Question: In a scenario where an MS roams out of a pool area and into the area of one or more SGSNs that do not know about the internal structure of the original pool area, how does the new SGSN determine the correct old SGSN?
Option 1: By utilizing the unique value range of the P-TMSI parameter within the pool area, the new SGSN resolves the ambiguity and determines the correct old SGSN from the P-TMSI (or the TLLI).
Option 2: The new SGSN sends a Location Update Request to all potential old SGSNs in the original pool area, and the correct one responds with an acknowledgement.
Option 3: The MS provides the identity of the previous SGSN during the attachment process to the new SGSN, which then directly communicates with the correct old SGSN.
Option 4: The new SGSN always selects the first available SGSN from its database as the old SGSN for simplicity, assuming minimal impact on network performance.
Explanation: This is because each SGSN serving a pool area has a unique value range of the P-TMSI parameter. When an MS roams out and into a new area, the new SGSN can use this uniqueness to identify and communicate with the correct old SGSN by resolving any ambiguity among multiple SGSNs in the original pool area through the P-TMSI (or TLLI) information. This mechanism ensures efficient routing without unnecessary signaling across the network.