RTLink: A TimeSynchronized Link Protocol for EnergyConstrained Multihop Wireless Networks

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RT-Link A Time-Synchronized Link Protocol for
Energy-Constrained Multi-hop Wireless Networks

• Anthony Rowe, Rahul Mangharam and Raj Rajkumar
• CMU
• SECON 2006

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Outline

• Introduction and Related works
• Protocol Overview
• Protocol Enhancements
• Implementation
• Evaluation
• Conclusion

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Introduction

• A MAC protocol for sensor network
• TDMA-based
• Hardware-based time synchronization
• Predictable lifetime (gt2 years)
• bounded end-to-end delay

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Keys for SensorNet MAC

• Lifetime
• Low duty cycles
• minimize the radio usage
• Delay
• Either keep sending
• Or co-operate very well (TDMA-based)
• Time synchronization is good
• But how to achieve with reasonable cost??

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Some MACs for SensorNet

• Asynchronous MAC B-MAC
• Berkeley MAC
• Carrier sense multiple access (CSMA) with low
  power listening (LPL)
• LPL
• Optional RTS-CTS

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Some MACs for SensorNet (Cont.)

• Loosely Synchronous
• S-MAC
• local sleep-wake schedules
• exchange synchronizing packets with neighbors
• Fully Synchronized
• TRAMA scheduled slot
• Assume time synchronization is an orthogonal
  problem
• Time-sync protocols for SensorNet
• Basic idea time packet exchange
• Suffer for link error rate (up to 50)

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Outline

• Introduction and Related works
• Protocol Overview
• Protocol Enhancements
• Implementation
• Evaluation
• Conclusion

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Protocol Overview
Communication cycle

• Each cycle
• 32 frames
• Each frame
• start by sync pulse
• 32 5ms slots
• Scheduled Slots (SS) for member nodes
• Contention Slots (CS) for guest nodes
• when a node is in transmission slot, its
  neighbors must listen to the channel
• Every member nodes must listen to all CS
• So, they need central control (Gateway)

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Protocol Operations
New Node
Send Hello to GW in CS
Software time-sync
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How to Enhance

• Basic idea of scheduling
• k-hop coloring
• Topology Control
• Degree high gt energy consumption high
• Gathering topology and prune nodes

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Interference-free Scheduling

• Observations
• Jammer-RX distance is critical
• Conclusion
• Jammer has no effect beyond 2(SRD)
• Concurrent transmitter may be placed 3(SRD)

Stable Reception distance (SRD)
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Coloring and Ordering

• Max concurrency vs delay sensitive
• Get minimum delay NP-complete

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Heuristic

• Dijkstras shortest path ? spanning tree
• BFS from G, assign smallest color that satisfy
  2-hop constraint
• Replace the leaves with larger value
• Inverts the numbers

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Outline

• Introduction and Related works
• Protocol Overview
• Protocol Enhancements
• Implementation
• Evaluation
• Conclusion

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Hardware Platform

• CMU FireFly 8-bit Micron IEEE802.15.4

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Time Synchronization outdoor

• WWVB atomic broadcast (Wiki)
• WWVB is a special NIST time signal radio station
  near Fort Collins, Colorado
• WWVB is the station that radio-controlled clocks
  throughout North America use to synchronize
  themselves.
• The signal transmitted from WWVB is a continuous
  60 kHz carrier wave, derived from a set of atomic
  clocks located at the transmitter site
• Off-the-shelf WWVB receiver

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Picture
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Time Synchronization Indoor

• Carrier-current AM broadcast
• Low power AM broadcasting for campus
• Not licensed in US
• Use building power lines as antenna

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Time Synchronization Indoor

• A modified FireFly receive outdoor atomic clock
• Transmit to AM station
• A (commercial AM receiver) (custom interface
  board) receive AM broadcast and sync the FireFly

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Performance

• 5 nodes
• Separate several hundred feet
• Hard wired

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TDMA Slot Mechanics
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Outline

• Introduction and Related works
• Protocol Overview
• Protocol Enhancements
• Implementation
• Evaluation
• Conclusion

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Energy Model over a frame
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Energy Model over a frame

• Tframe Tactive Tidle

Active time over a frame
Minimum energy consumption
Maximum energy consumption
Lifetime
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Lifetime Comparisons

• Sample Interval
• Event generation rate
• Check Interval
• Time between two consecutive LPL

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Lifetime Comparisons
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Lifetime Comparisons

• For high sample rate, hardware sync doesnt help

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Delay

• Simulation setup
• Topology
• Only leaf node generate traffic
• Total traffic fixed to 1000 1-byte packet
• Packet aggregation up to 100 bytes

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Delay -- Result
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Real Deployment
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Conclusion

• Hardware time sync, nice idea
• Not so difficult to implement
• High data rate
• maybe software time sync is enough
• Low data rate
• could software time sync achieve similar
  performance with hardware one? FireFly??