Question: What happens when a Serving GW receives downlink data and Idle State UE has activated Idle State Routing (ISR) in a network?
Option 1: The Serving GW sends downlink data notifications to the MME only.
Option 2: The Serving GW sends downlink data notifications to the SGSN only.
Option 3: The Serving GW establishes a direct connection with the UE.
Option 4: The Serving GW sends downlink data notifications to both MME and SGSN, which then start their paging procedures.
Explanation: When ISR is activated for an Idle State UE, the Serving GW has control connections with both MME (Mobility Management Entity) and SGSN (Serving GPRS Support Node). As a result, upon receiving downlink data, the Serving GW sends notifications to both nodes, which then initiate their respective paging procedures in parallel. This ensures that the UE is paged in its registered Routing Area (RA) and Tracking Area(s) (TA), regardless of whether it is camping on E-UTRAN, UTRAN, or GERAN.