FLUS session

Description

A FLUS session is a logical association established between a FLUS source and a FLUS sink, within which media content can be transmitted from the source to the sink. FLUS (which stands for Flexible Media Handling and is specified in 3GPP TS 26.238 and TS 26.939) provides a framework for handling media in a flexible manner, allowing media to be split, combined, or otherwise processed across different entities. The FLUS session is the container that binds a particular source to a particular sink for the purpose of media delivery.

The source in a FLUS session is the entity that originates or provides media content. The sink is the entity that receives and consumes it. The session defines the context in which media flows, including the identification of the endpoints and the rules governing the media transfer. Because it is a logical association rather than a physical connection, a FLUS session can span multiple transport paths and can be established, modified, and torn down independently of the underlying bearers.

FLUS was introduced in Release 15 as part of 3GPP's work on media handling for services such as Mission Critical Video (MCVideo) and Mission Critical Data (MCData), and it also supports enhanced media capabilities in other contexts. The framework allows media to be processed in ways that go beyond simple end-to-end delivery — for example, allowing a media source to send content to a sink that may apply further processing, or allowing multiple sources to contribute to a combined media stream. The FLUS session is the unit within which these operations are coordinated.

From an architectural standpoint, the FLUS session is established through signalling between the involved entities, which may include clients, media servers, and media functions. The session identifies the source and sink and defines the media content that will be sent. Once established, media can flow from source to sink according to the agreed parameters. The session may be long-lived or short-lived depending on the service, and it can be updated to reflect changes in media content or endpoint roles.

The concept is deliberately general. By defining a session as a logical association between a source and a sink, 3GPP allows FLUS to be applied to a variety of media handling scenarios without prescribing a single transport or encoding. This flexibility is important because mission-critical and other media services have diverse requirements, ranging from low-latency video to store-and-forward data. The FLUS session provides a common abstraction that can be adapted to these needs.

In the specifications, the FLUS session is described in TS 26.238 (FLUS) and TS 26.939 (media handling). The definition has remained stable since its introduction, with only minor editorial refinements. Its role is foundational: without a session binding a source to a sink, the flexible media handling operations that FLUS enables would have no defined context in which to occur.

Purpose & Motivation

The FLUS session exists to give structure to flexible media handling in 3GPP services. Before FLUS, media delivery in mission-critical and related services tended to be defined in terms of specific end-to-end flows, which made it difficult to introduce new media processing capabilities or to combine media from multiple sources. FLUS was created to provide a more general framework in which media can be handled flexibly — split, merged, transcoded, or otherwise processed — while still maintaining clear relationships between the entities involved.

The session abstraction solves the problem of context. When media is sent from one entity to another, there must be an agreed understanding of who is sending, who is receiving, and what content is involved. The FLUS session captures exactly this: a logical association between a FLUS source and a FLUS sink within which media content can be sent. This allows the framework to support a wide range of media operations without redefining the basic delivery relationship each time.

Introduced in Release 15, the FLUS session reflects the growing importance of media in mission-critical communications, where video and data complement voice. By providing a reusable session concept, 3GPP enables services to evolve their media handling capabilities without disrupting the underlying session model. The definition has proven stable, indicating that the abstraction was well chosen.