1. Multicast and Unicast Real-Time Video Streaming Over Wireless LANS April. 27 th, 2005 Presented by, Kang Eui Lee

2. 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 P e

3. 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

4. Coding for Packet-Erasure Channels (cont.) □ Partially-Synchronous version of ARQ ▪ Still requires low packet loss rate and low RTT

5. 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

6. 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

7. Streaming Video Over WLAN: A Single User Case-MDFEC(cont.) ‘i’ th (N, i ) RS code MR bitstream

8. Streaming Video Over WLAN: A Single User Case-MDFEC(cont.) Descriptions Layers(N, i ) RS codes

9. 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); }

10. Streaming Video Over WLAN: A 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

11. Streaming Video Over WLAN: A 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

12. Streaming Video Over WLAN: A Single User Case-Experiments

13. 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

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

15. Streaming Video Over WLAN: A Multi User Case(cont.) □ Maximal Regret Criterion ▪ Optimal coding scheme is the one that minimizes, ▫ E[d i ] min is the minimum ED for the ith client achieved by using the optimal coding scheme when it is the only client. ▫ E[d i (R)] is the ED for the particular coding scheme being used

16. Streaming Video Over WLAN: A Multi User Case(cont.) □ Constraints on solution ▪ Total rate constraint of the clients: R tot ▪ Total rate when MDFEC is used,

17. Streaming Video Over WLAN: A Multi User Case(cont.) ▪ Resource constraint ▪ Embedding constraint □ Proposed solution

18. 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

19. 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

20. 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.

21. Streaming Video Over WLAN: A Single User Case-MDFEC(extra) Marked MR bit-stream