Originating Mobile Station

Description

The Originating Mobile Station (Originating MS) is a fundamental concept in 3GPP group communication services, particularly within the Group Call System enablers defined in specifications such as 24.068, 24.069, 44.068, and 44.069. It refers to the mobile station (MS) that initiates or has initiated a group call. In the context of group communications, a group call is a service that allows multiple users to participate in a single call session, often used in mission-critical and public safety scenarios. The originating MS is the entity that triggers the setup of such a group call, either by sending a request to the network or by performing an action that leads to the establishment of the group call.

Architecturally, the Originating MS interacts with the network through the Group Call System (GCS) enablers, which may include functional entities such as the Group Call Server (GCS) and the Group Call Application Server (GCAS). When an MS initiates a group call, it sends a request to the network, which then processes the request and sets up the call for all group members. The originating MS may not always be the actual originator; in certain situations, an MS may assume the role of originating MS without having actually initiated the group call. This can happen due to network configurations, call forwarding, or other service interactions. The definition explicitly notes that there may be one or none originating MS for a given group call, meaning that a group call can exist without a specific originating MS, or with multiple MSs assuming that role.

The concept of the originating MS is crucial for call control and charging. The network needs to identify the originating MS to apply appropriate charging, policy control, and service authorization. For example, in mission-critical push-to-talk (MCPTT) services, the originating MS may be the one that initiates the group call, and the network can use this information to prioritize the call or to apply specific QoS parameters. Additionally, the originating MS may be involved in call termination procedures, such as when the call ends or when the originating MS leaves the group call.

In terms of implementation, the Originating MS is typically a user equipment (UE) that supports group communication services. It communicates with the network using protocols such as SIP (Session Initiation Protocol) for call setup and control, and may use other protocols for media transport. The MS may also interact with the Group Call Server to manage group membership and call parameters. The network may use identifiers such as the Group Call Identifier (GCI) to manage group calls and to associate the originating MS with a specific call.

The Originating MS concept has evolved over releases. In Rel-4, the definition included a note about the MS assuming the role without actually being the originator, and the possibility of one or none originating MS. In Rel-8, the wording was changed to remove the parenthetical note, simplifying the definition to just "mobile station initiating or having initiated the group call." This change likely reflected a simplification in the specification or a clarification that the note was no longer necessary. Subsequent releases carried the simplified definition unchanged, indicating stability in the concept. The term remains relevant in later releases, including Rel-19, where it continues to be used in group communication specifications.

Purpose & Motivation

The Originating Mobile Station concept exists to identify the entity that initiates a group call in 3GPP group communication services. This identification is essential for call control, charging, and service management. In group communications, multiple users participate in a single call, and the network must know who initiated the call to apply appropriate policies, such as prioritization, security, and charging. The concept also helps in managing call state and in handling scenarios where the call is initiated by a dispatcher or another entity on behalf of the group.

Historically, group communication services in 3GPP evolved from early push-to-talk over cellular (PoC) systems to more advanced mission-critical services. The need to identify the originating MS became apparent as these services required more sophisticated call control and charging mechanisms. In earlier systems, such as GSM, group calls were handled differently, and the concept of an originating MS was not as clearly defined. With the introduction of the Group Call System enablers in Rel-4, the Originating MS was formally defined to support these new services.

The definition's note about an MS assuming the role without actually being the originator addresses scenarios where the network or another entity may designate an MS as the originating MS for various reasons, such as when the actual originator is not available or when the call is initiated by a server. This flexibility is important for interoperability and for supporting different deployment scenarios. The possibility of one or none originating MS allows for group calls that are not initiated by any MS, such as those initiated by a dispatcher or an application server, and for calls where multiple MSs may act as originating MS due to network configurations.