Some questions about WMC and Media Extender

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I have some basic questions about WMC and Media Extender.

Can you make a comment for each question?

1. What is the main difference of technology between them?
(WMC : Using UPnP technology ?)
(Extender : UPnP + Remote UI ?)

2. SP2 is including WMC?
(When I update for SP2, then WMC is automatically downloaded?)

3. With WMC and DMRs can I watch some recorded TV programs?
(What is the difference using Media Ceneter PC and Media EXtender?)

4. In the case of Media center and Extender, Remote Desktop Protocol is for
Remote UI and UPnP AV is for AV streaming ? Is it right?

I really want to know the difference between them. Can you help me?

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