Mandatory, Basic and Supported Rates

"Mandatory rate", "basic rate", "minimum basic rate" and "supported rate" sound alike but mean different things, and vendors mix the words up. Build a rate set, see which clients can join, which rate every beacon, broadcast and ACK really goes out at, and what goes wrong when the minimum basic rate is a rate the standard doesn't make mandatory.

1 · Your rate set

Click a rate to change it: Basic → Supported → Disabled. The lowest Basic rate is the MBR.

Band

Try

Basic: every client must support it Supported: used if the client can Disabled MMandatory in the IEEE standard for this PHY

What the AP advertises in its beacons

Each rate is one byte in units of 500 kb/s; bit 7 set (value + 0x80) marks a Basic rate (IEEE 802.11-2024 9.4.2.3). Disabled rates aren't listed at all.

2 · Mandatory is not the same as Basic

"Mandatory" is fixed by the IEEE standard for each PHY. "Basic" is what you configure. They overlap, but only the overlap is safe for every client.

3 · Which clients can join

A client first has to decode a beacon (sent at the MBR), then support every Basic rate, or the AP refuses the association.

4 · Which rate each frame goes out at

5 · What to watch for in this rate set

Definitions

↗ opens a term's full entry in the Wi-Fi Glossary, which explains over 400 Wi-Fi terms with their IEEE 802.11 references.

Notes

Mandatory rates: OFDM (5 GHz): 6, 12 and 24 Mbps (IEEE 802.11-2024 17.1.1). ERP (2.4 GHz): 1, 2, 5.5, 11, 6, 12 and 24 Mbps (IEEE 802.11-2024 18.1.2). HR/DSSS (802.11b): 1, 2, 5.5 and 11 Mbps (IEEE 802.11-2024 16.2.3.4). Every other legacy rate is optional. A device has to be able to send and receive at the mandatory rates even when they aren't in the rate set it advertises (IEEE 802.11-2024 10.3.1).
Basic rates: every member of a BSS can send and receive at all the rates in the BSSBasicRateSet (IEEE 802.11-2024 10.3.1). An AP refuses a client that doesn't support them all with status code 18, REFUSED_BASIC_RATES_MISMATCH (IEEE 802.11-2024 Table 9-80). The same applies to the Basic HT-MCS, VHT-MCS and HE-MCS sets, which this tool doesn't model.
Beacons and group-addressed frames go at one of the Basic rates (IEEE 802.11-2024 10.6.5.1, IEEE 802.11-2024 10.6.5.4); which one is the vendor's choice. APs send beacons at the lowest (the MBR) A few controllers can set a separate beacon rate, such as Aruba's a-beacon-rate and g-beacon-rate, which Aruba documents for distributed antenna systems (DAS) only and warns may cause connectivity problems elsewhere (Aruba documentation). Cisco documents that multicast goes at "one of the configured mandatory data rates", and many community reports say it's the highest. Probe responses are individually addressed, but APs usually send them at the MBR too. With Multiple BSSID, beacons use a Basic rate common to all the SSIDs, or a mandatory rate if there is none (IEEE 802.11-2024 10.6.9).
ACK and CTS rate: the highest Basic rate that is not faster than the frame being answered; if there is none, the highest mandatory rate that is not faster (IEEE 802.11-2024 10.6.6.5.2). The answer must use the same modulation class as the frame being answered, and ERP-OFDM or OFDM for an HT or VHT frame, so this tool only compares OFDM rates with OFDM rates: a Basic 11 Mbps can't answer a 54 Mbps OFDM frame. HT, VHT and HE frames are converted to a non-HT reference rate from their modulation and coding rate (IEEE 802.11-2024 10.6.11, IEEE 802.11-2024 Table 10-10). The tool shows non-HT ACKs; a device may instead answer in an HT or HE PPDU, and a Compressed BlockAck follows the same rule.
An ACK can go below the MBR. The fallback uses the mandatory rates (6, 12 and 24 Mbps) even if they are Disabled. With the MBR at 24 Mbps and 6 to 18 Mbps Disabled, a client sending at MCS 0 gets its ACK at 6 Mbps, and at MCS 1 or MCS 2 it gets it at 12 Mbps. 9 and 18 Mbps are never used for ACKs unless they are Basic, because they aren't mandatory. Disabling legacy rates doesn't remove MCS 0 to 2. Some controllers can take low MCSs out of the rate set, which stops these ACKs but also shortens the range of HT, VHT and HE clients at the cell edge. The airtime cost is small anyway: an ACK is about 44 µs at 6 Mbps and 28 µs at 24 Mbps on 5 GHz.
Supported rates below the MBR are allowed. For example, the MBR is 24 Mbps and 9 Mbps is Supported (in the MBR 24 preset, click 9 twice to make it Supported). Beacons, broadcasts and multicasts still go at a Basic rate (24 Mbps). Unicast data can drop to 9 Mbps, because individually addressed frames may use any rate the receiver supports (IEEE 802.11-2024 10.6.5.8), and the ACK for that frame then goes at 6 Mbps. This only affects legacy (802.11a/g) frames; HT, VHT and HE clients use MCSs, which this list doesn't limit. The gain is small: a client that has fallen to 9 Mbps has usually stopped decoding the 24 Mbps beacons and is about to roam. Most designs disable every rate below the MBR.
Supported (non-Basic) rates are only for frames sent to one station, where the sender knows the receiver supports the rate. Beacons, broadcasts, multicasts and control responses use Basic rates (or the mandatory-rate fallback). There are exceptions: an HE AP may send group-addressed frames in an HE single-user (SU) PPDU at a mandatory HE-MCS with one spatial stream (IEEE 802.11-2024 26.15.6); some controllers send broadcast and multicast data at the lowest rate the associated clients are currently using (Aruba's Multicast Rate Optimization, rechecked every second; management frames stay at the lowest configured rate, Aruba documentation); and multicast-to-unicast conversion (Aruba Dynamic Multicast Optimization, Cisco VideoStream/Media Stream) sends each client its own unicast copy at any rate that client supports.
Minimum sensitivities are the worst a compliant receiver may be: OFDM IEEE 802.11-2024 Table 17-21 (20 MHz); DSSS −80 dBm, stated for 2 Mbps (IEEE 802.11-2024 15.4.6.2); HR/DSSS −76 dBm, stated for 11 Mbps (IEEE 802.11-2024 16.3.8.2). Real clients are usually a few dB better.
Client profiles: the "minimum" clients support only what the standard requires. The Class 2 ERP client is a 2.4 GHz device class that only has to support 1, 2 and 6 Mbps, yet has to advertise 1, 2, 5.5, 11, 6, 12 and 24 Mbps whether it supports them or not (IEEE 802.11-2024 18.1.2). Most real chipsets support every legacy rate, so an optional Basic rate won't lock out most clients. The standard just doesn't guarantee that it won't.
MBR is not an IEEE term. It's the industry name for the lowest Basic rate, usually set with every lower rate disabled. Vendor settings differ: Cisco calls Basic rates "Mandatory", and Meraki's Minimum bitrate becomes the only Basic rate once it is above the highest default Basic rate (Meraki documentation). Check what your controller actually puts in the beacon: the Frame Decoder shows it.
6 GHz isn't shown: 6 GHz BSSs use only OFDM and HE rates and have their own discovery rules. Their non-HT beacons still go at a Basic rate (IEEE 802.11-2024 10.6.5.1).