Intermediate GSN

Description

The Intermediate GSN (IM-GSN) is a functional role defined within the 3GPP GPRS core network architecture, specifically for handling inter-PLMN (Public Land Mobile Network) roaming. It is not a separate physical node but a logical function that can be implemented within a Gateway GPRS Support Node (GGSN) or a dedicated node. When a mobile subscriber roams into a visited network, their packet data sessions are anchored in their home network's GGSN (H-GGSN). The IM-GSN resides in the visited network and sits on the user plane path between the visited Serving GPRS Support Node (V-SGSN) and the H-GGSN. Its primary function is to manage and route the GTP (GPRS Tunnelling Protocol) tunnels for the subscriber's data traffic.

Operationally, during a Packet Data Protocol (PDP) context activation for a roaming subscriber, the V-SGSN establishes a GTP tunnel with the IM-GSN in the visited network, and the IM-GSN, in turn, establishes another GTP tunnel with the H-GGSN in the home network. All user plane packets traverse through the IM-GSN. This allows the IM-GSN to perform several key functions: it can apply local breakout policies (if supported), generate charging data records (CDRs) for the visited network operator's billing, and enforce quality of service (QoS) policies as agreed in inter-operator roaming agreements. The IM-GSN uses the GTP-C (control) protocol to manage the tunnels and the GTP-U (user) protocol to forward the actual IP packets.

Architecturally, the IM-GSN plays a crucial role in separating the control and charging responsibilities between the home and visited operators. The H-GGSN retains overall control of the PDP context and subscriber-specific policies, while the IM-GSN handles the local routing and access to the external packet data network (e.g., the internet) from the visited network's perspective. This model supports accurate inter-operator settlement, as the visited network can charge for the radio and core network resources used, while the home network charges for the overall service. The IM-GSN function is defined to ensure scalable and efficient roaming for packet-switched services, a cornerstone of global mobile data connectivity.

Purpose & Motivation

The IM-GSN was created to address the complexities of charging and traffic routing in GPRS/UMTS roaming scenarios. In early GPRS architectures, a simple model where the V-SGSN connected directly to the H-GGSN was possible, but this presented significant challenges for the visited network operator. The visited operator had limited visibility and control over the data traffic passing through its network, making it difficult to measure resource usage accurately for billing and settlement with the home operator.

The primary problem the IM-GSN solves is enabling fair and transparent inter-operator charging for roaming data services. By inserting an IM-GSN in the visited network, that operator can generate its own Charging Data Records (CDRs) based on actual data volume and session duration handled locally. This provides a clear basis for financial settlement between operators. Furthermore, it allows the visited network to apply its own local policies, such as traffic shaping or lawful interception, on the roaming subscriber's traffic without requiring changes to the home GGSN.

Historically, its specification in Release 99 was motivated by the commercial rollout of GPRS and the anticipation of widespread data roaming. It addressed the limitation of the initial, more simplistic roaming model which lacked a defined point for visited network charging and control. The IM-GSN concept provided the necessary architectural hook for commercial roaming agreements, ensuring that operators had the technical means to bill each other for carried traffic, which was essential for the business model of international mobile data. It also provided a foundation for more advanced roaming optimizations in later releases.

Key Features

• Acts as a GTP tunnel intermediary between V-SGSN and H-GGSN for roaming users
• Enables generation of visited network Charging Data Records (CDRs) for inter-operator settlement
• Provides a point for applying visited network QoS and traffic management policies
• Supports the logical separation of control (home network) and user plane (visited network) responsibilities
• Can be implemented as a functional subset within a GGSN or as a standalone node
• Facilitates scalable and manageable GPRS/UMTS roaming architecture

Evolution Across Releases

Defining Specifications