# Wireless (802.11)

Wi-Fi is a shared, lossy, half-duplex medium that behaves nothing like a cable. Most wireless "network problems" are capacity, airtime or interference problems wearing a connectivity costume.

## The moving parts

| Term | Meaning |
|---|---|
| **SSID** | The network name users see |
| **BSSID** | The MAC of one radio/AP — one SSID can have many BSSIDs |
| **Channel** | The slice of spectrum a radio uses |
| **Band** | 2.4 GHz, 5 GHz, 6 GHz (Wi-Fi 6E/7) |
| **Airtime** | The finite resource: one radio transmits to one client at a time per channel |
| **Roaming** | Moving a client between APs without dropping the session |

| Standard | Marketing name | Band | Notable |
|---|---|---|---|
| 802.11n | Wi-Fi 4 | 2.4/5 | MIMO, 40 MHz channels |
| 802.11ac | Wi-Fi 5 | 5 | Wider channels, MU-MIMO downlink |
| 802.11ax | Wi-Fi 6 / 6E | 2.4/5/6 | OFDMA, better behaviour with many clients |
| 802.11be | Wi-Fi 7 | 2.4/5/6 | Multi-link operation, 320 MHz |

## Why "more bars" is not "more speed"

Signal strength is one input. Throughput depends on:

- **Channel width** — 20/40/80/160 MHz. Wider is faster and consumes more spectrum, causing more co-channel interference.
- **Spatial streams** — MIMO count on both ends; a phone with 2 streams cannot use an AP's 8.
- **Airtime sharing** — 40 clients on one radio each get a slice; adding clients lowers everyone's throughput even at full signal.
- **Interference** — neighbouring networks, microwave ovens (2.4 GHz), radar (DFS channels), Bluetooth.
- **Retries** — lossy links retransmit; each retry is airtime spent twice.

A client at −55 dBm on a congested 2.4 GHz channel can be far slower than one at −70 dBm on a clean 5 GHz channel.

## Channels, briefly

| Band | Non-overlapping channels | Reality |
|---|---|---|
| 2.4 GHz | 1, 6, 11 | Only three; everything else overlaps |
| 5 GHz | Many, plus DFS | DFS channels must vacate on radar detection |
| 6 GHz | Many, no DFS legacy | Best density, shorter range, needs 6E clients |

"Set every AP to channel 1" is a configuration, not a plan.

## Security in one table

| Mode | Status |
|---|---|
| Open | No protection; captive portals only |
| WEP | Broken, never use |
| WPA (TKIP) | Broken, never use |
| WPA2-PSK | Acceptable for home; shared secret does not scale |
| WPA2/WPA3-Enterprise (802.1X) | Correct for organisations: per-user credentials, RADIUS |
| WPA3-SAE | Stronger handshake; protects against offline dictionary attacks |
| WPA3-Enterprise | Best current practice |

WPA2-PSK with a shared password means every departed employee still knows the key, and one leaked key compromises every device.

## Roaming and the three helper protocols

Roaming is the client's decision, which is why it is unpredictable.

- **802.11k** — neighbour reports, so the client knows what else is nearby.
- **802.11v** — network-initiated steering ("this AP is full, try that one").
- **802.11r** — fast transition, pre-authenticating the next AP to cut handover time.

Without them, a voice call drops as the client scans and re-authenticates. With them, roaming is usually under 50 ms.

## Where it goes wrong

- **Designing for coverage instead of capacity.** One AP per floor "covers" the floor and collapses under 60 laptops in a meeting room.
- **High density with narrow channels and no band steering.** Everyone sits on 2.4 GHz because it reaches furthest, which is exactly where there is least spectrum.
- **Hidden SSID as security.** It hides nothing from a scanner and breaks roaming.
- **Captive portals and IoT.** Devices without browsers cannot authenticate, which is why guest IoT networks usually need a separate PSK or MAC-based onboarding.
- **Assuming Wi-Fi is deterministic.** Retries, airtime and interference make it statistical; applications need to tolerate jitter, or the network gets blamed for the app.
- **Ignoring the wired side.** An AP with a 1 Gbps uplink serving 80 clients at 300 Mbps each is oversubscribed before the first download finishes.

## Teaching hook

**The party-line telephone analogy.** Everyone shares one line; only one person can talk at a time; the further from the speaker, the more you ask people to repeat themselves; and a loud conversation next door costs you throughput even if you can hear perfectly.

Demo: run a Wi-Fi scan (`iw dev wlan0 scan`, or a phone app) and show the channel overlap graph. Ask learners to pick a channel plan for four APs. Most pick 1/6/11 and then discover the neighbours already use them.

## Related Notes

- [[Switching-LAN/VLANs-Trunking.md]] — the wired side of an AP
- [[Security/Network-Security-Fundamentals.md]] — 802.1X and rogue APs
- [[Foundations/Network-Design-And-Topologies.md]] — capacity as a design input
- [[Automation-Observability/Telemetry-And-Monitoring.md]] — measuring airtime honestly