Allowable Public Land Mobile Network

Description

An Allowable PLMN is a concept in 3GPP mobility management that defines a PLMN which a User Equipment (UE) is allowed to access, based on the UE's list of forbidden PLMNs. The UE maintains a list of forbidden PLMNs, which includes networks that have been explicitly barred due to various reasons such as failed authentication, subscription restrictions, or regulatory requirements. Any PLMN not in this forbidden list is considered allowable. This concept is fundamental to the UE's network selection and reselection procedures.

The UE's forbidden PLMN list is stored in the SIM/USIM and can be updated by the network or the user. It typically includes PLMNs where the UE has attempted to register but was rejected, or PLMNs that are forbidden by the home operator. The list can be of different types: forbidden PLMNs for GPRS, for EPS, for 5GS, etc. The UE uses this list during the PLMN selection process to avoid attempting to access networks that are not permitted.

When the UE is roaming or in a border area with multiple PLMNs, it scans for available PLMNs and evaluates each against the forbidden list. Allowable PLMNs are then further prioritized based on other criteria such as whether they are the home PLMN (HPLMN), equivalent PLMN (EPLMN), or preferred PLMN. The UE then attempts to register on the highest priority allowable PLMN. If registration fails, the PLMN may be added to the forbidden list, and the UE will try the next allowable PLMN.

The concept of allowable PLMN is defined in 3GPP TS 21.905 (Vocabulary) and TS 25.304 (UE procedures in idle mode). It applies to all generations of 3GPP networks, from GSM to 5G. The definition has remained essentially the same since Rel-4, with only minor wording changes. This stability reflects the fundamental nature of the concept in mobility management.

In 5G, the concept is extended with the forbidden PLMN list for 5GS, and the UE may also consider Standalone Non-Public Networks (SNPNs) and their forbidden lists. However, the core idea remains: an allowable PLMN is one that the UE is not forbidden from accessing. This ensures that the UE does not waste resources attempting to access networks that will reject it, and it helps operators enforce roaming and access control policies.

Purpose & Motivation

The Allowable PLMN concept exists to streamline the UE's network selection process by excluding PLMNs that the UE is not permitted to access. Without this concept, the UE would attempt to register on any detected PLMN, leading to unnecessary signaling, delays, and potential service disruptions. The forbidden PLMN list allows the network and the user to control which networks the UE should avoid, based on past failures, subscription restrictions, or regulatory requirements.

Historically, as mobile networks evolved and roaming became common, the need to manage network selection efficiently grew. Early UEs would scan and attempt registration on any network, which was inefficient and could lead to repeated rejections. The introduction of the forbidden PLMN list and the allowable PLMN concept in 3GPP standards provided a standardized way to handle this. It allowed operators to push updates to the forbidden list via over-the-air (OTA) mechanisms, ensuring that UEs avoid problematic networks.

The concept also supports regulatory requirements, such as preventing UEs from accessing certain networks in specific countries. By maintaining a forbidden list, the UE can quickly exclude such networks and focus on allowable ones. This improves user experience by reducing registration delays and battery consumption. The allowable PLMN concept is thus a cornerstone of efficient mobility management in cellular networks.