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Wireless Networks
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Wireless Networks
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Now · 🎯 Learning Objectives
Wireless Networks
🎯 Learning Objectives
- Compare WiFi generations (802.11a/b/g/n/ac/ax)
- Explain CSMA/CA vs. CSMA/CD
- Understand 5G network architecture
- Identify IoT protocol tradeoffs
1. WiFi (IEEE 802.11)
1.1 Standards
| Standard | Year | Freq (GHz) | Max Rate | Range |
|---|---|---|---|---|
| 802.11b | 1999 | 2.4 | 11 Mbps | 35m |
| 802.11g | 2003 | 2.4 | 54 Mbps | 35m |
| 802.11n (WiFi 4) | 2009 | 2.4/5 | 600 Mbps | 70m |
| 802.11ac (WiFi 5) | 2014 | 5 | 3.5 Gbps | 35m |
| 802.11ax (WiFi 6) | 2020 | 2.4/5/6 | 9.6 Gbps | 30m |
| 802.11be (WiFi 7) | 2024 | 2.4/5/6 | 46 Gbps | 30m |
1.2 CSMA/CA (Collision Avoidance)
WiFi uses CSMA/CA instead of CSMA/CD (Ethernet) because radios can't detect collisions while transmitting.
- Listen before talk (Clear Channel Assessment)
- Wait random backoff time (DCF Interframe Space)
- Send RTS (Request to Send) → CTS (Clear to Send) → DATA → ACK
- If channel busy, double backoff window (exponential backoff) (Diagram)
2. 5G Cellular
2.1 Architecture
| Component | Function |
|---|---|
| UE | User Equipment (phone) |
| gNB | 5G base station (Next-Gen Node B) |
| AMF | Access & Mobility Management |
| UPF | User Plane Function (routing) |
| SMF | Session Management Function |
Key features: Network slicing, mmWave, massive MIMO, low latency (< 1ms)
3. Bluetooth & BLE
| Feature | Classic Bluetooth | BLE (Bluetooth Low Energy) |
|---|---|---|
| Rate | 1-3 Mbps | 125 kbps - 2 Mbps |
| Range | 10m | 100m |
| Power | 1W | 0.01-0.5W |
| Use | Audio, file transfer | IoT, beacons, wearables |
4. IoT Protocols
| Protocol | Layer | Range | Power | Rate |
|---|---|---|---|---|
| LoRaWAN | PHY/MAC | 10-15 km | Very low | 50 kbps |
| Zigbee | PHY/MAC | 100m | Low | 250 kbps |
| Z-Wave | PHY/MAC | 30m | Very low | 100 kbps |
| MQTT | Application | Internet | N/A | N/A |
| CoAP | Application | Internet | N/A | N/A |
5. Common Pitfalls
Pitfall: Assuming WiFi and Ethernet Have Same Performance
Correct approach: WiFi is half-duplex (shared medium), has higher latency, packet loss, and interference. CSMA/CA overhead reduces throughput vs. Ethernet.
6. Key Concepts Reference
| Concept | Definition |
|---|---|
| CSMA/CA | Carrier Sense Multiple Access with Collision Avoidance |
| OFDMA | Orthogonal Frequency Division Multiple Access (WiFi 6) |
| MIMO | Multiple Input Multiple Output (spatial streams) |
| Network slicing | Virtual networks on shared 5G infrastructure |
| Beamforming | Focused signal transmission directionally |
7. 📝 Practice Questions
Q1: Why does WiFi have lower throughput than Ethernet even at the same nominal rate?Answer: (1) CSMA/CA overhead (DIFS, backoff, RTS/CTS). (2) Half-duplex (shared channel). (3) Retransmissions due to interference. (4) Beacon frames and management overhead. (5) Variable signal quality reduces modulation rate. Real WiFi throughput is typically 40-60% of nominal. Q2: What is the hidden terminal problem in WiFi?Answer: Two stations (A and C) are both in range of AP (B) but not each other. A sends to B, C senses channel idle (can't hear A) and also sends to B — collision at B. Solution: RTS/CTS handshake. A sends RTS, B responds with CTS (heard by both A and C). C defers during A's data transmission. Q3: Compare LoRaWAN with NB-IoT for IoT connectivity.Answer: LoRaWAN: unlicensed spectrum, very low power, low data rate, public/community networks. NB-IoT: licensed cellular spectrum, better QoS, higher data rate,运营商-managed. LoRaWAN better for battery-critical, low-throughput sensors. NB-IoT better for mission-critical, higher-throughput IoT.
8. 🔗 Cross-References
- Week 4 - Data Link Layer: MAC protocols comparison
- Week 6 - Security: Wireless security (WPA3)
- BSCS4022 (OS): IoT operating systems Join Discord PreviousNetwork SecurityNextSocket Programming