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Voice Over Internet- Issues

Voice Over Internet- Issues. Dr. Abhay Karandikar Department of Electrical Engineering Indian Institute of Technology, Bombay. Dr. Abhay Karandikar Department of Electrical Engineering Indian Institute of Technology, Bombay. Characteristics of Audio / Voice over Internet.

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Voice Over Internet- Issues

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  1. Voice Over Internet- Issues Dr. Abhay Karandikar Department of Electrical Engineering Indian Institute of Technology, Bombay. Dr. Abhay Karandikar Department of Electrical Engineering Indian Institute of Technology, Bombay.

  2. Characteristics of Audio / Voice over Internet • Internet phone generates audio consisting of talk spurts and silent periods • Talk Spurt - 8 Kbytes/sec • Bytes into chunks every 20 msec, header is appended • UDP segment is sent

  3. Problem for Real Time Services • Packet Loss • Retransmission mechanism is not suitable • 1% to 20% packet loss can be tolerated depending upon application • FEC can help conceal Packet Loss • End To End Delay • Delay smaller than 150ms are not perceived by human • 150 - 400 ms delay acceptable • Delay jitter • Due to random queuing delays

  4. Handling Jitter at the Receiver • Append ‘Sequence Number’ • Append ‘Time Stamp’ • Use ‘Playback Buffer’

  5. Fixed Playout Delay Graph Packet Generation N/W delay Buffer Sequence Number Playback Time

  6. Adaptive Playback Buffer • Estimate the network delay and the variance of the delay • Adjust the playback delay at the beginning of each talk spurt • May cause sender’s silence period to be compressed and elongated

  7. Adaptive Playback Algorithm for Internet Phone Let ti = timestamp of the i packet ri = time of arrival of ith packet at the receiver pi = Playback time of the ith packet di = Estimated Average network delay upon reception of ith packet vi = Average deviation of the delay from the estimated average delay

  8. continued…. di and vi are updated as follows. di = ( 1-a )di-1 + a( ri - ti ) vi = ( 1-a )vi-1 + a | ri - ti - di | Playback time is calculated as pi = ti + di + bvi

  9. Playout Time of the next Packet in the Spurt If ith is the first packet of the talk spurt Let qi = ti - pi then pj = tj + qi

  10. Problem • How to determine first packet of a talk spurt? If ti - ti-1 > 20 ms then ith packet is first packet of the talk spurt • What if there is a packet loss? Take the help of Sequence Number

  11. Issues • Question • Who does • Appending Sequence Number? • Timestamping to audio/video chunks? • Answer • Real-time Transport Protocol

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