Forbidden Public Land Mobile Network

Description

The Forbidden Public Land Mobile Network (FPLMN) list is a crucial subscriber- and equipment-side data structure defined in 3GPP specifications, including TS 21.905 (vocabulary), TS 22.811 (USIM aspects), and TS 31.121 (USIM test specification). It is stored in the non-volatile memory of the Universal Subscriber Identity Module (USIM) or SIM card within the User Equipment (UE). Each entry in the list is a PLMN identifier, consisting of a Mobile Country Code (MCC) and a Mobile Network Code (MNC). The primary function of this list is to prevent the UE from wasting time and battery power repeatedly attempting to register with a network that has previously rejected it.

The mechanism works as follows: When a UE performs a registration (attach) or location update procedure with a visited PLMN (VPLMN), the network may respond with a rejection message. If this rejection carries a specific cause value indicating a permanent or semi-permanent failure (e.g., 'PLMN not allowed', 'GPRS not allowed in this PLMN', 'No suitable cells in location area'), the UE's mobility management layer will add that PLMN's identifier (MCC+MNC) to the FPLMN list stored on the USIM. During subsequent automatic network selection processes—such as when the UE is powered on, loses coverage, or returns from abroad—the UE's network selection algorithm is mandated to exclude any PLMN whose ID appears on the FPLMN list from its set of candidate networks for automatic selection.

The FPLMN list is managed by both the UE and the network. The UE adds entries based on rejection cause codes. Entries can be removed (the list is cleared) under specific conditions: 1) Upon a successful registration with a PLMN that is in the UE's Home PLMN (HPLMN) or Equivalent Home PLMN (EHPLMN) list. 2) Manually by the user via the device's network selection menu, which may trigger a 'search for all networks' operation. 3) Potentially through Over-The-Air (OTA) USIM updates from the home operator. The list has a finite size (minimum guaranteed by specification), and if full, the oldest entry may be replaced. This mechanism plays a vital role in radio resource efficiency, device power management, and user experience by steering the UE away from known-inaccessible networks.

Purpose & Motivation

The FPLMN list was created to solve the problem of inefficient and persistent registration attempts, which drain UE battery and create unnecessary signaling load on radio access and core networks. In early cellular systems, without such a list, a UE that was rejected by a network (e.g., due to lack of roaming agreement) might continuously re-attempt registration every time it scanned for networks or periodically updated its location, especially in border areas or places with overlapping coverage from multiple operators.

This behavior was wasteful and degraded user experience. The FPLMN concept introduced a form of 'memory' into the UE's network selection logic. By temporarily blacklisting a rejecting network, the system ensures operational efficiency. It addresses the limitation of a stateless selection process. The historical context includes its definition in GSM specifications and its carry-over into UMTS, LTE, and 5G NAS (Non-Access Stratum) procedures. The list is particularly important for international roaming, where a UE may encounter many networks, only a subset of which have valid roaming agreements with the home operator. It allows the UE to quickly 'learn' and avoid networks that will not serve it, streamlining the process of finding a permissible visited network. The conditions for clearing the list, especially upon contact with the home network, ensure the UE does not remain permanently barred from a network if the underlying reason for rejection (e.g., a temporary roaming agreement lapse) is later resolved.

Key Features

• Stored persistently on the USIM/SIM card within the UE
• Contains PLMN identifiers (MCC+MNC) of networks that have rejected registration
• Updated automatically by the UE based on specific rejection cause codes from the network
• Forbids automatic network selection/registration onto listed PLMNs
• Can be cleared manually by user-initiated network scan or automatically upon successful registration with HPLMN/EHPLMN
• Has a defined minimum storage capacity (e.g., 4 entries) to handle multiple forbidden networks

Evolution Across Releases

Defining Specifications