1. 1/12 Quantitative Characterization of Content-Based FEC Techniques on Interactive-Audio Transmissions over Wireless Networks José María González chema@cs.berkeley.edu IASTED CSN 2002

2. 2/12 Introduction Wireless Networks error behavior: –higher rates than wired channels –burstiness –handoffs => heavy losses (more burstiness) –causes: fading, environmental factors, interferences with other transmissions Problem: Bursty losses quickly degrade quality of audio streams over Wireless Networks

3. 3/12 Introduction (cont.) Interactive audio requirements: –bounded delay (~300 ms RTT max.) –large loss periods hamper effective communication –recovery from low-quality coding better than concealment from scratch Problem: Forget about retransmission! –ARQ infeasible due to delay RTT > maximum audio delay

4. 4/12 Error Concealment Take advantage of temporal redundancy –repeat the last correct sample –fade –insert silence Do not need extra information But, works bad with long errors

5. 5/12 Channel Speaker Listener Error Concealment

6. 6/12 Error Recovery Shuffled CB-FEC –send a (lower quality?) replica later, –delay playback point More resilient to losses But, increments BW and delay

7. 7/12 12345678910111213141516171819 0 Channel Speaker Listener 012 0 5 3 8 6 11 9 17 15 6 4 9 7 12 10 18 16 7 5 10 8 19 17 1256789101112 17 0 ??151634 Error Recovery

8. 8/12 Error Recovery Parameters Range: Bounded by the maximum delay tolerable by the users Coverage: Bounded by the maximum BW usable 1st and 2nd-class replicas quality: PCM, ADPCM, LPC,... Bounded by the BW Fragmentation: bursty period mean length ~ 4 ms, normally smaller than distance between packets

9. 9/12 Simulation Quantitative characterization via simulation of the tradeoffs in error recovery ns WaveLAN Channel model: used Nguyen’s traces (two-state empirical error) Audio CBR traffic (PCM, ADPCM, GSM, LPC)

10. 10/12

11. 11/12

12. 12/12 Future work Experimental Results (Subjective Quality) Go wired! Negotiation of Error Recovery configuration between communication peers Differently-weighted information (GSM) Multicast Asymmetric protocols Congestion control –multiple users in wireless channels –wired path