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UMTS network can be either ATM cell or IP based network. Assume that,
there is an UMTS network which is full of ATM cell based. Now, for the
communication between this UMTS network(which is a Packet switching
network) and the IP network, is it necessary to have a
media(&signalling) gateway in between these networks? If yes/no, why?

Could anybody clarify this? Thanks!

YOu need a router that convert UMTS/ATM to IP.For example you know the
router ADSL they convert with an ADSL modem the protocoll IPover ATM o
PPP over ATM to IP/Ethernet. I think you need the same thing but with
UMTS instead of ADSL.

Cristian