Mobile Visited Public Land Mobile Network

Description

The term 'mobile-vplmn' is an identifier used in 3GPP specifications, particularly in the context of IP Multimedia Subsystem (IMS) charging and policy control, to denote that a call or session was originated by a mobile device while it was registered in a Visited Public Land Mobile Network (VPLMN). The VPLMN is the network that the user is roaming on, which is not their home network. This term is used in specifications such as 24.229 (IMS call control protocol) and 24.503 (IMS charging) to differentiate between calls made while roaming versus those made in the home network.

In IMS, when a user initiates a session while roaming, the network needs to determine that the user is in a visited network. This information is crucial for charging, as roaming charges typically apply. The 'mobile-vplmn' indicator is included in charging information to specify that the call was generated by a mobile device in a VPLMN. This allows the charging system to apply the correct tariff and to generate appropriate Call Detail Records (CDRs). It also helps in policy control, as the visited network may have different policies or the home network may apply specific policies for roaming users.

The term is part of a set of indicators that describe the origin of a call. The counterpart is 'mobile-hplmn', which indicates that the call was generated by a mobile device in its home PLMN. These indicators are typically used in the IMS charging architecture, where the Charging Trigger Function (CTF) in the IMS nodes (e.g., P-CSCF, S-CSCF) generates charging events that include such information. The charging information is then sent to the Charging Data Function (CDF) or Charging Gateway Function (CGF) for processing and CDR generation.

The use of 'mobile-vplmn' is particularly relevant in roaming scenarios. When a user roams into a visited network, the session is anchored in the home network via the IMS. The home network's S-CSCF may need to know that the call originated in the VPLMN to apply the correct service logic. For example, certain services may be restricted while roaming, or different QoS parameters may be applied. The indicator helps in making these decisions.

In terms of implementation, the 'mobile-vplmn' indicator is often derived from the user's registration state. When the UE registers with IMS, it includes information about its location, such as the P-Access-Network-Info header, which can indicate the PLMN. The network can then determine if the PLMN is the HPLMN or a VPLMN. This information is then used in charging and policy decisions. The term is defined in 3GPP specifications and is used consistently across releases to ensure interoperability between different vendors' equipment.

Purpose & Motivation

The 'mobile-vplmn' term exists to provide a clear and standardized way to indicate that a call or session was originated by a mobile device in a Visited PLMN. This is essential for charging and policy control in IMS networks, as it allows operators to apply the correct roaming tariffs and service policies. Without this indicator, it would be difficult to distinguish between home and roaming scenarios, leading to incorrect charging and potential revenue loss. It also enables the home network to apply specific services or restrictions that are only applicable when the user is roaming.

Historically, as mobile networks evolved to support IMS and advanced services, the need for detailed charging information grew. The introduction of this term in Rel-7 coincided with the maturation of IMS and the need for granular charging in roaming scenarios. Before this, charging was often based on less specific indicators, which could lead to inaccuracies. The 'mobile-vplmn' and 'mobile-hplmn' terms provided a simple yet effective way to differentiate the two main scenarios.

Furthermore, the term supports regulatory requirements. In some countries, regulators require operators to provide detailed charging information for roaming users. The 'mobile-vplmn' indicator helps operators comply with these requirements by providing accurate data on where the call originated. It also aids in fraud detection, as unusual patterns of roaming usage can be flagged. Overall, the term is a small but important piece of the charging and policy framework in 3GPP networks.