1. 1 Lecture 17 – March 21, 2002 Content-delivery services. Multimedia services Reminder  next week individual meetings and project status report are due.

2. 2 Content-delivery services

3. 3 Multimedia Services Voice over IP Still images Audio broadcast Video broadcast Video on demand Teleconferencing Videoconferencing Remote instrumentation Telemedicine

4. 4 Analog and Digital Information Sampling – Nyquist theorem Quantization – quantization noise Examples: voice – sampling rate 8,000 samples/sec 8 bits/sample-256 quantization levels music – sampling rate 48,000 samples/sec 16 bits/sample color video – 10,000,000 samples/sec 24 bits/sample

5. 5 Timing requirements for data streaming Interactive service – voice over IP, teleconferencing  delay and jitter sensitive but loss tolerant Video streaming  rate sensitive and to some extent loss tolerant. 25 frames/sec Telemedicine and remote instrument control  delay and loss sensitive Stored media applications – video on demand  less strict/demanding

6. 6 Delay and Jitter Play-out delay End-to-end delay Delay at the sender Data transmission and propagation delay Delay at the receiver Jitter – variability of end to end delay For a normal conversation – play out delay < 100 msec  not noticeable < 400 msec  acceptable > 400 msec  not acceptable

7. 7 Delay and Jitter

8. 8 Removing the Jitter in Voice over IP Fixed play-out delay Adaptive play-out delay  takes advantage of silent periods between talk spurts Fill in a buffer at a variable rate and drain it at a constant rate.

9. 9 Data Streaming Data stream – sequence of bytes flowing out or into a process. Media server Media player Often they require a Web browser to connect to the server and download a metafile describing the media player.

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11. 11 Error Recovery Schemes Forward error correction – sender adds redundant information. Packet (i) contains the original information and a low-quality version of packet (i-1) Interleaving  the stream is scrambled. Packet (i) does not contain samples adjacent to each other.

12. 12 Real-time Protocol (RTP) RTP – application layer protocol Uses UDP; TCP congestion control and error control prevent delay or rate guarantees. RTP supports several audio formats including PCM, GSM, G.722, MPEG audio – MP3 video formats – JPEG, MPEG 1, and MPEG 2.

13. 13 RTP One may have multiple RTP connections; one for audio and one for video RTP supports unicast and multicast RTP packet: Header + Payload Header Payload type Magic number to identify the audio or video format Sequence number of the packet Time stamp Random number associated with the RTP session

14. 14 RTCP – RTP control protocol Senders – periodic source reports RTP session identifier Application generating the stream Address of source Receivers periodic reciver reports Number of packets lost Jitter Number of packets Received

15. 15 RTCP An application may use may use the data provided by RTCP to Modify transmission rates Synchronize multiple streams Diagnostics For multicast RTCP limits the frequency of receiver reports.

16. 16 Real-time Streaming Protocol (RTSP) Supports interactive applications Out-of-band channel

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18. 18 Data compression

19. 19 Data compression Text compression Still image compression Audio and Video compression

20. 20 Loseless compression Differential encoding – amplitudes of consecutive samples differ only slightly. Entropy encoding – removes the redundancy based upon the entropy Run Length Encoding Statistical Encoding

21. 21 Text Compression Huffman Encoding Produces optimal results only if the probability of occurrence of individual characters is a power of ½. Static Huffman Encoding Adaptive Huffman Encoding Lemplel-Ziv

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24. 24 Lempel-Ziv Construct a dictionary consisting of all the substrings in the input string Instead of transmitting a substring we transmit the index of the substring in the dictionary. Lempel-Ziv-Welsh  the dictionary is constructed dynamically.

25. 25 Compression factor for LZ N  # of characters in the text n  bits/character L  average length of a substring D  # of entries in the dictionary D=2 d compression = (N x n) / (N/L x d) = ( n x L) / d

26. 26 Still image compression GIF – Graphics Interchange Format Reduces the number of colors in the image from 2 24 to 2 8 Selects the 256 colors that match best the colors in the picture and builds a local color table Instead of sending 24- bit pixels we send 8-bit pixels that are indexes in the local color table JPEG – Joint Photographic Experts Group

27. 27 JPEG