Visited Public Land Mobile Network of the A subscriber

Description

VPLMNA is a network identifier used within the 3GPP call control and mobility management procedures, specifically defined in technical specification 23.018. It represents the Visited Public Land Mobile Network (VPLMN) where the A-subscriber—the party initiating a call or session—is physically located and registered at the time of the call establishment. This parameter is not a static identifier but a dynamic one, determined during the call setup signaling flow. When a mobile subscriber initiates a call, the serving MSC (Mobile Switching Centre) or equivalent core network node in the visited network includes the VPLMNA in the Initial Address Message (IAM) or other relevant signaling messages sent towards the network of the called party (B-subscriber). The value is typically the Mobile Country Code (MCC) and Mobile Network Code (MNC) of the visited network operator.

Architecturally, the VPLMNA is a data element carried within signaling protocols like ISDN User Part (ISUP) or Bearer Independent Call Control (BICC), and more recently within SIP (Session Initiation Protocol) headers in IP Multimedia Subsystem (IMS) networks. Its primary role is in the inter-network signaling between the Visited MSC (VMSC) of the A-subscriber and the Gateway MSC (GMSC) or other switching nodes in the home or transit networks. The inclusion of this parameter allows each network involved in the call path to identify the roaming context of the calling party. This is essential for functions like call routing, especially for optimal routing and call back procedures, and for generating accurate charging data records (CDRs) that reflect the roaming relationship.

From a network operation perspective, the VPLMNA enables several key processes. For billing, it allows the home network operator to apply specific tariff agreements for calls made while roaming in that particular visited network. For regulatory compliance, it helps in identifying the jurisdiction and applicable laws for the call origin. In terms of service logic, it can influence how services (like call barring or number presentation) are applied based on the subscriber's location. The parameter works in tandem with other identifiers like the MSISDN (Mobile Subscriber ISDN Number) and the IMSI (International Mobile Subscriber Identity) to provide a complete context for the call. Its consistent definition from Release 4 onwards ensures backward compatibility and stable inter-operator signaling, forming a foundational piece of the global roaming ecosystem.

Purpose & Motivation

The VPLMNA parameter was introduced to address the complexities of call handling and billing in a multi-operator, global roaming environment. Prior to standardized roaming signaling, identifying the exact network where a roaming subscriber originated a call was challenging, leading to issues in routing efficiency and accurate inter-operator settlement. The creation of VPLMNA provided a clear, machine-readable identifier that travels with the call setup signaling, solving these problems.

Its existence is motivated by the need for precise call routing. For instance, it enables optimal routing (also known as tromboning avoidance), where a call from a roaming subscriber to a party in their home country can be routed directly from the visited network to the destination, rather than being 'tromboned' back to the home network first. This reduces latency and network resource usage. Furthermore, it is fundamental for generating Transferred Account Procedures (TAP) records used for financial settlement between operators. By precisely identifying the visited network, the home operator can apply the correct roaming charge agreement and bill the subscriber accurately.

Historically, as GSM evolved into a global system, the signaling between different operators' networks required a common set of parameters to ensure interoperability. VPLMNA, along with its counterpart VPLMNB, became standardized elements in the Call Detail Records (CDRs) and signaling messages, providing the necessary context for every mobile-originated call. This solved the limitation of earlier systems where roaming call provenance was ambiguous, facilitating the automated, large-scale commercial roaming agreements that underpin modern mobile telecommunications.

Key Features

• Dynamic network identification for the calling (A) party during roaming
• Encoded as a PLMN ID (MCC+MNC) within call signaling protocols
• Essential for accurate inter-operator billing and settlement (TAP records)
• Enables optimal call routing to avoid unnecessary tromboning
• Supports roaming-specific service logic and policy enforcement
• A standardized parameter ensuring global interoperability between network operators

Evolution Across Releases

Defining Specifications