Supported Public Land Mobile Network

Description

A Supported PLMN is a Public Land Mobile Network (PLMN) belonging to a roaming partner with which the WLAN operator has established a direct roaming relationship. This concept is fundamental to 3GPP-WLAN interworking, allowing mobile subscribers to access 3GPP services through WLAN access networks. The term is defined in 3GPP specifications 23.234 and 24.234, which specify the architecture and procedures for WLAN interworking with 3GPP systems.

In the 3GPP-WLAN interworking architecture, the WLAN Access Network (WLAN AN) connects to the 3GPP network via the Wn interface to a 3GPP AAA server. The Supported PLMN concept ensures that only authorized PLMNs—those with which the WLAN operator has a direct roaming agreement—can be used for authentication and service access. When a user equipment (UE) attempts to access services through a WLAN, it provides its home PLMN identity. The WLAN network then checks whether this PLMN is among its Supported PLMNs. If so, the authentication request is forwarded to the appropriate 3GPP AAA server in the home PLMN.

The Supported PLMN list is typically configured in the WLAN Access Network and may be communicated to the UE during network discovery and selection procedures. This list helps the UE decide which WLANs to associate with and which PLMN to select for service. The concept is closely related to the Visited PLMN (VPLMN) and Home PLMN (HPLMN) concepts in roaming scenarios. In WLAN interworking, the Supported PLMN acts as an intermediary that enables the WLAN operator to offer 3GPP services without having a direct relationship with every possible home PLMN.

The direct roaming relationship implies that there is a business and technical agreement between the WLAN operator and the PLMN operator. This agreement covers aspects such as authentication, authorization, accounting, and service level agreements. The Supported PLMN concept thus ensures that only trusted partners can use the WLAN for 3GPP service delivery, maintaining security and proper billing.

From a technical standpoint, the Supported PLMN is identified by a PLMN ID consisting of a Mobile Country Code (MCC) and a Mobile Network Code (MNC). The WLAN network uses this identifier to route authentication requests to the correct 3GPP AAA server. The 3GPP AAA server then authenticates the subscriber and authorizes access to services. The Supported PLMN concept is essential for enabling seamless mobility and service continuity between 3GPP and WLAN access technologies.

Purpose & Motivation

The Supported PLMN concept was introduced to facilitate 3GPP-WLAN interworking by defining which PLMNs a WLAN operator can serve. Before this, WLAN access was largely separate from cellular networks, and there was no standardized way for mobile operators to extend their services over WLAN. The Supported PLMN mechanism allows WLAN operators to partner with specific PLMNs, ensuring that only authorized users from those PLMNs can access 3GPP services via WLAN. This enables roaming agreements and business models where WLAN operators act as access providers for 3GPP services.

The concept solves the problem of authentication and authorization in heterogeneous networks. Without a clear list of Supported PLMNs, a WLAN network would not know which 3GPP AAA server to contact for a given user, leading to failed authentications or security risks. By defining Supported PLMNs, the WLAN network can efficiently route authentication requests and enforce roaming policies. It also simplifies the user experience, as the UE can automatically select a WLAN that supports its home PLMN.

Historically, the Supported PLMN concept emerged in Release 6 as part of the initial 3GPP-WLAN interworking specifications. It addressed the need for a standardized framework to support roaming between cellular and WLAN networks, which was driven by the growing popularity of WLAN and the desire of mobile operators to offload traffic and offer new services. The concept has remained stable across releases, reflecting its fundamental role in interworking architectures.