Nobody Is In Charge
WiFi has no scheduler. Every radio decides for itself when to talk, because 802.11 was built for unlicensed spectrum where nothing can be coordinated. The whole design follows from that.
BSS, ESS, SSID, BSSID
- BSS — one access point plus its stations. Everything is built on this
- BSSID — the AP radio’s MAC address. A physical thing, one per radio
- SSID — the network name, up to 32 bytes. A label, deliberately not unique
- ESS — many BSSs sharing one SSID, so a building looks like one network
- Ad-hoc / mesh — no AP at all; mesh pays airtime twice per hop
The station decides when to roam, and the network cannot compel it. M11-L2 shows cellular doing the opposite.
A Radio Cannot Hear While It Shouts
Ethernet detects collisions. WiFi cannot: 20 dBm out against a −82 dBm wanted signal is M10-L4’s full-duplex barrier again. So a collision is found only by a missing ACK — after the whole frame is gone.
Listen, Wait, Hope
- DIFS = SIFS + 2 slots = 16 + 2×9 = 34 µs before anyone may start
- Backoff uniform on 0–15 slots, so on average 7.5×9 = 67.5 µs
- SIFS = 16 µs, shorter than DIFS, so the ACK always wins the channel
- No ACK → double the window: 15, 31, 63 … 1023
The random backoff is the only thing breaking ties. Politeness scales with load, and nobody measures the load.
The Hidden Node
- A and C both reach the AP but not each other, so both sense idle
- Both transmit; both frames die at the AP. Sensing worked perfectly
- Sensing describes the air at your antenna, not at the receiver
- RTS/CTS reserves through the AP — everyone hears the CTS
- Cost 40–60 µs per frame: worth it at 1500 bytes, absurd at 100
The mirror image is the exposed node, deferring to a transmission it could never have collided with (M9-L3).
Where the Airtime Goes
Faster PHY, Worse Efficiency
34 + 67.5 + 247.6 + 16 + 24.7 = 389.8 µs → 30.8 Mb/s. Eleven times the PHY rate buys 2.1× the throughput — so aggregation is not optional.
a, n, ac, ax, be
- 11a: 48×6×(3/4)/4.0 µs = 54 Mb/s ✓
- 11ac w2: 468×8×(5/6)×8/3.6 µs = 6933 ✓
- WiFi 6: 1960×10×(5/6)×8/13.6 µs = 9608 ✓
- WiFi 7: 3920×12×(5/6)×16/13.6 µs = 46 118 ✓
WiFi 6 Fixes the Protocol
- OFDMA — nine 26-tone units of 2.03 MHz share one overhead payment
- MU-MIMO up to 8 users; BSS colouring recovers exposed-node airtime
- TWT buys battery life; 1024-QAM adds 25% and wants ~35 dB SNR
- WiFi 7 — 320 MHz, 4096-QAM, and multi-link across two radios
- 2.4 GHz holds only 3 clean channels in 83.5 MHz: 1, 6, 11
320 MHz and 4096-QAM each cost about 3 dB of SNR, and they compound.
What you learned
- No referee: airtime is contended, not allocated
- No collision detection — 20 − (−82) = 102 dB in the way
- DCF costs 142 µs per frame: 57% at 54 Mb/s, 11.0% at 600
- Peak rates are just Nsd·log₂M·r·Nss/Tsym
- 2.4 GHz: 3 clean channels; WiFi 6 delivers 7.2 b/s/Hz per stream