IP Multimedia Session

Description

An IP multimedia session is a fundamental concept in the IP Multimedia Subsystem (IMS), defined by 3GPP as a set of multimedia senders and receivers along with the data streams flowing from senders to receivers. This definition, specified in 3GPP TS 22.228, establishes the session as the logical association between communicating parties, encompassing all media components such as audio, video, text, and shared content. The session is not merely a transport connection; it represents a rich, multi-party communication context that can be dynamically modified during its lifetime.

The IP multimedia session is supported by the IP Multimedia CN Subsystem (IMS), which provides the service control layer for establishing, managing, and terminating sessions. IMS uses SIP (Session Initiation Protocol) for session signaling, enabling features like session negotiation, modification, and termination. The actual media streams are transported over IP connectivity bearers, such as GPRS, EPS, or 5G, which provide the underlying packet-switched connectivity. This separation of control and bearer allows IMS to operate over various access technologies, ensuring service continuity and mobility.

A key characteristic of IP multimedia sessions is the ability for a user to invoke concurrent sessions. This means a single user equipment can participate in multiple independent multimedia sessions simultaneously, each with its own set of media streams and quality of service parameters. For example, a user might have an ongoing voice call while receiving a video stream and participating in an instant messaging session. The IMS core network manages the necessary resources and signaling to support such concurrency, ensuring that each session receives appropriate treatment.

The architecture enabling IP multimedia sessions includes several key components: the Call Session Control Function (CSCF) for SIP signaling, the Home Subscriber Server (HSS) for subscriber data, and various application servers for specific services. The session establishment involves SIP INVITE transactions, capability negotiation via SDP (Session Description Protocol), and bearer establishment through the policy and charging control (PCC) framework. The session can be modified using SIP re-INVITE or UPDATE methods, allowing dynamic changes to media types, codecs, or QoS parameters.

IP multimedia sessions are central to the IMS vision of converged multimedia services over IP. They enable operators to offer rich communication services (RCS), VoLTE, VoWiFi, and other advanced multimedia applications. The session concept also underpins interworking with circuit-switched networks and supports emergency services, lawful interception, and charging. As networks evolve towards 5G, the session concept remains relevant, with IMS continuing to provide voice and multimedia services over 5G NR and beyond.

Purpose & Motivation

The IP multimedia session concept was created to provide a standardized framework for delivering multimedia services over IP-based networks. Before IMS, multimedia services were often implemented in proprietary ways, leading to interoperability issues and limited service innovation. The session abstraction allows a consistent service experience across different access technologies and devices, enabling operators to offer converged services that combine voice, video, and data.

Historically, circuit-switched networks provided voice and limited data services, but they were not optimized for multimedia. The growth of the Internet and IP-based applications created a demand for similar services in mobile networks. 3GPP introduced IMS in Release 5 to address this, but the concept of an IP multimedia session was already outlined in Release 99 as part of the service requirements. The session concept solves the problem of managing multiple media streams with different QoS requirements within a single communication context, and it enables concurrent sessions, which is essential for multitasking users.

The IP multimedia session also facilitates charging and policy control by providing a clear unit for accounting and resource allocation. It allows operators to apply differentiated QoS and charging based on session characteristics. Furthermore, the session model supports mobility, as sessions can be maintained while the user moves between access networks, thanks to the separation of signaling and bearer. This has been crucial for the success of VoLTE and other IMS-based services.