Question: In UTRAN Iu interface (RANAP signalling), what happens when the RNC encounters an unknown RAB ID during RAB context transfer?
Option 1: The RNC silently ignores the unknown RAB ID and proceeds with known RAB IDs.
Option 2: The entire SRNS CONTEXT RESPONSE message is aborted, and a separate error message is sent.
Option 3: The unknown RAB ID is included in the SRNS CONTEXT RESPONSE message along with a 'Cause IE' (e.g., 'Invalid RAB ID').
Option 4: The UTRAN requests retransmission of the RAB context for the unknown RAB ID from the requesting node.
Explanation: When the RNC encounters an issue with a specific RAB, such as an unknown RAB ID during the transfer process, it efficiently manages this by including the problematic RAB ID in the response message and appending a Cause Information Element (IE). This approach allows for the identification of the exact issue ('Invalid RAB ID') without halting the entire process, thus facilitating targeted troubleshooting and minimizing disruptions to other RABs.