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Gradient Clock Synchronization in Wireless Sensor Networks. Philipp Sommer Roger Wattenhofer. Time Synchronization is a well-studied Problem. Time, Clocks, and the Ordering of Events in a Distributed System L. Lamport, Communications of the ACM, 1978.

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time synchronization is a well studied problem
Philipp Sommer, ETH Zurich @ IPSN'09Time Synchronization is a well-studied Problem
  • Time, Clocks, and the Ordering of Events in a Distributed SystemL. Lamport, Communications of the ACM, 1978.
  • Internet Time Synchronization: The Network Time ProtocolD. Mills, IEEE Transactions on Communications, 1991
  • Reference Broadcast Synchronization (RBS)J. Elson, L. Girod and D. Estrin, OSDI'02
  • Timing-sync Protocol for Sensor Networks (TPSN)S. Ganeriwal, R. Kumar and M. Srivastava, SenSys'03
  • Flooding Time Synchronization Protocol (FTSP)M. Maróti, B. Kusy, G. Simon and Á. Lédeczi, SenSys'04
  • and many more ...

State-of-the-art time syncprotocolforwirelesssensornetworks

preview ftsp vs gtsp
Philipp Sommer, ETH Zurich @ IPSN'09Preview: FTSP vs. GTSP
  • Gradient Time Synchronization Protocol (GTSP)
    • Details will followsoon
  • Network synchronizationerror(global skew)
    • Pair-wisesynchronizationerrorbetweenanynodes in thenetwork

NEW

FTSP (avg: 7.7 μs)

GTSP (avg: 14.0 μs)

preview ftsp vs gtsp 2
Philipp Sommer, ETH Zurich @ IPSN'09Preview: FTSP vs. GTSP (2)
  • NeighborSynchronizationerror(localskew)
    • Pair-wisesynchronizationerrorbetweenneighboringnodes
  • Synchronizationerrorbetweentwodirectneighbors

FTSP (avg: 15.0 μs)

GTSP (avg: 2.8 μs)

time in sensor networks
Philipp Sommer, ETH Zurich @ IPSN'09Time in Sensor Networks
  • Common time is essential formanyapplications:
    • Assigning a global timestamptosenseddata/events
    • Co-operationof multiple sensornodes
    • Preciseeventlocalization (e.g., shooterdetection)
    • Coordinationofwake-upandsleepingtimes (energyefficiency)
    • TDMA-based MAC layer

Global

Global

Local

Local

Local

outline
Philipp Sommer, ETH Zurich @ IPSN'09Outline
  • Introduction
  • ClockSynchronization Basics
  • Gradient Time Synchronization Protocol (GTSP)
  • Evaluation
  • Conclusions
sensor node clocks
Philipp Sommer, ETH Zurich @ IPSN'09Sensor NodeClocks
  • Eachnodehas a hardwareclockH(t)
    • Counter registerofthemicrocontroller
    • Crystal quartzoscillator (e.g., 32kHz, 7.37 MHz)
    • Subjecttoclockdrift (30 ppm)
  • Eachnodehas a logicalclockL(t)
    • Holdstheestimationofthecurrent global time
    • Computedas a functionofthecurrenthardwareclockH(t)
    • Logical clock rate
clock synchronization algorithm
Philipp Sommer, ETH Zurich @ IPSN'09ClockSynchronizationAlgorithm
  • Exchange messageswithcurrentclockvalue L(t) withothers
    • Adjustclockratesandoffset
    • Repeat thisprocessfrequently
  • Uncertainty (jitter) in themessagedelay
    • Varioussourcesoferrors (deterministicandundeterministic)
    • Can bereduced (but not eliminated) bytimestampingat MAC layer
theoretical bounds on the synchronization accuracy
Philipp Sommer, ETH Zurich @ IPSN'09TheoreticalBounds on theSynchronizationAccuracy
  • Two nodes u and v cannot be synchronized perfectly
    • Worst-case example:
  • Error increaseswithdistancefromthereferencenode
  • Lowerboundresultfromtheoreticalwork
    • Clockerrorbetweennodesdistanced apart depends on thenetworkdiameterD:
    • R. Fan and N. Lynch. Gradient Clock Synchronization. In PODC ’04: Proceedings of the twenty-third annual ACM symposium on principles of distributed computing, 2004.

u

0

1

2

3

μ

v

0

1

2

3

u

0

1

2

3

μ + ε

v

0

1

2

3

gradient clock synchronization
Philipp Sommer, ETH Zurich @ IPSN'09Gradient ClockSynchronization
  • Global property: Minimize clock error between any two nodes
  • Local (“gradient”) property: Small clock error between two nodes if the distance between the nodes is small.

