Question: In a congested UTRAN cell, what triggers the Radio Access Network Congestion Awareness Function (RCAF) to retrieve APN information for active PDN connections of served User Equipment (UEs)?
Option 1: Receipt of RAN user-plane congestion information (RUCI) from the Policy and Charging Rules Function (PCRF)
Option 2: Detection of congestion in a specific UTRAN cell, prompting the RCAF to determine serving PCRFs
Option 3: Regular updates from the Serving GPRS Support Node (SGSN) for all active PDN connections
Option 4: Change in the Routing Area Identity served by the congested UTRAN cell
Explanation: The RCAF initiates the APN information retrieval procedure upon detecting congestion in a specific UTRAN cell. This step is crucial for identifying the PCRFs serving the affected UEs, to whom the RCAF will report RAN user-plane congestion information (RUCI). This process directly relates to managing and mitigating network congestion based on real-time conditions within the RAN.