Question: In Inter RAT Handover (HO) involving E-UTRAN access, which of the following control interfaces exists at the CN level to enable backwards handover?
Option 1: Between MME/Serving Gateway and UE
Option 2: Between 2G/3G SGSN and corresponding RNC; - AMF/UPF and corresponding eNodeB
Option 3: Between E-UTRAN Node B and Target GERAN/UTRAN/NR Base Station
Option 4: Between: - 2G/3G SGSN and corresponding MME/Serving Gateway; - AMF/UPF and corresponding MME/Serving Gateway
Explanation: This is correct because, as per Inter RAT HO design principles for E-UTRAN, a control interface at the Core Network (CN) level is necessary for backwards handover when RAN level interfaces are not available. The specified interfaces in Option 4 accurately reflect the connections between the CN elements of different access systems, facilitating seamless mobility management across different radio access technologies.