“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 Hammer@ftw.at

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

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

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

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

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

Transmission Strategies IP 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

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

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

Results: No Header-Compression © ftw. 2003

Results: with Header-Compression © ftw. 2003

Packet Loss Analysis Use of corrupted payload avoids packet losses: 43.6% 33.6% 28.0% © ftw. 2003

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

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