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Control Mechanisms for Video Streaming Wireless

Links

Athina MarkopoulouElectrical Engineering Dept.

Stanford University

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Real-Time Multimedia over Packet Networks

Characteristics • Continuous Stream Playout, Real-Time

src

rcv

ideally

t

t

src

rcv

network

t

t

delay

loss jitter

Requirements• low loss, delay, delay jitter

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Multimedia - Networking

Problems at the interface between multimedia applications and underlying network

Control mechanisms • in the network and/or• at the end-systems

Challenges depend on underlying network(s)

Network

Application

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The Bigger Picture

wireless

backbone

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1

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Media Streaming over Wireless Last-Hop

Streaming to• laptops, cellphones, PDAs, wireless TV

displays

Challenges• limited resources, time variation• strict application requirements

Wireless

ServerMobile

TerminalAccess Point, Server/Proxy

Wireline

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Example

Tx

Rx

Play

IdeallyTx

Rx

Play

Over wireless

Tx

Rx

Play

Control at the RxTx

Rx

Play

Control at the Tx

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Problem Statement

Scenario• pre-stored media content at Tx.• interference i, according to a Markov chain with • deliver and play entire content

Objective • maximize the playout quality• minimize the power cost

N i

Tx Rx

rp

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System State and Controls

(p,r) = system controls in current time slot

(n,i,b, r’) = system state in current time slotn = remaining packets at Txi = channel interference b = available packets at Rxr’ = playout rate in previous slot

pn i b

Tx Rx

r

s(p,i)

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System Controls at Tx

Control p: transmission power in current slot• s(p,i) : probability of successful reception

• Power Cost: Φ = p

– battery lifetime– interference stress

0 2 4 6 8 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

SIR p/i

s(p

,i)

pn i

Tx s(p,i)

b

Rx

r

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Dynamic Programming Formulation

Define to be the minimum expected cost-to-go

(n,j,b-r;r)

(1-s(p,i))qij

(n,i,b;r’)

(n-1,j,b+1-r;r)

s(p,i)qij

(p,r)

System Evolution

… until n=0

from n=N….

Power Quality

·W

·W +

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Computing the Optimal Control

A stationary optimal solution (p*,r*) exists and can be obtained by value iteration

Optimal policy:• table p*(n,i,b;r’) and r*(n,i,b;r’)• obtain offline & store in lookup table

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Special Cases: Individual Controls

Similar formulations – obtain optimal policies Compare: no control, special cases, joint control

Scenario Control at Tx Control at RxNo control fix p fix r=R

Power Control adapt p fix r=R

Power Control & Re-buffering

adapt p r in {0, R}

Playout Control fix p r in {0, r1, r2…R}

Re-buffering fix p r in {0, R}

Joint Control adapt p r in {0, r1, r2…R}

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Power-Quality Tradeoff (1)

Better Performance

No control

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Power-Quality Tradeoff (2)

No control

Playout Only

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Power-Quality Tradeoff (3)

No control

Playout Only

Power OnlyPower+Rebuffering

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Power-Quality Tradeoff (4)

Power OnlyPower+Rebuffering

No control

Playout Only

Joint control

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Heuristics

Why heuristics? Justified vs. ad-hoc heuristics

• mimic properties of optimal control

Steps• Power-only heuristic • Playout-only heuristic• Joint power-playout heuristic

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Power Heuristic

in

Tx s(p,i)

b

Rx

rFix playout r=R

Optimal power:

Backlog pressure X(n,i,b) • has structural properties:

Heuristic: approximate • mimicking those properties

X̂b

X

n=1

n=N

p

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Playout - today

Purpose: choose r(b)

Fixed Threshold Heuristic • L thresholds for buffer occupancy b•

pn

Tx s(p,i)

b

Rx

ri

b

r

r=rl

Bl Bl+1

r=R

r=0BL

B2 B1

r=0r=R/2r=R

fix p

s(i)

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Playout Heuristic

Observation: channel not taken into account yet

Adaptive Threshold Heuristic• adapt rate • and adapt thresholds

b

Rx

riB2 B1

r=0r=R/2r=Rs(i)

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Joint Power-Playout Heuristic

i bp

n

Tx Rx

r

s(p,i)

Tx side: • compute X(n,b)• compute power p:

Rx side: • estimate i, compute p and s(p,i)• adjust thresholds, compute playout r• feedback to Tx

^

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Joint heuristic performs well

Joint OptimalJoint Heuristic

Optimal Playout

Optimal Power+Rebuffering

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Demo: no-control vs. joint heuristic

For the same interference scenario

For the same power consumption

Compare the playout quality

original

no control

Joint heuristic

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Comparison Details Controls off Joint Heuristic

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Wireless Video - Summary Contributions

• Joint power-playout control• Modeled in a dynamic programming framework• Developed simple, efficient heuristics

Extensions • Additional Channels and Responsive Interference • Additional Controls• Content-Aware Control • Apply to protocols (802.11h)

“Joint Power-Playout Control Schemes for Media Streaming over Wireless Links”, in IEEE Packet Video 2004, Markopoulou joint work with Y.Li, N.Bambos, J.Apostolopoulos

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Extension: adding more controls

Additional Controls:• Tx: control scheduling

– how many units to transmit and which to drop

• Rx: motion-aware playout– slowdown video scenes with low or no motion

Results:• trade-off: playout speed variation vs. distortion• effect of playout variation is less perceived

R(t)bn

Tx Rx

control scheduling

content-aware playout (r)

“Joint Packet Scheduling and Content-Aware Playout Control for Media Streaming over Wireless”, invited paper in IEEE MMSP 2005, A. Markopoulou joint work with Y.Li, N.Bambos, J.Apostolopoulos

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Example of Motion-Aware Playout

Motion-unaware playout

Motion-aware playout

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Future Directions Multimedia over IP

• Cross-layer optimization• Content distribution

Network Dependability • From traditional QoS to Reliability & Security• Measurements for diagnosis and control

Network shared by independent selfish entities• Network-adaptive applications• How bad is selfish routing? • Selfishness in other contexts?

Interaction …

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Appendix

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Responsive Interference - Setup

Primary Media link, background PCMA links Pairs of Tx-Rx randomly chosen from area

(500x500 wrapped in a torus) Background: geometric durations, Bernoulli

arrivals Free space path loss G~1/d^4, noise

1^(12) Estimate I using previous timeslot N=100, initial 5slots, buffer B=10

Heuristic gains: 60% in power, 66% in QoS

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Responsive Interference- Power

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Responsive Interference- Quality

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Adding mode control: (p, m, r)

Add a control m:• Packets transmitted in a time slot

Add a cost Psi(m) Modify Bellman equations

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Power-only heuristic Fix playout r=R and find optimal power

p*• similarly to [B&R (1997), B&K(2000), B&Li(2004)]

• where

Heuristic: approximate X, plug it in p.

soft backoff

hard backoff

aggressive

X

i

p*

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[Performance evaluation cont’d]

Simulated other channels Simulated responsive interference Found low sensitivity to r-

parameters