cars drive side by side in separate lanes.
Why it matters: many small packets per second (smart home devices, voice packets, IoT sensors) now travel in parallel instead of queueing. Airtime goes from "one at a time" to "many at once". In dense networks, that's the single biggest efficiency gain Wi-Fi 6 delivered.
OFDMA RUs come in fixed sizes: 26, 52, 106, 242, 484, 996 tones. A "tone" is one OFDM subcarrier, 78.125 kHz wide in Wi-Fi 6.
Per channel width, the RU count hierarchy is strict: 20 MHz splits into at most 9x 26-tone OR 4x 52 + 1x center OR 2x 106 + 1x center OR 1x 242. 40 MHz doubles this and adds 484 as a new size. 80 MHz triples it again and adds 996.
Why 26, 52, 106, 242, 484, 996? These numbers come from Wi-Fi 6's denser tone plan: each 20 MHz subchannel has 256 data+pilot+null subcarriers total, and the 26/52/106/242 sizes are the clean subdivisions that avoid straddling subchannel boundaries. The middle 26-tone RU is the exception and sits on the DC tone.
IEEE 802.11ax-2021 Clause 27 defines the RU set. Clause 27.3.2.2 and Figures 27-5/6/7 (Table 27-26) enumerate the tone-to-subchannel mapping for 20 / 40 / 80 MHz. Clause 28 adds 160 / 320 MHz and multi-RU (MRU) for 802.11be / Wi-Fi 7 — those are deferred for v2 of this visualizer.
The center-26 RUs are the non-mergeable slots. In 20 MHz, there is 1 center-26 (slot 4). In 40 MHz, there are 2 (slots 4 and 13). In 80 MHz, there are 5 (slots 4, 13, 18, 23, 32). These 26-tone RUs cannot combine with neighbors to form larger RUs — they sit on the DC tone or on subchannel gaps that break the larger RU's tone symmetry.
The DC-straddling exception at slot 18 in 80 MHz: this center-26 slot crosses the DC carrier between the two 40 MHz halves. 484-tone RUs on each half exclude it. A 996-tone RU spans the whole 80 MHz and includes it. This is the visual oddity the allocator demonstrates — try allocating 2x 484 and see slot 18 stays orphaned.
Multi-RU (MRU) in Wi-Fi 7. 802.11be (Section 36) allows a single STA to be assigned non-contiguous RUs — e.g., 106+26 tones in one transmission. This visualizer v1 does not model MRU; it stays in pure Wi-Fi 6 OFDMA territory.
Trigger frames. A Wi-Fi 6 AP signals OFDMA UL transmissions via a Trigger frame that specifies per-user RU allocations. The HE-SIG-B field of the DL OFDMA PPDU specifies per-user RU allocation on downlink. This visualizer shows allocation outcomes, not the Trigger/HE-SIG-B frames that encode them.
Wireshark filters. wlan.he.ru_allocation surfaces HE-SIG-B RU Allocation subfield. wlan.trigger.he matches Trigger frames. On EHT captures (Wi-Fi 7), wlan.eht.ehtsig carries the equivalent EHT-SIG RU allocation encoding.
OBSS considerations. OFDMA RUs are per-BSS — overlapping BSSs on the same channel do not share the RU allocation. This is where BSS Color and OBSS_PD Spatial Reuse come in to prevent one BSS's OFDMA transmission from triggering deferral on the other BSS's OFDMA transmission.
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