Sending Terminated

Description

Sending Terminated (ST) is a service capability defined within the 3GPP IP Multimedia Subsystem (IMS) framework, specifically related to session control and media handling. It operates as part of the broader Session Initiation Protocol (SIP) and Session Description Protocol (SDP) mechanisms used to establish, modify, and terminate multimedia sessions. ST is invoked when a user or network element decides to stop sending media in one direction within an ongoing session, which could be due to user action (e.g., muting a call), network conditions, or service logic. This capability is crucial for managing bidirectional media flows independently, allowing for scenarios like hold/resume or asymmetric media sessions where only one party transmits.

Architecturally, ST is implemented in IMS core elements such as the Serving-Call Session Control Function (S-CSCF) and Application Servers (AS), which process SIP signaling. When ST is triggered, the network sends specific SIP messages (e.g., re-INVITE or UPDATE) with SDP attributes indicating that media sending is terminated in a particular direction. This involves updating the session parameters to reflect the change, ensuring that the receiving party is informed and resources like bearers in the Packet Data Network (PDN) are adjusted accordingly. The process integrates with Policy and Charging Control (PCC) frameworks to enforce QoS and charging policies based on the modified media flow.

In practice, ST enables features such as call hold, where a user stops sending audio but may continue to receive it, or video streaming adjustments in response to bandwidth constraints. It works in conjunction with other IMS services like Supplementary Services and Multimedia Telephony (MMTel), providing granular control over media streams. Key components include the IMS core network, UE (User Equipment) supporting SIP, and interfaces like Gm (between UE and IMS) and ISC (between CSCF and AS). ST's role is to maintain session integrity while optimizing network resources, preventing unnecessary data transmission and aligning with user preferences or network policies.

Purpose & Motivation

ST was introduced to address the need for fine-grained control over media streams in IP-based multimedia services, particularly within IMS. Prior to its standardization, session management often lacked mechanisms to independently terminate sending in one direction without ending the entire session, leading to inefficiencies. For example, in early VoIP systems, putting a call on hold might require terminating and re-establishing the session, causing delays and potential service disruption. ST solves this by enabling asymmetric media handling, allowing networks to conserve resources and enhance user experience.

Historically, as 3GPP evolved from circuit-switched to packet-switched networks in Release 8, IMS became central for delivering rich communication services. ST emerged as part of this shift to support advanced features like multimedia conferencing and video calls, where dynamic media adjustments are common. It addresses limitations in earlier protocols that treated media flows as all-or-nothing, by providing a standardized way to modify sessions in real-time. This is motivated by the growing demand for flexible, efficient services that adapt to user actions and network conditions, such as in LTE and 5G deployments.

Moreover, ST facilitates service innovation by enabling operators to offer enhanced capabilities like selective media transmission, which is critical for applications such as remote surveillance or interactive gaming. It integrates with QoS and charging systems, ensuring that network policies are enforced when media flows change. By solving these problems, ST contributes to the overall goal of IMS: to deliver reliable, scalable multimedia services across diverse access technologies.

Key Features

• Enables termination of media sending in one direction within a session
• Integrates with SIP and SDP for signaling and session description
• Supports asymmetric media flows for features like call hold
• Interworks with IMS core elements (e.g., S-CSCF, AS) for service logic
• Aligns with Policy and Charging Control (PCC) for resource management
• Facilitates dynamic session modifications without full termination

Evolution Across Releases

Defining Specifications