WiFi (802.11, CSMA-CA, Topologies)

Context: FIT1047_MOC · wireless Ethernet (IEEE 802.11) — same layers, harder physics · security split into WiFi Security (WEP, WPA2)

Quick Revision

• 🎯 Objective: WLAN NICs ↔ Access Points ↔ wired LAN; medium is shared radio ⟹ CSMA/CA (Avoidance — you often CAN’T detect wireless collisions).
• 📦 Core Components: 2.4/5 GHz channels ➔ hidden node problem ➔ stop-and-wait ARQ ➔ BSS → ESS topologies ➔ placement/attenuation.
• ⚡ Key Constraint: 2.4 GHz channels are 22 MHz wide but 5 MHz apart ⟹ only 1, 6, 11 don’t overlap — the neighbour-channel puzzle is a standing exam item.

📝 Core

1. Basics (802.11)

• Why WiFi ➔ no cables (heritage buildings, rentals), flexible access, mobility (hospitals).
• Standards ladder ➔ 802.11a/b (1997–99) → g (2003, 54 Mbps) → n (2009) → ac (2013) → ax (2021); 5 GHz standards: more + wider channels (80 vs 20 MHz), up to ~1.73 Gbps, but shorter range ⟹ mesh systems.
• Frequencies ➔ 2.4 GHz and/or 5 GHz; higher frequency = more bandwidth but stronger attenuation (walls halve signal per 3 dB; worse at 5 GHz).

2. CSMA/CA and the Hidden Node

• CA not CD ➔ Carrier Sense, Multiple Access, Collision Avoidance — contrast Ethernet’s Detection (802.3).
• Hidden node problem ➔ two clients both in AP range but out of EACH OTHER’s range: neither senses the other ⟹ collisions at the AP that the senders can’t detect.
• Fixes ➔ stop-and-wait ARQ (every frame ACKed before the next) + controlled access via the AP.

3. Topologies

• Independent BSS (ad hoc) ➔ devices talk directly; rarely used now.
• Infrastructure BSS ➔ all traffic via one AP wired into the LAN — the normal case.
• ESS ➔ multiple overlapping BSSs; APs forward via the wired distribution system; devices roam between APs seeing one layer-2 network. (Roaming between ESSs needs higher-layer help, e.g. Mobile IP.)
• Planning ➔ tile coverage with minimal overlap, assign non-interfering channels (the 1/6/11 tessellation in the lecture’s 150 m × 100 m example).

4. Performance rules

• AP placement ➔ high/ceiling-mounted, few walls, never in a closet; kill interferers (Bluetooth, microwaves, cordless phones); directional antennas where needed.
• NEVER put a server on WiFi ➔ all traffic passes the AP on a shared medium ⟹ a WiFi server DOUBLES the radio traffic and waits between frames; servers belong on wired Ethernet.

⚠️ Common Mistakes

• 💡 Why CA, not CD ➔ a transmitting radio can’t hear a distant collision (hidden node) — detection is physically unreliable, so avoid + ACK instead.
• 💡 13 channels ≠ 13 usable ➔ overlap leaves three clean choices at 2.4 GHz (1, 6, 11); adjacent-number channels interfere WORSE than same-channel sharing in some layouts.
• 💡 5 GHz trade ➔ faster + cleaner spectrum, but attenuates harder — more APs, not fewer.

🧠 Active Recall

Explain the hidden node problem and the two 802.11 countermeasures.

Answer

• Short answer: Nodes A and C both reach AP B but not each other ⟹ simultaneous sends collide at B undetected; countered by stop-and-wait ARQ (per-frame ACKs expose losses) and AP-controlled access.
• Why: Detection needs shared hearing ➔ CSMA/CD assumes every sender hears every collision — false on radio, hence Avoidance.

Three neighbours run channels 1, 6, 6; you're on 11. Who must move, and to what?

Answer

• Short answer: One of the channel-6 pair moves — but the only non-overlapping options are 1, 6, 11, all taken ⟹ the two 6s must share, or the mover picks whichever of 1/6/11 is most distant physically.
• Why: Three-colour constraint ➔ 2.4 GHz gives exactly three orthogonal channels; beyond three overlapping cells, sharing is unavoidable — plan geometry, not just numbers.