Question: In a 3GPP network, when does an MS (Mobile Station) transition from PMM-CONNECTED to PMM-IDLE state in relation to its PS signalling connection with the 3G-SGSN?
Option 1: When the MS successfully completes a routing area update procedure.
Option 2: When the 3G-SGSN initiates a GPRS attach procedure for the MS.
Option 3: Upon receipt of a new RAI (Routing Area Identity) in the MM system information, prompting an MS-initiated update.
Option 4: When the PS signalling connection to the 3G-SGSN is released, broken, or fails with specific causes like 'IMSI unknown in RNC'.
Explanation: The correct transition from PMM-CONNECTED to PMM-IDLE due to PS signalling connection issues aligns with the network's need to manage mobile state based on connectivity and authentication status. This is crucial for resource allocation, billing, and ensuring seamless service delivery. Specific causes like 'IMSI unknown in RNC' indicate a break in the necessary handshake between network elements, necessitating a state change to PMM-IDLE to re-establish or correct the connection.