General Information About Streaming Media


What are RTSP, PNM/PNA and MMS?

RTSP (Real Time Streaming Protocol) is an Open Standard protocol for streaming media, used by the majority of streaming vendors, including Apple. MMS is Microsoft’s own solution, but Microsoft will replace MMS with RTSP in future releases. PNA and PNM are RealNetworks’ legacy streaming protocols and are not used anymore. It is important for network administrators not to block RTSP protocols and connections in their firewalls.Downloading vs Streaming Content
Multimedia content downloaded from the Internet, or received by email can indeed contain a virus, therefore you should install Anti-Virus software on your mail servers and firewalls. Streaming Media on the other hand, is the opposite of downloading: content is streamed directly to the Player, and is not placed on your harddrive. There are no known viruses that can distribute nor activate themselves using Streaming Media content, servers or players. There are some minor and major known vulnerabilities in Windows Media Player, please use Windows Update to patch these, and please read the EULA for all players carefully.

What is the difference between downloading and streaming?
Downloading is exactly what it says: you load a copy of a file from a server to your local computer. Basic error correction technology built in TCP makes sure that you receive every bit and byte. The error correction is bad for audiovisual content since it requires a large overhead in the connection. Another problem is that you have to wait until the complete file is on your computer before you can use it. Some players fake streaming by measuring how fast a file is downloaded and start playing it during the download, this is called “Fast Start”. This results in those annoying hickups. Another problem is that webservers aren’t capable of sending a lot of simultaneous audio and video files.

Streaming Server only sends streaming data right to the streaming client. No data is saved to the harddisk in this process. The rtsp and mms protocols uses UDP which has no error correction. (The player and server use better error correction technology which prevents data loss as far as possible but don’t require a large amount of overhead). The stream is buffered at the client side so possible dips can be overcome. This is just an example of the smart technology used in streaming software. Since server and player software are optimized for realtime content distribution, the overall experience using streaming is *far superior* opposed to downloading. Note: streaming servers can tunnel mms/rtsp streams through HTTP, so content still can’t easily be downloaded, but will still get through firewalls. Some other servers (Icecast) fake a stream and do send a regular file over HTTP (progressive download).

What are Multirate Files? Windows Media Technology
Multi-bitrate streams can contain several video tracks and one audio track to allow viewers with different internet connection speeds to view the encoded media as best as can be available to them. The Windows Media Player determines the best connection rate with the Windows Media server, and then streams at the best bit rate available to that connection speed. Multicast normally only sends data to a client without receiving any from a client, there is no way for the player and server to negotiate bandwidth. Therefore, the server only transmits the highest bit rate stream from the stream. Multirate technology requires you to use streaming server software.

QuickTime uses another similar solution but with another technology. You encode multiple streaming files in several bitrates and sizes. You also produce a reference movie. This reference movie looks at the technical details of the system configuration of the end-user (like system power, network connection, software version, etc) and selects the appropriate streaming file. This technology gives you the most control over the content but also has a downside, because a player can’t automatically switch between bitrates during playback.

Content from streaming servers can’t be downloaded
First, optimized streaming servers don’t run http services such as Apache or IIS. When streaming content to a player, no data is written to the harddisk, so no copy is placed on the users’ PC. So yes, your content is safer on a streaming server than on a regular webserver. But, several rtsp and mms recorders are available that pretend to be a media player and try to rebuild a media file from a stream. These rebuilt files aren’t exact clones of the original file and always have some level of degradation. Another way to capture a stream is to connect the PC to a VCR, tapedeck or digital recording device. But in order to redistribute the content, one has to re-encode the captured file again, resulting in huge quality losses. A third way is to write virtual drivers or virtual audio and video cards, and then save the stream to a file. This workaround is also used to bypass DRM security. Our conclusion: content is never 100% safe, but you shouldn’t worry too much about (and invest too much in!) a minority of some smart kids that get a kick out of ripping some streams.

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