Timing-sync Protocol for Sensor Networks by Saurabh Ganeriwal, Ram Kumar, Mani B. Srivastava

Presented by: Chinh VuInstructor: Dr. Yingshu Li

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OutlineIntroduction

System model

The protocol

Error analysis

Experiment result

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IntroductionTiming-sync Protocol for Sensor Networks - “sender-receiver” timing synchronization

Comparison with “Reference Broadcast Synchronization (RBS)”, “receiver-receiver” timing synchronization.

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System modelThe network is “always-on”

Every node maintains 16-bit register as clock

Sensor has unique ID

Build hierarchical topology for the network

Node at level i can connect with at least one node at level i-1.

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The protocolLevel discovery: trivial

Synchronization phase: pair-wise sync is performed along the edge of hierarchical structure

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Two-way message exchange

✴T1: A is sender, starting sync by sending synchronization_pulse packet to B: T2 = T1 + ∆ + d where

•∆ is the clock drift•d is propagation delay

✴T2: B replies acknowledgement containing T1,T2,T3✴T4: A receive ack and T4 = T3 + ∆ + d. So:

•∆ = [(T2-T1) - (T4-T3)]/2•d = [(T2-T1) + (T4-T3)]/2

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Root node initiating the phase by broadcasting a time_sync packet.

Node at level 1 wait for some random time, and use two-way message exchange with the root node.

Nodes belonging to level 2 will overhear this message exchange.

Nodes in level 2 back off for some random time and continue process

The protocol

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Some special situationsNode does not have level:

Wait for some period.

Broadcasts a level_request message

Nodes at level i-1 die randomly

Nodes at level i would retransmit the synchronization_pulse for some random times.

if not success, do the “local level discovery” again.

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Decomposition of Packet Sender:

Send time: Application layer → MAC layer

Access time: wait for accessing the channel (MAC contention).

Transmission time: at physical layer

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Decomposition of Packet Propagation: packets traverse wireless link

Receive:

Reception time: Physical layer → MAC layer

Receive time: MAC layer → Application layer

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Error Analysis

Ti: local time

ti: global/real/ideal time

SA: send packet (send + access + transmission time) at node A

PA→B:propagation time between node A, B

RB: Receive packet (reception time + receive time) at node B

Dt: Drift between A and B at time t.11Sunday, September 28, 2008

Error Analysis (cont)Similarly: thus:

We further have:

(3)-(4):

Where:

Thus:12Sunday, September 28, 2008

Comparing with RBSTPSN error:

RBS error:

Where A, B receive the common packet from the same sender C and:

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Experiment - Sync Pair of MotesBuild on Berkeley Mote with TinyOS

Two nodes started randomly and are synced.

Result:

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A Pair: Synchronization error

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Multihop: Synchronization errorChain: A→B→C→D→E

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Auxiliary Benefits of TPSN

An average accuracy of synchronizing a pair of motes < 20 µs:

Average ranging error ≅(Speed of sound) * (Average timing synchronization error)≅ (345m/s)*(20µs) = 0.69cm

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