Inter-system change

Description

Inter-system change is a fundamental mobility procedure in 3GPP networks that governs how a mobile station (MS) or user equipment (UE) transitions its radio access between distinct radio access technologies (RATs). Unlike intra-system handover, which occurs between cells of the same RAT, inter-system change involves a change of the underlying radio interface technology — for example, moving from a GSM EDGE Radio Access Network (GERAN) to a UMTS Terrestrial Radio Access Network (UTRAN), or from UTRAN to Evolved UTRAN (E-UTRAN/LTE). The procedure is defined primarily in TS 23.060 for the packet-switched domain and is referenced across a broad set of charging and management specifications including 32.240, 32.250, 32.251, 32.272, 32.273, 32.277, 32.278 and 32.295.

Architecturally, inter-system change involves coordination between the UE, the source and target radio access networks, the core network entities (such as the SGSN, MME, GGSN, PGW and HLR/HSS), and the policy and charging functions. When a UE moves from one RAT to another, the serving core network node may change — for example, from an SGSN to an MME in the case of a UTRAN/GERAN-to-E-UTRAN transition. The procedure requires the transfer of session context, bearer or PDP context information, security associations, and subscriber profile data between the involved nodes. In EPS, this is realized through interfaces such as S3 (SGSN–MME), S4 (SGSN–SGW), and S5/S8 (SGW–PGW), which allow the anchoring of user-plane traffic at the PGW to remain stable while the access leg changes.

The procedure can be triggered by several conditions: radio conditions degrading in the source RAT, coverage boundaries, load balancing policies, service requirements (e.g., a need for higher throughput available only in LTE), or operator preferences configured via ANDSF or access network discovery policies. Depending on whether the UE is in idle or connected mode, the change may be executed as a handover (with explicit signalling and resource reservation in the target cell) or as a cell/TAU reselection followed by a routing area update or tracking area update.

From a service continuity perspective, inter-system change is critical for maintaining voice and data sessions. For voice, techniques such as Single Radio Voice Call Continuity (SRVCC) and, more recently, EPS fallback and VoNR-to-VoLTE transitions rely on inter-system change mechanisms to hand over an ongoing call from LTE to UTRAN/GERAN or between 5G NR and LTE. For data, the maintenance of IP address continuity and bearer/QoS parameters ensures that applications do not experience session drops during the transition.

Charging and management specifications (32.240, 32.250, 32.251, 32.272, 32.273, 32.277, 32.278, 32.295) define how inter-system change events are recorded, correlated and billed. This includes the generation of CDRs at the SGSN, MME, SGW and PGW, and the correlation of partial records across RAT boundaries so that a single subscriber session can be accurately charged even when it spans multiple access technologies. The term has remained stable in its broad meaning since Rel-4, though its scope expanded from GSM/UMTS to include LTE and later 5G access.

Purpose & Motivation

Inter-system change exists to solve the fundamental problem of heterogeneous network coverage and capability. Early mobile networks were built as islands of technology — GSM coverage in one area, UMTS in another — and without a standardized mechanism to move active sessions between them, users would experience dropped calls and broken data sessions whenever they crossed a technology boundary. The procedure was created to provide service continuity across these islands, allowing operators to deploy new RATs incrementally while leveraging existing coverage.

Historically, before inter-system change was formalized, mobility was largely confined within a single access technology. As operators began overlaying UMTS on GSM networks in the late 1990s and early 2000s, the need for standardized inter-RAT mobility became acute. 3GPP addressed this in Rel-4 by defining inter-system change as a change of radio access between different RATs such as GSM and UMTS. This allowed dual-mode terminals to hand over between the two technologies, preserving voice calls and data sessions.

The motivation continued to evolve with LTE. When LTE was introduced in Rel-8, inter-system change was extended to cover GSM, UMTS and LTE, enabling operators to use LTE as a capacity layer while falling back to UMTS or GSM for coverage or voice. Later releases added support for 5G NR interworking, ensuring that the same conceptual framework could support transitions between NR and LTE. In essence, inter-system change provides the mobility glue that makes multi-RAT networks function as a single coherent service environment.