Thanks for the subject matter,Conwyn. I scoured for some more info, if i got it right, Frame relay over ATM is usually referred to Frame relay encapsulation that is encapusalated into ATM network( ie frame relay "frames" are converted into ATM "cells" through the ATM internetwork) and back while it is being forwarded to other end of frame relay router, cells are reverted back to frames.
Regarding why this is preferred...I guess this type of ATM implemenation is designed since ATM is much more flexible(due to multiplexing layer) with better QOS and much faster
Correct me if i am wrong.
Some background info for you
VPNs is just a generic term, MPLS is a type of VPN implementation;
Service providers may have to route the traffic across existing networks, a lot of which may be existing ATM network equipment
ATM was meant to be the solution to everything. It really came from Voice world in the days when Data was not particully important. The Telcos invested heavily in it. I had PCs with ATM-25Mb cards, and large computers with 155Mb cards. You can run pure (classical) IP over it but generally you ran LAN emulation so it was both expensive and technically complex. Ethernet speeds increased and the hardware was much cheaper so ATM generally is only found in Telco world. I do not know if it is included in the Cisco certification programme anymore. I still have a few ATM/Ethernet bridges in the US. So you may encounter one in your career. Obviously most people experience it nowadays with PPPoA in the DSL environment.