3rd Generation Serving GPRS Support Node

Description

The 3G-SGSN (3rd Generation Serving GPRS Support Node) is a fundamental component of the UMTS (Universal Mobile Telecommunications System) packet-switched core network, defined from 3GPP Release 99 onwards. It serves as the primary network node responsible for delivering packet data services to User Equipment (UE) within its geographical service area. The 3G-SGSN performs critical functions including mobility management, session management, user authentication and authorization, and packet routing and transfer. It interfaces with the UMTS Terrestrial Radio Access Network (UTRAN) via the Iu-PS interface, with other SGSNs via the Gn interface (within the same PLMN) or Gp interface (to SGSNs in other PLMNs), and with the Gateway GPRS Support Node (GGSN) via the Gn/Gp interface to access external packet data networks (PDNs) like the internet or private corporate networks.

Architecturally, the 3G-SGSN contains several logical functional units. The mobility management function tracks the location of the UE, whether it is in IDLE, STANDBY, or READY state, and manages procedures like Attach, Detach, Routing Area Updates, and inter-SGSN handovers. The session management function is responsible for establishing, maintaining, and terminating Packet Data Protocol (PDP) contexts, which are logical associations between the UE and a specific GGSN that provide a data session with a defined QoS profile. The 3G-SGSN also interfaces with the Home Location Register (HLR) or Home Subscriber Server (HSS) via the Gr interface to authenticate the user (using algorithms like AKA - Authentication and Key Agreement) and retrieve subscriber profile data, and with the Equipment Identity Register (EIR) via the Gf interface for IMEI checking.

From a data plane perspective, the 3G-SGSN receives user data packets from the UTRAN, encapsulates them using the GPRS Tunneling Protocol for the user plane (GTP-U), and tunnels them towards the appropriate GGSN based on the PDP context. Conversely, it receives downlink GTP-U packets from the GGSN, decapsulates them, and forwards them to the correct Radio Network Controller (RNC) serving the UE. It also performs functions like charging data collection (generating Charging Data Records - CDRs), lawful interception, and may enforce certain policy controls. The 3G-SGSN was designed to support higher data rates and more sophisticated QoS mechanisms compared to its 2G GPRS predecessor, aligning with the capabilities of the UMTS radio interface.

Purpose & Motivation

The 3G-SGSN was created as part of the 3GPP UMTS architecture to provide a robust and scalable packet-switched core network capable of supporting the advanced data services envisioned for third-generation mobile networks. Prior to 3G, 2G GPRS networks introduced the SGSN concept for basic packet data, but these systems were limited in data throughput, QoS support, and efficiency. The transition to UMTS with its Wideband CDMA (WCDMA) radio interface promised significantly higher data rates (up to 2 Mbps initially) and more complex services like video calling and rich multimedia. The existing 2G-SGSN was not optimized for the new Iu-PS interface protocol stack (using RANAP over ATM/AAL5) or the enhanced QoS requirements of UMTS.

The 3G-SGSN was developed to address these limitations and serve as the anchor for packet data mobility and session management in the new 3G ecosystem. It solved the problem of efficiently managing packet data sessions for users moving at high speeds across cells and routing areas within a UTRAN. It provided the necessary interfaces to integrate with the new authentication infrastructure (using USIM cards and stronger AKA procedures) and to support multiple, simultaneous PDP contexts per user, enabling services with different QoS requirements (e.g., a voice-over-IP session and a web browsing session) to run concurrently. Its creation was motivated by the industry's shift from voice-centric to data-centric mobile networks, requiring a core network element that could handle the anticipated growth in mobile data traffic and service complexity while maintaining seamless mobility and robust security.

Key Features

• Mobility Management for 3G packet data users (Attach, RAU, handovers)
• Session Management for PDP context activation, modification, and deactivation
• User authentication and authorization via HLR/HSS using AKA procedures
• Packet routing and tunneling between UTRAN and GGSN using GTP-U
• Charging data collection for packet data sessions
• Interfaces with UTRAN (Iu-PS), GGSN (Gn), HLR (Gr), and other SGSNs (Gn/Gp)

Evolution Across Releases

Defining Specifications