Fine-Grained Network Time Synchronization Fine-Grained Network Time Synchronization using Reference Broadcastsusing Reference Broadcasts

Jeremy Elson, Lew Girod, and Deborah Estrin

OSDI 2002 - Boston, MA

Speaker : hsiwei-Chen

OutlineOutline

IntroductionIntroductionReference-Broadcast SynchronReference-Broadcast Synchron

ization(RBS)ization(RBS) Implementation on Berkeley MoImplementation on Berkeley Mo

testesConclusions and FutureWorkConclusions and FutureWork

Sources of Time Sources of Time Synchronization Error(1/2)Synchronization Error(1/2)

Send Time Send Time Time spent at the sender to construct the Time spent at the sender to construct the

message, e.g. context switches and system message, e.g. context switches and system call overheadcall overhead

The time required to transfer the message The time required to transfer the message from the host to its network interface.from the host to its network interface.

Access TimeAccess Time Wait for the channel to be clear before Wait for the channel to be clear before

transmittingtransmitting

Sources of Time Sources of Time Synchronization Error(2/2)Synchronization Error(2/2)

Propagation TimePropagation Time The time needed for the message to transit frThe time needed for the message to transit fr

om sender to receiversom sender to receivers Receive TimeReceive Time

Time to receive the message and notify the hoTime to receive the message and notify the host of its arrivalst of its arrival

Send time & access time are most noSend time & access time are most non-deterministicn-deterministic

Reference-Broadcast SynchronizReference-Broadcast Synchronization(RBS)ation(RBS)

Traditional protocols Traditional protocols Synchronize the sender of a message Synchronize the sender of a message with its receiverwith its receiver

RBSRBS A set of nodes synchronize with each otA set of nodes synchronize with each other (not with the sender)her (not with the sender)reduces non-determinismreduces non-determinism

Critical Path AnalysisCritical Path Analysis

Traditional RBS

Send Time Access

Time PropagationDelayReceiveTime

RBS Critical PathRBS Critical Path

Remove the send time & access time from Remove the send time & access time from the critical paththe critical path

Receive time is independent of the sender or netwReceive time is independent of the sender or network loadork load

Propagation time can be ignored in a short distancPropagation time can be ignored in a short distancee

RBS Critical Path (Cont.)RBS Critical Path (Cont.)

Use broadcast as a relative time referUse broadcast as a relative time referenceence Broadcast packet does NOT include timestamBroadcast packet does NOT include timestam

p generated by sender p generated by sender Any broadcast, e.g., RTS/CTS, route discoverAny broadcast, e.g., RTS/CTS, route discover

y packetsy packets

Simple RBSSimple RBS One transmitter broadcasts a reference packet One transmitter broadcasts a reference packet

to two receiversto two receivers Each receiver records the time the reference pEach receiver records the time the reference p

acket arrived according to the local clockacket arrived according to the local clock Receivers exchange the timestampsReceivers exchange the timestamps We consider the Propagation Time to be effectWe consider the Propagation Time to be effect

ively 0ively 0 Sensitiving to Sensitiving to differences differences in the receive time of in the receive time of

messages within a set of receivers.messages within a set of receivers.

Simple RBSSimple RBS

Sender

Receiver 1

Receiver 2

Broadcast

Broadcast

exchange the tiexchange the timestampsmestamps

Distribution of Inter-Receiver Distribution of Inter-Receiver Phase OffsetPhase Offset

Pairwise Difference in Packet Reception Time (usec)

Number of Trials

Phase offset recorded by a logic analyzer for1,478 broadcast packets(total)(sender emitted 160 broadcast packets)

Gaussian distribution= 0, = 11.1usConfidence = 99.8%

5 motes: receivers1 mote: transmitter

Estimation of Phase OffsetEstimation of Phase Offset

A sender broadcasts A sender broadcasts mm reference packets reference packets Each of the Each of the nn receiver records the time at which receiver records the time at which

the reference was receivedthe reference was received Receivers exchange Receivers exchange timestampstimestamps Receiver Receiver ii computes its phase offset to any computes its phase offset to any

other receiver other receiver jj as the average of as the average of mm differences differences

m

kkjki TT

mjiOffsetnjni

1,, )(

1],[:,

Estimation of Clock SkewEstimation of Clock Skew

Clock skew due to crystal oscillators errorsClock skew due to crystal oscillators errors Frequency errorFrequency error

Difference between the expected and actual Difference between the expected and actual frequenciesfrequencies

