1, 2, 5.5 and 11 Mbps are 802.11b rates from 1999. They only exist on 2.4 GHz, and leaving them enabled costs airtime in three ways: slow beacons, slow clients that hold everyone back, and protection frames in front of every modern frame. Pick the lowest basic rate and see what each one costs.
Every AP sends one beacon per SSID about ten times a second, always at the lowest basic rate so every client can hear it. On 2.4 GHz only three channels don't overlap, so a client or AP hears many APs on its own channel.
BeaconWaiting before each beacon (DIFS)Left for everything else
One beacon takes
Airtime used by beacons
Compared with a 12 Mbps basic rate
2 · One slow client slows everyone down
Wi-Fi gives every station an equal chance to transmit, not equal time. When an old 802.11b device sends a frame, the air is busy for as long as that frame takes, and the modern client simply waits.
The old client's frame usesof each round's airtime
3 · Protection frames in front of every modern frame
An 802.11b device can't decode OFDM frames, so it can't tell the channel is busy. As soon as one is associated, the AP turns on protection: every OFDM sender first transmits a short CTS-to-self at an 802.11b rate that reserves the air. The 802.11b device doesn't even have to be sending anything.
CTS-to-self sent at
802.11g 54 Mbps, no protection1,500-byte frames, one client
With protection
Throughput lostjust because an 802.11b device is associated
4 · Bigger cells, more interference
The lower the basic rate, the further away a beacon can still be decoded. A 1 Mbps beacon reaches much further than a 12 Mbps one, so each AP's cell grows: clients stay attached to distant APs (see Sticky Clients and Minimum Basic Rate) and every AP interferes with more of its neighbours on the same channel.
That multiplies panel 1: the more APs each client can hear, the more beacons fill the air.
What to do
Disable 1, 2, 5.5 and 11 Mbps (and usually 6 and 9) on 2.4 GHz. Devices that only speak 802.11b are almost extinct.
Set the lowest basic rate to 12 or 24 Mbps. Check that the MBR edges of neighbouring APs still overlap.
Keep SSIDs to a minimum: every extra SSID adds a beacon per AP, ten times a second.
Consider 2.4 GHz for coverage only, with lower AP power and some 2.4 GHz radios turned off, and put capable clients on 5 and 6 GHz.
Notes and assumptions
802.11b (DSSS) frames start with a 192 µs long preamble and header, then the frame at 1, 2, 5.5 or 11 Mbps. A short 96 µs preamble exists for 2–11 Mbps, but 1 Mbps always uses the long one and many networks keep long preambles while 802.11b devices are allowed; this tool uses the long preamble throughout.
802.11g (ERP-OFDM) frames take 20 µs of preamble and signal field, 4 µs per symbol, and a 6 µs signal extension on 2.4 GHz. 802.11n frames (mixed format, 20 MHz, 1 stream, MCS 7, 800 ns guard interval = 65 Mbps) take a 36 µs preamble.
Timing: SIFS 10 µs. While 802.11b devices are allowed, stations use the 20 µs long slot (DIFS 50 µs); without them, the 9 µs short slot (DIFS 28 µs). Average backoff is half the minimum contention window: 15.5 slots for the 802.11b client, whose rates are only 1, 2, 5.5 and 11 Mbps (CWmin 31), and 7.5 slots for 802.11g/n stations (CWmin 15), per IEEE 802.11-2024 (Table 18-5, aCWmin(0) = 31 and aCWmin(1) = 15). ACKs are 14 bytes at the rate the network uses for control frames: the client's DSSS rate for the 802.11b client, 24 Mbps OFDM otherwise.
Beacons: one per SSID per AP every 100 TU (102.4 ms), each preceded by a DIFS. Beacon size depends on the SSID length and the features advertised; 250–400 bytes is typical for enterprise APs.
Throughput is the MAC-level result of two stations taking turns (panel 2) or one station alone (panels 2 and 3); real networks add retries, more clients and aggregation. 802.11n aggregation (A-MPDU) would make the modern client much faster on its own, which makes the drop caused by the slow client even bigger.