Explicit Communication Transfer Session Identifier Uniform Resource Identifier

Description

The ECT Session Identifier URI is a specialized Public Service Identity (PSI) defined within the IP Multimedia Subsystem (IMS) architecture to support the Explicit Communication Transfer (ECT) supplementary service. ECT allows a user to transfer an existing communication session (such as a voice call) to a third party. The ECT Session Identifier URI is created and inserted by the ECT Application Server (AS) and is designed to resolve back to the AS itself. This URI acts as a unique identifier for the transfer session, enabling the AS to maintain control and state information throughout the transfer process.

Architecturally, the ECT AS resides within the IMS application layer and interacts with the Serving Call Session Control Function (S-CSCF) via the ISC interface. When a user invokes the ECT service, the ECT AS generates a PSI in the form of a SIP URI (e.g., sip:[email protected]). This URI is inserted into the signalling messages, such as the REFER request, to uniquely identify the transfer session. Because the URI resolves to the ECT AS, any subsequent requests or responses related to the transfer are routed back to the AS, allowing it to manage the session state, apply policies, and coordinate the transfer between the parties.

The ECT Session Identifier URI is crucial for several reasons. First, it provides a means to correlate multiple SIP messages that belong to the same transfer session, ensuring that the AS can track the progress of the transfer and handle any errors or timeouts. Second, it enables the AS to remain in the signalling path for the duration of the transfer, which is necessary for charging, lawful interception, and service control. Third, it allows the AS to distinguish between different transfer sessions, especially in scenarios where a user may have multiple active transfers.

In terms of operation, when a user initiates a transfer, the ECT AS creates the ECT Session Identifier URI and includes it in the REFER request sent to the transferee. The transferee then uses this URI to send a NOTIFY message back to the AS, indicating the status of the transfer. The AS can also use the URI to send further instructions or to terminate the session if needed. The URI is typically a SIP URI with a unique user part, ensuring global uniqueness within the IMS domain.

The ECT Session Identifier URI is specified in 3GPP Technical Specifications 24.429 (for the ECT service), 24.529 (for the ECT service in the IP Multimedia Subsystem), and 24.629 (for the ECT service in the IMS). These specifications define the procedures, message flows, and the format of the URI. The term has been stable since its introduction in Release 7, with only a minor wording change in Release 12 to correct the article from "a ECT AS" to "an ECT AS". This stability reflects the maturity of the ECT service and its underlying architecture.

Purpose & Motivation

The ECT Session Identifier URI exists to solve the problem of session identification and routing in the context of the Explicit Communication Transfer supplementary service. Before its introduction, there was no standardized way for an ECT Application Server to uniquely identify and control a transfer session within the IMS. Without such an identifier, the AS could not reliably correlate the various SIP messages involved in a transfer, leading to potential misrouting, charging errors, and an inability to enforce service policies. The ECT Session Identifier URI provides a standardized mechanism for the AS to insert itself into the signalling path and maintain state for each transfer session.

The motivation for creating this identifier was driven by the need for a robust and scalable ECT service in IMS networks. As IMS deployments grew, operators required a way to offer supplementary services like call transfer with the same level of control and reliability as traditional circuit-switched networks. The ECT Session Identifier URI enables the AS to act as a back-to-back user agent (B2BUA) for the transfer, ensuring that all signalling related to the transfer passes through the AS. This allows for advanced features such as charging based on transfer duration, lawful interception, and policy enforcement.

Furthermore, the ECT Session Identifier URI addresses the challenge of interoperability between different vendors' equipment. By standardizing the format and usage of the URI, 3GPP ensures that an ECT AS from one vendor can interoperate with an S-CSCF from another, facilitating multi-vendor IMS deployments. The identifier also supports the requirement for the AS to remain in the signalling path even after the transfer is completed, for example, to receive notifications about the transferred session. Overall, the ECT Session Identifier URI is a fundamental building block for the ECT service, providing the necessary identification and routing capabilities to enable a reliable and feature-rich call transfer experience.