GPRS Support Node

Description

The GPRS Support Node (GSN) is a generic term for the two primary packet-switched core network elements in GPRS and UMTS architectures: the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). The SGSN is responsible for the delivery of data packets to and from mobile stations within its geographical service area. Its functions include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The SGSN communicates with the Radio Access Network (e.g., BSC in 2G, RNC in 3G) via the Gb or Iu-ps interfaces and with other SGSNs and GGSNs via the Gn/Gp interfaces using the GPRS Tunneling Protocol (GTP).

The GGSN acts as a gateway between the GPRS/UMTS core network and external packet data networks (PDNs), such as the public internet or a corporate intranet. It contains routing information for attached mobile stations and is responsible for IP address assignment, often via DHCP. The GGSN performs the role of a router, forwarding packets between the external PDN and the SGSN serving the user. It also handles policy enforcement, packet filtering, and is a primary point for charging data collection (e.g., for volume-based billing). The connection between the SGSN and GGSN is a GTP tunnel, which encapsulates the user's IP packets to provide mobility transparency.

Together, the SGSN and GGSN form the backbone of the packet-switched core, enabling always-on connectivity. The SGSN manages the user plane (data forwarding) and control plane (session and mobility signaling) towards the access network, while the GGSN anchors the user's session and provides connectivity to the outside world. This architecture separated the packet-switched domain from the circuit-switched domain for voice, allowing for efficient data services. In later 4G EPC and 5G Core networks, the functions of the SGSN and GGSN are evolved and distributed into entities like the MME, SGW, and PGW.

Purpose & Motivation

The GSN architecture was created to introduce packet-switched data capabilities into existing 2G GSM networks, which were originally designed only for circuit-switched voice and SMS. Before GPRS, data services like Circuit Switched Data (CSD) were inefficient, tying up a full traffic channel for the duration of a connection regardless of actual data transfer. The GPRS Support Nodes were developed to provide an 'always-on' IP-based data service, allowing statistical multiplexing of multiple users on shared radio resources and enabling efficient bursty data traffic like web browsing and email.

The separation into SGSN and GGSN served specific purposes. The SGSN, being closer to the radio network, efficiently manages radio resource connections and mobility for a local area. The GGSN provides a stable anchor point to the external IP world, hiding the user's mobility from external networks and simplifying routing and charging. This division allowed for scalable network deployment and independent evolution of access and core network functions. The GSN framework laid the groundwork for mobile broadband, evolving through 3G UMTS and into the foundations of the 4G Evolved Packet Core.