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

2. 6/21/2015Nirupama Bulusu2 Source Acknowledgements Alberto Cerpa and Deborah Estrin Alec Woo and David Culler Jerry Zhao and Ramesh Govindan

3. 6/21/2015Nirupama Bulusu3 Outline IEEE 802.15.4 Wireless Communication Standard Single Hop packet loss characteristics –Axes Environment, distance, transmit power, temporal correlation, data rate, packet size

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. 6/21/2015Nirupama Bulusu18 Outline IEEE 802.15.4 Wireless Communication Standard Single Hop packet loss characteristics –Axes Environment, distance, transmit power, temporal correlation, data rate, packet size

19. 6/21/2015Nirupama Bulusu19 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

20. 6/21/2015Nirupama Bulusu20 Indoor is the Harshest

21. 6/21/2015Nirupama Bulusu21 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

22. 6/21/2015Nirupama Bulusu22 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??

23. 6/21/2015Nirupama Bulusu23 ChipCon Radio (Cerpa et al.) 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 Mica On Ceiling

24. 6/21/2015Nirupama Bulusu24 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?

25. 6/21/2015Nirupama Bulusu25 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)

26. 6/21/2015Nirupama Bulusu26 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

27. 6/21/2015Nirupama Bulusu27 Packet Loss vs. RSSI Low packet loss => good RSSI –But not vice versa –Too high a threshold limits number of links Network partition??

28. 6/21/2015Nirupama Bulusu28 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

29. 6/21/2015Nirupama Bulusu29 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

30. 6/21/2015Nirupama Bulusu30 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?