CS 410/510 Sensor Networks Portland State University

1. CS 410/510 Sensor NetworksPortland State University Lecture 3 Wireless Communication

2. Source Acknowledgements • Alberto Cerpa and Deborah Estrin • Alec Woo and David Culler • Jerry Zhao and Ramesh Govindan Nirupama Bulusu

3. Outline • IEEE 802.15.4 Wireless Communication Standard • Single Hop packet loss characteristics • Axes • Environment, distance, transmit power, temporal correlation, data rate, packet size Nirupama Bulusu

4. IEEE 802.15.4: Why the need? • Sensor and Personal Area Networks require • Low Power Consumption • Minimal Installation Cost • Low Overall Cost • Existing Technologies • Wired • 802.11 (WiFi) and Bluetooth

5. History • Combination of Two Standards Groups • ZigBee Alliance: “an association of companies working together to enable reliable, cost-effective, low-power, wirelessly networked, monitoring and control products based on an open global standard.” • IEEE 802 Working Group 15 • Task Group 4 formed in December 2000 • Low-rate Wireless Personal Area Network

6. System Layering

7. High-Level Characteristics

8. Network Layer Guidelines • 802.15.4 Specification does not address Network Layer • Expected to be self-organizing and self-maintaining to minimize cost to user • Two Network Topologies Supported: • Star Topologies • Peer-to-Peer Topologies

9. Topology Formations

10. Data Link Layer • Two Parts • Logical Link Control (LLC) • Standard among many 802.x standards • Communicates with MAC through SSCS • Proprietary LLC’s can communicate directly • MAC Sublayer • Data Service - Common Part Sublayer • Management Service – Management Entity

11. MAC Frame Format

12. Superframe Beacons • Time between beacons divided in 16 time slots • Can be used to provide bandwidth guarantees • Contention-free period and duration of superframe announced in beacon

13. Additional MAC Features • Channel Access Mediums • Slotted CSMA-CA • Unslotted CSMA-CA • Acknowledgements • Security • No security • Access Control Lists • Symmetric Key Security

14. Physical Layer • Two Potential Physical Layers • 868/915Mhz • 2.4Ghz • Direct Sequence Spread Spectrum • Same Packet Structure • 27 Frequency Channels Total • Dynamic Channel Selection left to network layer

15. Physical Layer Packet Structure

16. Other Physical Layer Features • Modulation • 868/915 – Binary Phase Shift Keying • 2.4 – Offset Quadrature Phase Shift Keying • Sensitivity and Range • 868/915  -92 dBm • 2.4  -85 dBm • 10-20m typical range

17. MicaZ and Sun SPOT Platforms

18. Outline • IEEE 802.15.4 Wireless Communication Standard • Single Hop packet loss characteristics • Axes • Environment, distance, transmit power, temporal correlation, data rate, packet size Nirupama Bulusu

19. Zhao’s Study of Packet Loss • Hardware • Mica, RFM 433MHz • MAC • TinyOS Mac (CSMA) • Encoding • Manchester (1:2) • 4b/6b (1:1.5) • SECDED (1:3) • Environment • Indoor, Open Structure, Habitat Environment Nirupama Bulusu

20. Indoor is the Harshest Nirupama Bulusu

21. Indoor is the Harshest • Linear topology over a hallway (0.5/0.25m spacing) • 40% of the links have quality < 70% • Lower transmit power • yields smaller tail distribution • SECDEC • significantly helps to lower the heavy tail Nirupama Bulusu

22. Packet Loss and Distance • Gray/Transitional Area • ranges from 20% to 50% of the communication range • Habitat has smaller communication range? • Other evidence (Cerpa et al., Woo et al.) • RFM: BAD RADIO?? Nirupama Bulusu

23. ChipCon Radio (Cerpa et al.) Mica On Ceiling • Higher transmit power doesn’t eliminate transitional region • Range in (a) and (b) are the same? • Indoor RFM result is worst than that in Zhao’s work • cannot even see the effective region Nirupama Bulusu

24. Can better coding help? • SECDED is effective if start symbol is detected but does not increase “communication range” • Bit error rate (BER) is higher in transitional region • Missing start symbol is fatal • Better coding for start symbol? Nirupama Bulusu

25. Loss Variation (Cerpa et al.) • Variation over distance and over time • binomial approximation for variation over time? • Zhao shows that SECDED helps decrease the variation over distance (but very large SD here) Nirupama Bulusu

26. Packet Loss vs. Workload • Packet loss increases as network load increases • But what is the network load? • How many nodes are in range? • Not sure! • Is 0.5 packets/s already in saturation? • Difficult to observe is it hidden node terminal Nirupama Bulusu

27. Packet Loss vs. RSSI • Low packet loss => good RSSI • But not vice versa • Too high a threshold limits number of links • Network partition?? Nirupama Bulusu

28. Other Findings • Correlation of Packet Loss • correlation at the gray (transitional) region for indoor • Habitat: much less • Independent losses are reasonable • 50%-80% of the retransmissions are wasted • Neighbor = hear a node once • Asymmetric links are common • > 10% of link pairs have link quality difference > 50% • Cerpa et al. • Moving a little bit doesn’t help • Swap the two nodes, asymmetrical link swaps too • i.e. not due to the environment Nirupama Bulusu

29. Packet Size (Cerpa et al.) • Loss over distance is relatively the same for different packet size (25 bytes and 150 bytes) at different transmit power Nirupama Bulusu

30. Lessons to Take Away • Who to blame? • Radio? • Similar results found over RFM and ChipCon radio • Hardware calibration! Yeah!  • Base-band radio • Multi-path will remain unless spread-spectrum radio is used • But 802.11 is also not ideal (Decouto et al. Mobicom 03) • What is the effective communication range? • What does it mean when you deploy a network • What defines a neighbor? • Why study high density sensor network? Nirupama Bulusu