Question: In the UTRAN Iu interface's ATM Transport Option towards the packet switched domain, what protocol stack is utilized for the User Plane, from top to bottom?
Option 1: GTP-U over TCP over IP over AAL2 over ATM
Option 2: GTP-U over UDP over IP over AAL5 over ATM
Option 3: GTP-U over SCTP over IP over AAL5 over ATM
Option 4: GTP-U over ICMP over IP over AAL2 over ATM
Explanation: The correct protocol stack for the User Plane in the ATM Transport Option is GTP-U (General Packet Radio Service Tunnelling Protocol - User Plane) over UDP for efficient, connectionless transport, over IP for routing, over AAL5 suitable for packet data transfer, and finally over ATM. This stack ensures reliable, packet-based communication between the UTRAN and the packet switched domain of the Core Network.