Streaming involves sending movies (included audio and video) from a server to a client over a network such as the Internet. The server breaks the movie into packets that can be sent over the network. At the receiving end, the packets are reassembled by the client and the movie is played as it comes in. Aseries of related packets is called a stream.
Streaming is different from simple file transfer, in that the client plays the movie as it comes in over the network, rather than waiting for the entire movie to download before it can be played. In fact, a streaming client may never actually download a streaming movie; it may simply play the movie’s packets as they come in, then discard them.
Quick Time the best in multimedia system included (video or audio). QuickTime Streaming extends the QuickTime software architecture to support the creation, transmission, and reception of multimedia streams. QuickTime programmers to create applications that receive multimedia in real time, and to create authoring and editing tools that work with streaming content. Existing applications that play QuickTime movies can play real-time streaming movies with little or no code change.
QuickTime movies can be streamed using a variety of protocols, including :
■ HTTP (Hyper Text Transport Protocol)
■ FTP (File Transfer Protocol)
■ RTP (Realtime Transport Protocol).
HTTP and FTP are essentially file transfer protocols. Any QuickTime movie saved using the QuickStart option can be streamed using these protocols because the QuickTime client software is able to start playing the movie before the entire file has arrived.
RTP is used for real time streaming. The movie packets are sent in real time, so that a one-minute movie is sent over the network in one minute. The packets are time-stamped, so they can be displayed in time-synchronized order. Because packets are sent in real time, RTP streaming works with live content in addition to previously-recorded movies. It can even carry a mixture of the two. Real-time streams can be sent one-to-one (unicast) or one-to-many (multicast).
Unicast streaming
In a unicast, the client contacts the server to request a movie using RTSP (Real Time Streaming Protocol). The server then replies to the client over RTSP with information describing the movie as a streaming session. A streaming session consists of one or more streams of data, such as a video stream and an audio stream. The server tells the client how many streams to expect and gives details on each stream, such as the media type and codec. The actual streams are then sent to the client over RTP. When a QuickTime movie is streamed over RTP.
A stream can contain live content, such as a stock ticker or a radio broadcast, or stored content, such as a video track from a QuickTime movie. When a client is receiving unicast streams from stored content, the client’s movie controller includes a “thumb” that allows the user to jump to any point in the movie. This gives the client random access to long movies without having to download an entire movie or store it locally.
Multicast streaming
In a multicast, one copy of each stream is sent over each branch of a network. This reduces the amount of network traffic required to send the streams to large numbers of clients.Aclient receives the streams by “joining” the multicast.
The client finds out how to join the multicast by opening an SDP (Session Description Protocol) file. The SDP file contains the information needed to join the multicast, such as group address and port number, as well as the stream description information that would come over RTSP for a unicast. See Figure, SDP files are commonly posted on web servers to announce upcoming multicasts.
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