Access Leg

Description

The Access Leg is a fundamental concept in 3GPP's IMS Centralized Services (ICS) and Voice Call Continuity (VCC) architectures, defined primarily in TS 23.292 and TS 23.206. It represents the call control signaling relationship between the User Equipment (UE) and the Service Centralization and Continuity Application Server (SCC AS). In a typical IMS session, the overall communication is decomposed into two distinct legs: the Access Leg and the Remote Leg. The Access Leg carries the access-specific signaling between the UE and the SCC AS, while the Remote Leg connects the SCC AS to the remote party. This decomposition is what enables the SCC AS to serve as an anchor point for session control, allowing it to coordinate and translate between different access technologies.

The Access Leg is established when the UE initiates or receives a session. For an originating session, the UE sends its session setup request toward the SCC AS, which terminates the Access Leg and then generates a corresponding request on the Remote Leg toward the remote endpoint. For a terminating session, the SCC AS receives the incoming request on the Remote Leg and then establishes the Access Leg toward the UE. The SCC AS maintains the correlation between the two legs, acting as a back-to-back user agent (B2BUA) that can independently manipulate each leg. This architecture is essential for enabling features such as mid-call access transfer, where the UE's access leg can be moved from one access network to another (e.g., from CS to PS or vice versa) without affecting the Remote Leg and therefore without disrupting the session from the remote party's perspective.

In the context of IMS sessions that use CS media, as described in TS 23.292, the Access Leg definition extends to cover scenarios where the media itself is carried over the Circuit Switched domain. In such cases, the call control signaling on the Access Leg may traverse the CS domain via a Mobile Switching Center (MSC) enhanced with ICS capabilities, while the SCC AS still maintains overall session control. This allows IMS services to be delivered to UEs that are attached via CS access, ensuring service consistency regardless of the underlying access technology. The Access Leg therefore abstracts the access-specific details from the service layer, enabling a unified service experience.

The concept of the Access Leg is closely tied to the Session Initiation Protocol (SIP) as the call control protocol. The Access Leg typically consists of SIP dialog(s) between the UE and the SCC AS. The SCC AS uses the information in these SIP messages, along with stored service profiles and policies, to determine how to handle the session. For example, it may decide to route the session over IMS, CS, or to perform a transfer. The Access Leg also carries the necessary parameters for media negotiation, such as SDP offers and answers, which the SCC AS may modify or relay depending on the access type.

From a network architecture perspective, the Access Leg is not a physical interface but a logical concept. It is realized through a combination of SIP signaling, possibly interworked with CS signaling (e.g., ISUP/BICC) when CS access is involved. The SCC AS is the pivotal node that terminates the Access Leg and manages the session. Other network elements, such as the P-CSCF, S-CSCF, and MSC, may be involved in transporting the Access Leg signaling, but the leg itself is defined by the endpoints: the UE and the SCC AS. This logical separation is what gives the architecture its flexibility and is a key enabler for Voice over LTE (VoLTE) and Voice over NR (VoNR) service continuity features.

Purpose & Motivation

The Access Leg concept was created to solve the problem of service continuity and centralization in multi-access networks. Before its introduction, voice calls were tied to the specific access technology used (e.g., CS domain for 2G/3G). When a user moved between access types or when operators wanted to offer IMS services over CS access, there was no standardized way to anchor the session and transfer it seamlessly. The Access Leg, together with the Remote Leg, provides a clean architectural separation that allows the SCC AS to act as an anchor point, independent of the access technology.

This separation addresses several limitations of previous approaches. First, it enables Voice Call Continuity (VCC) between CS and PS domains, allowing a call to survive when the UE moves out of coverage of one access and into another. Second, it supports IMS Centralized Services (ICS), where IMS services are provided to users regardless of whether they are attached via PS or CS access. Third, it simplifies the introduction of new access technologies, as the core IMS service logic remains in the SCC AS and only the Access Leg needs to be adapted for the new access. This modularity reduces the impact of access-specific changes on the overall service architecture.

Historically, the Access Leg was first defined in Release 7 as part of the VCC work (TS 23.206), where it was described as the call control leg between the VCC UE and the Domain Transfer Function (DTF). In Release 8, with the evolution towards ICS (TS 23.292), the definition was updated to reference the SCC AS instead of the DTF, reflecting the architectural shift from a VCC-specific entity to a more generic service centralization function. This change marked the broadening of the concept from a VCC-specific mechanism to a foundational element of IMS service delivery across accesses.