Wireless Networks
Wireless networks communicate via radio waves, eliminating the need for physical cables. This file covers Wi-Fi, Bluetooth, cellular, and other wireless technologies.
Wi-Fi (IEEE 802.11)
Standards Evolution
| Standard | Year | Band | Max Speed | Key Technology |
|---|---|---|---|---|
| 802.11b | 1999 | 2.4 GHz | 11 Mbps | DSSS |
| 802.11a | 1999 | 5 GHz | 54 Mbps | OFDM |
| 802.11g | 2003 | 2.4 GHz | 54 Mbps | OFDM |
| 802.11n (Wi-Fi 4) | 2009 | 2.4/5 GHz | 600 Mbps | MIMO, 40 MHz channels |
| 802.11ac (Wi-Fi 5) | 2014 | 5 GHz | 6.9 Gbps | MU-MIMO, 160 MHz, 256-QAM |
| 802.11ax (Wi-Fi 6/6E) | 2020 | 2.4/5/6 GHz | 9.6 Gbps | OFDMA, 1024-QAM, TWT |
| 802.11be (Wi-Fi 7) | 2024 | 2.4/5/6 GHz | 46 Gbps | 320 MHz, 4096-QAM, MLO |
Key Technologies
OFDM (Orthogonal Frequency-Division Multiplexing): Divide the channel into many narrow subcarriers. Each carries data. Robust against multipath fading.
OFDMA (Wi-Fi 6): Multiple users share subcarriers simultaneously. Like cellular LTE but for Wi-Fi. Better in dense environments.
MIMO (Multiple-Input Multiple-Output): Multiple antennas for parallel data streams. 2×2, 4×4, 8×8 configurations. Linear throughput increase.
MU-MIMO (Multi-User MIMO): Serve multiple clients simultaneously using spatial multiplexing.
Beamforming: Focus the signal toward specific clients using antenna phase control. Better range and throughput.
TWT (Target Wake Time): Schedule wake times for IoT devices. Saves power — device sleeps until its scheduled transmission time.
MLO (Multi-Link Operation, Wi-Fi 7): Use multiple bands simultaneously (e.g., 2.4 + 5 + 6 GHz). Higher throughput and lower latency.
2.4 GHz vs 5 GHz vs 6 GHz
| Band | Range | Channels | Congestion |
|---|---|---|---|
| 2.4 GHz | Long (~100m) | 3 non-overlapping | High (many devices) |
| 5 GHz | Medium (~50m) | 25 non-overlapping | Moderate |
| 6 GHz | Short (~30m) | 59 non-overlapping | Low (Wi-Fi 6E+ only) |
6 GHz (Wi-Fi 6E): 1200 MHz of new spectrum. Less congestion. Wide channels (160/320 MHz). No legacy devices.
Bluetooth
Short-range wireless for personal area networks.
| Version | Max Speed | Range | Key Feature |
|---|---|---|---|
| 1.0 | 1 Mbps | 10m | Basic voice/data |
| 2.0+EDR | 3 Mbps | 10m | Enhanced data rate |
| 4.0 (BLE) | 1 Mbps | 100m | Low Energy — IoT |
| 5.0 | 2 Mbps | 300m | Long range, broadcast |
| 5.2 | 2 Mbps | — | LE Audio, LC3 codec |
| 5.3 | 2 Mbps | — | Channel classification, connection subrating |
BLE (Bluetooth Low Energy): Designed for IoT. Very low power (coin cell battery lasts years). Short bursts of data. Used in: fitness trackers, beacons, smart home, medical devices.
Bluetooth profiles: A2DP (audio streaming), HFP (hands-free), HID (keyboard/mouse), GATT (BLE services).
Other Short-Range Wireless
Zigbee (802.15.4): Low-power, low-data-rate mesh networking. 250 kbps. Smart home (lights, sensors, locks). Mesh routing enables large networks.
Z-Wave: Proprietary. Low-power mesh for home automation. 100 kbps. Sub-GHz band (less interference).
