Question: When a GGSN detects congestion for a specific Access Point Name (APN) and initiates overload control, what action may the Serving GPRS Support Node (SGSN) take if it receives a 'GGSN back-off time' indication for that APN?
Option 1: The SGSN must always reject new PDP context requests for the specified APN during the indicated 'back-off time', without exception.
Option 2: The SGSN may select an alternative GGSN for the same APN instead of rejecting new PDP context requests, unless there's an existing PDP context to the same APN for the Mobile Station (MS).
Option 3: The SGSN will automatically redirect all affected UEs (User Equipment) to a different APN to alleviate congestion.
Option 4: The SGSN ignores the 'GGSN back-off time' and continues to process new PDP context requests as usual.
Explanation: This is correct because, according to overload control mechanisms, when a GGSN indicates APN congestion by specifying a 'GGSN back-off time', the SGSN has flexibility to either reject new PDP context requests for the specified APN during this time or select another GGSN for that APN, provided there isn't an existing PDP context to the same APN for the MS, helping manage network congestion while minimizing user impact.