Ryad Ben-El-Kezadri, Giovanni PauNetwork Research Lab, UCLA

TimeRemap: Stable & Accurate Time in Vehicular Networks

REVE Workshop - 2010

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Outline

• Problem & Application

• Available clocks on nodes

• TimeRemap algorithm

• Testbed & Results

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Outline

• Problem & Application– Problem

– Application & constraints

– Scenarios

• Available clocks on nodes

• TimeRemap algorithm

• Testbed & Results

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Problem & Application > Problem

• General Problem

Vehicular nets Comm. : Off-the-shelf HW

– 802.11 NIC

Sync. : No extra HW – GPS receiver

Synch. clocks

Environment & constraints

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Problem & Application > Appli.

Network functioning

Contextualization/Distributed Sensing :

Evt ↔Time & Place

Network monitoring

Pollution monitoring

Time constraints >

Comm. constraints >

Synchronization

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Problem & Application > Constraints

• Time constraints– Inter-packet/2 : ~100 s

– Slot/2 : ~5 sPkt

Sync signalingimpossible

• Comm. constraints– No contact between

trackers

Nodes to track

Tracker

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Problem & Application > Scenarios

Google car

The largest RT system to attack

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Outline

• Problem & Application

• Available clocks on nodes

– Packet timestamping

– OS & NIC characterization

– Key idea

• TimeRemap algorithm

• Testbed & Results

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Available clocks > Timestamping

• A packet event e, a timestamping system S– When did e come at the antenna twalle / fS(twalle) ?

• Clock model

– fS(twalle) = a twalle + b + error

• Available systems : OS, NIC

OS

e

NIC

e

GPS

Probabilistic errorEx. Time to read the clock

Wallclock time modified

- fOS(twalle) = twalle + errorant-OS

- fNIC(twalle) = at twalle + bt

No good

No good

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Available clocks > OS characterization

1) Difference between OS time and wallclock for each e (phase)

0) Generate periodic e

Stable but,…

Bad precision

11σ

Available clocks > OS characterization

1) Difference between OS time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

0) Generate periodic e

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Available clocks > OS characterization

1) Difference between OS time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

0) Generate periodic e

σσ

Allan variation plot

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Available clocks > OS characterization

1) Difference between OS time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

0) Generate periodic e

2σ

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Available clocks > OS characterization

1) Difference between OS time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

0) Generate periodic e

2σσ

Allan variation plot

2σ

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Available clocks > OS characterization

1) Difference between OS time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

0) Generate periodic e

3σ

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Available clocks > OS characterization

1) Difference between OS time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

0) Generate periodic e

3σσ

Allan variation plot

2σ 3σ

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Available clocks > NIC characterization

1) Difference between NIC time and wallclock for each e (phase)

Good precision but,…

Drift

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Available clocks > NIC characterization

1) Difference between NIC time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

σ

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Available clocks > NIC characterization

1) Difference between NIC time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

σ

Allan variation plot

2σ 3σ

σ

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Available clocks > NIC characterization

1) Difference between NIC time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

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Available clocks > NIC characterization

1) Difference between NIC time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

σ

Allan variation plot

2σ 3σ2σ

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Available clocks > NIC characterization

1) Difference between NIC time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

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Available clocks > NIC characterization

1) Difference between NIC time and wallclock for each e (phase)

2) Compute phase variations over interval (freq.) & observe freq variations

σ

Allan variation plot

2σ 3σ3σ

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Available clocks > Summary & Idea

OS-stable-not precise

Construct a 3rd clockwhich leverages : -the stability of the OS -the precision of the NIC

NIC-not stable-precise

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Outline

• Problem & Application

• Available clocks on nodes

• TimeRemap algorithm

– Steps

– Zoom on regression

– Detection of group of outliers

– The TimeRemap “clock”

• Testbed & Results

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TimeRemap Algo > Steps

Capture OS & NIC timestamps

e’s

NIC tstampsOS tstamps

e’s

NIC tstampsOS tstamps

Data segmentation1 2

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TimeRemap Algo > Steps

e’s

NIC tstampsOS tstamps

Extract NIC & OS supports (regression)

for each segment3

e’s

NIC tstampsOS tstamps

Remap NIC tstamps to OSsupport for each segment

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supports

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TimeRemap Algo > Regression

1st Regression line (for selection)

NIC tstamps

OS tstamps

2nd Regression line y=ax+b (for remapping)

NIC tstamps

OS tstamps

☻☻

☻

Outlier suppression

3.a Compute support conversion parameters (a,b)

3.b

A two step regression over each segment

☻

☻☻

Timestampingdelay (probablistic)

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TimeRemap Algo > Group of outliers

Error between OS tstamps :

Error b/w remapped NIC tstamps :

Error b/w remapped NIC tstamps with detection of group of outliers:

e’s OS NIC

GPS

OS NIC

GPS

Group of outliers (GPS fix loss,…)

Reuse the support conversion parameters of previous segment

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TimeRemap Algo >Timeremap clock

TimeRemap service

NIC clock

TimeRemap “clock”

What time is it?

Global time (GT)

Local time (LT)

The TimeRemap “clock” can be used by

the OS to convert the NIC tsamps to GT

the NICs to convert a LT to GT & share it with other NICs

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Outline

• Problem & Application

• Available clocks on nodes

• TimeRemap algorithm

• Testbed & Results

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Testbed & Results > Testbed

GPS/PPS electric pulse

Internet NTP time

C C C C C C C C C C

Chrony+shmpps

OLSR ‘VANET’ +Madwifi NIC

Comm.

Sync.

See http://sites.google.com/site/gpssync/for our GPS/PPS multiplexer specs

GPS receiver Garmin 18 LVC

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Testbed & Results > Results

3 scenarios No car stressed All cars stressed Half cars stressed, Half cars not stressed

Compare timestamps b/w cars (nodes)• OS vs TimeRemap tstamps• Performance metrics

Mean sync error Std dev Outlier ratio (Outlier if error>30 s)

Some cars are stressed with the stressed linux command

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Testbed & Results > Results

Timeremap does not produce any outlier

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Conclusion

• TimeRemap leverages – the stability of the OS – the precision of the NIC

• Performance Mean sync error reduced to 3sec

No outlier

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Perspective

• Deployment on the road

• Better stability