Voice Over IP - PowerPoint PPT Presentation

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Voice Over IP

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  1. Voice Over IP Developing IPHONE Jeremy Stanley CS 460 section 1

  2. VoIP Advantages • No long distance fees • No dedicated circuit • More efficient use of networks • 64 Kbps raw audio vs. 13 Kbps compressed • Data need not be transmitted during silence

  3. VoIP Issues • Latency • Packetization • Network jitter • Quality of Service • Real-time requirements • Addressing Issues • IPv6 is a pratical necessity

  4. Voice Encoding • PCM (pulse code modulation) • ADPCM - adaptive delta PCM • Fast 4:1 Compression, Good quality • Vocoders (LPC, CELP, GSM) • Slower, 12-30x compression common • Voice only - Music does not work well

  5. Introduction to IPHONE • Win32 application • Uses Windows Multimedia and Sockets APIs for audio and communication • Two-way, full duplex voice channel (PC to PC) • Runs over UDP or TCP, supports data compression and silence detection

  6. IPHONE Design

  7. Screen Shot

  8. The Naïve Algorithm • Receive thread • Receive a packet • Decode and play audio • Return to step 1 • Send thread • Record audio • Encode, packetize, and send • Return to step 1

  9. Problems • Sounds like talking into a fan • Latency quickly rises to intolerable levels

  10. Timing Issues

  11. Solutions • Asynchronous sound processing • Encode and send previous packet while recording current packet • Receive and decode next packet while playing current packet • At least two send and receive buffers required

  12. Revised Algorithm

  13. Network Jitter • The double-buffered solution last described works well on a LAN • Delay still accumulates when receive buffer empties due to delayed packets • Increasing receive buffer only does so much - it will eventually be exhausted

  14. Jitter Solutions • Drop late packets • Imperceptibly increase playback speed • Shorten delays between words • Use silence detection • IPHONE does this

  15. Silence Detection • Do not send audio chunks whose maximum amplitude is below a silence threshold • Phone conversations are 60% silence • Turns voice into bursty data

  16. Comfort Noise • Total silence construed as dead line • Substitute pre-recorded "open mic" • Play low-volume white noise • Record and repeat periods of silence during conversation

  17. Demonstration/Questions