Voice Over IP Developing IPHONE Jeremy Stanley CS 460 section 1

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

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

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

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

IPHONE Design

Screen Shot

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

Problems  Sounds like talking into a fan  Latency quickly rises to intolerable levels

Timing Issues

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

Revised Algorithm

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

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

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

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

Demonstration/Questions