Advanced Technology Laboratories Practical Considerations for Smoothing Multimedia Traffic over Packet- Switched Networks Christos Tryfonas [email protected]

Advanced Technology LaboratoriesAdvanced Technology Laboratories

Practical Considerations for Smoothing Practical Considerations for Smoothing Multimedia Traffic over Packet-Switched Multimedia Traffic over Packet-Switched

NetworksNetworks

Christos [email protected]

April 2002

Sprint Advanced Technology LaboratoriesBurlingame, California


Problem StatementProblem Statement

Objective: Compute a rate schedule to send a video stream that maximizes some network parameter (e.g. utilization)

Constraints: Avoid decoder buffer under/overflows

NetworkInterface

Packet-SwitchedNetwork

Video Server

Stream Decoder (Buffer)

Sender Receiver


Work-Ahead Smoothing AlgorithmsWork-Ahead Smoothing Algorithms

Most smoothing algorithms compute maximum constant rate segments

Differences in the selection of the starting point of next segment

Size of frame 4

Decoder Buffer Size

Underflow Envelope

Overflow Envelope

Time

Rec

eive

d D

ata

Sche

dule

J it t

e r


OutlineOutline

Practical Observations Smoothing under variable network delays Sender-Based Smoothing Clock-Aware Smoothing Renegotiation Failure Decisions Conclusions


Practical ObservationsPractical Observations

Network delay is variable Current smoothing algorithms compute

schedule based on the receiver side Not all data sent by the sender is utilized by

the receiver Some applications require clock to be

recovered at the receiver (e.g. Broadcast TV)

Consideration of Renegotiation Failures


Smoothing under Variable Network DelaysSmoothing under Variable Network Delays

Jitter usually a function of the reserved rate (e.g. Fair Queueing schedulers)

Currently optimization done with a pre-specified worst-case network jitter, i.e., not optimal

Overflow envelope is a function of the decision, not fixed The burstiness of the source at the selected rate defines the

ambiguity zones


Our ApproachOur Approach

Optimization should be done by considering the ambiguity zones resulting from the varying reserved rates

Existing optimization algorithms should consider the variable ambiguity zones for their decisions

Size of frame 4

Decoder Buffer Size

Underflow Envelope

Overflow Envelope

Time

Rec

eive

d D

ata

Sche

dule

– A

mbi

guity

Zon

es


The Case of Rate DecreaseThe Case of Rate Decrease

New rate corresponds to higher jitter Lower envelope is changed to

accommodate the increase in jitter

Schedule Ambiguity Envelope

Time

Rec

eive

d D

ata

Jitter 2

Jitter 1

Jitter 1 < Jitter 2

2


The Case of Rate IncreaseThe Case of Rate Increase

New rate corresponds to less jitter A peak-rate segment is inserted before the

new ambiguity zone with the new jitter

Schedule Ambiguity Envelope

Time

Rec

eive

d D

ata

Jitter 2

Jitter 1

Jitter 1 > Jitter 2

2

Peak Rate segment


Two approaches:

1. Rate Adaptation

2. Forward Schedule computation

Sender-Based SmoothingSender-Based Smoothing

Not all data transmitted is inserted in the decoder buffer

Sequence Header

GOPHeader

FrameHeader

FrameCoding

Extension

FrameHeader

FrameCoding

ExtensionFrame Data Frame Data

Actual Received Data

after Rate Adaptation

Time

Rec

eive

d D

ata

Received Data function as

computed by optimization


Clock-Aware SmoothingClock-Aware Smoothing

Time

Rec

eive

d D

ata

Sche

dule

Time

Rec

eive

d D

ata

Clock

-Aware

Sch

edul

e

Clo

ck

Time

Clo

ckTime


Renegotiation Failure DecisionsRenegotiation Failure Decisions Dynamic Requantization Use of scalable-encoded streams New Approach: Pre-computed alternate schedules with

convergence points

Time

Rec

eive

d D

ata

Main Schedule

Alternate Schedule

RenegotiationFailure

IntersectionPoint

ConvergencePoint


ConclusionsConclusions

Current smoothing algorithms produce infeasible schedules in practice

Optimization has to consider:• Variable Network Jitter a function of the reserved rate

• Unused data sent by the sender but not utilized by the receiver/decoder

• Clock-awareness

• Renegotiation Failures

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Documents

Advanced Technology Laboratories Practical Considerations for Smoothing Multimedia Traffic over Packet- Switched Networks Christos Tryfonas [email protected]