department of communication technology

Video Streaming over 802.11b LANWireless channel unreliability : managing the

starvation phenomenon

Mohamed Ali Ben Abid

Monday, 28 June 2004

Department of Communication Technology

Supervisors CensorsSupervisors Censors

Frank H.P. Fitzek Karsten ThygesenHans Peter Schwefel Thomas Toftegaard Nielsen

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Actual Concept

802.11b LAN: mobility, high data speed

Video Streaming: more and more expanded in the wired network

Video Streaming over 802.11b LAN, a promising combination.

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Project Presentation (1)

Goal : Optimizing the video client’s resources while maintaining a good video quality.

Means : Managing the Playout Buffer of the video. Estimating a buffer compensation for the

wireless channel unreliability.

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Outline• Background

The 802.11b LAN Video Streaming

• The StudyProblem SettingScenarioMethodologyResultsConclusion

Project Presentation (2)

Background The 802.11b LANThe 802.11b LAN

Video StreamingVideo Streaming

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802.11b LAN - Architecture

• different BSS, different MN

• 1 BSS controlled by 1 AP

The 802.11b LANThe 802.11b LAN

Access Mechanism

Layers

Errors

Architecture

Background

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802.11 layers

• PHY layer : data transmission

• 802.11 MAC : fragmentation, Ack

• 802.2 : packets retransmission

The 802.11b LANThe 802.11b LAN

Access Mechanism

Layers

Errors

Architecture

Background

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CSMA/CA Access Mechanism (1)

802.11b LAN802.11b LAN

Access Mechanism

Layers

Architecture

Background

Errors

IFS

SIFS : separate transmissions, 28 μs

DIFS : station to start transmission, 128 μs

Positive Acknowledgement

Virtual Carrier Sense

• hidden node problem

• RTS/CTS

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CSMA/CA Access Mechanism (2)

The 802.11b LANThe 802.11b LAN

Access Mechanism

Layers

Architecture

Background

Errors

The access method is Distributed Coordination Function (DCF)

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CSMA/CA Access Mechanism (3)

The 802.11b LANThe 802.11b LAN

Access Mechanism

Layers

Architecture

Background

Errors

• The Backoff algorithm :

• Contention window from CW_min (16) to CW_max (1024).

• m = maximum transmissions times.

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Errors in the channel

The 802.11b LANThe 802.11b LAN

Access Mechanism

Layers

Errors

Architecture

Background

Main Types of errors : frame loss / erroneous frames.

Causes of errors due to the channel :

Shadowing

Multipath fading

PHY layer adjusting the sending rate.

Detection/Correction Mechanisms :

if CRC failed, frame discarded

each MAC frame ACKnowledged (unicast)

ARQ (Send and Wait)

FEC (adds redundant bits)

BackgroundThe 802.11b LANThe 802.11b LAN

Video StreamingVideo Streaming

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Video Structure

Video StreamingVideo Streaming

Real-time Requirements

Streaming principle

Video structure

Background

Protocol Stack

def:

Video frame = Picture

• e.g. QCIF compression format : 1 picture = 176*144 pixels

• with YUV representation, 1 pixel : 3Bytes

Gives frame size (Byte)

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Streaming principle (1)

Video StreamingVideo Streaming

Real-time Requirements

Streaming principle

Video Structure

Background

Protocol Stack

Why is frame size variable ?

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Streaming principle (2)

Video StreamingVideo Streaming

Real-time Requirements

Streaming principle

Video Structure

Background

Protocol Stack

• Example of frame size PDF (Friends 2x16)

here, the total number of frames is 32455

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Video Requirements

• Burstiness of video + wireless channel unreliability Packet losses & delays

Video StreamingVideo Streaming

Real-time Requirements

Streaming principle

Video Structure

Background

Protocol Stack

Tradeoff : number of Data Link retransmission Nr / delay introduced.

FER < 8/100

Nr_max = 4 (unicast)

= 0 (multicast)

UDP traffic (no layer 4

retransmission)

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Protocol Stack

Video StreamingVideo Streaming

Real - time Requirements

Streaming principle

Video Structure

Background

Protocol Stack

The Study Problem Setting

ScenarioScenario

MethodologyMethodology

ResultsResults

ConclusionConclusion

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Problem Setting (1)

Main ProblemMain Problem

PBO constraints

definitions

ε dependences

PBO/IBO

The Study

Playout Buffer Occupancy (PBO) :

Intitial Buffer Occupancy (IBO) =

T_start(display) – T_start(buffer filling)

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Problem Setting (2)

Main ProblemMain Problem

PBO constraints

definitions

ε dependences

PBO/IBO

The Study

• θ ?

