The purpose of enabling PGW Restart Notification (PRN) in S4-SGSN is to provide a simple and optimized solution for handling the signaling overload on the SGSN when a PGW failure occurs. Until release 10, the SGW used to send Delete Bearer Request for every PDN connection activated through the failed PGW. This results in signaling overload on the SGSN. From 3GPP Release 10 specifications onwards it is possible for a SGW to indicate a PGW failure through a single PRN message to the SGSN.
When the SGW detects that a peer PGW has restarted or it is not reachable, it deletes all the PDN connections associated with that peer node and releases all the internal resources associated with those PDN connections.
The SGW sends a PGW Restart Notification only to the SGSNs that have configured advertisement of PGW restart notification in echo request/response messages. When the S4-SGSN receives this message, according to the control plane IP address of the restarted PGW and the control plane IP address of the SGW on the S4 interface included in the message, the S4-SGSN deletes all PDN connections associated with the SGW and the restarted PGW. The SGSN also releases any internal resources associated with those PDN connections.
The S4-SGSN sends a PGW Restart Notification Acknowledge message in response to the PGW Restart Notification message sent by the SGW.