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Fleet capacity

Every frame costs more airtime than its data: a wait, a preamble, and an acknowledgement. Describe what each vehicle sends and see how many vehicles one channel can carry.

1. The radio

Data rate at this setting: 86.03 Mbps.

2. What each vehicle sends

3. The fleet and the ceiling

4. Airtime and capacity

In plain words

One vehicle's traffic takes 1.574% of the channel's airtime at 86 Mbps, counting the waits, preambles, and acknowledgements each frame needs.

63 vehicles like this would fill the channel completely, which no channel can carry in practice. Set the utilization ceiling your vehicle maker or design guide uses.

Only 17% of each of its exchanges is the data itself; the rest is fixed overhead. For small frames like these, a faster data rate barely helps, and fewer, larger frames would.

Airtime per vehicle

1.574 %

Vehicles that would fill the channel: 63

TrafficFrames per secondUp, µs per exchangeDown, µs per exchangeAirtime
Control I/O50161.9152.91.574%
One exchange of the first traffic (vehicle to network), drawn to scale
PartµsShare
AIFS (fixed wait)3421%
Backoff (average)13.58%
Preamble43.227%
Data27.217%
SIFS1610%
Acknowledgement2817%
Total161.9100%

What this model leaves out

  • Collisions between stations that pick the same backoff slot.
  • Beacons, probes, and other management frames.
  • Other clients, other networks, and anything else on the same channel.
  • RTS and CTS, transmit opportunity bursts, and multi-user transmissions (OFDMA and MU-MIMO).
  • Rate changes as a vehicle moves, scans, and roams.
  • Interference that is not Wi-Fi.

Each of these takes more airtime, so treat the result as a floor. And airtime is an average: a channel with room to spare can still hold one frame long enough to break a timeout. The worst gap is what the timing budget checks.

Where these numbers come from

The calculator uses the default timing every Wi-Fi radio follows. An access point can advertise other values, and your clients may not use every option, so measure your own airtime before you commit to a fleet size.

Slot time, SIFS and signal extension

Slot time, OFDM (5 GHz, 6 GHz, and 2.4 GHz with short slot)
9 µs
Slot time, 2.4 GHz long slot
20 µs
SIFS at 5 GHz
16 µs
SIFS at 6 GHz
16 µs (one source)
SIFS at 2.4 GHz
10 µs
Signal extension after an OFDM frame at 2.4 GHz
6 µs

Slot time is 9 µs on OFDM radios. At 2.4 GHz the slot stays at 20 µs unless every station in the cell supports the short slot. SIFS is 16 µs at 5 GHz. At 2.4 GHz SIFS is 10 µs and each OFDM frame adds a 6 µs signal extension, so the Linux kernel's frame-duration code counts 16 µs for the two together. The 6 GHz SIFS rests on one network simulator, which runs 802.11ax outside 2.4 GHz on the same OFDM timing as 5 GHz. The European harmonised standards for 5 and 6 GHz set observation slots of at least 9 µs, with a fixed wait of 16 µs at 5 GHz and at least 14 µs at 6 GHz; those are regulatory floors for listen-before-talk, not the 802.11 SIFS.

EDCA defaults for stations (WMM)

AIFS
AIFSN × slot time + SIFS
Contention window unit
slots
VO (voice)
AIFSN 2, CWmin 3, CWmax 7, TXOP limit 1.504 ms
VI (video)
AIFSN 2, CWmin 7, CWmax 15, TXOP limit 3.008 ms
BE (best effort)
AIFSN 3, CWmin 15, CWmax 1023, TXOP limit 0
BK (background)
AIFSN 7, CWmin 15, CWmax 1023, TXOP limit 0
TXOP limit 0
one MSDU per access, plus an optional RTS/CTS or CTS-to-self

These are the defaults a station uses until its access point advertises other values, and the access point may change them at any beacon. Contention windows are shown for OFDM radios, where aCWmin is 15 and aCWmax is 1023; 802.11b uses larger windows and longer TXOP limits. Europe's harmonised standards for 5 and 6 GHz list the same four sets as minimum values for client devices.

EDCA defaults for access points (WMM)

Contention window unit
slots
VO (voice)
AIFSN 1, CWmin 3, CWmax 7, TXOP limit 1.504 ms
VI (video)
AIFSN 1, CWmin 7, CWmax 15, TXOP limit 3.008 ms
BE (best effort)
AIFSN 3, CWmin 15, CWmax 63, TXOP limit 0
BK (background)
AIFSN 7, CWmin 15, CWmax 1023, TXOP limit 0

The access point's own recommended set differs from the one it advertises to stations. Voice and video wait one slot instead of two, and best effort backs off to at most 63 slots instead of 1023. The WMM specification recommends this set rather than requiring it, so your access point may run other values; check its configuration before you rely on these.

