Unregistered State (IMS Public Identity State)

Description

The Unregistered State is a service state defined in the 3GPP IP Multimedia Subsystem (IMS) architecture for a user's Public Identity (also called Public User Identity, IMPU). In IMS, a Public Identity can exist in one of several states, and the Unregistered State occupies a middle ground between full registration and complete absence from the network. In this state, the Public Identity is not registered — meaning the user has not performed an IMS registration and there is no active contact address (IP address/port) bound to the identity — yet a serving Call Session Control Function (S-CSCF) is still assigned to that identity. This assigned S-CSCF holds the user profile and is capable of executing Unregistered state services.

The assignment of an S-CSCF in the Unregistered State typically arises from one of three triggers. First, a terminating request destined for the user (for example, an incoming session or message) may cause the Interrogating CSCF (I-CSCF) to query the Home Subscriber Server (HSS) and select an S-CSCF, which then stores the user profile even though the user is not registered. Second, an originating request from an Application Server (AS) acting on behalf of the user may trigger S-CSCF assignment. Third, an S-CSCF may simply be keeping the user profile stored from a previous interaction. In all cases, the S-CSCF becomes the serving node for that Public Identity while it remains unregistered.

The key functional value of the Unregistered State is that it enables Unregistered state services. These are services that can be delivered to a user who is not currently registered, most notably terminating services such as incoming calls being routed to voicemail, call forwarding, or other AS-based treatments. Because the S-CSCF has the user profile, it can apply the correct service logic and filter criteria (iFCs) even without an active registration. This allows the network to handle incoming requests gracefully rather than rejecting them outright.

Architecturally, the Unregistered State is managed through the Cx interface between the I-CSCF/S-CSCF and the HSS, and through the Sh interface between the HSS and the AS. The HSS maintains the registration state of each Public Identity and the identity of the assigned S-CSCF. When a terminating request arrives, the I-CSCF queries the HSS; if the HSS indicates an S-CSCF is already assigned (even in unregistered state), the request is routed there. The S-CSCF then retrieves or uses the stored profile to execute the appropriate service.

It is important to distinguish the Unregistered State from the Not Registered state. In the Not Registered state, no S-CSCF is assigned and no user profile is stored, so the network cannot execute any services for that identity. The Unregistered State, by contrast, is an active service state with an assigned S-CSCF. This distinction is fundamental to IMS service continuity and is documented in 3GPP TS 29.228 (IMS Cx and Dx interfaces) and TS 29.562 (IMS restoration procedures). The concept has been stable since Release 8 and remains part of the IMS architecture through Release 20.

Purpose & Motivation

The Unregistered State exists to solve a fundamental problem in IMS service delivery: how to handle incoming requests for users who are not currently registered but who still have services configured in the network. Without this state, a terminating call to an unregistered user would either be rejected or would require the network to treat the user as completely unknown, losing the ability to apply personalized service logic such as call forwarding, voicemail routing, or barring services.

Before this concept was formalized, IMS networks had only binary treatment of identities — either registered or not registered. This meant that services configured for a subscriber could not be executed when the subscriber was offline. Operators needed a mechanism to keep a serving node (the S-CSCF) associated with a Public Identity so that terminating services and AS-originated requests could still be processed. The Unregistered State provides exactly this: a persistent association between a Public Identity and an S-CSCF, along with the stored user profile, enabling service execution without an active registration.

The state was introduced as part of the foundational IMS architecture in Release 8 and has remained unchanged in definition through Release 20. Its stability reflects its role as a core architectural concept rather than a feature subject to frequent revision. It addresses the limitations of purely registration-based service delivery and supports important use cases such as terminating call handling for offline users, AS-initiated services, and profile retention for service continuity.