Question: Over the Nq' interface, what is the primary purpose of the APN Information Retrieval Procedure initiated by the RCAF (RAN Congestion Awareness Function) towards the SGSN (Serving GPRS Support Node)?
Option 1: To notify the SGSN about changes in the RAN's user plane congestion status for specific IMSIs.
Option 2: To retrieve a list of all available Access Point Names (APNs) supported by the SGSN.
Option 3: To obtain from the SGSN, the APNs currently having PDP contexts activated for each provided International Mobile Subscriber Identity (IMSI).
Option 4: To request the SGSN to modify the Quality of Service (QoS) settings for certain subscribers.
Explanation: The RCAF uses this procedure to retrieve from the SGSN the APNs currently having PDP contexts activated for each provided IMSI, crucial for managing and mitigating user plane congestion in the RAN.