What is the difference between fast dormancy and DRX in 3G

Hello, as I’m stuyding 3g energy consumption,

I get confused between two featuer “Fast Dormancy” and “DRX”

Can you guys let me know what is the difference between these two operations?

I remember fast dormancy is related to change from cell dch to cell fach and then to cell pch/ura pch faster, and DRX is to configure the UE to wake up intermittently to save energy also. Not sure but I believe it was something like that.