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

CS 410/510 Sensor Networks Portland State University

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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. Outline. IEEE 802.15.4 Wireless Communication Standard - PowerPoint PPT Presentation

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Page 1: CS 410/510 Sensor Networks Portland State University

CS 410/510 Sensor NetworksPortland State University

Lecture 3

Wireless Communication

Page 2: CS 410/510 Sensor Networks Portland State University

04/19/23 Nirupama Bulusu 2

Source Acknowledgements

• Alberto Cerpa and Deborah Estrin

• Alec Woo and David Culler

• Jerry Zhao and Ramesh Govindan

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Outline

• IEEE 802.15.4 Wireless Communication Standard

• Single Hop packet loss characteristics– Axes

• Environment, distance, transmit power, temporal correlation, data rate, packet size

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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

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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

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System Layering

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High-Level Characteristics

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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

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Topology Formations

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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

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MAC Frame Format

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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

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Additional MAC Features

• Channel Access Mediums– Slotted CSMA-CA– Unslotted CSMA-CA

• Acknowledgements

• Security– No security– Access Control Lists– Symmetric Key Security

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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

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Physical Layer Packet Structure

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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

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MicaZ and Sun SPOT Platforms

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Outline

• IEEE 802.15.4 Wireless Communication Standard

• Single Hop packet loss characteristics– Axes

• Environment, distance, transmit power, temporal correlation, data rate, packet size

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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

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Indoor is the Harshest

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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

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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??

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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

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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?

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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)

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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

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Packet Loss vs. RSSI

• Low packet loss => good RSSI– But not vice versa– Too high a threshold limits number of links

• Network partition??

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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

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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

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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?