IIS Smooth Streaming

Smooth Streaming, an IIS Media Services extension, was created for playing video over the internet and reducing the amount of time a client would wait for buffering on slow connections. The format can include video compressed at several bit rates. The client can automatically switch between the available streams when the bandwidth changes.

If the bandwidth increases, it will start playing higher quality video.
If the bandwidth decreases, it will start playing lower quality video.

This automatic video switching is built-in into the Silverlight video player. So, all you need to do is put the resulting video on the web site and Silverlight will automatically pick the video stream that matches the available bandwidth.

This format can use WMV or H264 video compression and WMA or AAC audio compression

See Playing Smooth Streaming Content in Silverlight and Using the LEAD SSF Writer for more details.