Thread: IPv6-based mesh networking for IoT. Built on 802.15.4. Used by Matter (smart home standard).
Matter: Application-layer protocol for smart home interoperability. Works over Wi-Fi, Thread, Ethernet. Backed by Apple, Google, Amazon, Samsung.
Long-Range Wireless
LoRa / LoRaWAN: Long range (2-15 km urban, 45+ km rural), very low data rate (0.3-50 kbps), very low power. For IoT sensors, agriculture, smart cities.
Sigfox: Ultra-narrow band. Very long range. Very low data rate (100 bps). For simple sensor data.
NB-IoT (Narrowband IoT): Cellular-based. Operated by carriers. Better coverage than LoRa in urban areas.
LTE-M (LTE for Machines): Cellular IoT. Higher data rate than NB-IoT (1 Mbps). Supports voice.
Cellular Networks
Generations
| Gen | Year | Peak Speed | Key Technology |
|---|---|---|---|
| 2G (GSM) | 1991 | 14.4 kbps (voice focus) | TDMA/FDMA |
| 3G (UMTS) | 2001 | 2 Mbps → 42 Mbps (HSPA+) | CDMA, WCDMA |
| 4G (LTE) | 2009 | 100 Mbps → 1 Gbps (LTE-A) | OFDMA, MIMO |
| 5G NR | 2019 | 20 Gbps (theory) | mmWave, massive MIMO, beamforming |
5G
Three pillars:
- eMBB (enhanced Mobile Broadband): High speed (streaming, VR). mmWave bands (24-100 GHz).
- URLLC (Ultra-Reliable Low-Latency): <1ms latency. Autonomous vehicles, remote surgery.
- mMTC (massive Machine-Type Communication): Millions of IoT devices per km².
mmWave: Very high bandwidth but very short range (~200m). Easily blocked by obstacles (walls, trees, rain). Requires dense small cell deployment.
Sub-6 GHz: Better range than mmWave. Good balance of speed and coverage. Most common 5G deployment.
Mobile IP and Handoff
Handoff (handover): Transfer an active connection from one cell tower to another as the user moves.
Hard handoff: Break connection, then reconnect. Brief interruption. Used in 2G/3G.
Soft handoff: Connect to new tower before disconnecting from old. No interruption. Used in 3G CDMA, 4G, 5G.
Ad-Hoc and Mesh Networks
Ad-Hoc Networks
Decentralized — no access point. Devices communicate directly with each other.
MANET (Mobile Ad-hoc Network): Nodes are mobile. Topology changes dynamically. Routing protocols: AODV, DSR, OLSR.
Mesh Networks
Nodes relay traffic for each other. Self-configuring, self-healing. If one node fails, traffic routes around it.
Examples: Google Nest Wi-Fi (home mesh), Thread (IoT mesh), military tactical networks.
Mesh routing: Each node is both a client and a router. Multi-hop paths to the gateway.
Wireless Challenges
Hidden terminal problem: A can hear B. C can hear B. But A can't hear C. A and C transmit simultaneously → collision at B.
Solution: RTS/CTS (Request to Send / Clear to Send). Virtual carrier sensing.
Exposed terminal problem: B transmits to A. C wants to transmit to D (away from A). C hears B and defers — unnecessarily.
Multipath fading: Reflected signals arrive at different times, causing constructive/destructive interference. OFDM and MIMO mitigate this.
Near-far problem: A nearby transmitter drowns out a distant transmitter at the receiver. Power control needed.
Applications in CS
- Mobile apps: Understanding Wi-Fi vs cellular characteristics helps design responsive apps.
- IoT: Choose protocol based on range, power, data rate (BLE for wearables, LoRa for agriculture, Wi-Fi for cameras).
- Smart home: Matter/Thread for interoperability. Zigbee mesh for reliability.
- Cloud/edge: 5G MEC (Multi-access Edge Computing) places compute at cell towers.
- AR/VR: Wi-Fi 7 (low latency, high bandwidth) enables wireless VR headsets.
- Industrial IoT: Time-sensitive networking over 5G URLLC for factory automation.