Information and communication technologies principles and perspectives

1. Information and Communication
Technologies(ICT)
Principles and Perspectives
Lecture : Distributed multimedia systems
Study guide(17) essential reading (7)

2. Beyond best effort
• Best effort IP protocol
• Variable network delay
• This variable packet network delay within the same media stream is called
jitter

3. Approaches
• Integrated services philosophy
• Fundamental changes required in internet architecture
• Laissez-faire
• No change required. QoS should be managed at application layer
• Differentiated services philosophy
• Compromise between above 2. Fewer changes enough to support QoS

4. Delay/jitter
• Audio is generating 8000 bytes/sec.
• UDP chunk is being prepared after 20 millisec
• Number of bytes in a chunk
(8000 bytes/sec) x (20 millisec) = 160 bytes
• After every 20 millisec packet is sent and reached to client after end to end
delay

5. Packet jitter

6. End to end delay
• processing delay
• Intermediate routers for checking bit error and output link
• Queuing delay
• Waiting at router’s output link for transmission. Congestion level etc.
• Transmission delay
• Time needed to send the 160 bytes onto the router’s output link

7. End to end delay
• propagation delay
• actual physical distance of the links travelled by the audio packets from the server to
the client application
• end-system processing delay:
• the time spent decompressing and further processing the audio stream on the client
side.

8. Removing jitter
• Prepending each chunk with a timestamp
• Sequence numbers
• Delaying playout of chunks at the receiver
• This is essentially is removing jitter on client

9. Packet Loss
• By product of computer networks for using unreliable protocols
• UDP Vs TCP
• Loss occurs
• Packet arrives too late
• Network congestion ( router buffer delay)

10. Adaptive playout delay

11. Recovering from Packet loss
• A packet is lost either if it never arrives at the receiver or if it arrives after its
scheduled playout time
• No use to resend in real time conversational applications
• Loss anticipation
• FEC, interleaving, error concealment, CDN

12. Loss recovery schemes
• Forward error correction (FEC)
• Interleaving
• FEC
• Send an extra chunk for every n chunks.
• Sends n+1 chunks , increase bandwidth factor 1/n (for n=3, increase is 33%)
• Paylout delay for n+1
• To use bandwidth efficiently if we increase n, chances of more than 1 chunk losing increase

13. FEC
• 2nd FEC approach is to send lower
resolution audio stream as
redundant

14. FEC
• To avoid consecutive packet loss append (n-1) and (n-2) with nth chunk

15. Interleaving
• Does not add bandwidth
• Original chunks into smaller units, and then interleaved them into audio
stream such that one big chunk will contain smaller units of different chunks
• Increase payload as receiver has to wait for all chunks
• Used in streaming but not efficient for interactive applications

16. Interleaving

17. Error concealment
• Attempt to produce a replacement for a lost packet that is similar to the
original
• these techniques work for relatively small loss rates (less than 15 percent),
and for small packets (4–40 msecs).
• When the loss length approaches the length of a phoneme (5–100 msecs)
these techniques break down, since whole phonemes may be missed by the
listener

18. Content Distribution Networks ( CDN)
• Replicate content on multiple locations
• Hundred of locations as memory and processing power is cheap
• Chances that client app will find content closer to it
• client avoids the enhanced probability with which impairments such as loss
and delay occur over longer paths