Registered State

Description

In the IP Multimedia Subsystem (IMS), the Registered State is a condition of a Public User Identity (PUI) that indicates the identity is actively registered with the network and has a Serving Call Session Control Function (S-CSCF) assigned to it. This state is fundamental to the operation of IMS services, as it determines whether a user can originate or receive sessions. The concept was introduced in 3GPP Release 8 and has been maintained across all subsequent releases up to Rel-20. It is defined in specifications such as 29.228 (IP Multimedia (IM) Subsystem Cx and Dx interfaces; Signalling flows and message contents) and 29.562 (IMS restoration procedures).

When a user registers a Public User Identity, the IMS core network performs a series of procedures to authenticate the user and assign an S-CSCF. The S-CSCF acts as the registrar and session control entity for that identity. Upon successful registration, the identity is considered to be in the Registered State. This state is stored in the Home Subscriber Server (HSS) and is used by various network functions to route session requests. For example, when an incoming session request arrives for a registered identity, the Interrogating-CSCF (I-CSCF) queries the HSS to find the assigned S-CSCF, which then delivers the request to the user. If the identity is not in the Registered State, the session request may be forwarded to a voicemail server or rejected, depending on the user's subscription.

The Registered State is dynamic and can be transitioned to other states, such as the Unregistered State. A Public User Identity can become unregistered if the user explicitly deregisters, if the registration expires, or if the network initiates a deregistration (e.g., due to administrative actions or network failures). In the Unregistered State, the identity still exists in the HSS but does not have an S-CSCF assigned. However, the user may still have services that can be delivered in the Unregistered State, such as voice mail or instant messaging, depending on the subscription. The Registered State is therefore a key enabler for real-time communication services.

The state information is maintained in the HSS and is updated via the Cx interface (between I-CSCF/S-CSCF and HSS) and the Sh interface (between application servers and HSS). The HSS stores the registration status as part of the user profile. When a user registers, the S-CSCF sends a Server-Assignment-Request (SAR) to the HSS, which updates the state to registered and stores the S-CSCF name. When the user deregisters, the S-CSCF sends a Server-Assignment-Request with a deregistration indication, and the HSS updates the state accordingly. These procedures are specified in 29.228.

The Registered State is also important for IMS restoration procedures. If an S-CSCF fails, the HSS may need to reassign a new S-CSCF for registered identities. The restoration procedures defined in 29.562 ensure that the Registered State can be recovered or the user can be re-registered. This ensures high availability of IMS services.

In summary, the Registered State is a core concept in IMS that represents the active registration of a Public User Identity with an assigned S-CSCF. It is essential for session establishment and service delivery, and its management is standardized across all 3GPP releases since Rel-8.

Purpose & Motivation

The Registered State was defined to provide a clear and standardized way to manage user registration in IMS. Before its introduction, IMS registration handling was less formalized, leading to inconsistencies in how different network elements interpreted the registration status. The Registered State solves this by explicitly linking a Public User Identity to an assigned S-CSCF, ensuring that session requests can be routed correctly and efficiently. It also enables the network to know whether a user is available for terminating sessions and to apply appropriate services based on the subscription. The state is crucial for enabling seamless mobility and service continuity, as it allows the network to track which S-CSCF is serving the user. Furthermore, it supports restoration procedures by allowing the HSS to maintain a record of registered identities, which can be used to recover from failures. Over the releases, the definition has remained stable, indicating the maturity of the concept. Minor wording changes in Rel-17 and Rel-20 were purely editorial, reflecting a refinement of the specification text without altering the meaning.