Equivalent PLMN list

Description

The Equivalent PLMN (EPLMN) list is a list of Public Land Mobile Network (PLMN) identities that the UE considers equivalent to the currently registered PLMN for the purposes of cell selection, cell reselection, and handover. The list is provided by the network to the UE via Non-Access Stratum (NAS) signalling, typically during the attach procedure or a routing area update. The EPLMN list is used by the UE's Access Stratum (AS) to determine which cells are suitable for camping and which are considered equivalent for mobility procedures. The concept of equivalent PLMNs allows a network to treat multiple PLMN codes as a single network for mobility management, which is useful in scenarios such as network sharing, national roaming, and multi-operator core networks.

The EPLMN list is distinct from the EHPLMN list, which is stored on the USIM and used for PLMN selection. The EPLMN list is dynamic and provided by the serving network, while the EHPLMN list is static and controlled by the home operator. The EPLMN list is used for cell selection and reselection, while the EHPLMN list is used for PLMN selection (i.e., which network to register on). Both lists can coexist and serve different purposes. For example, a UE may have an EHPLMN list that includes several home PLMNs, and the serving network may provide an EPLMN list that includes additional PLMNs that are considered equivalent for cell selection. The UE will use the EPLMN list to determine if a cell belonging to a different PLMN can be considered as belonging to the registered PLMN for the purpose of cell reselection and handover.

The EPLMN list is provided to the UE in the EPS Mobility Management (EMM) or 5G Mobility Management (5GMM) messages, such as ATTACH ACCEPT, TRACKING AREA UPDATE ACCEPT, or REGISTRATION ACCEPT. The list contains up to 15 PLMN codes (in LTE) or more in 5G. The UE stores the list and uses it until it is updated or until the UE deregisters. The list is used by the AS layer when evaluating cells for reselection: a cell belonging to a PLMN in the EPLMN list is considered equivalent to the registered PLMN, and the UE can reselect to that cell without performing a PLMN reselection. This enables seamless mobility across PLMNs that are part of the same network sharing arrangement.

The EPLMN list also plays a role in manual CSG selection. In Release 8, the definition was updated to include manual CSG selection, which allows the user to manually select a closed subscriber group (CSG) cell. The EPLMN list is used to determine if a CSG cell belonging to a different PLMN is considered equivalent. This is important for CSG deployments where multiple PLMNs may share the same CSG cell.

The EPLMN list is specified in 3GPP TS 25.304 (UTRAN), TS 25.367 (UTRAN), TS 36.304 (E-UTRAN), and TS 38.304 (NR). The definition has evolved over releases. In Release 6, it was defined as 'List of PLMNs considered as equivalent by the UE for cell selection, cell reselection and handover according to the information provided by the NAS.' In Release 7, MBSFN Cluster selection and reselection were added. In Release 8, manual CSG selection was added. In Release 9, the wording was changed to remove the comma before 'and handover' and to clarify that the list is used for cell selection, cell reselection, MBSFN cluster selection, MBSFN cluster reselection, and handover. This wording remained stable through Release 19, with minor punctuation changes.

The EPLMN list is a key enabler for network sharing and national roaming. It allows multiple operators to share a radio access network while maintaining separate core networks, and it allows UEs to move between PLMNs without losing service. It is also used in multi-operator core network (MOCN) and multi-operator radio access network (MORAN) sharing scenarios. The list is managed by the network and can be updated dynamically based on the network's configuration and the UE's location.

Purpose & Motivation

The EPLMN list was created to support network sharing and national roaming scenarios where a UE needs to treat multiple PLMN codes as equivalent for mobility purposes. In traditional networks, a UE would only consider cells belonging to its registered PLMN as suitable for camping. However, in shared networks, multiple PLMNs may be broadcast in the same cell, and the UE needs to be able to reselect to cells of other PLMNs without performing a full PLMN reselection. The EPLMN list solves this problem by providing the UE with a list of PLMNs that are considered equivalent to the registered PLMN. This allows the UE to treat cells of those PLMNs as if they were cells of the registered PLMN, enabling seamless mobility.

The EPLMN list also supports manual CSG selection, which was added in Release 8. In CSG deployments, a CSG cell may be shared by multiple PLMNs, and the UE needs to know which PLMNs are considered equivalent when the user manually selects a CSG cell. The EPLMN list provides this information. The list is also used in MBSFN (Multicast Broadcast Single Frequency Network) cluster selection and reselection, which was added in Release 7. MBSFN is used for broadcast services such as mobile TV, and the EPLMN list allows the UE to consider MBSFN clusters of equivalent PLMNs as equivalent for selection and reselection.

The EPLMN list is a dynamic list provided by the network, which means it can be tailored to the specific sharing arrangement and can be updated as the UE moves. This flexibility is essential for operators that have complex sharing agreements. The list is also used in 5G networks for network slicing and multi-operator scenarios. Overall, the EPLMN list is a fundamental enabler for network sharing and roaming, and it has been widely deployed in commercial networks.