Download - Department of Communication Technology
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Video Streaming over 802.11b LANWireless channel unreliability : managing the
starvation phenomenon
Mohamed Ali Ben Abid
Monday, 28 June 2004
Department of Communication Technology
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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)
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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)
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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
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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
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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.
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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