Live Internet/Video/TV Broadcast: Technology Behind
Nowadays, the Internet world is no longer a passageway for communication among people with text contents. Over the last few years there has been a dramatic improvement in the quality of IP-based network media technologies. Obviously, streaming media technologies continue to be cast into new applications, extending services so far reserved for the domains of the PC, home entertainment and mobile cellular technologies. The server, protocol and network architectures for streaming content to multiple devices find their evolution within services among webconferencing and webcasting. Whether for personal desktop or widespread delivery, the serving device contrives a system for conveying messages and information.
Some Basic Concepts
In order to play smoothly, video data needs to be available continuously and in the proper sequence without interruption. Until fairly recently, it had to be downloaded in its entirety to the PC before it could be played. With streaming, the file remains on the server. The initial part is copied to a buffer on the PC and then, after a short delay, called pre-roll, starts to play and continues as the rest of the file is being pulled down. Streaming provides a steady method of transmission controlled by interaction between the PC and the server. The server regulates the stream according to network congestion and thereby optimizes the presentation on the PC.
There are three software components involved in streaming:
- The Player: The client software that must be loaded on the PC in order to play back the stream. This may be run as a stand-alone application or launched through a plug-in for the browser.
- The Video Server: The software that handles the distribution of streams. The video server is I/O intensive so the machine to support it needs to have fast disc access and plenty of free storage capacity to hold the content.
- The Encoder: The software that compresses an audio/video source into a file or live stream that can be streamed over the network. Encoding is CPU-intensive so the machine that supports it needs to have a fast processor and sufficient memory.
Content can be On-demand or Broadcast
On-demand content delivery is controlled by the client. The user can select a pre-recorded stream and also freely choose when to view it. Furthermore the user can control the video stream - pausing, jumping ahead/back, restarting, etc. – just as with a video recorder. On the other hand, broadcast content delivery is controlled and scheduled by the server. The content is only made available for viewing at selected times. The viewer can only watch the stream as it is being transmitted without any control over it, just as with a television or radio broadcast. Broadcast content can be scheduled to come from an archived file or can be a live transmission from an external audio/video device such as a camera or video recorder.
Microsoft Windows streaming solutions
The following diagram illustrates the key elements for broadcasting live/VOD contents to designating clients (Windows Media Streaming solution - Microsoft Inc.)
Understanding unicast vs. multicast streaming
Windows Media Services uses the terms unicast and multicast when describing how clients receive data packets from a Windows Media server (See below).
A unicast is a point-to-point connection between the client and server. Point-to-point means that each client receives a distinct stream from the server. A unicast stream is sent to only the client that requested it.
A multicast is a content stream delivered over a multicast-enabled network; all clients on the network share the same stream. The biggest advantage of streaming ASF (Advanced Systems Format) content in this manner is that it saves network bandwidth. Multicast produces the most efficient use of bandwidth when enabled for several clients, making it well-suited to services such as video-over-IP. Multicasting uses the same bandwidth for dozens of clients as it does for one; however, it may require that all routers in the path have software, firmware or, in some cases, hardware updates. Furthermore, for multicast delivery, the entire path must be multicast-enabled, which is frustrating on public Internet domain.
MMS (Microsoft Media Server) protocol is the proprietary Microsoft network streaming protocol and is used extensively by Microsoft media player software. MMS protocol can be used on top of TCP (Transmission Control Protocol) and UDP (Universal Datagram Protocol) transport protocols over any network medium, with its primary use being the streaming of live or prerecorded audio and video to computers that do not require downloading a file before playing it.
While the Internet cloud remains a best efforts network, there are Internet protocols that address streaming media delivery. UDP, for example, has been important for streaming video because it is more permissive of dropped packets than is TCP. RTP (Real-Time Protocol) and RTSP (Real-Time Streaming Protocol) have been developed to enhance the sequencing, synchronization, and interactive capabilities of both UDP and TCP.
RTSP is a control or communication protocol used between client and server; RTP is the data protocol used by the server to send data to the client. Rather than first downloading a file to the client, RTP plays it in real time, which differentiates it from HTTP (Hyper-Text Transfer Protocol) and FTP (File Transfer Protocol). Often, the real-time protocols are indicated as one, shown as RTSP/RTP. Some services, such as Real System Server, will use RDT, its own proprietary data channel, for the delivery of content to Real ONE players.
Much of research into future technology involves offering Video over wireless networks. High bandwidth-fluctuation and high bit-error rates require new video coding formats (i.e. H264 / MPEG-4-AVC) being able to deliver the video over wireless networks smoothly with limited bandwidth. Along with more compression and transport technology evolving, developing suitable environment for conveying high definition streaming contents could be dominant to the marketplace.
- e-Video: Producing Internet Video as Broadband Technologies Converge – H. Peter Alesso (Addison Wesley)
- Microsoft TechNet (Microsoft Inc.): Microsoft’s Streaming Media Technologies: Windows Server 2003 R2
- Technology Review – MIT: http://www.technologyreview.com