Group call anchor Mobile Switching Centre

Description

The Group Call Anchor MSC is a key network element in GSM/UMTS group call services, specifically for Voice Group Call Service (VGCS) and Voice Broadcast Service (VBS). It is defined as the MSC responsible for managing and maintaining a particular voice broadcast call. The anchor MSC is determined as the one controlling the cells of the group call area. For a voice broadcast call where the group call area exceeds an MSC area, the group call anchor MSC is predefined in the network. In a RANflex configuration with group call redundancy, the group call anchor MSC redundancy pool (instead of a single anchor MSC) is predefined for a given voice broadcast call.

The anchor MSC acts as the central control point for the group call. It handles call setup, resource allocation, and coordination with other MSCs (called relay MSCs) that serve cells within the group call area. When a group call is initiated, the anchor MSC is responsible for paging group members, establishing the call, and managing the distribution of the call to the relevant cells. It also interfaces with the Group Call Register (GCR) to retrieve group definitions and call-related data.

The concept of an anchor MSC is necessary because a group call area can span multiple MSC areas. In such cases, a single MSC must be designated as the anchor to ensure consistent call control. The anchor MSC is typically the MSC that controls the majority of the cells in the group call area, or it may be explicitly configured in the network. The relay MSCs are responsible for the cells in their areas and act under the coordination of the anchor MSC.

In a RANflex configuration, which allows for redundancy and load sharing, the anchor MSC function can be distributed across a pool of MSCs. This enhances reliability and scalability. The redundancy pool ensures that if one MSC fails, another can take over the anchor role, minimizing service disruption.

The Group Call Anchor MSC is primarily used in GSM/UMTS networks. In LTE and 5G, group call services are handled by different entities, such as the MCPTT Server in the Mission Critical Services architecture. However, the concept of a central control point for group calls remains relevant, albeit implemented differently.

Purpose & Motivation

The Group Call Anchor MSC exists to provide centralized control for group calls that span multiple MSC areas. In GSM/UMTS networks, an MSC serves a specific geographic area. When a group call needs to be delivered to members across a wider area, a single MSC must coordinate the call to avoid conflicts and ensure efficient resource usage. The anchor MSC fulfills this role by managing the call and instructing relay MSCs to distribute the call to their cells.

Without an anchor MSC, group calls spanning multiple MSCs would require complex peer-to-peer coordination, which could lead to inconsistencies and inefficiencies. The anchor MSC simplifies the architecture by providing a single point of control for call setup, maintenance, and teardown. It also enables features like call priority and pre-emption, which are critical for public safety and emergency communications.

The introduction of the anchor MSC concept in Rel-4 coincided with the standardization of VGCS and VBS. These services required a robust mechanism to handle group calls across large areas. The anchor MSC, along with the Group Call Register, formed the foundation for group call services in GSM/UMTS. Later releases added enhancements like RANflex support, which introduced redundancy and improved reliability.