Forschungszentrum Telekommunikation Wien

Reducing Packet Loss by Employing Damaged Speech Data

Florian HammerHammer@ftw.at

ETSI STQ Workshop on“Compensating for Packet Loss in Real-Time Applications“

February 11th, 2003

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Overview

Signal Processing and Networking Techniques Three Strategies for VoIP Transport Simulations Conclusions

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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)?

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Signal Processing

Quality related algorithms:- Speech coding- Packet loss concealment- De-jitter buffers- Unequal level protection ULP; E.g., 3GPP Adaptive

MultiRate (AMR) codec

A CB

Perceptual importanceHigh Low

Speech data bits (ULP)

81 Bits 163 Bits 60 Bits

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Avoid packet drops caused by bit errors RTP/UDP/IP

Networking

IP BARTPUDP C

UDP checksum coverage

IP BARTPUDP C

UDP checksum coverage

RTP/UDPlite/IP

Bit error -> entire packet lost!

Error at less important bits -> packet saved!

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Robust Header Compression (ROHC)

Headers -> huge overhead RTP/UDP/IP (12/8/20 Bytes)

Headers BA C

BA C

ROHC (e.g., 4 Bytes)

330 : 256* [Bits]

42 : 256* [Bits]

cHdrs

*AMR 12.2 kb/s, 1 frame per packet, RTP bandwidth efficient mode

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Transmission Strategies

IP BARTPUDP C

Strategy 1

Strategy 3

Drop

Keep

Keep

Strategy 2

Drop

Drop

Keep

Part corrupted Strategy 1

Header Drop

Class A data Drop

Class B/C data Drop

Strategy 3

Strategy 2

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Simulations

Codec DecoderMatLab

Simulator

SpeechDatabase

Evaluation(PESQ)

Estimatedspeech-quality[PESQ-MOS]

Bit error rate Bitstreamsfor 3 strategies

Referencespeechsample

Degradedspeechsamples

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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

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Results: No Header-Compression

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Results: with Header-Compression

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Packet Loss Analysis

Use of corrupted payload avoids packet losses:

43.6%

33.6%28.0%

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Packet Loss Analysis Contd.Compressed Headers

Less than 5% packet loss at a BER of 10-3

25%

11.5%

4.1%

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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