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Reducing Packet Loss by Employing Damaged Speech Data

ETSI STQ Workshop on “Compensating for Packet Loss in Real-Time Applications“ February 11 th , 2003. Reducing Packet Loss by Employing Damaged Speech Data. Florian Hammer Hammer@ftw.at. Overview. Signal Processing and Networking Techniques Three Strategies for VoIP Transport Simulations

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Reducing Packet Loss by Employing Damaged Speech Data

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  1. ETSI STQ Workshop on “Compensating for Packet Loss in Real-Time Applications“ February 11th, 2003 Reducing Packet Loss by Employing Damaged Speech Data Florian Hammer Hammer@ftw.at

  2. Overview • Signal Processing and Networking Techniques • Three Strategies for VoIP Transport • Simulations • Conclusions

  3. Dropping Damaged IP-packetsConsidered Harmful • Links with bit errors currently result in IP-packets being dropped • Quality of speech signals is severely degraded by dropped packets • Approach: Save corrupted speech data • Combination of signal processing and networking techniques • AMR codec provides uneven level protection (ULP) • UDP-Lite allows for arbitrary checksum coverage • What can be gained by using RObust Header Compression (ROHC)?

  4. Speech data bits (ULP) A B C High Low Perceptual importance Signal Processing • Quality related algorithms: • Speech coding • Packet loss concealment • De-jitter buffers • Unequal level protection ULP; E.g., 3GPP Adaptive MultiRate (AMR) codec 81 Bits 163 Bits 60 Bits

  5. IP UDP RTP A B C UDP checksum coverage IP UDP RTP A B C UDP checksum coverage Networking • Avoid packet drops caused by bit errors • RTP/UDP/IP Bit error -> entire packet lost! • RTP/UDPlite/IP Error at less important bits -> packet saved!

  6. Robust Header Compression (ROHC) • Headers -> huge overhead • RTP/UDP/IP (12/8/20 Bytes) Headers A B C 330 : 256* [Bits] • ROHC (e.g., 4 Bytes) cHdrs A B C 42 : 256* [Bits] *AMR 12.2 kb/s, 1 frame per packet, RTP bandwidth efficient mode

  7. Strategy 2 Strategy 3 Transmission Strategies IP UDP RTP A B C Strategy 1

  8. Simulations Bit error rate Bitstreams for 3 strategies MatLab Simulator Codec Decoder Reference speech sample Evaluation (PESQ) Degraded speech samples Speech Database Estimated speech-quality [PESQ-MOS]

  9. Bit Error Model • Simulation of • Additive White Gaussian Noise Channel (xDSL) • Binary Symmetric Channel (UMTS) • Number of bit errors X within actual packet: • Binomial distribution X » B(N,p) • N...Packet size [Bits] • p...Bit Error Rate • Error locations LX within the packet: • Uniform distribution

  10. Results: No Header-Compression

  11. Results: with Header-Compression

  12. Packet Loss Analysis • Use of corrupted payload avoids packet losses: 43.6% 33.6% 28.0%

  13. Packet Loss Analysis Contd.Compressed Headers • Less than 5% packet loss at a BER of 10-3 25% 11.5% 4.1%

  14. Conclusions • Dropping packets degrade the speech-quality much more than bit errors • Using techniques like UDP-Lite and ROHC improves the speech-quality by reducing packet losses • For example, using traditional IP-transport with a BER 10-4 we get a PESQ-MOS of 3.0, while using UDP-Lite and ROHC the quality increases to 3.7

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