Terminating Access Domain Selection

Description

Terminating Access Domain Selection (T-ADS) is a network-based service capability within the IP Multimedia Subsystem (IMS) architecture, defined to handle the routing of terminating (incoming) voice or video sessions (e.g., SIP INVITEs) for users who are simultaneously registered over both Circuit-Switched (CS) and Packet-Switched (PS) access networks. This scenario is common in early IMS deployments and during network transitions, where a user might have a CS registration (via MSC Server) for legacy voice and an IMS registration over LTE/5G NR for VoLTE/VoNR. The core problem T-ADS solves is deciding whether to route the incoming call to the user's CS domain (e.g., as a normal CS call) or to their PS/IMS domain (as a VoIP call).

Architecturally, T-ADS logic is typically implemented within the Serving-Call Session Control Function (S-CSCF) or can be delegated to an Application Server (AS). When an incoming SIP session request arrives for a user, the S-CSCF invokes the T-ADS procedure. The S-CSCF queries the Home Subscriber Server (HSS) to retrieve the user's current registration status across domains, stored as part of the user's IMS Subscription Profile. This includes the user's CS and IMS registration states, the associated contact addresses (e.g., MSC server number for CS, SIP URI for IMS), and any operator-defined preferences or capabilities. Based on this information, along with potentially additional criteria like the called party's preferences, the calling party's capabilities, and network policies, the T-ADS function makes a selection.

The selection algorithm follows standardized rules and priorities. Common criteria include: preferring the PS/IMS domain if the user is IMS-registered and supports the required service (e.g., VoLTE), falling back to CS if IMS registration is unavailable or unsuitable; considering the user's explicit communication service identifier (e.g., ICSI) or implicit service requirements; and applying operator-configured policies for load balancing or service continuity. Once the selection is made, the S-CSCF routes the session accordingly—either forwarding the SIP request within the IMS core or initiating CS redirection via mechanisms like Service Centralization and Continuity (SRVCC) or interworking with the CS network. T-ADS is crucial for enabling smooth service continuity and efficient network resource utilization in a converged multi-access network environment.

Purpose & Motivation

T-ADS was developed to address the challenge of delivering voice and multimedia services in a converged network environment where users can be reachable via both traditional circuit-switched mobile networks (like 2G/3G) and packet-switched IMS networks. Prior to IMS and T-ADS, a dual-registered user (e.g., a UE with both GSM and IMS capabilities) could face call delivery failures or suboptimal routing if the network could not intelligently choose the best domain. T-ADS solves this by providing a standardized, policy-driven selection mechanism that ensures the user experience is maintained regardless of the access network used. It was motivated by the transition from pure CS voice to Voice over LTE (VoLTE) and the need for seamless service continuity. Without T-ADS, an incoming call might be routed to an unavailable domain, causing missed calls or poor user experience. T-ADS enables operators to optimize network resources and user experience by selecting the most appropriate domain based on real-time registration and policy data.