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CS 410/510 Sensor Networks Portland State University Lecture 3 Wireless Communication Source Acknowledgements • Alberto Cerpa and Deborah Estrin • Alec Woo and David Culler • Jerry Zhao and Ramesh Govindan 5/25/2017 Nirupama Bulusu 2 Outline • IEEE 802.15.4 Wireless Communication Standard • Single Hop packet loss characteristics – Axes • Environment, distance, transmit power, temporal correlation, data rate, packet size 5/25/2017 Nirupama Bulusu 3 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 History • Combination of Two Standards Groups – ZigBee Alliance: “an association of companies working together to enable reliable, costeffective, 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 System Layering High-Level Characteristics Network Layer Guidelines • 802.15.4 Specification does not address Network Layer • Expected to be self-organizing and selfmaintaining to minimize cost to user • Two Network Topologies Supported: – Star Topologies – Peer-to-Peer Topologies Topology Formations 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 MAC Frame Format 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 Additional MAC Features • Channel Access Mediums – Slotted CSMA-CA – Unslotted CSMA-CA • Acknowledgements • Security – No security – Access Control Lists – Symmetric Key Security 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 Physical Layer Packet Structure 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 MicaZ and Sun SPOT Platforms Outline • IEEE 802.15.4 Wireless Communication Standard • Single Hop packet loss characteristics – Axes • Environment, distance, transmit power, temporal correlation, data rate, packet size 5/25/2017 Nirupama Bulusu 18 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 5/25/2017 Nirupama Bulusu 19 Indoor is the Harshest 5/25/2017 Nirupama Bulusu 20 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 5/25/2017 Nirupama Bulusu 21 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?? 5/25/2017 Nirupama Bulusu 22 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 5/25/2017 Nirupama Bulusu 23 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 Nirupama Bulusu – Better coding for start symbol? 5/25/2017 24 Loss Variation (Cerpa et al.) • Variation over distance and over time – binomial approximation for variation over time? • Zhao shows that SECDED helps decrease the 5/25/2017 Nirupama Bulusu variation over distance (but very large SD here)25 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 5/25/2017 Nirupama Bulusu 26 Packet Loss vs. RSSI • Low packet loss => good RSSI – But not vice versa – Too high a threshold limits number of links • Network partition?? 5/25/2017 Nirupama Bulusu 27 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 5/25/2017 Nirupama Bulusu 28 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 5/25/2017 Nirupama Bulusu 29 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? 5/25/2017 Nirupama Bulusu 30