Media Flow

Description

A media flow is a logical grouping of one or more packet flows that together transport a single media instance, such as an audio stream, video stream, or text, within an IP multimedia session. It is a key concept in the IP Multimedia Subsystem (IMS) and the 5G System (5GS) for describing media transport between endpoints. The term is defined in 3GPP TS 23.228 for IMS and TS 23.503 for policy control, and it is used by the Application Function (AF) to request QoS treatment for specific media.

In the IMS context, a media flow is defined as one or more IP flows carrying a single media instance. This includes both media plane information (e.g., RTP audio) and control signalling (e.g., RTCP or SIP). The media flow concept allows the network to apply appropriate QoS and policy control to each media component of a session. For example, an audio and a video stream in a video call would be two separate media flows, each potentially with different QoS requirements.

With the introduction of 5GS, the definition was updated to be more generic: 'one or more packet flows that are used for the transfer of a single media of a specific Media Type in an AF session.' This new definition covers not only IP-based flows but also non-IP packet flows, reflecting the broader capabilities of 5G. It also explicitly ties the media flow to an AF session and a Media Type as defined in TS 29.514. A media flow describes a service data flow from the perspective of the AF, meaning it is the AF's view of the traffic that should receive a certain QoS treatment.

The media flow concept is used in policy and charging control (PCC) to map application-level media descriptions to IP flows and then to QoS flows in 5G or bearers in 4G. The AF can provide media flow descriptions (e.g., IP addresses, port numbers, protocol) to the Policy Control Function (PCF), which then creates the appropriate PCC rules. In 5GS, these rules are enforced by the Session Management Function (SMF) and User Plane Function (UPF) to ensure the media gets the requested QoS.

Media flows are essential for enabling differentiated QoS for multimedia services. They allow the network to treat each media component according to its needs, such as low latency for voice or high throughput for video. They also enable charging based on media type and facilitate lawful interception and policy enforcement. The evolution from IP-only to generic packet flows ensures that the concept remains relevant as networks move towards all-IP and non-IP transports.

Purpose & Motivation

The media flow concept was created to provide a standardized way to describe and manage individual media components within a multimedia session. In early IMS deployments, it became clear that a single session could contain multiple media streams with different characteristics and QoS requirements. Without a common terminology, it was difficult for the network to apply the right policies and for applications to request appropriate resources.

Before the media flow definition, media was often described at the IP flow level, which was too granular and did not capture the application-level notion of a media instance. The media flow abstraction groups related IP flows (e.g., RTP and RTCP for a single audio stream) into a single logical unit. This simplifies policy control and QoS management because the network can treat the entire media component as one entity.

As networks evolved to 5G, the original IP-centric definition became limiting because 5G supports non-IP packet flows and a wider variety of media types. The Rel-19 update redefined media flow in terms of packet flows and Media Types, making it applicable to any type of media transport. This ensures that the concept can be used uniformly across IMS, 5GS, and future systems, supporting new services like immersive media and machine-type communications.

Thus, media flow solves the problem of mapping application media to network transport, enabling efficient QoS, policy, and charging control in a multi-media, multi-access environment.