【 Weak current College 】 streaming media technology principle and play mode-Power By 【 China power house network 】

First, the principle of streaming media technology
Streaming implementations need to cache. Because the Internet is based on the packet transmission for disjoint asynchronous transfer, on a live A/V source or storage of A/V files, they are in transit is broken down into many packages, since the network is dynamic, various package selection, the route may vary, they reach the client's time delay would not, even the first packet is also possible after arrival. To do this, use the cache system to compensate for the delay and jitter, and packet in the correct order, so that the media data can be continuously output, not because the network is temporarily congested to play there. Usually cache capacity is not required because the cache using a circular linked list data structure to store: by discarding has played content, flow can reuse empty out of cache space to cache the content of the follow-up is not playing.
Streaming requires the appropriate transport protocol. Since TCP requires more overhead, so it is not suitable for the transmission of real-time data. In streaming implementation scenarios, generally use HTTP/TCP to transfer control information, and RTP/UDP to transfer audio data in real time.
Streaming process generally goes like this: the user to select a top media, Web browsers and Web servers using HTTP/TCP exchange control information, so that the required transmission of real-time data to retrieve from the original information; then the client's Web browser to start A program, use the/VHelper HTTP retrieval from a Web server related parameters on Helper program initialization. These parameters may include directory information, A/V data encoding type or with the A/V retrieval server address.
A/VHelper program and A/V server running real-time streaming protocol (RTSP), in exchange for A/V transmission required control information. And CD players or VCRs provides functionality similar to the RTSP provides control playback, fast forward, fast reverse, pause, and record commands. A/V Server using RTP/UDP protocol to A/V data is transferred to the A/V client (usually think customer program is the equivalent of a Helper program), once A/V data arrives at a client, A/V customer program to play the output.
It is important to note that in streaming, using RTP/RTSP/UDP and TCP are two different communication protocols and A/V Server networking is to enable the server to redirect output to a different than running A/client VHelper program is intended to address. Implement streaming generally require a dedicated server and player.
Second, the way streaming media playback
1. unicast
Client and media server requires a separate data access from one server to send out each packet can only be delivered to a client, this way is called unicast transmission. Each user must send to the media server, a separate query and media server must be sent to each user of the application a copy of the packet. This huge redundancy first heavy burden caused the server to respond to the need for a long time, or even stop playback; managers are forced to buy hardware and bandwidth to ensure a certain quality of service.
2. multicast
IP multicast technology to build a network with multicast capability, allows the router to a packets to multiple channels. Using multicast mode, a single server to hundreds of thousands of clients at the same time send a continuous stream of data without delay. Media Server only needs to send a packet, instead of multiple; all the requesting client to share the same packet. Information can be sent to any address of the client, reduce the network transfer the total amount of packets. Network utilization efficiency is greatly improved, costs are reduced.
3. on-demand and broadcast
On-demand connection is a client-server proactive connection. In on-demand connections, users select the content item to initialize the client connection. Users can start, stop, rewind, fast forward, or pause the stream. On-demand connections provide maximum control of convection, but this way because each client's respective connected server, but will quickly run out of network bandwidth.
Broadcasting means users passive reception. In broadcasting, the client receiving the stream, but cannot control the flow. For example, the user will not be able to pause, fast forward or rewind the stream. Broadcast packets in a separate copy will be sent to all users on the network. Use unicast for sending packets that need to be copied multiple copies multiple point-to-point way both to those users that need it, and send using broadcast, a copy of the individual data packets are sent to all users on the network, regardless of whether the user is needed, the above two transmission modes would be a waste of network bandwidth. Multicast incorporates both outgoing strengths to overcome these weaknesses in the way of sending a packet of separate a copy sent to the needs of those customers. Multicast will not replicate multiple copies of data packets to the network, do not send the packet to those customers who require it to ensure that the network multimedia applications consume network bandwidth of minimum.