Question: What mechanism ensures forwards and backwards compatibility in UTRAN's Node B Application Part (NBAP) signalling protocol?
Option 1: Periodic updates to all User Equipment (UE) to match the latest Node B software versions.
Option 2: Inclusion of Id and criticality fields in a standard, unchanging format within all messages, Information Elements (IEs), or groups of related IEs.
Option 3: Implementation of a backwards compatibility mode in Node B that can temporarily revert to older signalling protocols.
Option 4: Mandatory support for all legacy NBAP messages in all future Node B deployments.
Explanation: The correct answer, Option 2, highlights the specific technical mechanism (inclusion of Id and criticality fields in a standard format) that ensures both forwards and backwards compatibility. This approach allows for seamless decoding of message components regardless of the standard version, facilitating interoperability between different versions of the protocol without requiring updates to existing infrastructure or devices. The other options either imply unnecessary complexity, temporary workarounds, or unrealistic expectations about legacy support, which do not accurately reflect the efficient, scalable solution provided by the Id and criticality fields mechanism.