Multicast session

Description

A multicast session represents the group construct through which a specific multicast service is provided to its members. Defined in 3GPP Release 8 and maintained through Release 19, the multicast session is the mechanism by which end users signal their interest in receiving a particular multicast service and become members of the associated delivery group. When a user joins a multicast session, they indicate to the network their desire to receive the multicast content, and the network tracks this membership to determine where and how to deliver the service. This concept operates at a level below the service subscription: while subscription establishes a user's eligibility for a multicast service, joining a multicast session represents the user's active participation in consuming that service.

The multicast session is closely related to the concept of the multicast group. In 3GPP terminology, a multicast session can be understood as the group that provides the same multicast service. When an end user joins the multicast session, they become a member of the multicast session, which is functionally equivalent to joining the multicast group associated with the specific multicast service. This joining operation is a dynamic process—users can join and leave sessions as their consumption needs change, and the network adjusts resource allocation accordingly. The session concept thus provides a level of indirection between the service (which is relatively static, tied to subscription) and the actual delivery resources (which are dynamically allocated based on current membership).

From an architectural perspective, the multicast session is managed through signaling between the user equipment and the network, as well as within the core network between functional entities. In the MBMS architecture, session initiation and management involves the BM-SC (Broadcast Multicast Service Center) and other core network nodes. The session start triggers resource allocation in the radio access network, while session stop releases those resources. User joining and leaving operations influence the network's decision about whether to use point-to-multipoint or point-to-point bearers in specific cells, depending on the number and distribution of members.

The distinction between multicast service and multicast session is important for understanding the layered model of 3GPP multicast delivery. A multicast service defines what content is offered and the rules for accessing it, while a multicast session represents an active instance of that service being consumed by a set of joined members. Multiple sessions could theoretically be associated with the same service (for example, different scheduled instances), and users interact with sessions to receive the service content. The session-context nature means that session membership is more transient than service subscription.

In later releases, the multicast session concept has been extended and refined, particularly with the introduction of 5G MBS (Multicast/Broadcast Service) in Release 17, where session management takes on new architectural forms while preserving the fundamental notion of a group context for service delivery. The multicast session remains a foundational concept for understanding how group-based content delivery is managed and controlled in 3GPP networks across generations.

Purpose & Motivation

The multicast session concept was introduced to provide a dynamic, session-level mechanism for managing user participation in multicast services, complementing the more static subscription model. Prior to this, the distinction between being authorized for a service and actively receiving it was not clearly formalized, which created ambiguity in resource management and signaling. The multicast session solves this by creating an explicit group membership construct that users join when they want to receive content and leave when they are done, allowing the network to know precisely which users are actively consuming the service at any given moment.

This distinction is crucial for efficient resource allocation. If the network only knew about subscriptions, it would have to provision resources for all subscribers at all times, which is wasteful. By tracking session membership, the network can allocate multicast resources only where and when users have actually joined, optimizing spectrum usage and backhaul capacity. This is particularly important for the radio access network, where the decision to use point-to-multipoint bearers (efficient for many users) versus point-to-point bearers (efficient for few users) depends on the number and location of active session members.

The concept also provides a clean interface for user-initiated control. Users can dynamically join and leave multicast sessions through well-defined signaling procedures, giving them control over their content consumption without requiring changes to their service subscriptions. This decoupling of subscription (a commercial/administrative relationship) from session participation (a technical/dynamic process) is a key architectural principle that has proven valuable across multiple generations of 3GPP technology, from the original MBMS in UMTS through LTE eMBMS and into 5G MBS.