Question: In UTRAN's Iur interface, what is the primary purpose of the Message Structure Information Element (IE) in RNSAP signalling, as it relates to error reporting?
Option 1: To provide a flat list of all possible error codes in the network.
Option 2: To specify the protocol version used for the current session.
Option 3: To indicate the priority level of the User Equipment (UE) experiencing the error.
Option 4: To hierarchically describe the message structure from top level down to the lowest level above the reported error.
Explanation: The correct answer, Option 4, accurately reflects the role of the Message Structure IE in providing a hierarchical overview of the message structure, stopping just above the level where an error occurred. This is crucial for efficient error diagnosis and troubleshooting in the UTRAN's Iur interface, as it contextualizes the error within the broader message structure. The other options are incorrect because they do not relate to the described function of the Message Structure IE in the context of error reporting and RNSAP signalling.