Video Codecs
Codecs (the software or hardware used to COmpress or DECompress digital video) are an integral part of streaming technology. To fit uncompressed video down a 28.8kbps modem line would require around 12,000:1 compression! The increased use of broadband technology, such as ADSL (see below for details) enables far faster download times but it is best to assume, in the UK especially, that most end users will be viewing the video with a 28.8kbps, or at best 56kbps modem on an ordinary telephone line.

We will look at Codecs in relation to their specific players and encoders in greater detail next week.

UDP and TCP
Traditionally the term 'streaming media' applied only to content that was streamed real-time through UDP (User Data gram Protocol). More recently though, the term streaming media has been applied to any video that is played on the web. There are two methods for streaming data over the Net - TCP (Transmission Control Protocol) or UDP. TCP has error-correction that makes sure all packets of data that are sent are put back together again in the right order. The problem with this is that you might have to wait for a packet to arrive before you can continue playing the data. UDP, however, just throws the data at the client end without any check-ups, so if there is an error, you might get a blur on the video or a bit of hiss on the music instead of it stopping to catch up. In summary, TCP will basically give you 100% accurate playback, but it might skip or stop. UDP won't give you 100% accurate playback, but it won't stop or skip.

UDP is really the preferential method, as slight inaccuracies are normally not noticed. The problem is that an awful lot of firewalls block UDP packets as these can be used for denial-of-service attacks.

MBone
Columbia University provides a comprehensive description of the MBone also known 'Multicast Backbone'. On the Internet, MBone is an extension to the Internet to enable it to support IP multicasting. This is a two-way transmission of data between different sites. The TCP/IP (Transmission Control Protocol/Internet Protocol) protocol used by the Internet divides messages into packets and sends each packet independently of the others. Packets can travel over various different routes to their destination. This means that they can arrive in any order, and with sizable delays between the first and last packets. Also, each client or end user machine requires that separate packets be sent from the source to the destination. This works fine for static information such as text and graphics, but it doesn't work well for real-time audio and video.

With MBone, a single packet can have multiple destinations and isn't split up until the last possible moment. This means that it can pass through several routers before it needs to be divided in order to reach its final destinations. The outcome of this is a much more efficient transmission. It also ensures that packets reach multiple destinations at roughly the same time.

The MBone is an experiment to upgrade the Internet to handle live multimedia messages with around 3,000 MBone servers on the Internet in March 1997.

The main difference between these two forms is that the MBone works in a similar way to broadcasting in that it 'pushes' the signal onto the net (via the MBone). Therefore, the 'viewer' must 'tune in' at the appropriate scheduled time, there must also be enough bandwidth to play the video in real time. The other, more familiar form of streaming, transfers the information at whatever speed is available using the HTTP (Hyper Text Transfer Protocol). This form of streaming, often called 'progressive download', downloads as much of the file to your disk as it needs to act as a buffer before it can play back. The advantage of 'progressive download' is that the end user will receive a high quality film but the delay could prove prohibitive.

Mbone on Wikipedia
http://en.wikipedia.org/wiki/Mbone

Next we will take a look at some of the different players and encoders that are available for viewing and creating streaming media on the Web.