Multimedia Streaming. Multimedia Retrieval Architecture

Transcription

1 Multimedia Streaming

2 Some Example Applications Common multimedia applications on the Internet: Streaming stored audio and video. Streaming live audio and video. Real-time interactive audio and video. All the above have common characteristics: Delay sensitivity. End-to-end packet delay. Delay jitter :: variability of packet delay within the same packet stream.

3 Application QoS Categories Hard QoS: Guaranteed service(also called hard (QoS) This is an absolute reservation of network resources for specific traffic. Hard QoS describes the process during which the devices on the network through signaling can negotiate, request and adjust priority levels for different types of traffic based on the previously agreed values. Hard QoS includes protocols such as Integrated Services/Resource Reservation Protocol. The application may malfunction if the QoS constraints cannot be met. Typical examples: Critical patient monitoring systems.

4 Differentiated service (also called soft QoS). Soft QoS, means ensuring resources for applications based on available bandwidth. Differentiated Service (DiffServ) is an example for this type of QoS service. Some traffic is treated better than the rest(faster handling, more average bandwidth, and lower average loss rate). Functionally application performs correctly. Typical examples: Most multimedia applications.

5 Streaming Stored Multimedia Basic Concept: The compressed Audio Video file is stored at the source. The file is transmitted to the client when requested. Compressed Audio Video file is downloaded. Client plays the file.

6 Streaming Stored Multimedia A/V files are requested by client machine through browser to the web server 2. Web server responds by send the file 3. Client plays the file using Media player.

7 Stored Audio Video files Limitation: The file is bulky even after compression say 300 Mb for VCR quality. one-hour video using MPEG1 Takes time based on size of the file and bandwidth of the link Occupies large space.

8 Streaming Stored Multimedia Basic Concept: The meta file is stored at the source. The file is transmitted to the client when Requested. Compressed A/V file is downloaded. The client starts playing the media before the whole of it is transferred. Central concept to streaming. Minimum continuous rate of transfer to be maintained for jitter-free playback.

9 Streaming Stored Multimedia using Metafile Browser sends request for meta file (contains information of actual file) to web server Web server sends as response. Media player uses this metafile and send request for A/V file to web server Web server streams the A/V file

10 Client playing a part of the video, and sever sending the later part, are carried out in overlapped fashion. Advantage: Storage requirement in the client is small. Disadvantage: Browser and Media player uses HTTP to fetch Metafile and Audio/Video file that runs over TCP. TCP is not suitable for streaming since TCP performs Flow control, error control.

11 Seperate Media Server

12 Advantages of using separate streaming server. Provides the best performance. Uses non-http protocols between server and media player. Can also use UDP instead of TCP

13 Real time Interactive Multimedia Basic concept: Interactive in the sense that the content to be transmitted is decided by the end parties dynamically. Typical examples: IP telephony Video conferencing On-line games

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15 End-to-end delay requirements are important. Includes application-level and also network delays. About 200 msec considered to be good enough for audio. Beyond 500 msec, audio may be unacceptable.

16 Some Issues in Streaming Use of client buffering Allows to compensate for network-added delay and delay jitter. Whether to use TCP or UDP... TCP Transfer rate fluctuates due to TCP congestion control. Better quality because no packets are lost. More delay variations due to retransmission.

17 UDP Server sends data at rate appropriate for client. Does not depend on network congestion. Send rate = encoding rate = constant rate Short playout delay (2-5 seconds) to compensate for network delay jitter.

18 Variability in client rates How to handle variations in client receive rate capabilities? 33 Kbps dialup 2 Mbps leased line 100 Mbps Ethernet Common solution: Server stores multiple copies of the content (say, video), that have been encoded at different rates.

19 Real Time Streaming Protocol RTSP Gives user much better control over streaming media. RTSP is... A client-server application-layer protocol. Provides control to the user: Pause, play, rewind, forward, repositioning, etc.

20 Using a Media server and RTSP

21 A Typical Metafile <title>trailer</title> <session> <group language=en> <switch> <track type=audio e="pcmu/8000/1" src = "rtsp://stream.com/trailer/audio.en/lofi"> <track type=audio e="dvi4/16000/2" pt="90 DVI4/8000/1" src="rtsp://stream.com/trailer/audio.en/hifi"> </switch> <track type="video/jpeg" src="rtsp://stream.com/trailer/video.en"> </group> </session>