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Adaptive Dual-layered Bucket Synchronization

Control for Distributed Virtual Environment

JaeYoun Kim, Seokhee Lee and JongWon Kim2005/8/23

{jykim, shlee, jongwon}@netmedia.gist.ac.kr

Networked Media LaboratoryDept. of Information & Communications

Gwang-Ju Institute of Science & Technology

APAN NRW 2005

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Contents

BackgroundRelated WorkMotivations & ContributionsOverall FrameworkSimulation & Experiment ResultsConclusion

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Background

In these daysAdvances in computer graphics, parallel/distributed system and high-speed networkingDemands on building distributed virtual environments (DVEs) system

ConsistencyEnsure consistent view for each participant over time-varying Network

Tradeoff between responsiveness and consistency

Synchronization algorithmPlayout delay control

Network

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Playout Delay Control

Adaptation efficient playout delay

Event Generation

Event Execution

Small Playout Delay

Large Playout Delay

Large Network

Delay

Network delayNetwork delay

Synchronization delaySynchronization delay

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Related Work

Y. Ishibashi et al. Enhance a virtual-time rendering (VTR) algorithm for DVEs.The synchronization maestro

When the network load is heavy, gradually increases (i.e., delays) the target output time. When the network load becomes lighter, gradually decreases the target output time.

D. Lee et al.Add dynamic playout delay control scheme in bucket synchronization.

Current network state is estimated by measuring how many events have been lost.

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Motivations and Contributions

MotivationsEvent loss rate can not guarantee correct estimation of network state. Event loss when decreasing the playout delay because of skipping.

ContributionsAdaptive dual-layered bucket

Estimates network state based on one-way transmission delay and event loss rate.Decrease the playout delay for silence period in order to prevent from skipping (event loss).

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Synchronization System Model

< System model > < Block diagram >

System modelAll participants use a multicast address for event and report (Tmax)

Each participant is based on bucket-based synchronization layerAdaptation master among participants determine reference playout delay (Rdelay)

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Reference Playout DelayAdaptation master

Group maximum transmission delay (Gmax)Maximum value of Tmax

Smoothed current Gmax ( Cdelay )Filter out short-term fluctuations caused by jitter

Upper target threshold (UTT), Lower target threshold (LTT)Reference playout delay based on UTT and event loss rate

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Dual-layered Bucket Management

Each participantReceives the new playout delay (Rdelay) from adaptation master.

However, does not apply it directly.Prevent event loss when decreasing the playout delay

If there is sending idle time like silence period in DVE systems, apply the playout delay.

ERdelay

Virtual layer

Real layer

S 1 S 2

CRdelay

play out

play out

Event Buffer

Sending idle time

< Dual-layed Bucket management >

Virtual layer: provide guideline for possible playout delay

Candidate playout delay (CRdelay)

Real layer: detect sending idle time like silence period

Established playout delay (ERdealy)

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Dual-layered Bucket Management(cont’)

Insert event

CRdelay is equal to ERdelay

Locate event by ERdelay

on the real buffer layer

End of inserting eventto the real buffer layer

yes

no

CRdelay > ERdelay

Gradually extending ERdelay to CRdelay

yes

no

TRdelay := ERdelay ? BperiodTRdelay is equal

to CRdelay

yes

TRdelay := TRdelay ? Bperiod

Locate event by TRdelay

no

End of inserting eventto the virtual buffer layer

ERdelay : established playout delayCRdelay : candidate playout delay

TRdelay : temporal palyout delay Bperiod: bucket period

Locate event by TRdelay

on the virtual buffer layer

Adapitationcontroller

ERdelay := ERdelay + Bperiod

CRdelay == ERdelay

End of adaptation controller

Calculating new_ERdelay ,

ERdelay := new_ERdelay

CRdelay > ERdelay

no no

yes

Detecting empty bucket

yes

yes

Updating ERdelay

no

Inserting events coming from network

Updating new reference playout delay

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Simulation and Experiment

SimulationGenerate CBR traffic and TCP traffic (FTP) in NS-2 DVE systems( 4 nodes) communicate through single multicast addressCompare playout delay

ExperimentGenerate background UDP traffic (20Mbps~70Mbps) by iperfDVE systems( 2 nodes) communicate through multicast router(CISCO 2600)Compare skipping rate

Topologies

< Experiment>< Simulation NS-2>

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Simulation Result

<Comparison of playout delay>

Network state estimation based on delay and loss

(Proposed scheme)

Network state estimation based on loss rate

Proposed schemeProvides more correct estimation of current network stateGuarantees responsiveness and interactivity of DVEs

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Experiment Result

Comparison of playout delay Similar to simulation result

Event loss rateProposed scheme provides smaller loss rate

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Conclusions

Dual-layered Bucket ManagementProvides more correct network state estimation to determine playout delay.Reduces event loss in event buffer for consistent global state.

Future workImprove the proposed scheme for haptic-based DVEs.

Haptic interface : 1 kHz I/O rate.Haptic interaction : more sensitive to delay and jitter (about 30 ~ 6

0 ms).Current scheme can not guarantee consistency in haptic-based DV

Es.

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Thank You…

Question ?