5. Wireless Personal Area Networks (WPANs)
Definition: WPANs are short-range wireless networks (typically ≤ 10 m) designed for interconnection of personal devices (e.g., phones, laptops, sensors) with low power consumption and low cost.
5.1 Bluetooth
Overview:
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Short-range (2.4 GHz ISM band), FHSS (Frequency-Hopping Spread Spectrum) technology.
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Versions: Classic Bluetooth (BR/EDR), Bluetooth Low Energy (BLE, 4.0+).
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Data Rates: Classic: 1–3 Mbps; BLE: 0.27–2 Mbps.
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Topologies: Piconet (basic) and Scatternet (interconnected piconets).
Piconet:
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Master: Controls synchronization, clock, and hopping sequence.
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Slaves: Up to 7 active slaves (parked mode allows up to 255 devices).
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Communication: Master–slave only; slaves communicate via master.
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Range: ~10 m (Class 2), up to 100 m (Class 1).
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Applications: File transfer, audio streaming (headsets), peripherals.
Scatternet:
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Multiple piconets interconnected via bridge devices (device acts as master in one piconet, slave in another).
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Complexity: Requires time-division multiplexing across piconets; no standard scheduling.
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Scalability: Higher than piconet, but limited by interference and protocol overhead.
Comparison: Piconet vs. Scatternet
| Feature | Piconet | Scatternet |
|---|---|---|
| Scalability | Max 7 active devices | Higher (via multiple piconets) |
| Coverage | Limited to single master range | Extended via inter-piconet links |
| Device Interaction | Simple master-slave | Complex (role switching, sync) |
| Synchronization | Single master clock | Multiple clocks, challenging |
| Throughput | Higher (no inter-piconet overhead) | Lower due to time-sharing |
[!TIP] Exam Focus: Bluetooth scatternet is rarely implemented due to scheduling complexity; most real-world use is single piconet or BLE mesh (different from scatternet).
5.2 IEEE 802.15 WPAN Standard
Overview:
IEEE 802.15 defines standards for WPANs with low power, low cost, short-range communication. Key task groups:
| Task Group | Focus | Data Rate | Key Applications |
|---|---|---|---|
| 802.15.1 | Bluetooth (legacy) | 1–3 Mbps | Audio, file transfer |
| 802.15.3 | High-Rate WPAN (HR-WPAN) | Up to 480 Mbps | Multimedia, high-speed IoT |
| 802.15.4 | Low-Rate WPAN (LR-WPAN) | 250 kbps (2.4 GHz) | Sensor networks, automation |
| 802.15.6 | Body Area Networks (BAN) | ≤ 10 Mbps | Healthcare, wearables |
Key Characteristics of 802.15.4 (Zigbee/IEEE 802.15.4 basis):
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Frequency Bands: 2.4 GHz (global), 915 MHz (Americas), 868 MHz (Europe).
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Modulation: O-QPSK (2.4 GHz), BPSK (sub-GHz).
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Topologies: Star, peer-to-peer, cluster-tree, mesh.
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Device Types:
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Full-Function Device (FFD): Can be coordinator, router, or end device.
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Reduced-Function Device (RFD): Simple end device, limited functionality.
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MAC Layer: CSMA/CA, beacon-enabled/non-beacon modes, guaranteed time slots (GTS) for QoS.
[!TIP] Common Pitfall: 802.15.4 is the PHY/MAC standard; Zigbee builds on it with network/application layers. Do not confuse "802.15" with "Bluetooth" (802.15.1).
5.3 Zigbee Technology
Overview:
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Based on IEEE 802.15.4 (PHY/MAC) with added network (NWK) and application (APL) layers (Zigbee Alliance, now Connectivity Standards Alliance).
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Target: Low-power, low-data-rate, long-battery-life applications (months–years).
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Data Rate: 250 kbps (2.4 GHz), 40 kbps (915 MHz), 20 kbps (868 MHz).
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Range: 10–100 m (line-of-sight), extended via mesh routing.
Network Topologies:
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Star: Single coordinator; all devices communicate directly with coordinator.
- Use: Simple, low-latency, central control.
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Tree: Coordinator with routers forming hierarchical structure.
- Use: Scalable, multi-hop coverage.
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Mesh: Peer-to-peer, any FFD can route.
- Use: Robust, self-healing, high reliability.
Device Roles:
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Coordinator: Forms network, stores network info, may be mains-powered.
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Router: Extends network, relays data, may be mains/battery-powered.
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End Device: Sleeps most of time, wakes to transmit/receive; battery-powered.
Key Features:
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Low Power: Duty cycling, sleep modes (end devices).
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Large Network Size: Up to 65,000+ nodes (theoretical).
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Security: AES-128 encryption, integrity checks, access control lists.
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Latency: Low for star (≤ 30 ms), higher for multi-hop mesh.
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Cost: Low (simple RF, minimal protocol overhead).
Applications:
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Home automation (lighting, HVAC).
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Industrial monitoring (sensor networks).
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Smart metering, asset tracking.
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Healthcare (patient monitoring).
Comparison: Zigbee vs. Bluetooth (Classic/BLE)
| Feature | Zigbee (802.15.4) | Bluetooth Classic/BLE |
|---|---|---|
| Data Rate | Low (250 kbps max) | Higher (Classic: 3 Mbps; BLE: 2 Mbps) |
| Power | Very low (years on battery) | Moderate (BLE: months; Classic: days) |
| Network Size | Large (thousands) | Small (piconet: 7–8; BLE mesh: 32,000) |
| Latency | Higher (multi-hop) | Lower (direct) |
| Complexity | Simple, lightweight | Complex (FHSS, protocols) |
| Use Case | Sensor/automation networks | Audio, intermittent data |
[!TIP] Exam Key: Zigbee excels in low-power, large-scale sensor networks; Bluetooth is for intermittent, higher-data-rate device connectivity. BLE mesh (Bluetooth 5+) now competes with Zigbee in IoT but differs in protocol stack.
Summary for UNIT 5:
WPANs enable personal device connectivity. Bluetooth uses piconet/scatternet for ad-hoc audio/data. IEEE 802.15 provides the standard foundation (task groups for different needs). Zigbee (on 802.15.4) targets low-power, large-scale IoT with mesh topologies. Always distinguish between the standard (802.15) and implementations (Bluetooth, Zigbee).