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.

| 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.