A Cross-Layer Methodology for Design and Optimization of ...
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CROSS LAYER DESIGN
CMPT 820: Multimedia Systems
Kaushik Choudhary
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Outline
Problem Definition
Motivation for Optimal Strategy
Categorization of Cross Layer Solutions MAC Layer Retransmission Limit Adaptation
Joint Application-MAC Cross LayerOptimization
Impact of Cross Layer Strategies
Conclusion
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Problem Definition
Autonomous Wireless Stations (WSTAs) = M,
Available wireless resources = R R+,
Channel condition experienced by WSTA i = SNRi ,
Video source characteristics = i ,
Current state information (private information) of WSTA
i = xi = (SNRi, i )
Resource allocation = T(R ) = [t1,,tM] R+M
Time allocation by resource coordinator to WSTA i = ti
(0 <= ti <= tSI) (PCF or other contention)
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Problem Definition
Given the above constraints, the cross layerdesign problem may be formulated as anoptimization with objectives like maximizinggoodput or minimizing consumed power etc.
If si is the cross layer strategy available toWSTA i from a set Si then given xi and ti, si willresult in a utility ui(ti,si,xi)
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Problem Definition
The optimal cross layer strategy can be found
by maximizing the video quality in terms of
perceived quality or PSNR
Mathematically,
siopt = arg max ui(ti,si,xi)
si S
i
such that Delay(ti,si,xi) <= Delayimax
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Outline
Problem Definition
Motivation for Optimal Strategy
Categorization of Cross Layer Solutions MAC Layer Retransmission Limit Adaptation
Joint Application-MAC Cross LayerOptimization
Impact of Cross Layer Strategies
Conclusion
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Motivation for Optimal Strategy
The above functions ui(ti,si,xi) and Delay(ti,si,xi)are non-deterministic, non-linear, complex and
multi-variate optimization.
The strategies to solve them must take intoaccount different practical considerations likebuffer sizes, modulation schemes etc. and shouldbe procedurally grouped and ordered for crosslayer optimization.
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Outline
Problem Definition
Motivation for Optimal Strategy
Categorization of Cross Layer Solutions MAC Layer Retransmission Limit Adaptation
Joint Application-MAC Cross LayerOptimization
Impact of Cross Layer Strategies
Conclusion
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Categorization of Cross Layer Solutions
Possible solutions based on the order in which
cross layer optimization is performed:
± Top-down approach
± Bottom-up approach
± Application-centric approach
± MAC-centric approach
± Integrated approach
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Top-down approach
Higher layer protocols optimize their
parameters and strategies at the next lower
layer.
Very widely used for example in systems
where APP dictates MAC parameters and MAC
selects optimal PHY parameters.
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Bottom-up approach
Lower layers try to insulate higher layers from
losses and bandwidth variations.
Not optimal due to incurred delays and
throughput reductions.
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Application-centric approach
APP layer optimizes parameters of lower
layers one at a time in either top-down or
bottom-up manner.
Not optimal since APP layer operates on
slower timescales and coarser data
granularities.
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MAC-centric approach
MAC layer decides which APP layer packets
should be transmitted with which delay alongwith selecting PHY layer parameters.
MAC layer is unable to perform adaptivesource channel coding
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Integrated approach
Strategies are determined jointly across
various protocols.
Complex and introduces delays.
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Outline
Problem Definition
Motivation for Optimal Strategy
Categorization of Cross Layer Solutions MAC Layer Retransmission Limit Adaptation
Joint Application-MAC Cross LayerOptimization
Impact of Cross Layer Strategies
Conclusion
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MAC Layer Retransmission Limit
Adaptation
To maximize video quality, minimize the MAC
packet loss rate (PLR).
MAC packet losses occur due to: ± Link erasures
± Buffer overflows
Define a strategy to optimally select
retransmission limit R that minimizes the
overall MAC packet loss.
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MAC Layer Retransmission Limit
Adaptation
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MAC Layer Retransmission Limit
Adaptation
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MAC Layer Retransmission Limit
Adaptation
Fig 1: MAC PLR under f ix ed- and RTRO-based retransmi ssion strategi es.
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MAC Layer Retransmission Limit
Adaptation
From Fig 1 we note that the optimal R is
located at the point where pB(R)=pL(R)(intersection)
Thus, optimal R = arg minR |pB(R)-pL(R)| (3)
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MAC Layer Retransmission Limit
Adaptation
Li et. al. [2] performed empirical analysis using
M/G/1 queuing model and proposed real-time
retransmission limit optimization (RTRO)
algorithm:
1. The network queue and the MAC layer monitor
the overflow rate pB(R) and the packet error rate
pL(R).2. If pB <pL, then R is increased; if pB >pL, then R
should be decreased
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Outline
Problem Definition
Motivation for Optimal Strategy
Categorization of Cross Layer Solutions
MAC Layer Retransmission Limit Adaptation
Joint Application-MAC Cross LayerOptimization
Impact of Cross Layer Strategies
Conclusion
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Joint Application-MAC Cross Layer
Optimization
By associating different retransmission limits
to different priority packets the MAC-layerRTRO optimization can be jointly optimized by
the APP layer.
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Joint Application-MAC Cross Layer
Optimization
Tolerable MAC packet loss rates of all video
layers = PV = [PV1 PV2 PVN]
Video quality = Q To maximize Q Unequal Error Protection (UEP)
must be provided.
To provide UEP multiple priority queues aremaintained with a common absolute Priority-
Queuing (PQ) discipline.
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Joint Application-MAC Cross Layer
Optimization
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Joint Application-MAC Cross Layer
Optimization
The above model can be further extended to
include a multiqueue system based on which asystematic retry-limit configuration method
for MAC can be determined to optimize video
quality.
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Outline
Problem Definition
Motivation for Optimal Strategy
Categorization of Cross Layer Solutions
MAC Layer Retransmission Limit Adaptation
Joint Application-MAC Cross LayerOptimization
Impact of Cross Layer Strategies
Conclusion
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Impact of Cross Layer Strategies
Deployed Strategies Visual Score
No optimization at MAC and application 1.4
MAC-layer optimization (RTRO) 1.9
Application layer optimization 3.8
Joint application-MAC cross-layer
optimization
4.6
Table 1: Subject iv e vi deo qual i ty e x per i ment.
Very
Annoying
Annoying Slightly
annoying
Perceptible
but not
annoying
Imperceptib
le
1 2 3 4 5
Table 2: Decod i ng the vi sual scores i n Table 1.
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Outline
Problem Definition
Motivation for Optimal Strategy
Categorization of Cross Layer Solutions
MAC Layer Retransmission Limit Adaptation
Joint Application-MAC Cross LayerOptimization
Impact of Cross Layer Strategies
Conclusion
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Conclusion
Cross layer design can be modeled as amultivariate optimization problem expressed inthe form of various strategies to maximizeperceived quality and improve user experience.
Cross layer solutions can be categorized intovarious approaches based on a layer dictatingstrategies and parameters for other layers.
MAC-layer optimization yields improved video
quality. MAC-APP layer optimization performs even
better.
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References
1. P. A. Chou and M. van der Schaar.Multimedia over IP and Wireless Networks,Academic Press, ISBN 10: 0-12-088480-1, pp.
351-360, 2007.2. Q. Li and M. van der Schaar. Providing
Adaptive QoS to Layered Video over WirelessLocal Area Networks through Real-Time RetryLimit Adaptation, IEEE Trans. on Mult i med i a,vol. 6, no. 2, pp. 278290, April 2004.