Transcript Voice Over IP

Voice Over IP
Developing IPHONE
Jeremy Stanley
CS 460 section 1
VoIP Advantages
No long distance fees
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No dedicated circuit
More efficient use of networks
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64 Kbps raw audio vs. 13 Kbps
compressed
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Data need not be transmitted during
silence
VoIP Issues
Latency
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Packetization
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Network jitter
Quality of Service
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Real-time requirements
Addressing Issues
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IPv6 is a pratical necessity
Voice Encoding
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PCM (pulse code modulation)
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ADPCM - adaptive delta PCM
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Fast 4:1 Compression, Good quality
Vocoders (LPC, CELP, GSM)
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Slower, 12-30x compression common
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Voice only - Music does not work well
Introduction to IPHONE
Win32 application
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Uses Windows Multimedia and Sockets
APIs for audio and communication
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Two-way, full duplex voice channel
(PC to PC)
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Runs over UDP or TCP, supports
data compression and silence
detection
IPHONE Design
Screen Shot
The Naïve Algorithm
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Receive thread
1.
Receive a packet
2.
Decode and play audio
3.
Return to step 1
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Send thread
1.
Record audio
2.
Encode, packetize, and send
3.
Return to step 1
Problems
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Sounds like talking into a fan
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Latency quickly rises to intolerable
levels
Timing Issues
Solutions
Asynchronous sound processing
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Encode and send previous packet while
recording current packet
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Receive and decode next packet while
playing current packet
At least two send and receive
buffers required
Revised Algorithm
Network Jitter
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The double-buffered solution last
described works well on a LAN
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Delay still accumulates when
receive buffer empties due to
delayed packets
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Increasing receive buffer only does
so much - it will eventually be
exhausted
Jitter Solutions
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Drop late packets
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Imperceptibly increase playback
speed
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Shorten delays between words
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Use silence detection
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IPHONE does this
Silence Detection
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Do not send audio chunks whose
maximum amplitude is below a
silence threshold
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Phone conversations are 60%
silence
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Turns voice into bursty data
Comfort Noise
Total silence construed as dead line
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Substitute pre-recorded "open mic"
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Play low-volume white noise
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Record and repeat periods of silence
during conversation
Demonstration/Questions