1 Control Mechanisms for Video Streaming Wireless Links Athina Markopoulou Electrical Engineering...
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Transcript of 1 Control Mechanisms for Video Streaming Wireless Links Athina Markopoulou Electrical Engineering...
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Control Mechanisms for Video Streaming Wireless
Links
Athina MarkopoulouElectrical Engineering Dept.
Stanford University
2
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
3
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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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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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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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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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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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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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*