Cross Layer Design 1

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CROSS LAYER DESIGN

CMPT 820: Multimedia Systems

Kaushik Choudhary

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

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)

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)

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)

such that Delay(ti,si,xi) <= Delayimax

Outline

Problem Definition

Conclusion

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.

Outline

Problem Definition

Conclusion

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

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.

Bottom-up approach

Lower layers try to insulate higher layers from

losses and bandwidth variations.

Not optimal due to incurred delays and

throughput reductions.

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.

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

Integrated approach

Strategies are determined jointly across

various protocols.

Complex and introduces delays.

Outline

Problem Definition

Conclusion

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.

Adaptation

Fig 1: MAC PLR under f ix ed- and RTRO-based retransmi ssion strategi es.

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)

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

Outline

Problem Definition

MAC Layer Retransmission Limit Adaptation

Conclusion

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.

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.

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.

Outline

Problem Definition

Conclusion

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

Table 1: Subject iv e vi deo qual i ty e x per i ment.

Annoying

Annoying Slightly

annoying

Perceptible

but not

annoying

Imperceptib

1 2 3 4 5

Table 2: Decod i ng the vi sual scores i n Table 1.

Outline

Problem Definition

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.

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.

Cross Layer Design 1

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