Question: What is the primary purpose of the 'Old-Serial-Number' parameter in the context of Cell Broadcast Service (CBS) as defined in telecom standards?
Option 1: To uniquely identify a specific existing CBS message from a particular source/type, enabling its update or cancellation.
Option 2: To authenticate the User Equipment (UE) before receiving CBS messages.
Option 3: To indicate the priority level of CBS messages to the Base Station Controller (BSC)/Radio Network Controller (RNC).
Option 4: To notify the UE about changes in the CBS message format.
Explanation: The 'Old-Serial-Number' parameter's key function is to identify a specific existing CBS message, crucial for updating or cancelling messages from a particular source/type, ensuring the integrity of broadcasted messages in telecom networks.