Question: In the context of SMS, what is the primary purpose of the Short Message Identifier (SMI) exchanged between the Short Message Transfer Layer (SM-TL) and the Short Message Relay Layer (SM-RL)?
Option 1: To authenticate the sender's identity in each TPDU transmission.
Option 2: To prioritize TPDUs based on their urgency across the network.
Option 3: To keep track of TPDUs and associate them with reports about earlier transfer requests.
Option 4: To encrypt the content of TPDUs for secure transmission.
Explanation: The Short Message Identifier (SMI) is crucial for tracking TPDUs and linking them with subsequent reports, ensuring reliable message transfer by correctly associating responses or acknowledgments with their corresponding requests, which is vital for maintaining the integrity of SMS services.