Question: When interworking with a Core Network (CN) node/Radio Network Controller (RNC) that only supports Asynchronous Transfer Mode (ATM) transport, what must an RNC/CN-node supporting Internet Protocol (IP) transport additionally support for compatibility?
Option 1: Only an IP-ALCAP protocol for direct conversion
Option 2: At least one of the following: ATM&IP dual stack or an Interworking Function (IWF)
Option 3: Mandatory external Interworking Function (IWF) with specific protocol stacks
Option 4: Upgrade to a unified transport protocol not based on ATM or IP
Explanation: For seamless interworking between an RNC/CN-node that supports IP transport and one that only supports ATM transport, flexibility in the interworking mechanism is crucial. Supporting at least one of the mentioned mechanisms (ATM&IP dual stack for simultaneous support of both protocols without needing an IP-ALCAP protocol, or an Interworking Function which can be either internal or external to facilitate compatibility) ensures adaptability and efficient communication between networks with different transport options. This approach accommodates various network architectures and ensures backward compatibility.