Should I reach out to my future advisor before accepting PhD admission? I have been offered PhD admission which I have not accepted yet.I have already been assigned advisors though.

I want to reach out to my potential advisor to discuss future research and maybe even dissertation topics ? Was wondering if that is okay for to do ? i am little hesitant because I am not sure if this is the norm for how things are done ?
Yes. If you're certain about the advisor you'll be working with, feel free to get in contact with them. Even if you change advisors before beginning the program - or even if you don't join that program - it will not hurt to introduce yourself and to start planning.