Legacy OFDM PHY (802.11a/g, control frames)

Training fields (L-STF and L-LTF)
16 µs
SIGNAL field
4 µs
Preamble plus SIGNAL
20 µs
Symbol
4 µs
SERVICE field
16 bits
Tail
6 bits
Data symbols
ceil((16 + 8 × PSDU bytes + 6) / data bits per symbol)
Data bits per symbol at 6 Mbps
24
Data bits per symbol at 9 Mbps
36
Data bits per symbol at 12 Mbps
48
Data bits per symbol at 18 Mbps
72
Data bits per symbol at 24 Mbps
96
Data bits per symbol at 36 Mbps
144
Data bits per symbol at 48 Mbps
192
Data bits per symbol at 54 Mbps
216

Data bits per symbol equal the rate in Mbps times 4. The data field rounds up to whole 4 µs symbols, so every legacy frame costs 20 µs plus a whole number of symbols, before any SIFS or signal extension.

HT and VHT PHY (802.11n and 802.11ac)

L-STF
8 µs
L-LTF
8 µs
L-SIG
4 µs
HT-SIG
8 µs
HT-STF
4 µs
HT-LTF, each
4 µs
VHT-SIG-A
8 µs
VHT-STF
4 µs
VHT-LTF, each
4 µs
VHT-SIG-B
4 µs (one source)
LTFs for 1, 2, 3 or 4, 5 or 6, 7 or 8 streams
1, 2, 4, 6, 8
Symbol with 0.8 µs guard interval
4.0 µs
Symbol with 0.4 µs guard interval
3.6 µs
Data subcarriers at 20 MHz
52
Data subcarriers at 40 MHz
108
Data subcarriers at 80 MHz (VHT)
234
Data subcarriers at 160 MHz (VHT)
468

Sources disagree on VHT-SIG-B in single-user frames. One simulator adds it only to multi-user frames, and one packet dissector computes the VHT preamble as 32 µs plus 4 µs per stream. A published analysis draws a 4 µs VHT-SIG-B in the single-user frame, and a test-equipment maker describes VHT-SIG-B as one symbol, the last field before the data. Published single-stream VHT preamble totals differ by 4 µs: 40 µs in one study and 36 µs in another. Stream counts of 5 to 8 apply to VHT only; HT allows at most 4 LTFs.

HE PHY (802.11ax, Wi-Fi 6 and 6E)

L-STF
8 µs
L-LTF
8 µs
L-SIG
4 µs
RL-SIG
4 µs
HE-SIG-A (SU)
8 µs
HE-STF (SU)
4 µs
HE-LTF 1x
3.2 µs, used with a 0.8 or 1.6 µs guard interval (one source)
HE-LTF 2x
6.4 µs, used with a 0.8 or 1.6 µs guard interval; 7.2 µs counted with a 0.8 µs guard interval
HE-LTF 4x
12.8 µs, used with a 0.8 or 3.2 µs guard interval (one source)
Data symbol
12.8 µs plus guard interval
Data symbol with 0.8, 1.6, 3.2 µs guard interval
13.6, 14.4, 16.0 µs
Data subcarriers at 20 MHz
234 (one source)
Data subcarriers at 40 MHz
468 (one source)
Data subcarriers at 80 MHz
980
Data subcarriers at 160 MHz
1960 (one source)

HE data symbols last 12.8 µs plus the guard interval, four times the 3.2 µs base of HT and VHT. One modeling tool gives HE-LTF durations of 3.2, 6.4 and 12.8 µs and pairs each with its allowed guard intervals; one analysis counts a 2x HE-LTF with a 0.8 µs guard interval as 7.2 µs. One simulator fixes every HE-LTF at 8 µs and marks that as a placeholder. Data subcarrier counts for 20, 40 and 160 MHz come from one simulator; one vendor's white paper confirms 980 at 80 MHz. One analysis counts one HE-LTF per spatial stream; no source read here gives the full stream-to-LTF mapping for HE.

MAC framing overhead

QoS data MAC header
26 bytes
LLC/SNAP header
8 bytes
FCS
4 bytes
CCMP
16 bytes (8 header + 8 MIC)
CCMP-256
24 bytes (8 header + 16 MIC)
GCMP-256
24 bytes (8 header + 16 MIC)
ACK frame
14 bytes
Compressed Block Ack (64-frame bitmap)
32 bytes
RTS frame
20 bytes
CTS frame
14 bytes
A-MPDU delimiter
4 bytes
A-MPDU subframe padding
to a multiple of 4 bytes

Add the MAC header, LLC/SNAP, security and FCS bytes to each frame's payload, and a delimiter plus padding to each subframe of an aggregate. Published analyses size the MAC header differently: 26 bytes in one, 28 bytes in another and 320 bits in a third, so check which optional fields your calculation includes. One analysis counts 30 bytes for a Block Ack covering 64 frames and 54 bytes for one covering 256 frames. The GCMP lengths come from code that defines one GCMP header and MIC length for the GCMP family.