SQL & PL/SQL

Hi..could some one explain why db file sequential wait event appears continually while table is accessing through full table scan method.

i know sometime even in full table scan mode oracle fetch single blocks if it's segment block.undo block but not contiguously but in my case i haven't seen any other wait event except db file sequential wait event why it is so?.

Addition to this i have used parallel hint also but unfortunately all parallel servers current event is db file sequential event that means even parallel servers are reading single blocks.

Any information would be deeply appreciated