Remote Leg

Description

The Remote Leg is a term used in 3GPP specifications to describe the call control leg between the Service Centralization and Continuity Application Server (SCC AS) and the remote party, from the subscriber's perspective. It is defined in TS 23.206, TS 23.237, and TS 23.292, and was introduced in Release 7. The Remote Leg represents the portion of a session that connects the SCC AS to the other end of the call (the remote party), as opposed to the Access Leg, which connects the UE to the SCC AS. This distinction is fundamental to IMS (IP Multimedia Subsystem) service continuity mechanisms, such as Single Radio Voice Call Continuity (SRVCC) and IMS Centralized Services (ICS).

In an IMS session, the SCC AS acts as a back-to-back user agent (B2BUA) or an application server that anchors the session. When a UE initiates or receives a call, the SCC AS establishes two legs: the Access Leg (between the UE and SCC AS) and the Remote Leg (between SCC AS and the remote party). The Remote Leg remains stable even if the Access Leg changes due to mobility or access transfer (e.g., from LTE to CS). This allows the session to survive handovers between different access technologies. The Remote Leg is controlled by the SCC AS using SIP (Session Initiation Protocol) signaling, and it carries the media (voice, video) between the SCC AS and the remote party.

The definition of Remote Leg has evolved. In Rel-7, it was defined as 'This is the call control leg between the DTF and the remote party from the VCC subscriber's perspective.' Here, DTF stands for Domain Transfer Function, which was the entity responsible for VCC (Voice Call Continuity) in early specifications. In Rel-8, the wording changed to 'This is the call control leg between the SCC AS and the remote party from the subscriber's perspective; also see TS 23.292 [5] for the definition of Remote Leg for IMS sessions which use the CS media.' This change reflected the architectural evolution from DTF to SCC AS and the broader scope of IMS centralized services. The new definition also included a reference to TS 23.292 for CS media scenarios.

From Rel-8 onward, the definition remained unchanged through Rel-19. The Remote Leg concept is critical for enabling seamless service continuity. For example, in SRVCC, when a UE moves from LTE to a CS network, the Access Leg is transferred from the LTE access to the CS access, while the Remote Leg remains anchored at the SCC AS. This ensures that the remote party does not experience a dropped call. Similarly, in ICS, the Remote Leg allows IMS services to be provided over CS access, with the SCC AS managing the session.

The Remote Leg is also relevant for other services like RCS (Rich Communication Services) and VoLTE (Voice over LTE). In VoLTE, the SCC AS may not always be involved, but when service continuity is required, the Remote Leg concept applies. The term is defined in the vocabulary specification TS 21.905 and is used throughout 3GPP specifications for IMS and service continuity. Understanding the Remote Leg is essential for engineers working on IMS, VoLTE, and mobility management, as it underpins the architecture that allows sessions to be anchored and transferred without interruption.

Purpose & Motivation

The Remote Leg concept was created to solve the problem of session continuity in IMS networks. Before its introduction, when a user moved between access networks (e.g., from WLAN to cellular), ongoing sessions could be dropped because the session state was tied to the access network. The Remote Leg, anchored at the SCC AS, provides a stable anchor point that remains unchanged during access transfers. This allows the session to be maintained even as the Access Leg changes.

The evolution from DTF to SCC AS in the definition reflects the maturation of the architecture. In early VCC (Rel-7), the DTF was a specific function for voice call continuity. As IMS centralized services expanded, the SCC AS became the central entity for service continuity and centralization. The updated definition in Rel-8 aligned with this architectural shift and included references to CS media scenarios, broadening the applicability.

The Remote Leg is essential for enabling seamless mobility and service continuity, which are key requirements for operators. It allows users to roam between different access technologies without dropping calls, improving user experience. It also enables the delivery of IMS services over CS access, extending the reach of IMS. The concept has been stable since Rel-8, indicating its fundamental importance and correct design.