GSM EDGE Radio Access Network

Description

The GSM EDGE Radio Access Network (GERAN) is the collective term for the radio network infrastructure that implements the GSM, GPRS, and EDGE radio technologies as defined by 3GPP. Architecturally, GERAN is composed of Base Station Subsystems (BSS). Each BSS consists of two main elements: the Base Transceiver Station (BTS), which houses the radio transceivers and antennas for one or more cells, and the Base Station Controller (BSC), which manages radio resources, handles handovers, and controls the BTSs. The BSC performs critical functions like frequency allocation, power control, and the management of traffic and signaling channels (e.g., SDCCH, TCH).

GERAN connects to the core network via two key interfaces. For circuit-switched (CS) voice and SMS services, it uses the A-interface to link the BSC with the Mobile Switching Center (MSC). For packet-switched (PS) data services via GPRS and EDGE, it uses the Gb-interface to connect the BSC (or a dedicated Packet Control Unit - PCU) with the Serving GPRS Support Node (SGSN). The radio interface, known as the Um interface, uses a combination of FDMA (dividing the spectrum into 200 kHz carriers) and TDMA (dividing each carrier into 8 time slots). EDGE (Enhanced Data rates for GSM Evolution) introduced advanced modulation (8-PSK alongside GMSK) and coding schemes, significantly boosting data rates within the same TDMA structure.

Within a multi-RAT (Radio Access Technology) environment, GERAN's role evolved from being the sole RAN to interworking with UMTS (UTRAN) and LTE (E-UTRAN). Specifications define mechanisms for handover and cell reselection between GERAN and these newer networks, allowing for service continuity. The BSC manages both CS and PS resources, and with the introduction of EDGE, it also handles the adaptive modulation and coding that defines EDGE's higher throughput. GERAN specifications cover everything from the physical layer (modulation, burst structures) in the 45-series specs, to layer 2 and 3 protocols (e.g., RLC/MAC, BSSGP over Gb) in the 44 and 48-series, and to performance requirements in the 51-series.

Purpose & Motivation

GERAN was formally defined as a term to encompass the evolved 2G radio access network that included both the original GSM standard and its packet-data enhancements, GPRS and EDGE. Its creation solved the problem of providing a clear architectural definition for the radio network that supported both voice-centric circuit-switched services and emerging internet-based packet-switched data services under a single umbrella. Before this consolidation, specifications often referred separately to GSM radio and GPRS radio aspects.

The historical motivation was the need to evolve the successful GSM network to handle data efficiently, leading first to GPRS (a packet overlay) and then to EDGE (a radio enhancement). GERAN represents the culmination of this evolution, maximizing the utility of the deployed 200 kHz carrier TDMA infrastructure. It addressed the limitation of slow circuit-switched data (CSD) by introducing always-on packet connectivity (GPRS) and then significantly boosting its speed (EDGE), enabling early mobile internet experiences. As 3G (UMTS) emerged, defining GERAN allowed for clearer specifications on how this legacy network would interwork and coexist with the new WCDMA-based UTRAN, ensuring a smooth transition and global roaming across multiple generations of technology.