In-Call Modification

Description

In-Call Modification (ICM) is a service feature within the IP Multimedia Subsystem (IMS) and Circuit-Switched (CS) domains that permits the parameters of an established, active call or session to be altered dynamically. This is achieved by re-negotiating the session description, which is carried within SIP signaling messages (for IMS) or within appropriate Bearer Independent Call Control (BICC)/ISUP signaling (for CS). The process is initiated by one of the participating user endpoints (UE) or by a network application server. In IMS, the UE sends a SIP re-INVITE or UPDATE request within the existing dialog, containing a new Session Description Protocol (SDP) offer that proposes the modified media characteristics.

The core network elements, primarily the Serving-Call Session Control Function (S-CSCF), process this request, applying any necessary service logic and forwarding it to the other party. The remote party responds with a SIP 200 OK containing an SDP answer, confirming the acceptance of the new parameters. This triggers the bearer plane to be updated accordingly; for example, the Packet Data Network Gateway (PGW) and Policy and Charging Rules Function (PCRF) may be engaged to modify the dedicated bearer for the media stream with new QoS parameters. The architecture ensures that the modification is seamless, maintaining the core signaling association (dialog) and, where possible, avoiding media interruption.

ICM supports a wide range of modifications. These include adding a new media component (e.g., upgrading a voice call to a video call), removing a media component (e.g., stopping video to conserve bandwidth), changing the codec (e.g., from AMR to EVS for better quality), adjusting bandwidth, or changing session timers. It is also the fundamental mechanism used to add a third party to a point-to-point call, transforming it into a multi-party conference call, often managed by a Multimedia Resource Function Controller (MRFC). The feature relies on the capability of endpoints and network nodes to support SDP offer/answer model re-negotiation and corresponding bearer management procedures.

Purpose & Motivation

ICM was developed to overcome the rigidity of traditional telephony calls, where the characteristics of a call were fixed at establishment. In early cellular systems, any change to a call (like adding a video stream) required terminating the existing call and setting up a new one, which was disruptive and provided a poor user experience. As networks evolved to support rich multimedia services, there was a clear need for sessions to be adaptable to changing user needs, device capabilities, or network conditions during their lifetime.

Standardized in 3GPP Release 5 alongside IMS, ICM was a key enabler for the "any media, any time" promise of IMS. It solved the problem of service flexibility, allowing operators to offer features like "Voice add Video" or seamless conference setup. From a network efficiency perspective, it also allows for dynamic adaptation to network congestion by negotiating a lower-bitrate codec without dropping the call. Its creation was motivated by the convergence of telephony and data services, requiring a signaling framework (SIP/SDP) that inherently supported such mid-session modifications, which was a significant advancement over the static nature of traditional circuit-switched signaling.

Key Features

• Dynamic re-negotiation of session parameters using SIP re-INVITE or UPDATE
• Modification of media streams (add, remove, change codec, change bandwidth)
• Support for upgrading a voice call to a video call (and vice versa)
• Fundamental procedure for adding participants to create a conference call
• Integration with policy control (PCRF) for bearer QoS updates
• Applicable in both IMS and legacy Circuit-Switched network domains

Evolution Across Releases

Defining Specifications