Forschungszentrum Telekommunikation Wien
Reducing Packet Loss by Employing Damaged Speech Data
Florian [email protected]
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)?
© ftw. 2003
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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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