synchoronous optical network

I have some querries about sonet(synchoronous optical network)..i want to know about that in sonet architecture, 8000 frames transmitt per sec .my question is that.willl the 8000 frames are allocated for one user or for many users within one network..in short how 8000 frames are distriuted among the users.?

AND

where we use and preffered virtual tributries(vt’s) over sts frame?

kindly look into the matter and reply..
thank you