Selected Public Land Mobile Network

Description

The Selected PLMN is defined in 3GPP TS 21.905 and referenced in TS 24.234, TS 25.304, TS 25.367, TS 36.304, and TS 38.304 as the PLMN that has been selected by the non-access stratum (NAS), either manually or automatically, according to the PLMN selection procedures. PLMN selection is a fundamental mobility function: before a UE can access any service, it must decide which network to roam on or register with. The selected PLMN then becomes the reference network for cell selection, reselection, and registration procedures.

In manual mode, the UE presents the user with a list of available PLMNs discovered during the scan, and the user chooses one. In automatic mode, the UE applies a priority list — typically stored in the (U)SIM or managed by the home operator — to select the best PLMN according to criteria such as home PLMN first, then equivalent home PLMNs, then preferred visited PLMNs, and finally other PLMNs if no preferred one is available. The NAS layer performs this decision and communicates the result to the AS (access stratum), which then searches for a suitable cell belonging to the selected PLMN.

Once selected, the PLMN identifier (MCC + MNC) guides the AS in cell selection. The AS evaluates candidate cells based on radio quality, and for each candidate it checks whether the cell's broadcast PLMN identity matches the selected PLMN (or an equivalent PLMN). Cells that do not belong to the selected PLMN are normally not considered suitable for camping, except in limited cases such as emergency calls or when the UE is in a limited service state. This tight coupling between NAS PLMN selection and AS cell selection ensures the UE never registers with an unintended network.

The Selected PLMN also plays a role in inter-system and inter-RAT mobility. When the UE moves between 2G, 3G, 4G, and 5G coverage, the same selected PLMN concept applies across all radio technologies. Specifications such as TS 25.304 (UTRAN), TS 36.304 (E-UTRAN), and TS 38.304 (NR) each define cell selection and reselection rules relative to the selected PLMN. This cross-generation consistency is important for seamless service continuity and for roaming agreements that span multiple access technologies.

For network operators and roaming engineers, the Selected PLMN is a central concept for steering and for troubleshooting. Operator-controlled PLMN lists and equivalent PLMN lists enable steering of roaming, while the UE's selected PLMN is visible in signalling and can be used to diagnose why a subscriber attached to a particular network. In 5G, the concept remains largely unchanged, although the interaction with network slicing and the 5G NAS adds complexity. Understanding the Selected PLMN is therefore essential for anyone working on mobility management, roaming, or UE protocol stacks.

Purpose & Motivation

The Selected PLMN concept exists to formalise and standardise how a mobile device decides which network to use. In a multi-operator, multi-technology environment, a UE may see many PLMNs, and it needs a deterministic, operator-controllable procedure to pick one. Without a clear definition, devices could behave inconsistently, register with unintended networks, or fail to respect roaming agreements.

By defining the Selected PLMN as the PLMN chosen by NAS — either manually by the user or automatically via operator-defined priorities — 3GPP separates the high-level network choice from low-level radio cell selection. This separation gives operators a steering mechanism through preference lists while allowing the access stratum to focus on radio suitability. It also provides a common vocabulary across all radio access technologies.

Historically, PLMN selection was standardised early in GSM and carried into UMTS, LTE, and NR with only minor wording adjustments. The persistence of the definition across releases reflects how fundamental and stable this concept is: despite major changes in radio technology and core network architecture, the problem of choosing a network remains and the same conceptual solution continues to serve it.