1. “Compensating for Packet Loss in Real-Time Applications“
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

2. Overview Signal Processing and Networking Techniques
Three Strategies for VoIP Transport
Simulations
Conclusions
© ftw. 2003

3. Dropping Damaged IP-packets Considered 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

4. Signal Processing Quality related algorithms: A C B
Speech coding
Packet loss concealment
De-jitter buffers
Unequal level protection ULP; E.g., 3GPP Adaptive MultiRate (AMR) codec A C B
Perceptual importance
High
Low
Speech data bits (ULP)
81 Bits
163 Bits
60 Bits
© ftw. 2003

5. Networking Avoid packet drops caused by bit errors RTP/UDP/IP
UDP checksum coverage
Bit error -> entire packet lost!
RTP/UDPlite/IP IP B A
RTP
UDP C
UDP checksum coverage
Error at less important bits -> packet saved!
© ftw. 2003

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
© ftw. 2003

7. Transmission StrategiesIP
UDP
RTP A B C
Strategy 1
Strategy 2
Strategy 3
Part corrupted
Strategy 1
Header
Drop
Class A data
Class B/C data
Strategy 2
Drop
Keep
Strategy 3
Drop
Keep
© ftw. 2003

8. Simulations Bit error rate Bitstreams for 3 strategies MatLab Codec
Simulator
Codec
Decoder
Reference
speech
sample
Evaluation
(PESQ)
Degraded
speech
samples
Speech
Database
Estimated
speech-quality
[PESQ-MOS]
© ftw. 2003

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
© ftw. 2003

10. Results: No Header-Compression
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11. Results: with Header-Compression
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12. Packet Loss Analysis Use of corrupted payload avoids packet losses:
43.6%
33.6%
28.0%
© ftw. 2003

13. Packet Loss Analysis Contd. Compressed Headers
Less than 5% packet loss at a BER of 10-3
25%
11.5%
4.1%
Old curves in grey!!
Packet loss is more severe than bit errors
=> Due to reduced packet loss rates => ROHC ... Better performance in perceived speech-quality
© ftw. 2003

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
© ftw. 2003