• M ? Overflow ?

• T0, T’ ?

• Starvation, interruption ?

Playout Buffer Occupancy

(PBO)free in an error free channel

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Problem Setting (3)

• P9, P10…still not in buffer

• e.g. if F4 = P8, F4 displayed, buffer empty : starvation

. Then, e.g. if F5 = (P9,P10)

& if P9, P10 did not arrive

interruption in display

Main ProblemMain Problem

PBO constraints

definitions

ε dependences

PBO/IBO

The Study

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Problem Setting (4)• θ = Initial buffer occupancy (error free

channel)• ε = Buffer compensation to the

wireless channel unreliability• Initial_Buffer = θ + ε

0 <(a) PBO = PBOfree + ε < M+ ε <(b)S (a) = no interruption (b) = no buffer overflow

Main ProblemMain Problem

PBO constraints

Variables definition

ε dependences

PBO/IBO

The Study

Project focus : (a)

given wireless scenario/ given video

Chose an appropriate ε

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Problem Setting (5)

ε depends on the following parameters :

Wireless conditions• N = number of MNs• Distance(s) laptop(s)/AP• Competing traffic(s)• FER (must be < 8%)• NLoS• Interference (neglected)• Handovers (not here)

Video Features• Θ, T’A priori estimation : ε < 5%* Θ Main ProblemMain Problem

PBO constraints

Variables definition

ε dependences

PBO/IBO

The Study

The StudyProblem SettingProblem Setting

ScenarioScenario

MethodologyMethodology

ResultsResults

ConclusionConclusion

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Scenario (1)

• Server : desktop, P3-800MHz, 256MB RAM, 100Mbps Ethernet Card, 10/100 BaseT cable

•AP is Nokia A032 and cards are Nokia C110

•MN = 1 laptop P4-2.2GHz, 256MB RAM, WinXP

ScenarioScenario

4 scenariii

Main features

Experiment Scheme

The Study

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Scenario (2)

layer 3 fragmentation threshold :

1475 B No L3 fragmentation

layer 2 fragmentation threshold :

2346 B No L2 fragmentation

• UDP datagram size = 1460 B

ScenarioScenario

4 scenariii

Main features

Experiment Scheme

The Study

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Scenario (3)

• Video modelized by the traffic (Friends 2x16)

duration :1300 s mean rate : 759486 bit/s

Iperf generated traffic is UDP traffic sent with a rate of 759486 bit/s for 1300s.

ScenarioScenario

4 scenariii

Main features

Experiment Scheme

The Study

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Scenario (4)

ScenarioScenario

4 scenariii

Main features

Experiment Scheme

The Study

• Unicast / Multicast

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Scenario (5)

• Channel : Non overlapping conditions

Automatically choosed channel is number 10, but experiments made again with channel 1, 7, 13 (no difference / no interference problem)

ScenarioScenario

4 scenariii

Main features

Experiment Scheme

The Study

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Scenario (6)

• 4 scenarii :

ScenarioScenario

4 scenariii

Main features

Experiment Scheme

The Study

(*) UDP traffic sent at 759486 bps from time 0s to 1300s.

& competing TCP traffic sent at 4.38 Mbps from time 360s to 960s.

The Study Problem SettingProblem Setting

ScenarioScenario

MethodologyMethodology

ResultsResults

ConclusionConclusion

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Methodology (1)

Data is sent by the server with the CBR : λArrival Times delivered by Ethereal

cumulative data volume V(t) can be plotted:

MethodologyMethodology

Deducing ε

Plotting the margin

Definitions

The Study

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Methodology (2)

• The Cumulative (receiving) throughput,

Λ(t) = V(t)/t < λ ; (t>0)

• The margin function μ(t) :

μ(t) = [ λ - Λ(t) ]*t

= λ*t – V(t) > 0 ; (t>0)MethodologyMethodology

Deducing ε

Plotting the margin

Definitions

The Study

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Methodology (3)

the difference gives μ(t)

MethodologyMethodology

Deducing ε

Plotting the margin

Definitions

The Study

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Methodology (4) – deducing ε

then, plotting :

the Probability Density Function (PDF)

of the margin μ the Cumulative Distribution Function

(CDF) of the margin μ

MethodologyMethodology

Deducing ε

Plotting the margin

Definitions

The Study

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Methodology (5) – deducing ε

• Also, using the PBO of the video (during the time T’

