Description
The UE Reachability Request Parameter for SGSN (URRP-SGSN) is a specific information element defined within the 3GPP specifications for the S3 interface, which connects the Serving GPRS Support Node (SGSN) of the 2G/3G packet-switched core to the Mobility Management Entity (MME) of the 4G Evolved Packet Core (EPC). Its primary function is to act as a trigger and control mechanism for UE reachability procedures when a UE is registered in the EPC but its last known serving node was an SGSN. When the SGSN receives downlink data for a UE that is in an idle state (e.g., PMM-IDLE in GPRS) but is known to be registered with the EPC, it cannot page the UE itself over the GERAN/UTRAN if the UE's last known location is managed by the MME. Instead, the SGSN includes the URRP-SGSN parameter in a message (specifically, a Context Request procedure) sent to the MME over the S3 interface. This parameter informs the MME that the SGSN is requesting the MME to perform a network-initiated service request procedure to re-establish the UE's connection. The MME, upon receiving this request, will then initiate paging of the UE in its registered tracking area(s) within the E-UTRAN. This mechanism is crucial for ensuring that downlink data can be delivered to a mobile device that has moved between 2G/3G and 4G coverage areas without performing a full Tracking Area Update (TAU) back to the 4G network. It optimizes signaling by allowing the SGSN to delegate the paging responsibility to the node that currently holds the most accurate location information for the UE in the target radio access technology. The parameter is part of a broader set of inter-RAT mobility and reachability procedures that maintain seamless service continuity as defined in 3GPP TS 23.060, "General Packet Radio Service (GPRS); Service description."
Purpose & Motivation
The URRP-SGSN was introduced to solve a specific problem in the interworking between 2G/3G GPRS networks and the new 4G Evolved Packet System (EPS). Prior to EPS, paging and mobility were handled within a single core network domain (GPRS). With the introduction of the EPC and the S3 interface for inter-core connectivity, a mechanism was needed to handle downlink data arrival when a UE's last known attachment point was in the legacy system but its current registration was in the new system. Without URRP-SGSN, an SGSN receiving data for a UE known to be EPS-registered might attempt to page it in the GERAN/UTRAN unsuccessfully, leading to data loss or excessive, inefficient signaling as it tried to locate the UE. The parameter provides a standardized, efficient way for the SGSN to signal to the MME that the MME should take action to re-establish a connection with the UE, leveraging the MME's knowledge of the UE's EPS registration state and location. This was a critical component in achieving seamless mobility and service continuity for early LTE deployments that relied on inter-RAT handovers and idle mode mobility with existing GSM/UMTS networks.
Key Features
- Triggers MME-initiated service request procedure for EPS-registered UEs
- Used in S3 interface signaling between SGSN and MME
- Enables efficient downlink data delivery during inter-RAT idle mode mobility
- Part of the Context Request/Response procedure
- Prevents unnecessary paging in the wrong RAT
- Supports network optimization by delegating paging to the correct core network node
Evolution Across Releases
Initially introduced as part of the EPS architecture defined in 3GPP Release 9. It established the parameter within the S3 interface procedures (Context Request) to allow an SGSN to request an MME to perform UE reachability procedures, forming a foundational mechanism for inter-RAT idle mode signaling between GPRS and EPC networks.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.060 | 3GPP TS 23.060 |