Calling user (Broadcast Call Control entity)

Description

The term "Calling user" is defined in 3GPP specifications as the Broadcast Call Control (BCC) entity within a Mobile Station (MS) that initiates or has initiated a broadcast call. This concept is central to broadcast call services, such as Voice Group Call Service (VGCS) and Voice Broadcast Service (VBS), which were introduced in GSM and later extended to UMTS. The calling user is the originator of a broadcast call, and its identity is used by the network to manage call setup, resource allocation, and charging. The BCC entity resides in the mobile station and is responsible for the broadcast call control procedures, including initiation, termination, and participation in group communications.

In the architecture of broadcast call services, the network distinguishes between the calling user (the originator) and the called users (the recipients). The calling user initiates the call by sending a request to the network, which then sets up the necessary resources and notifies the group members. The BCC entity in the calling user's MS handles the signaling exchange with the network, ensuring that the call is established according to the service requirements. This includes the selection of the appropriate service (VGCS or VBS), the provision of the group identity, and the management of the voice or data stream.

The calling user concept is defined in several 3GPP specifications, including 22.060 (General Packet Radio Service (GPRS); Service description; Stage 1), 24.068 (Voice Group Call Service (VGCS) - Stage 3), 24.069 (Voice Broadcast Service (VBS) - Stage 3), 44.068 (Voice Group Call Service (VGCS) - Stage 3 for GERAN), and 44.069 (Voice Broadcast Service (VBS) - Stage 3 for GERAN). These specifications detail the procedures and protocols for broadcast call services, with the calling user being a fundamental entity in the call control state machine.

The calling user's role is not limited to call initiation; it also participates in the call as a speaker or listener, depending on the service. In VGCS, the calling user typically becomes the initial talker, while in VBS, the calling user is the source of the broadcast. The network uses the calling user's identity to authenticate and authorize the call, and to apply the appropriate charging and policy rules. The BCC entity in the MS maintains the state of the call and interacts with other entities such as the Group Call Register (GCR) and the Mobile Switching Center (MSC) to ensure proper operation.

Over the releases, the definition of "Calling user" has remained largely unchanged, reflecting the stability of the broadcast call service architecture. However, the term continues to be relevant in modern networks, especially in mission-critical push-to-talk (MCPTT) and other group communication services that have evolved from the original GSM broadcast call concepts. Understanding the calling user is essential for engineers working with group communication features in cellular networks.

Purpose & Motivation

The concept of the "Calling user" exists to identify the originator of a broadcast call within the broadcast call service architecture. Broadcast call services, such as VGCS and VBS, were developed to provide efficient group communication over cellular networks, allowing a single user to initiate a call to a predefined group of users. The calling user is the entity that triggers the call setup, and its identification is crucial for the network to manage the call, allocate resources, and apply the correct service logic.

Before the introduction of broadcast call services, group communication over cellular networks was not standardized, and users relied on separate systems or manual conference calls. The need for a standardized approach led to the development of VGCS and VBS in GSM, which required a clear definition of the roles of the participants. The calling user was defined to distinguish the initiator from the recipients, enabling the network to handle call setup, authentication, and charging appropriately. This distinction also allows the network to provide different treatment to the calling user, such as priority or special charging.

The calling user concept solves the problem of identifying the source of a group call in a multi-party communication scenario. It enables the network to route the call to the correct group, to manage the talker arbitration (in VGCS), and to ensure that only authorized users can initiate calls. The definition has remained consistent across releases, demonstrating its fundamental importance to the service. As group communication evolves towards LTE and 5G, the principles established by the calling user concept continue to influence the design of mission-critical communication services.