Available Public Land Mobile Network

Description

Available PLMN is a concept in 3GPP specifications that defines a Public Land Mobile Network (PLMN) that a User Equipment (UE) has discovered and from which it has successfully read the PLMN identity. This is a fundamental step in the network selection process, which occurs when a UE is powered on, moves into a new area, or loses coverage. The UE scans for cells, and for each cell it detects, it attempts to read the system information, including the PLMN identity broadcast by the cell. If the UE can read the PLMN identity, that PLMN is considered 'available' to the UE.

The availability of a PLMN does not guarantee that the UE can register or obtain service from it; it simply means that the UE has found a cell belonging to that PLMN and has decoded its identity. The UE may still be barred from accessing the PLMN due to various reasons, such as the cell being reserved for operator use, the UE not having a subscription, or the PLMN not being in the UE's allowed list. However, the list of available PLMNs is used by the UE to make decisions about which PLMN to select for service, based on priority and other criteria.

The process of determining available PLMNs is specified in 3GPP TS 25.304 for UTRAN and TS 25.367 for E-UTRAN, as well as TS 21.905 for vocabulary. The UE performs a cell search and reads the Master Information Block (MIB) and System Information Blocks (SIBs) to obtain the PLMN identity. In LTE and 5G, the PLMN identity is broadcast in SIB1. The UE maintains a list of available PLMNs, which may include the Home PLMN (HPLMN), Equivalent PLMNs (EPLMNs), and other PLMNs. The UE also considers the radio access technology (RAT) of the cell, as different PLMNs may be available via different RATs.

The concept of Available PLMN is closely related to other PLMN states, such as 'Registered PLMN' (the PLMN on which the UE is currently registered) and 'Equivalent PLMN' (a PLMN considered equivalent to the registered PLMN for selection purposes). The UE uses the list of available PLMNs during the PLMN selection procedure, which can be automatic or manual. In automatic mode, the UE selects the highest-priority PLMN from the available list that it is allowed to access. In manual mode, the user is presented with a list of available PLMNs and can choose one.

The Available PLMN concept has been stable since its introduction in Release 4, with no changes to its definition through Release 19. This stability reflects the fundamental nature of the concept in cellular network selection. As networks evolve towards 5G and beyond, the concept remains applicable, although the specific procedures for reading PLMN identity may be adapted to new radio technologies. The term is essential for understanding how UEs discover and select networks, and it forms the basis for mobility management and roaming.

Purpose & Motivation

The Available PLMN concept exists to provide a standardised way for UEs to identify which networks are present in their vicinity and can potentially be used for service. It solves the problem of network discovery in a multi-operator, multi-technology environment. Without a clear definition of availability, UEs would not know which networks to consider for registration, leading to inefficient scanning and poor user experience.

Historically, as cellular networks proliferated and roaming became common, the need for UEs to automatically select the best network became critical. The Available PLMN concept was introduced to enable automatic and manual network selection procedures. It allows the UE to build a list of candidate networks, which can then be filtered based on priority, subscription, and access restrictions. This is particularly important for international roaming, where a UE may find multiple visited PLMNs and must choose the one with which its home operator has a roaming agreement.

The concept also supports regulatory requirements, such as emergency calls, where the UE may need to select any available PLMN to place an emergency call, even if it does not have a subscription. By defining what constitutes an available PLMN, the standards ensure that UEs can reliably discover networks and make informed selection decisions, enhancing the overall mobility experience.