Allowing Errors in Speech over Wireless LANs Ian Chakeres, Hui Dong, Elizabeth Belding-Royer, Allen Gersho and Jerry Gibson University of California, Santa Barbara Ian Chakeres
Outline Motivation Related Work System model Design Evaluation Conclusions Ian Chakeres
Motivation Wireless Erroneous channel Retransmissions Congestion Voice Significant error concealment available Improve capacity and maintain quality Ian Chakeres
Related Work Voice encoding improvements Bit rate, error correction, error and loss concealment Additional protection Redundant transmissions, retransmissions, packet size, wireless data rate UDP Lite [Larzon 99] Partial UDP checksum Requires lower layer support Wireless usage not examined Ian Chakeres
System Model Digitized Voice IEEE 802.11 Gilbert Error Model Ian Chakeres
Voice over IP Encoder/decoder Encoder Bit Rate Bit stream G.711 64 kbps Packetization 5.3, 6.3 Quality G.723.1 kbps Delay, jitter, loss Ian Chakeres
IEEE 802.11 Distributed Coordination Function (DCF) RTS-CTS-Data-ACK Data-ACK MAC layer retransmission Collisions and errors Up to 7 times Ian Chakeres
Gilbert Error Model P gb Two state Markov model P bg Good Bad Bursty errors Experimentally Model ABER determined values Avg 1% Average (Avg) Noise Limited (NL) NL 1% Interference Limited (IL) IL 8% Frequency Selective Fading Limited (FSFL) FSFL 0.3% Ian Chakeres
Design - CRCs and Checksums MAC Header MAC CRC IP Header Protection methods UDP Header MAC CRC: Entire packet IP checksum: IP header Voice Data UDP checksum Headers + payload or disabled Typical CRC = T-CRC Ian Chakeres
New Designs MAC Header MAC CRC IP Header MAC Header CRC = M-CRC UDP Header MAC CRC - MAC header only Voice Data UDP checksum off Entire Header CRC = E-CRC MAC CRC - All headers UDP checksum off Ian Chakeres
Experiments Node Node 1 Wireless Wireless Communication Communication n Network Topology Ian Chakeres
Voice Traffic Voice Bytes Encoding Bit Rate per Packet per Packet G.723.1 6.3 kbps 30 ms 24 G.711 64 kbps 20 ms 160 G.711 64 kbps 25 ms 200 G.711 64 kbps 30 ms 240 G.711 64 kbps 35 ms 280 G.711 64 kbps 40 ms 320 Ian Chakeres Ian Chakeres
Voice Traffic Voice Bytes Encoding Bit Rate per Packet per Packet G.723.1 6.3 kbps 30 ms 24 G.711 64 kbps 20 ms 160 G.711 64 kbps 25 ms 200 G.711 64 kbps 30 ms 240 G.711 64 kbps 35 ms 280 G.711 64 kbps 40 ms 320 Ian Chakeres
General Results 100 1.5 Packet Delivery Ratio (%) 95 90 1.0 T-CRC Delay (sec) 85 M-CRC 80 0.5 E-CRC 75 70 0 1 2 3 4 1 2 3 4 Number of Calls Number of Calls 2.0 100 Transmissions per 1.8 80 Packet Delivered 1.6 60 CDF 1.4 40 1.2 20 1.0 0 1 2 3 4 0 1 2 3 4 5 Number of Calls % Bits in Error E-CRC Ian Chakeres
Efficient Voice Encoder 100 1.5 Packet Delivery Ratio (%) 95 90 1.0 T-CRC Delay (sec) 85 M-CRC 80 0.5 E-CRC 75 70 0 1 2 3 4 5 6 7 8 9 101112 1 2 3 4 5 6 7 8 9 101112 Number of Calls Number of Calls 2.0 Transmissions per 1.8 Packet Delivered G.723.1 1.6 1.4 6.3 kbps 1.2 1.0 1 2 3 4 5 6 7 8 9 101112 Number of Calls Ian Chakeres Ian Chakeres
Voice Frame Size 100 0.25 Packet Delivery Ratio (%) T-CRC 99 0.20 M-CRC Delay (sec) 98 0.15 E-CRC 97 0.10 96 0.05 95 0 20 25 30 35 40 20 25 30 35 40 Frame Size (ms) Frame Size (ms) 2.0 Transmissions per 1.8 Packet Delivered G.711 1.6 1.4 3 Calls 1.2 1.0 20 25 30 35 40 Frame Size (ms) Ian Chakeres
Harsh Wireless Conditions - IL 100 1.5 Packet Delivery Ratio (%) 95 90 1.0 T-CRC Delay (sec) 85 M-CRC 80 0.5 E-CRC 75 70 0 1 2 3 4 1 2 3 4 Number of Calls Number of Calls 2.0 100 Transmissions per 1.8 80 Packet Delivered 1.6 60 CDF 1.4 40 1.2 20 1.0 0 1 2 3 4 0 5 10 15 20 25 30 Number of Calls % Bits in Error E-CRC Ian Chakeres
Conclusions Allowing bit errors results in Lower delay More capacity Acceptable call quality Future work User studies Multihop networks Video and other multimedia Ian Chakeres
Questions? Ian Chakeres idc@cs.ucsb.edu Research funded by Intel & NSF Ian Chakeres
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