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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

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source acknowledgements
Source Acknowledgements
  • Alberto Cerpa and Deborah Estrin
  • Alec Woo and David Culler
  • Jerry Zhao and Ramesh Govindan

Nirupama Bulusu

outline
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

ieee 802 15 4 why the need
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
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
network layer guidelines
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
data link layer
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
superframe beacons
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
Additional MAC Features
  • Channel Access Mediums
    • Slotted CSMA-CA
    • Unslotted CSMA-CA
  • Acknowledgements
  • Security
    • No security
    • Access Control Lists
    • Symmetric Key Security
physical layer
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
other physical layer features
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
outline1
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

zhao s study of packet loss
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

indoor is the harshest
Indoor is the Harshest

Nirupama Bulusu

indoor is the harshest1
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

packet loss and distance
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

chipcon radio cerpa et al
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

can better coding help
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

loss variation cerpa et al
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

packet loss vs workload
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

packet loss vs rssi
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

other findings
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

packet size cerpa et al
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

lessons to take away
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