Multicast and Unicast RealTime Video Streaming Over Wireless LANS

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Multicast and Unicast Real-TimeVideo Streaming
OverWireless LANS

• April. 27th 2005
• Presented by Kang Eui Lee

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Packet-Erasure Model for IEEE 802.11 LANs

• Two lower layers
• Physical layer
• Data link layer
• On the application level
• Can not access to those two layers
• User application see the wireless channel as
  an IP
• packet channel with erasures
• Simplest Model
• Erasures are i.i.d with probability of Pe

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Coding for Packet-Erasure Channels
Automatic Repeat reQuest (ARQ)
Asynchronous Reliable but with unbounded
delay Works well for data communication
Forward Error Correction (FEC) Synchronous
Protect data using parity packets No feedback
channel Original data can be recovered
perfectly
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Coding for Packet-Erasure Channels (cont.)

• Partially-Synchronous version of ARQ

Still requires low packet loss rate and low RTT
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Coding for Packet-Erasure Channels (cont.)
Reed-Solomon Code (n k) n is the
length of codeword k is the number of data
symbols in codeword RS code can be used for
correction and erasures Correct any (n-k)
erasures out of n
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Streaming Video Over WLANA Single User
Case-MDFEC
MDFEC(Multi Description FEC) Transcoding
mechanism to convert a prioritized MR
bitstream into a nonprioritized bitstream using
efficient FEC The progressive
bitstream is marked at N different
positions. (forms N resolution layers) ith
layer is split into i equal parts and (Ni) RS
code is applied to it to form the N
descriptions
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Streaming Video Over WLANA Single User
Case-MDFEC(cont.)
MR bitstream
ith (Ni) RS code
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Streaming Video Over WLANA Single User
Case-MDFEC(cont.)
Descriptions
Layers
(Ni) RS codes
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Streaming Video Over WLANA Single User
Case-Hybrid ARQ
Hybrid ARQ To combine the reliability of ARQ
and bounded delay of FEC
Algorithm main() send(first k data
packets) while(ARQ is not received Timeout
is not expired) send(n-k RS parity
packets) send(next k data packets)
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Streaming Video Over WLANA Single User
Case-Throughput
Throughput 1. FEC R.V. that represents
the number of packet erasures
in a group of n packets Probability of
packet erasure
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Streaming Video Over WLANA Single User
Case-Throughput(cont.)
2. ARQ E R.V. that represents the total number
of packets sent in a successful
transmission of k packet
3. Hybrid ARQ
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Streaming Video Over WLANA Single User
Case-Experiments
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Streaming Video Over WLANA Multi User Case
ARQ vs. FEC ARQ based schemes are less
appropriate Too many ACKs Different
user requires retransmission of different
packets Goal in the multicast scenario
Maximize some composite delivered quality
criterion given the total rate
constraint and the transmission profile
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Streaming Video Over WLANA Multi User
Case(cont.)
Definitions Rate Partition
Rate-Distortion function for rate r
Transmission profile probability of the
ith client receiving j out of N Expected
Distortion(ED) where E is the
source variance
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Streaming Video Over WLANA Multi User
Case(cont.)
Maximal Regret Criterion Optimal coding
scheme is the one that minimizes Edimin
is the minimum ED for the ith client achieved
by using the optimal coding scheme when it is
the only client. Edi(R) is the ED for the
particular coding scheme being used
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Streaming Video Over WLANA Multi User
Case(cont.)

• Constraints on solution
• Total rate constraint of the clients Rtot
• Total rate when MDFEC is used

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Streaming Video Over WLANA Multi User
Case(cont.)

• Resource constraint
• Embedding constraint
• Proposed solution

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Streaming Video Over WLANA Multi User
Case(cont.)

• Proposed solution
• Assuming that rate-distortion function is
  convex
• is
  also convex
• Since infimum/supremum of convex is also
  convex is convex
• Finding the minimax regret becomes convex
• optimization

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Streaming Video Over WLANA Multi User
Case(cont.)

• Proposed solution
• For 2 clients
• Since is convex and minimum of
  we
• choose R where
• For more than 2 clients
• Analyzing the users pairwise
• choose the highest point of

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Packet-Erasure Model
Erasures generally model two types of
events An unfortunate noise sequence that the
underlying error correcting code could not
correct Collisions at either an intermediate
node in a network (packet drop) or over the
shared comm. medium Recovery of
erasures Knowledge of the erasure comes back to
the TX. through either an acknowledgment packet
or by the transmitter observing the packet
getting mangled over the link.
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Streaming Video Over WLANA Single User
Case-MDFEC(extra)
Marked MR bit-stream