Question: What is the primary trigger for detecting a Transfer Syntax Error in E-UTRAN's S1 Application Protocol (S1AP)?
Option 1: Receipt of an unknown protocol message from a neighboring cell.
Option 2: Failure to decode a received physical message during ASN.1 decoding process.
Option 3: Expiration of a timer for a pending response in a signaling procedure.
Option 4: Detection of a mismatch in the version number of the S1AP protocol.
Explanation: A Transfer Syntax Error fundamentally occurs due to an issue with decoding the message structure, which is directly related to the Abstract Syntax Notation One (ASN.1) decoding process. This process is crucial for interpreting the format and content of messages exchanged over the S1 interface in E-UTRAN. Therefore, the primary trigger for detecting such errors is the failure to successfully decode a received physical message during this ASN.1 decoding process, highlighting a problem with the message's syntax or structure.