CS 410/510 Sensor Networks Portland State University

1
CS 410/510 Sensor NetworksPortland State
University

• Lecture 3
• Wireless Communication

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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 LLCs 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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Zhaos Study of Packet Loss

• Hardware
• Mica, RFM 433MHz
• MAC
• TinyOS Mac (CSMA)
• Encoding
• Manchester (12)
• 4b/6b (11.5)
• SECDED (13)
• 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 lt 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.)
Mica On Ceiling

• Higher transmit power doesnt eliminate
  transitional region
• Range in (a) and (b) are the same?
• Indoor RFM result is worst than that in Zhaos
  work
• cannot even see the effective region

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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 gt 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
• gt 10 of link pairs have link quality difference
  gt 50
• Cerpa et al.
• Moving a little bit doesnt 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?