GSM Core Network

Description

The GSM core network is the central part of a GSM system that handles call routing, subscriber management, and data transport. It is composed of two main domains: the Circuit-Switched (CS) domain, which includes the Mobile Switching Center (MSC), Visitor Location Register (VLR), Home Location Register (HLR), Authentication Center (AuC), and Equipment Identity Register (EIR); and the Packet-Switched (PS) domain, which includes the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). These elements work together to provide voice, SMS, and data services to mobile subscribers.

The CS domain is responsible for real-time services such as voice calls and circuit-switched data. The MSC acts as the primary switching node, routing calls and managing mobility through signaling with the VLR and HLR. The HLR is a central database that stores subscription information and location data for each subscriber, while the VLR temporarily stores data for subscribers roaming in its area. The AuC provides authentication and encryption keys, and the EIR maintains a list of valid and stolen devices.

The PS domain, introduced with GPRS, enables packet-based data services. The SGSN routes data packets to and from mobile stations within its service area, performing mobility management, authentication, and session management. The GGSN acts as the gateway to external packet data networks such as the Internet or corporate intranets, handling IP address allocation and policy enforcement. Together, these nodes form the GPRS backbone infrastructure.

The GSM core network has evolved over releases to support interworking with other access technologies, including UMTS, LTE, and 5G. It provides the foundation for mobility management, security, and service control that has been adapted and extended in later generations. Despite the advent of newer technologies, the GSM core network remains relevant for legacy support, roaming, and certain IoT applications.

Key interfaces within the core network include the A interface to the Base Station Subsystem (BSS), the Gb interface to the GPRS radio access, and various internal interfaces such as MAP, CAP, and GTP. These protocols ensure seamless operation and interoperability between network elements and with external networks.

Purpose & Motivation

The GSM core network was created to provide a standardized, scalable architecture for mobile telephony and data services. Before its introduction, mobile networks were fragmented and lacked interoperability. The GSM core network solved this by defining common interfaces and procedures, enabling subscribers to roam across networks and use services consistently. It also introduced a clear separation between circuit-switched and packet-switched domains, allowing efficient handling of both voice and data traffic.

The evolution from purely circuit-switched to include packet-switched capabilities with GPRS addressed the growing demand for mobile data. The core network's modular design allowed operators to incrementally upgrade and scale their networks. Its robust subscriber management and security features set the stage for future generations, which have built upon and integrated with the GSM core network to ensure backward compatibility and smooth migration paths.