FTSP

GTSP

gradient time synchronization protocol gtsp
Philipp Sommer, ETH Zurich @ IPSN'09Gradient Time Synchronization Protocol (GTSP)
  • Synchronizewithallneighboringnodes
    • Broadcast periodic time beacons, e.g., every 30 s
    • Noreferencenodenecessary
  • Howtosynchronizeclockswithouthaving a leader?
    • Follow thenodewiththe fastest/slowestclock?
    • Idea: Go totheaverageclockvalue/rate of all neighbors (includingnodeitself)
drift and offset compensation in gtsp
Philipp Sommer, ETH Zurich @ IPSN'09Drift and Offset Compensation in GTSP
  • Update ruleforthelogicalclock rate:
  • Update ruleforthelogicalclockoffset:
    • Note: We will jump directlyto a higherclockvalueiftheoffsetexceeds a certainthreshold, e.g., 20 μs.
experimental evaluation
Philipp Sommer, ETH Zurich @ IPSN'09Experimental Evaluation
  • Mica2 platformusingTinyOS 2.1
    • System clock: 7.37 MHz (crystalquartz)
    • Hardware clock: System clockdividedby 8 = 921 kHz
    • Clockgranularityof 1 microsecond (1 clock tick ≈ 1 μs)
  • Testbedof 20 Mica2 nodes
    • Base stationtriggersexternaleventsbysending time probe packets
    • Ring topologyisenforcedbysoftware
experimental results
Philipp Sommer, ETH Zurich @ IPSN'09Experimental Results
  • Network synchronizationerror(global clockskew)
    • 7.7 μs with FTSP, 14.0 μs with GTSP
  • FTSP needsmore time tosynchronize all nodes after startup

FTSP

GTSP

experimental results 2
Philipp Sommer, ETH Zurich @ IPSN'09Experimental Results (2)
  • Neighborsynchronizationerror(localclockskew)
    • 5.3 μs with FTSP, 4.0 μs with GTSP

FTSP

GTSP

neighbor synchronization error ftsp vs gtsp
Philipp Sommer, ETH Zurich @ IPSN'09NeighborSynchronization Error: FTSP vs. GTSP
  • FTSP has a large clockerrorforneighborswith large stretch in thetree (Node 8 andNode 15)

FTSP

GTSP

multi hop time synchronization in practice
Philipp Sommer, ETH Zurich @ IPSN'09Multi-Hop Time Synchronization in Practice
  • Is thisreally a problem in practice?
    • Ring topologyof 20 nodesseemstobe „artificial“!?
  • Finding a tree-embeddingwithlowstretchishard
    • In a n = m*m gridyou will havetwoneighborswith a stretchofat least
    • Example: FTSP on a 5x4 gridtopology
    • Node 2 and 7 have a distanceof 13 hops!
simulation results
Philipp Sommer, ETH Zurich @ IPSN'09Simulation Results
  • Simulation of GTSP for larger networktopologies
    • Network errorof ~1 msfor 100 nodes in a linetopology
    • Neighborerrorbelow 100 μs forthe same topology
conclusions and future work
Philipp Sommer, ETH Zurich @ IPSN'09Conclusionsand Future Work
  • Gradient Time Synchronization Protocol (GTSP)
    • Distributed time synchronizationalgorithm (noleader)
    • Improvesthesynchronizationerrorbetweenneighboringnodeswhile still providingprecisenetwork-widesynchronization
    • Bridgingthegapbetweentheoryandpractice
  • Is there a „perfect“ clocksynchronizationprotocol?
    • Goal: Minimizinglocaland global skewatthe same time