Transcript Cross-Layer Wireless Multimedia

Cross-Layer Wireless Multimedia
Presented by Scott Kristjanson
CMPT-820
Multimedia Systems
Instructor: Dr. Mohamed Hefeeda
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Outline
 Challenges and requirements for wireless transmission of
multimedia
 Need for cross-layer optimization
 Short summary of 802.11 wireless LAN standard and impact
on wireless multimedia
 Example of cross-layer impact on throughput efficiency
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Introduction
 Evolution of different wireless technologies
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Challenges and Requirements for
Wireless Transmission of Multimedia
 Wireless Networks Exhibit a large Variation in Channel Conditions
 Noise, Mobility, Multipath fading, Cochannel interference, Handoff,
…
 variability of wireless resources leads to unsatisfactory user
experience
 High bandwidths
 transmission bit rates of several Mbps.
 High-definition TV
 Very stringent delay constraints:
 delays of less than 200 ms are for interactive applications
 delays of 1–5 s for multimedia streaming applications
 Quality of Service (QoS) issues becomes essential
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Need for Cross-Layer Optimization
 Normal Design
 Multimedia compression and streaming algorithms do not
consider the mechanisms provided by the lower layers
 Resource management, adaptation, and protection strategies
available in the lower layers of the OSI optimized without
explicitly considering the specific characteristics of the
multimedia applications
 Simpler implementation, but local optimization of all layers
may not lead to global optimization.
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802.11 wireless LAN standard
 Wireless version of Ethernet
 Specifications for the physical layer and the media access
control (MAC) layer
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Functionalities Provided by 802.11
 PHY layer:
 Several modulation and coding schemes
 to adapt to changing channel conditions, varying code rates can be employed
 802.11a PHY provides eight different PHY modes with different modulation
schemes and code rates
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Functionalities Provided by 802.11
 MAC layer:
 Control of access to the shared wireless medium
 Access to the shared wireless medium is paramount For
transmitting delay-sensitive multimedia
 Mechanisms
 Distributed coordination function (DCF)
 Point coordination function (PCF)
 Enhanced Distributed Channel Access (EDCA)
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Distributed Coordination Function (DCF)
 DCF provides basic access service
 Best-effort data transfer
 All stations contend for access to medium and relinquishe control
after transmitting a single packet
 CSMA-CA
 Ready stations wait for completion of transmission
 All stations must wait Interframe Space (IFS)
DIFS
 Distributed, fair access to the
Contention
window medium
wireless
 Not appropriate when dealing with real-time multimedia
DIFS
applications
that exhibit different delay deadlines and bandwidth
SIFS
requirements
Busy medium
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Defer access
Next frame
Wait for
reattempt time
Time
Enhanced Distributed Channel Access
 Four levels of priorities or access categories (AC)
 Higher priority → shorter maximum back-off time →
higher priority wins access to the medium more frequently
than the lower priority
 Provides (DiffServ) QoS
 Nondeterministic nature → not possible to guarantee
parameters such as bandwidth, jitter, and latency → not
suitable for multimedia streaming
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Point Coordination Function (PCF)
 Designed to support delay-sensitive applications
 Contention free access to the wireless medium - controlled
by a point coordinator (PC)
 based on a poll-and-response protocol: all stations are polled
for a certain amount of time during a service interval →
provides real-time applications a guaranteed transmission
time (opportunity, no actual guarantee)
DIFS
Contention
window
PIFS
DIFS
SIFS
Busy medium
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Defer access
Next frame
Wait for
reattempt time
Time
Example of Cross-Layer Impact on Throughput,
Efficiency, and Delay for Video Streaming
 Assumptions:
 Polling based mode of MAC standard (PCF)
 Adaptive retransmission at MAC layer
 Reed-Solomon (RS) codes at application layer
 Video packets size: La bytes
 Packets are not fragmented in any of the lower layers
 Overhead of higher layers: O bytes
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Average Packet Transmission Duration
m
m
Davm ( L, R)  Davm , succ ( L, R) Psucc
( L, R)  Davm ,unsucc ( L, R)(1  Psucc
( L, R))
 Davm ,succ (L, R) : The average transmission duration for a packet
with an L-byte payload, given that the transmission is
successful with the retransmission limit of R
m
 Dav,unsucc ( L, R): The average transmission duration for a packet
with an L-byte payload, given that the transmission is
successful with the retransmission limit of R
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Average Packet Transmission Duration
 Good Cycle: successful packet transmission
 Tgood: average transmission duration for a good cycle
 Bad Cycle: retransmission due to packet or ACK error
 Tbad: average transmission duration for a good cycle
 Probability of a successful transmission



m
L  1 Pem,ack 1 Pem,data L
Pgood_cycle
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Average Packet Transmission Duration
 Average successful transmission duration:
( i| L )
P
m
m
Davm ,succ ( L, R)   m succ [iTbad
( L)  Tgood
( L)]
i 0 Psucc ( L, R)
R
 the probability that the packet with L-byte data payload is transmitted
successfully after the ith retransmission
m
m
i m
Psucc
(i | L)  [1  Pgood
(
L
)]
Pgood _ cycle(L)
_ cycle
 the probability that the packet with L-byte data payload is transmitted
successfully after the ith retransmission
m
m
R1
Psucc
(L, R)  [1  Pgood
(
L
)]
_ cycle
 Average unsuccessful transmission duration
m
Davm ,unsucc (L, R)  (R 1)Tbad
(L)
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Throughput Efficiency
 (N,K)RS, decoder can correct up to N-k packet erasure.
 Probability of error after RS decoding:
m
RS
P
 1
N K
 C ( N , i)( p
) (1  prm ) N i
m i
r
i 0
 Where the error probability of data packet after R
retransmission is:
m
PRS
 1  Psucc ( L, R)
 Throughput efficiency:
m
ERS
( La , R, N , K ) 
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8La ( K (1  P ) 
m
RS
N
k
C ( N , i )( prm )i (1  prm ) N i )

N
i  N  k 1
m
NDav
( La  O, R) DR(m)
( N  i)
Impact of Cross-Layer Optimization on
Video Quality
 Optimal packet size to maximize video quality
 Given RS code, retransmission limit
L*a  arg max QE m
RS
La
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Summary
 QoS is essential for multimedia transmission over wireless
networks
 Local optimization of all layers may not lead to global
optimization. Even poor performance when wireless
resources are limited.
 Cross layer design has impact on performance such as
throughput efficiency.
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References
 Schaar and Chou (editors), Multimedia over IP and
Wireless Networks: Compression, Networking, and
Systems, Elsevier, 2007
 http://en.wikipedia.org/wiki/ReedSolomon_error_correction
 http://technet.microsoft.com/enus/library/cc757419%28WS.10%29.aspx
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