MethodologyMethodology

Deducing ε

Plotting the margin

Definitions

The Study

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Methodology (6) – deducing ε

MethodologyMethodology

Deducing ε

Plotting the margin

Definitions

The Study

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Methodology (7) – deducing ε

• Choosing an appropriate ε ?Simple method : (e.g) ε = μ / CDF(μ) =0.9More judicuous:

Pstarvation = (Pr (B + < x) . fμ (x). dx < 10-4

 where, B = PBOfree and x from to infinity

(FB (x - ) . fμ (x). dx < 10-4

  ( CDF [PBOfree(x - )] *

PDF [(x)]. dx < 10-4

MethodologyMethodology

Deducing ε

Plotting the margin

Definitions

The Study

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The Study

Problem SettingProblem Setting

ScenarioScenario

MethodologyMethodology

ResultsResults

ConclusionConclusion

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Remembering Scenarii

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Results (1)

• For Friends 2x16, θ = 6.79 Mbyte 5 % * θ ~ 0.3 MByte• Using the simple method:

Scenario 1 : ε = 0.25 MByte Scenario 2 : ε = 0.30 MByte Scenario 3 : ε = 2.75 Mbyte !!! (need to use the second method found 1.4 Mbyte with method 2)Scenario 4 : ε = 0.31 MByte

ResultsResults

Problems Managing

SEQuence number

Found ε /scenario

The Study

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Results (2)

• Ethereal : IP ID field SEQ numbers of missing packets

ResultsResults

Problems Managing

SEQuence number

Found ε /scenario

The Study

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Results (3)

ResultsResults

Problems Managing

SEQuence number

Found ε /scenario

The Study

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Results (4)

ResultsResults

Problems Managing

SEQuence number

Found ε /scenario

The Study

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Results (5)

ResultsResults

Problems Managing

SEQuence number

Found ε /scenario

The Study

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Results (6)

• Pb 1 : μ(t) sometimes negative ?!?

μ(t) = = λ*t – V(t) > 0 ; (t>0)

e.g : scenario 2

ResultsResults

Problems Managing

SEQuence number

Found ε /scenario

The Study

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Results (7)

Choice of origin !!

ResultsResults

Problems Managing

SEQuence number

Found ε /scenario

The Study

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Results (8)

• Pb2 : Why cumulative loss data is different from the maximum value of μ ?

e.g. (scenario 2) respectively 0.17 Mbit & 2.4 Mbit

AP adjusting the sending rate :

AP sends with λAP < λ

& λAP is variable (VBR)ResultsResults

Problems Managing

SEQuence number

Found ε /scenario

The Study

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Results (9)

• Future possible corrections: study λAP (Sniffer near AP)

Suppress the time in the wired network

• Ter (wired) = Temission-reception

Temission = 1460*8/10*106 (10Mbps) =1.17ms

Tpropag = 5*2/200000 = 0.085 ms (neglected)

T traitment , Tqueues (negleted)

ResultsResults

Problems Managing

SEQuence number

Found ε /scenario

The Study

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Results (10)

• Ter (wired) ~ 1.17 ms

• mean IAT = 1460*8/ λ = 15 ms

ResultsResults

Problems Managing

SEQuence number

Found ε /scenario

The Study

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The Study

Problem SettingProblem Setting

ScenarioScenario

MethodologyMethodology

ResultsResults

ConclusionConclusion

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Conclusion (1)

• Tradeoff between wireless channel unreliability and Video Streaming stringent QoS requirements

• ε defined as buffer compensation

to manage the starvation phenomenom

• ε depends both on the wireless conditions and the video features

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Conclusion (2)

• Video features :PBOfree , T’ and θ• Wireless parameters : distance

AP/laptop, Mode, traffic duration, datagram lengths, mean rate, competing traffic, NloS…

• CBR λ, volume V(t)• Margin function defined :μ(t) = λ*t – V(t) > 0 ; (t>0)

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Conclusion (3)

• ε is deduced from the PBOfree (video features) and μ (wireless conditions)

e.g : ε / ( CDF [PBOfree(x - )] * PDF [(x)]. dx < 10-4

ε ~ 5% θ (unicast)

ε ~ 20% θ !! (multicast) to be reviewed

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Conclusion (4)

• Future work : solve origin problems consider λAP instead of λ

(use of sniffer in air interface)mobility/handoversDifferent laptops with different traffics at

different starting times

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THANK YOU

Mohamed Ali Ben Abid

Supervisors

Frank H. P. Fitzek

Hans Peter Schwefel

Censors

Karsten Thygesen

Thomas Toftegaard Nielsen