Subscriber A

Description

Subscriber A is a specific role defined in 3GPP specifications for supplementary services, particularly the Explicit Call Transfer (ECT) service. ECT allows a subscriber to transfer an established call to a third party, effectively connecting two other parties and releasing the subscriber's own connection. In this scenario, Subscriber A is the served mobile subscriber—the one who has the subscription to the ECT service and actively invokes it. Subscriber A is typically the party who initiates the transfer, either by pressing a sequence of keys or using a service code.

The ECT service is specified in 3GPP TS 22.091 (Stage 1), TS 23.093 (Stage 2), and TS 24.093 (Stage 3). Subscriber A must have a subscription to ECT, which is provisioned in the Home Subscriber Server (HSS) or Home Location Register (HLR). When Subscriber A invokes ECT, the network checks the subscription profile to ensure the service is allowed. If authorized, the network performs the transfer, which involves signaling between the mobile station and the network, and between network entities such as the Mobile Switching Center (MSC) or the IP Multimedia Subsystem (IMS) in later releases.

In the call transfer process, Subscriber A is connected to two other parties: Subscriber B and Subscriber C. Subscriber A initiates the transfer, and the network then connects Subscriber B and Subscriber C together, while Subscriber A is released. The term "Subscriber A" is used to unambiguously identify the served subscriber in the specification text, distinguishing them from the other parties. This is important for defining the precise signaling flows and service interactions.

The concept of Subscriber A is not limited to ECT; it appears in other supplementary service specifications such as 22.072 (Call Deflection) and 22.093 (Completion of Calls to Busy Subscriber). In each case, Subscriber A is the served subscriber who invokes the service. This consistent naming convention helps engineers and readers understand the roles in various service scenarios.

From an implementation perspective, the network must handle Subscriber A's request, authenticate and authorize it, and then execute the transfer. This involves interactions with the HSS/HLR for subscription data, the MSC or IMS for call control, and possibly the Service Control Point (SCP) for intelligent network services. The ECT service can be implemented in both circuit-switched (CS) and packet-switched (PS) domains, with IMS providing a more flexible platform for such supplementary services.

In summary, Subscriber A is a role-based term that identifies the served mobile subscriber who invokes a supplementary service, particularly ECT. It is essential for specifying the behavior of supplementary services and ensuring that the correct subscriber is billed and authorized.

Purpose & Motivation

The term Subscriber A exists to provide a clear and unambiguous reference to the served mobile subscriber who invokes a supplementary service, such as Explicit Call Transfer. In complex call scenarios involving multiple parties, it is crucial to distinguish the subscriber who has the service subscription and initiates the service from other parties who may be affected by the service. This clarity is essential for writing precise specifications and for implementing network and device behavior correctly.

Historically, as supplementary services were introduced in GSM and later in UMTS and LTE, the need arose to define roles for the various parties involved. The ECT service, for example, involves three parties: the served subscriber (Subscriber A), the first remote party (Subscriber B), and the second remote party (Subscriber C). Without a consistent naming convention, specifications would be ambiguous, leading to interoperability issues. The term Subscriber A was thus adopted to denote the served subscriber who has the subscription and invokes the service.

The concept also helps in defining service interactions and charging. Since Subscriber A is the one who invokes the service, any charges associated with the service invocation are typically applied to Subscriber A's account. This aligns with the general principle that the subscriber who requests a service is responsible for its cost. Furthermore, the authorization check is performed against Subscriber A's subscription profile, ensuring that only subscribers who have subscribed to the service can use it.