Description
A/Gb mode is a fundamental architectural concept in 3GPP specifications that defines how a mobile system or subsystem operates when the functional division between the radio access network (RAN) and the core network (CN) follows the use of either the A interface or the Gb interface. The A interface is the circuit-switched interface between the Base Station Subsystem (BSS) and the Mobile Switching Center (MSC), while the Gb interface is the packet-switched interface between the BSS and the Serving GPRS Support Node (SGSN). This mode is primarily associated with GSM, GPRS, and EDGE radio access technologies, collectively known as GERAN.
In A/Gb mode, the RAN (specifically the Base Station Subsystem) connects to the core network through these standardized interfaces. The A interface carries circuit-switched traffic such as voice calls, supporting the traditional GSM circuit-switched domain. The Gb interface carries packet-switched data traffic, enabling GPRS and later EDGE for mobile data services. The functional split means that radio resource management, ciphering, and some mobility functions reside in the BSS, while call control, session management, and subscriber management are handled by the core network nodes (MSC/VLR for CS, SGSN/GGSN for PS).
A key characteristic of A/Gb mode is its independence from the support of both interfaces simultaneously. For example, an SGSN operating in A/Gb mode uses only the Gb interface, as it is a packet-switched core node. Similarly, an MSC uses only the A interface. This flexibility allows network operators to deploy circuit-switched and packet-switched services independently or together, depending on their needs. The mode is defined in 3GPP TS 21.905 (Vocabulary) and referenced in numerous specifications including TS 23.060 (GPRS), TS 43.051 (GERAN overall description), and others.
The A/Gb mode is contrasted with Iu mode, which is used in UMTS (3G) networks where the RAN connects to the core network via the Iu interface (Iu-CS and Iu-PS). The Iu mode provides a different functional split, with more centralized control in the core network and different protocol stacks. The coexistence of A/Gb mode and Iu mode allowed for smooth migration from 2G to 3G and enabled dual-mode operation in early 3G deployments.
In the context of 3GPP specifications, the term "A/Gb mode" is often used as a qualifier in clause headings or text to indicate that the described functionality applies only to systems operating in this mode. This is crucial for multi-mode specifications that cover both GERAN and UTRAN access. The definition has remained largely stable across releases, with only minor editorial changes, reflecting the mature and stable nature of this architectural concept. Even in later releases (up to Rel-19), the term is maintained for backward compatibility and to support legacy GERAN deployments and interworking scenarios.
Purpose & Motivation
The A/Gb mode concept was created to provide a clear architectural framework for GSM and GPRS networks, which were the first widely deployed cellular systems. It solved the problem of defining a standardized functional split between the radio access network and core network, enabling multi-vendor interoperability and allowing operators to deploy circuit-switched and packet-switched services in a modular fashion. Before this definition, early GSM implementations had varied interfaces, leading to interoperability issues.
The introduction of GPRS in 3GPP Release 97/98 required a packet-switched core network, and the Gb interface was defined to connect the BSS to the SGSN. The A/Gb mode concept formalized the coexistence of the A and Gb interfaces, allowing a single BSS to support both CS and PS domains. This was essential for the evolution from pure voice-centric GSM to data-capable networks.
As 3GPP evolved towards UMTS and LTE, the A/Gb mode remained relevant for legacy support and interworking. It provided a stable reference for dual-mode terminals and networks, ensuring that GSM/GPRS/EDGE access could continue to operate alongside newer technologies. The term also helps specification writers clearly delineate requirements that apply only to GERAN access, avoiding confusion in multi-access specifications.
What changed in its specifications
The latest release-to-release comparison of specifications that define A/Gb mode. Each summary describes the whole specification, not only this term.
Rel-18 brings only minor changes to TS 21.905: 0 clauses changed, with no change attributed to a release feature.
Full comparison in TR 21.905 →Rel-19 brings only minor changes to TS 23.060: 0 clauses changed, with no change attributed to a release feature.
Full comparison in TS 23.060 →Release Timeline
Evolution Across Releases
An overview written from the defining specifications, not a verified change history. For the exact text, open the specifications listed below.
Initial definition introduced: 'indicates that this (sub)clause or paragraph applies only to a system or sub-system which operate in A/Gb mode of operation, i.e. with a functional division that is in accordance with the use of an A or a Gb interface between the radio access network and the core network. This definition is consitent with the A/Gb mode definition for the RAN in 3G 43.051 [74]. Note that A/Gb mode is independent of the support of both interfaces, e.g. an SGSN in A/Gb mode uses only the Gb interface.'
Wording changed: removed the note about independence of interfaces ('Note that A/Gb mode is independent...') and corrected 'consitent' to 'consistent' and '3G 43.051' to 'TS 43.051'. The definition became: 'indicates that this clause or paragraph applies only to a system or sub-system which operate in A/Gb mode of operation, i.e. with a functional division that is in accordance with the use of an A or a Gb interface between the radio access network and the core network. This definition is consistent with the A/Gb mode definition for the RAN in TS 43.051 [74].'
Defining Specifications
3GPP specifications that define or reference A/Gb mode, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj20 | 3GPP Terminology and Definitions | Rel-19 read |
| TS 23.060 vj00 | GPRS Stage 2 Service Description | Rel-19 read |
| TS 23.202 vj00 | CS Bearer Services Architecture in UMTS | Rel-19 read |
| TS 23.205 vj00 | Bearer Independent CS Core Network Stage 2 | Rel-19 read |
| TR 23.910 v550 | CS Bearer Services in UMTS | Rel-5 read |
| TS 24.022 vj00 | Radio Link Protocol (RLP) for Circuit Switched Data | Rel-19 read |
| TS 27.001 vj00 | Terminal Adaptation Functions for PLMN | Rel-19 read |
| TS 27.002 vj00 | Terminal Adaptation Functions for Asynchronous Services | Rel-19 read |
| TS 27.003 vj00 | Terminal Adaptation Functions (TAF) for Synchronous Services | Rel-19 read |
| TS 27.060 vj00 | TE-MT Interworking for Packet Domain | Rel-19 read |
| TS 29.007 vj00 | PLMN-PSTN/ISDN Interworking Requirements | Rel-19 read |
| TS 29.061 vk10 | Packet Domain Interworking Requirements | Rel-20 read |
| TS 43.059 vj00 | GERAN LCS Stage 2 Specification | Rel-19 read |
| TS 43.129 vj00 | PS Handover in GERAN A/Gb and GAN Modes | Rel-19 read |
| TS 43.130 vj00 | Iur-g Interface Overview | Rel-19 read |
| TS 44.118 vg00 | Radio Resource Control (RRC) Protocol for Iu Mode | Rel-16 read |