Lecture 17 – March 21, 2002

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

2. Content-delivery services

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

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. 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. 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. Delay and Jitter

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. 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.

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

18. Data compression

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

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. 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

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. 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=2d compression = (N x n) / (N/L x d) = ( n x L) / d

26. Still image compression • GIF – Graphics Interchange Format • Reduces the number of colors in the image from 224 to 28 • 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. JPEG