Subscriber C

Description

Subscriber C is a conceptual term in 3GPP specifications (22.091, 23.093) representing the forwarded-to-party in call forwarding scenarios. It is the user of Mobile Station C (MS C) and is associated with network elements such as MSC C, VLR C, and HLR C. This term is part of a naming convention that includes Subscriber A (calling party), Subscriber B (called party), and Subscriber C (forwarded-to party), enabling clear description of multi-party call flows.

In call forwarding, when Subscriber A calls Subscriber B, and Subscriber B has activated call forwarding to Subscriber C, the call is redirected to Subscriber C. Subscriber C then becomes the terminating party. The specifications use Subscriber C to denote this role, and the associated network elements (MSC C, VLR C, HLR C) are those serving Subscriber C. This allows precise description of signaling and routing procedures, as the call must be routed from the originating network to the forwarding network and then to the forwarded-to network.

The term is not a physical entity but a modeling abstraction. It helps in defining the behavior of supplementary services like Call Forwarding Unconditional (CFU), Call Forwarding Busy (CFB), Call Forwarding No Reply (CFNRy), and Call Forwarding Not Reachable (CFNRc). For each scenario, the specifications describe how the call is handled, with Subscriber C as the final destination. The network elements associated with Subscriber C are involved in the forwarding process: HLR C stores the subscription data, VLR C handles the visitor location, and MSC C switches the call to MS C.

Understanding Subscriber C is essential for engineers implementing call forwarding features. It clarifies the roles of different parties and the network entities involved, facilitating the design, testing, and troubleshooting of these services. The term has remained unchanged since Release 4, underscoring its fundamental role in service description.

The definition also implies that Subscriber C may be in a different network than Subscriber B, requiring inter-network signaling. This highlights the importance of standardized interfaces and protocols to ensure seamless call forwarding across networks. Subscriber C thus represents both a subscriber role and a set of network functions that must interoperate to deliver the service.

Purpose & Motivation

Subscriber C exists to provide a standardized reference for the forwarded-to-party in call forwarding scenarios within 3GPP specifications. Without this term, describing the destination of a forwarded call would be ambiguous, especially when multiple parties are involved. By defining Subscriber C as the user of MS C and associating it with specific network elements (MSC C, VLR C, HLR C), the specifications enable precise and consistent documentation of call forwarding procedures.

The motivation for this term arises from the complexity of call forwarding services, which involve redirection of calls from the originally called party to another party. This requires coordination between the originating network, the forwarding network, and the terminating network. Subscriber C represents the final destination and the network elements that serve it. The term helps in delineating the responsibilities of each network component and in defining the signaling flows necessary to complete the forwarded call.

Historically, as call forwarding features became more sophisticated, the need for a clear naming convention for the forwarded-to party became evident. The introduction of Subscriber C in Release 4 provided this clarity. It has been carried forward unchanged, demonstrating its effectiveness. It solves the problem of referencing the forwarded-to party in a way that is independent of the specific forwarding condition or network configuration, thereby facilitating the specification of interoperable services across different vendors and networks.