If SONET/SDH can carry IP, and ATM rides on SONET/SDH, then why do we need to encapsulate IP inside ATM?
That makes "IP over ATM over SONET/SDH". What's its benifit over "IP over SONET/SDH"?
Why do we need this extra layer? For transport? But doesn't SONET/SDH already provides us the transport network?