How Does Streaming Video Work?
Most Internet data transfers are similar, no matter whether you are downloading a webpage or a computer program. The data is broken up into small packets which are encoded with their origin, destination, and position in the datastream.
This allows each individual data packet to take the most efficient route to its destination. Packets can arrive in any order because they will not be reassembled until all of the data has reached its destination.
Video streaming also works by dividing the datastream into packets, but it is different from downloading because the packets have to arrive in a specific order. Also, it is less critical to the video stream if some packets are lost en route. This is in contrast to downloading which requires every single packet to arrive intact before the file can be reassembled.
So how does video streaming work?
Video streaming is possible because of a set of streaming protocols. Without these protocols, the entire video file would have to be downloaded before it could be viewed. Downloading can take a long time - several hours for a large video file.
Streaming protocols allow the video to be viewed as it is downloading. The delay for streaming is measured in seconds rather than minutes or hours. The small delay between hitting the play button and seeing the video is due to buffering - creating a small reserve of video in case the datastream slows down.
Real Time Streaming Protocol
There are several streaming protocols, but the most popular for streaming video is Real Time Streaming Protocol (RTSP). This protocol allows viewers to remotely control the datastream so that the video can be paused, rewound, or fast forwarded.
There are also two protocols which handle the actual delivery of the datastream. These are Real Time Transport Protocol (RTTP) and Real Time Control Protocol (RTCP).
Webcasting
Most video streams are offered as Video On Demand (VOD) which means that anybody can view them at any time. There is another form of streaming video called webcasting. This is a live video stream which can only be viewed at the time of creation, just like a TV broadcast.
Webcasting is more resource intensive because there may be hundreds of viewers watching at the same time. Since each viewer requires a certain amount of bandwidth, the number of viewers is limited by the bandwidth resources of the server.
There is a way to get around this limitation, with the use of a protocol called multicast. This allows one datastream to be viewed simultaneously by a potentially unlimited number of people. The problem with multicast is that it is often blocked by firewalls, so its usefulness is often limited to private networks.
Video Conferencing
Another form of webcasting is video conferencing in which several parties can communicate in real time. This is even more resource intensive than webcasting because of the need for multiple two-way connections.
When video conferencing is done on a private network, the multicast protocol is very useful in cutting down on bandwidth. Each participant sends out a multicast video stream while at the same time receiving a multicast from the other participants. This is the preferred method of video conferencing whenever multicast is available.
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