Generic Access Network Mode

Description

GAN Mode, or Generic Access Network Mode, is a term used in 3GPP specifications to refer to procedures that apply to both GAN A/Gb mode and GAN Iu mode. The Generic Access Network (GAN) is a technology that allows mobile devices to connect to cellular core networks via unlicensed wireless networks, such as Wi-Fi, extending coverage and improving indoor service. GAN was originally known as Unlicensed Mobile Access (UMA) and was standardized by 3GPP in Rel-8. The term GAN Mode is used when a procedure is common to both variants of GAN: A/Gb mode, which connects to the GSM/EDGE core network (2G), and Iu mode, which connects to the UMTS core network (3G). This distinction is important because the core network interfaces differ, but many procedures, such as mode selection, are similar.

The architecture of GAN involves several key components. The UE connects to a Generic Access Network Controller (GANC) via an IP-based interface (Up interface) over a Wi-Fi or other unlicensed network. The GANC then connects to the core network using either the A/Gb interface (for 2G) or the Iu interface (for 3G). The GANC appears to the core network as a Base Station Subsystem (BSS) or Radio Network Subsystem (RNS), respectively. The UE can operate in GAN mode, meaning it is registered with the GANC and can make voice calls, send SMS, and use data services over the unlicensed network. The term GAN Mode is used in specifications like TS 43.129, TS 43.318, and TS 44.060 to describe procedures that are common to both A/Gb and Iu modes, such as mode selection in multi-mode terminals.

In operation, a multi-mode terminal can support both GAN and traditional cellular access. The mode selection procedure determines whether the UE should use GAN or cellular access based on factors like signal strength, user preferences, and network policies. The term GAN Mode is used to describe this procedure when it applies to both A/Gb and Iu variants. For example, clause 8.1 of the relevant specification discusses the mechanism of mode selection in multi-mode terminals, and this applies to both GAN modes. The GAN Mode concept simplifies specification by allowing common text to cover both variants, avoiding duplication. It also ensures that procedures are consistent regardless of the core network interface used.

The importance of GAN Mode lies in its role in enabling seamless mobility and service continuity between cellular and unlicensed networks. GAN was one of the early technologies for offloading cellular traffic to Wi-Fi, and it paved the way for later developments like LTE-WLAN Aggregation (LWA) and 5G access traffic steering. The term GAN Mode helps standardize the behavior of multi-mode terminals, ensuring interoperability. Although GAN is a legacy technology, its concepts are still relevant in the context of 5G and beyond, where integration with non-3GPP access is a key feature. The term has been present since Rel-8 and has been carried forward through all subsequent releases without changes to its definition, indicating its stable meaning.

In summary, GAN Mode is a term that encompasses procedures applicable to both GAN A/Gb and GAN Iu modes. It is defined in 3GPP TS 21.905 and used in specifications for GAN. It facilitates the specification of common procedures for multi-mode terminals, ensuring consistency across different core network interfaces. While GAN itself is not widely deployed today, the term remains part of the 3GPP vocabulary, and its definition has remained unchanged since Rel-8.

Purpose & Motivation

GAN Mode exists to provide a common term for procedures that apply to both GAN A/Gb mode and GAN Iu mode. The Generic Access Network was developed to allow cellular operators to offer services over unlicensed networks like Wi-Fi, improving coverage and capacity. However, GAN can connect to either 2G (A/Gb) or 3G (Iu) core networks, leading to two variants. Many procedures, such as mode selection, are similar regardless of the core network interface. The term GAN Mode allows specifications to refer to both variants collectively, avoiding duplication and ensuring consistency. It solves the problem of having to specify separate procedures for each variant, simplifying the standards. The motivation was to enable seamless integration of unlicensed access into cellular networks, and the term has been maintained for backward compatibility and clarity.