Question: What is a key characteristic of the username part in an Alternative Network Access Identifier (NAI) as per telecom network specifications?
Option 1: It must be derived from the International Mobile Subscriber Identity (IMSI).
Option 2: It shall not be derived from the IMSI and must be a non-null string.
Option 3: It can be any string but the REALM part must vary accordingly.
Option 4: It is always a null string to ensure anonymity.
Explanation: In telecom networks, the Alternative NAI's username part is designed for specific use cases where anonymity or non-traceability to the IMSI is required. By not deriving the username from the IMSI and ensuring it is not a null string, the network can maintain functionality while protecting user identity in certain scenarios.