Question: What triggers the initiation of an Error Indication procedure due to a Transfer Syntax Error in the UTRAN Iu-BC interface?
Option 1: Receipt of an unknown SABP message type
Option 2: Failure to authenticate the sender's identity
Option 3: Failure to decode the received physical message during ASN.1 decoding
Option 4: Expiration of a timer for an expected response
Explanation: A Transfer Syntax Error occurs when there is an issue with decoding the message's structure, related to the Abstract Syntax Notation One (ASN.1) decoding process. This process ensures messages are formatted correctly for interpretation by the receiving entity. Thus, the correct trigger for initiating an Error Indication procedure due to a Transfer Syntax Error is the failure to successfully decode the received message during this ASN.1 decoding step.