clock sync 1

7/24/2019 Clock Sync 1

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Wireless Sensor Networks

Clock Synchronization

Professor Jack Stankovic

University of Virginia


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Why is clock sync important

Spatial-emporal System Where an! whenevents occ"r

o !isting"ish m"ltiple events# comp"telifetime of event# coor!inate sensing#coor!inate control actions $for wake-"p#for act"ators# %& # %

Signal processing $local clock may 'eeno"gh&

Comp"te velocity


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Co"l! we have a WSN witho"t clocksync() *f all operations are logical

+r Use ,PS) Power h"ngry

) ./pensive per no!e


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+"tline

NP $Network ime Protocol& ) for*nternet

01S $0eference 1roa!cast Sync&

PSN $ime-sync Protocol for SensorNetworks&

2SP $2loo!ing ime Sync Protocol&


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What is Clock Sync

he clocks of each no!e in the WSNsho"l! rea! the same time withinepsilon an!remain that way

) Sync clocks

) 3an!le clock !rift $"p to 45 microsec per

secon! on 6ica platform7 89:8 63z clock& Sol"tion; 0e-sync $e/pensive&

Sol"tion; .stimate clock !rift an! compensate


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3ow 'a! is 45 microsec>sec(

45 / ?5 @ A#455 microsec>min

A#455 / ?5 @ B44#555 microsec>hr

B44#555 / A4 @ :#4?#555 microsec>!ay


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,oals for Clock Sync inWSN

6eet re="ire! precision

Dow comm"nication 1W

0o'"st against no!e an! link fail"res

6"lti-hop $large scale&


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NP

Network ime Protocol $NP& on *nternet

) *ncl"!e! in +S co!e) Simple NP $SNP& e/ists for PCs

) 0"ns as 'ackgro"n! process) ,et time from E'estF servers

2in!s those with lowest Gitter>latency


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NP

Not $!irectly& "sa'le for WSN) Comple/ ) see 5 page !oc"ment

) Contin"o"s cost

) Darge co!e size

) ./pensive in messages>energy

) Non-!eterminism at 6HC layer $per hop&

if NP was implemente! in WSN can a!! B55sms !elay


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Clock Sync


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Clock Sync -


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Clock Sync -


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*!eal

0ea!Clock 5 5

SetClock

Hll Iero Costs>


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What if


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01S

0eference messageis 'roa!cast 0eceivers recor! their local time when message is

receive!) imestamps are +ND on the receiver si!e

) his eliminates access an! sen! times No!es e/change their recor!e! times with each

other AK9B microsec acc"racy for B hop case

No transmitter si!e non-!eterminism

Not e/ten!e! to large m"lti-hop networks


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01S

5

6

7

Docal ime

Sen! 6S,

No Clock

6,7

5,7

5,6

./change H!G"st

Plus 2

Plus 1

OK

Propagation ime @ 5


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What can ca"se the Ei!ealF case of allthree no!es getting the clock messageat the EsameF time to 'e false(

) No!e t"rns off interr"pts Co!e e/plicitly t"rns off interr"pts

nesC atomic statement

) No!e t"rns off ra!io $sleep mo!e&


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PSN

Creates a spanning tree

Perform pairwise sync along e!ges ofthe tree

6"st 'e symmetric links) 0ecall ra!io irreg"larity paper

Consi!ers EentireF system) 01S looks at one hop

./change two sync messages withparent no!e


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Synchronization Phase


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./ample

T1= 5.0

T2=5.35 T3=6.0

T4=5.75

Node A

Node B

P = ((T2-T1)+(T4-T3))/2


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PSN

No 'roa!casting so PSN is e/pensive imestamp the message in the 6HC

layerm"ltiple times an! average those

times $improves acc"racy over 01S 'y Atimes&

$B?9K microsec acc"racy for B hop&

No clock !rift corrections


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0ea! Clock in 6HC Dayer Uncertainties of ra!io message !elivery


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PSN

Hss"mes topology !oes not change) Not ro'"st to fail"re of a no!e

No!e fails ) m"st re!o the spanning tree


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2loo!ing ime SyncProtocol $2SP&

*mplemente! on 6ica platform

LB 6icrosec acc"racy

6HC-layer timestamp Skew compensation with linear

regression $acco"nts for !rift&

Perio!ic floo!ing ) ro'"st to fail"resan! topology changes

3an!les large scale networks


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0emove Uncertainties

.liminate Sen! Uncertainty) ,et time in the 6HC layer

.liminate Hccess ime) ,et time after the message has access to

the channel

.liminate 0eceive ime

) 0ecor! local time message receive! at the6HC layer


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0emaining

$6ostly&


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1asic *!ea

When to time stamp the message

sender

receiver

6icaA "ncertainties;

*nterr"pt;"s# :5"s$MA&.nco!eO!eco!e;BB5"s -BBA"s

1yte align; 5"s ) :?"s

Propagation; B"s

,et timestamp

Set timestamp

0ea!y


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1asic *!ea When to time stamp the message

) 0a!io layer# after the secon! SN sent o"t#? timestamps in row# take the average an!sen! only B timestamp

Normalize and then

take Average of these

timestamps for 6 bytes

of data


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Why take ? samples(

1eca"se of all the "ncertainties as!escri'e! in a previo"s sli!e


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2SP

0oot maintains glo'al time for system Hll others sync to the root No!es form an a! hoc str"ct"re rather

than a spanning tree 0oot 'roa!casts a timestamp for the

transmission time of a certain 'yte

.very receiver time stamps receptionof that 'yte Hcco"nt for !eterministic times


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S"mmary of 2SPSo 2ar

$1elow& 6HC-layer timestamp

Correct sen!er timestamp to acco"ntfor known !elays) Comp"te final offset error

0es"lt; B94 microsec acc"racy for B hop


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Clock


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1asic *!ea

entry linear regression ta'le toestimate clock skew $each entry !erive! fromB clock sync protocol e/ec"tion&

./ample

B "s offsetA "s offset: 8 "s offset

4 4 "s offset 4 "s offset? "s offset8 8 "s offset "s offset

"s overSay a B5 sec

0esync perio!

Compensate 9 "s.ach sec

10 se#o$ds

1 se#


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Performance Single hop# stop sync after :: min"tes

$where sync was r"n every :5 secs&

./ample; *f clock sync stops at time :: an! error of B5

6icrosec is +Q7 then 8 min"te re-sync cycle is +Q


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6"lti-3op ime Sync

Create a fi/e! spanning tree for clocksync !issemination like in PSN $notro'"st&

Use more a! hoc techni="e) Hss"me every no!e has "ni="e *0eference Point

0oot>0eference Point

Step B

Step A sync msgsto performDinear regression

3ow to han!le

messages fromm"ltiple no!es


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Sync 6essage 2ormat

imestamp ) glo'al time estimate oftransmitter when 'roa!cast

0oot*

clock sync 1

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