Well beyond the scope of AB......
It can be done either dynamically (i.e. the player chooses the most appropriate bit rate for the user) or, as you tube does it.... gives the user the choice... either way,
You need to learn action script, there is not a simple answer to this... it took me several weeks to get variable bitrate streaming to work properly and it's not going to be possible to cover it in a couple of paragraphs!
start with adobe guru websites and spend a lot of time on google, it's the only way you are going to do this!



