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  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

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 – ms delay acceptable  Delay jitter –Due to random queuing delays

Handling Jitter at the Receiver  Append ‘Sequence Number’  Append ‘Time Stamp’  Use ‘Playback Buffer’

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

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

Adaptive Playback Algorithm for Internet Phone Let t i = timestamp of the i packet r i = time of arrival of i th packet at the receiver p i = Playback time of the i th packet d i = Estimated Average network delay upon reception of i th packet v i = Average deviation of the delay from the estimated average delay

continued…. d i and v i are updated as follows. d i = ( 1-  )d i-1 +  ( r i - t i ) v i = ( 1-  )v i-1 +  | r i - t i - d i | Playback time is calculated as p i = t i + d i +  v i

Playout Time of the next Packet in the Spurt If i th is the first packet of the talk spurt Let q i = t i - p i then p j = t j + q i

Problem  How to determine first packet of a talk spurt? If t i - t i-1 > 20 ms then i th packet is first packet of the talk spurt  What if there is a packet loss? Take the help of Sequence Number

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