Clock Synchronization in Sensor Networks*

Clock Synchronization in Sensor Networks*

**Timing-sync Protocol for Sensor Networks (SenSys 2003)Saurabh Ganeriwal, Ram Kumar & Mani B. Srivastava

**Fine-Grained Network Time Synchronization using Reference Broadcasts Jeremy Elson, Lewis Girod & Deborah Estrin (OSDI 2002)

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IntroductionIntroduction

Sensor nodes maintain local clocks Typically lower precision clocks (crystal

oscillators) Clocks need to be synchronized to determine

Exact timing of events Chronology of events

Clock synchronization Global : all nodes are synchronized to a single

clock Example: NTP (often used in LANS)

Local : a set of “ localized” nodes are synchronized Reference Broadcast Synchronization (RBS)

Post-facto Synchronization

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ChallengesChallenges

Scalability Must be scalable to a large number of nodes

Energy Efficiency Limited messaging GPS technology cannot be employed

Requirements are more stringent Microsecond precision desirable Local : a set of “ localized” nodes are

synchronize Clock skew is large

Two node clocks might drift 1–100µsec per second

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Clock Synchronization Models Clock Synchronization Models

Maintain relative notion of time Generate the right chronology of events Clocks need not be synchronized

Relative Clocks Nodes maintain individual clocks Maintain relative drift information with other

clocks RBS

“Always On” Model Every clock is synchronized to a reference clock Superset of the above models TSPN

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Today’s TalkToday’s Talk

TSPN : Time-Sync Protocol for Sensor Networks Sender-Receiver synchronization (classical

approach) needs MAC level integration Always-On

RBS : Reference Broadcast Synchronization Receiver-Receiver synchronization Could be implemented at user-level Relative Sync

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Packet Delay (Uncertainty) Model Packet Delay (Uncertainty) Model

Sender Uncertainty Send, Access, Transmission

Propagation Uncertainty Negligible Speed thru medium = 1foot/nanosecond =

c Receiver Uncertainty

Packet reception Packet processing

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TSPNTSPN

Clock Sync Algorithm involves 2 steps Level Discovery phase Synchronization phase

TSPN makes the following assumptions Sensor nodes have unique identifiers Node is aware of its neighbors Bi-directional links Creating the hierarchical tree is not

exclusive to TSPN

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TSPN: Level DiscoveryTSPN: Level Discovery

Algorithm Root node is assigned level i = 0

broadcasts “level discovery pkt.” Neighbors get packet and assign level i+1 to

themselves Ignore discovery pkts. once level has been

assigned Broadcast “level discovery pkt.” with own level

STOP when all nodes have been assigned a level

Optimization Use minimum spanning trees instead of flooding

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TSPN: Synchronization phaseTSPN: Synchronization phase

Synchronization using handshake between a pair of nodes (sender-initiated)

Level #Value of T1 Level #

T1, T2, T3

Assuming no clock drift and propagation delay do not change

Clock drift

Delay

A can now synchronize with B

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TSPN: SynchronizationTSPN: Synchronization

Algorithm Root node initiates the protocol

broadcasts “time sync pkt.” Nodes at level = 1

Wait for a random time, initiate time-sync with root On receiving ACK .. Synchronize with root

Nodes at level = 2 overhear sync from nodes at level 1

Do a random back-off, initiate time-sync with level 1 node

Nodes sends ACK only if it has been synchronized If a node does not receive an ACK, resend time-sync

pulse after a timeout.

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RBS: Reference Broadcast SynchronizationRBS: Reference Broadcast Synchronization

Eliminate “non-deterministic latency at the sender NIC delay Protocol processing

Critical path length reduced

Packets do not need to store explicit timing information

Relative time synchronization

RBS

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RBS: How it Works?RBS: How it Works?

Characterize Receiver Error Determine Phase Offset between nodes Determine Clock Skew between nodes Empirically determined

With knowledge of phase offset and clock skew, the clocks of any two nodes can be synchronized

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RBS: Receiver Error CharacterizationRBS: Receiver Error Characterization

Histogram showing the distribution of inter-receiver phase offsets recorded for 1,478 broadcast packets, grouped into 1µsec buckets. The curve is a plot of the best-fit Gaussian parameters. (µ = 0;s = 11:1µsec, confidence=99.8%)

Accuracy can be increased by increasing the number of sync messages

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RBS: Phase Offset EstimationRBS: Phase Offset Estimation

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RBS: Phase Offset EstimationRBS: Phase Offset Estimation

Analysis of expected group dispersion (i.e., maximum pair-wise error) after reference-broadcast synchronization. Each point represents the average of 1,000 simulated trials. The underlying receiver is assumed to report packet arrivals with error conforming to last figure. Mean group dispersion, and standard deviation from the mean, for a 2-receiver (bottom) and 20-receiver (top) group.

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RBS: Phase Offset with SkewRBS: Phase Offset with Skew

Each point represents the phase offset between two nodes as implied by the value of their clocks after receiving a reference broadcast. A node can compute a least-squared error fit to these observations (diagonal line), and convert time values between its own clock and that of its peer. Synchronization of the Mote’s internal clock

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RBS: Multi-hop Time SynchronizationRBS: Multi-hop Time Synchronization

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RBS: Multi-hop Time SynchronizationRBS: Multi-hop Time Synchronization

Example, we can compare the time of E1(R1) with E10(R10) by transformingE1(R1) ► E1(R4) ► E1(R8) ► E1(R10).Conversions can be made automatically by using the shortest path algorithm

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Error Analysis: TPSN & RBSError Analysis: TPSN & RBS

Analyze sources of error for the algorithms

Compare TPSN and RBS Trade-Offs

Level #Value of T1 Level #

T1, T2, T3

Clock drift

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Error Comparison: TPSN & RBSError Comparison: TPSN & RBS

TPSN

RBS

Comparison/Discussion:

1. Sender delays2. Receiver delays3. Relative Skews

Clock Synchronization in Sensor Networks*

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