InstabilityInstabilityShort-term: Environmental factors, e.g., variations Short-term: Environmental factors, e.g., variations

in temperature, supply voltage, and shockin temperature, supply voltage, and shockLong-term: Oscillator agingLong-term: Oscillator aging

Least square method (linear regression)Least square method (linear regression)Assume phase error is changing at constant rateAssume phase error is changing at constant rateContinuously adjust frequency based on recent Continuously adjust frequency based on recent

observations relating the local oscillator to a observations relating the local oscillator to a referencereference

Implementation on Berkeley Implementation on Berkeley MotesMotes

One sender, five receiversOne sender, five receivers1000 trials1000 trials

Receiver Group DispersionReceiver Group Dispersion

#Reference packet broadcasts

Rec

eive

r G

roup

Dis

pers

ion

(us)

25 broadcasts can reducethe dispersion below 10us

20 Receivers

2 Reeivers

Effect of Clock Skew on RBSEffect of Clock Skew on RBS

Time (sec)

Pha

se O

ffse

t (u s

)

Fit

Err

o r (

u s)

Performance ComparisonsPerformance Comparisons

Compare to NTPCompare to NTP UNIX implementation of RBS for fair comparisonsUNIX implementation of RBS for fair comparisons

User space UNIX daemonUser space UNIX daemon UDP datagramUDP datagram StrongArm-based Compaq IPAQ PDA’sStrongArm-based Compaq IPAQ PDA’s

““Familiar” Linux (2.4 kernel)Familiar” Linux (2.4 kernel) Lucent Technology 11Mbit 802.11 wireless Ethernet aLucent Technology 11Mbit 802.11 wireless Ethernet a

dapters dapters

300 trials,300 trials, 每個每個 trialtrial 間格間格 88 秒鐘秒鐘 ,Total time is 40 ,Total time is 40 minutersminuters

Synchronization Error with Light Synchronization Error with Light Network LoadNetwork Load

Error (us)

Cum

ulat

ive

Err

o r P

rob.

RBS

NTP

NTP-Offset

Sync. Error between two IPAQ’s

Synchronization Error with Light Synchronization Error with Light Network LoadNetwork Load

RBS: 6.29 +/- 6.45 usRBS: 6.29 +/- 6.45 us NTP: 51.18 +/- 53.30 usNTP: 51.18 +/- 53.30 us NTP-Offset: 204 +/- 599.44 usNTP-Offset: 204 +/- 599.44 us RBS works better due to the reduced non-RBS works better due to the reduced non-

determinismdeterminism

Synchronization Error with Synchronization Error with Heavy Network LoadHeavy Network Load

Error (us)

Cum

ulat

ive

Err

o r P

rob.

RBS

NTP

NTP-Offset

Synchronization Error with Synchronization Error with Heavy Network LoadHeavy Network Load

RBS: 8.44 +/- 9.37 usRBS: 8.44 +/- 9.37 us With kernel support which timestamps at With kernel support which timestamps at

interrupt time, 1.85 +/- 1.28 usinterrupt time, 1.85 +/- 1.28 us NTP: 1542.27 +/- 1192.53 usNTP: 1542.27 +/- 1192.53 us NTP-Offset: 5139.08 +/- 6994.58 usNTP-Offset: 5139.08 +/- 6994.58 us RBS is not affected by Medium Access RBS is not affected by Medium Access

DelayDelay

Multi-Hop Time SynchronizationMulti-Hop Time Synchronization

1 2

3 4

1. 在 node A 面積 local 中 ,node 1 與 node 4 同步

2. 再以 node 4 當 getway ,與 node B 面積 local 中 ,node 4 與 node 7 同步

3. 即可達到 Global 同步 .

A5

6 7

B

Multi-Hop RBS PerformanceMulti-Hop RBS Performance

Hop Mean Error( ) 1 1.85 2 2.73 3 2.73 4 3.68

sec

Conclusions and FutureWorkConclusions and FutureWork

RBSRBS provides more precise, flexible, and res provides more precise, flexible, and resource-efficient network time synchronization tource-efficient network time synchronization than traditional algorithmshan traditional algorithms

But such as automatic, dynamic election of thBut such as automatic, dynamic election of the set of nodes to act as beacon senders e set of nodes to act as beacon senders ??

SummarySummary

本篇作者去除掉了本篇作者去除掉了 Send Time and Acces Send Time and Acces

TimeTime 同步同步 errorerror 的兩個來源的兩個來源 以相對的時間代替絕對的時間 以相對的時間代替絕對的時間

Thank you