Multicast and Unicast Real-TimeVideo Streaming Over

Wireless LANSApril. 27th, 2005

Presented by, Kang Eui Lee

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

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

Coding for Packet-Erasure Channels (cont.)

□ Partially-Synchronous version of ARQ

▪ Still requires low packet loss rate and low RTT

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

Streaming Video Over WLAN:A 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)

▪ ‘i’th layer is split into ‘i’ equal parts and (N,i) RS code is applied to it to form the N descriptions

Streaming Video Over WLAN:A Single User Case-MDFEC(cont.)

1 2 ... i ... N ...

1 i...32 FEC FECFECFEC

(N-i)

R1 RNRN-1RiRi-1R2

‘i’th (N,i) RS code

MR bitstream

1 i...32

Streaming Video Over WLAN:A Single User Case-MDFEC(cont.)

i... FEC FECFECFEC

1 FECFECFECFECFECFECFEC FEC

2 2 FECFECFECFECFECFEC FEC

N ...NN ............ N

iii

1 2 N... ...ii-1 Description 1

Descriptions

1

2

i

N

...

...

Layers (N,i) RS codes

FEC 2 N...... ii-1 Description 2

... FECFEC FEC

FEC

FEC

FEC

FEC

N...

N

i... FEC

Description N

Description i

Streaming Video Over WLAN:A 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);

}

Streaming Video Over WLAN:A Single User Case-Throughput

neI

timeTotal

data truesend torequired timeActual

eP

Throughput:

1. FEC :R.V. that represents the number of packet

erasures in a group of n packets :Probability of packet erasure

d

d

FEC ln

lkT

)Pr( knIn

k ne

kn

j

jne

je jnj

nPP

n

k

0 )!(!

!)1(

Streaming Video Over WLAN:A Single User Case-Throughput(cont.)

n

ke ad

dARQ eE

elel

klT )Pr(

)!1()!(

)!1()1()Pr( ,

kke

ePPeEwhere ke

ek

e

2. ARQE: R.V. that represents the total number of packets

sent in a successful transmission of k packet

3. Hybrid ARQ

n

ke ad

dHARQ eE

lel

klT )Pr(

Streaming Video Over WLAN:A Single User Case-Experiments

Streaming Video Over WLAN:A 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

Streaming Video Over WLAN:A Multi User Case(cont.)

} ,... , , ,{ 321 NRRRRR

)(rD)(Nq ji

□ Definitions▪ Rate Partition, ▪ Rate-Distortion function for rate ‘r’, ▪ Transmission profile,

▫probability of the ‘i’th client receiving j out of N▪ Expected Distortion(ED),

where ‘E’ is the source variance

N

jjjiii RDqEqRdE

10 )()]([

Streaming Video Over WLAN:A Multi User Case(cont.)

□ Maximal Regret Criterion▪ Optimal coding scheme is the one that minimizes,

▫ E[di]min is the minimum ED for the ith client achieved by using the optimal coding

scheme when it is the only client. ▫ E[di(R)] is the ED for the particular coding scheme being used

)][)]([(max)( miniii

dERdER

Streaming Video Over WLAN:A Multi User Case(cont.)

□ Constraints on solution▪ Total rate constraint of the clients: Rtot ▪ Total rate when MDFEC is used,

NN

RRN

RRN

RRN

RR NN

t

)(...

3

)(

2

)(

1123121

1 and ,...,2,1for )1(

where,1

Nj

N

jjj Nj

jj

NR

1 2 ... i ... N ...

R1 RNRN-1RiRi-1R2

1 i...32

Streaming Video Over WLAN:A Multi User Case(cont.)

▪ Resource constraint

▪ Embedding constraint

□ Proposed solution

(1) tott RR

(2) ...321 NRRRR

(2) and (1) subject to )(min

generally,or

(2) and (1) subject to )]([min

R

RdE

R

iR

Streaming Video Over WLAN:A 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

min][)]([)( iii dERdER

)(R

Streaming Video Over WLAN:A 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

)(Ri

)()( 21 RR 0)( Ri

)()( RR ji

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.

Streaming Video Over WLAN:A Single User Case-MDFEC(extra)

1

2

i

Split

...

Layer 1 (R1)

1

RS (N,i)

1 part (R1)

1 FEC FEC

Total, N(R1)

Layer 2 (R2-R1)

2 2

2 parts (R2-R1)/2 Total, N(R2-R1)/2

Layer i (Ri-Ri-1)

i...i

2 2 FEC FEC ... FEC

i i ... iFEC

FEC

...FEC

i parts (Ri-Ri-1)/i Total, N(Ri-Ri-1)/i

NN

RRN

RRN

RRN

RR NN

t

)(...

3

)(

2

)(

1123121

1 2 ... i ... N ...

R1 RNRN-1RiRi-1R2Marked MR bit-stream