Home Public Land Mobile Network of the C subscriber

Description

The Home Public Land Mobile Network of the C subscriber (HPLMNC) is a specialized identifier used in telecommunications scenarios involving three or more parties. The 'C-subscriber' typically refers to a third party introduced into a communication session, most commonly in call forwarding, call transfer, or multi-party conference calling. For example, if subscriber A calls subscriber B, and B has unconditional call forwarding active to subscriber C, then C is the C-subscriber. The HPLMNC is the home network of this forwarded-to or transferred-to party. Its identification is necessary for the network to properly route the call to C, authenticate and authorize C's participation, and apply the correct service logic and billing for this third leg of the call.

Architecturally, the need for HPLMNC arises within network nodes that handle complex call state models, such as Service Switching Functions (SSF) in traditional circuits, MSC servers, or the Media Resource Function Controller (MRFC) and Application Servers (AS) in IMS. When a call is to be forwarded or a conference is initiated, the controlling network element must perform routing analysis for the new target (C-subscriber). This involves extracting or determining the HPLMNC from C's MSISDN or IMSI. The node may then need to query the HLR or HSS in the HPLMNC to obtain a roaming number (MSRN) or to check service authorization, similar to the process for a B-subscriber but now in a secondary, derived call leg.

The operational role of HPLMNC is deeply tied to supplementary services and intelligent network features. In a CAMEL-controlled environment, the detection point for call forwarding may trigger a dialogue with an SCP. The HPLMNC is a valuable piece of information in this dialogue, as the SCP's logic might depend on the relationship between the networks (e.g., whether forwarding is to an on-net or off-net number, which affects cost). In IMS, for a conference call, the HPLMNC of each participant (beyond the first two) must be considered by the MRFC and AS when managing the session and potentially when applying participant-specific policies. The handling of HPLMNC ensures that the network can scale service logic beyond simple point-to-point calls, supporting the rich set of supplementary services that define modern telecommunications.

Purpose & Motivation

The purpose of defining HPLMNC is to extend the principle of home network identification and control to all parties involved in a complex communication service, not just the primary two. Early call forwarding was a simple switch-based rerouting. However, with the introduction of subscriber-controlled services, prepaid billing, and inter-operator settlements, it became critical to identify and treat each leg of a communication independently. The C-subscriber is not merely a routing endpoint but a full subscriber with their own HPLMN, service profile, and billing relationship.

This addresses specific technical and commercial challenges. Technically, it allows the correct service logic for the C-subscriber to be invoked. For instance, if C has a 'do not disturb' service active in their HPLMN, the network needs to identify HPLMNC to apply that barring. Commercially, it enables accurate charging. The cost of a call forwarded to an international number (C-subscriber) is very different from one forwarded locally. By identifying the HPLMNC, the network can determine the appropriate interconnect charges for the leg to C. The concept formalizes the handling of tertiary parties within signaling standards, ensuring interoperability between operators when complex call scenarios span multiple networks.

Key Features

• Identifies the home network of a third party (C-subscriber) in multi-leg communication services
• Used in call forwarding (CFU, CFB, CFNRy), explicit call transfer, and conference call setups
• Enables routing queries to the C-subscriber's HLR/HSS to obtain a current serving node address
• Provides context for intelligent network (CAMEL) service logic affecting the C-party leg
• Supports accurate billing and inter-operator settlement for the additional call leg to the C-subscriber
• Ensures the C-subscriber's own subscription services and policies can be applied when they are introduced into a session

Evolution Across Releases

Defining Specifications