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

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

3/12 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

4/12 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

5/12 Channel Speaker Listener Error Concealment

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/ Channel Speaker Listener ?? Error Recovery

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

9/12 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)

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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