Not Registered State

Description

The Not Registered State is a fundamental concept in the IP Multimedia Subsystem (IMS) architecture, defined in 3GPP specifications such as TS 29.228 and TS 29.562. In IMS, a Public Identity (also known as an IMPU, IP Multimedia Public Identity) represents a user's identity that can be used for communication, such as a SIP URI or a Tel URI. Each Public Identity can be in one of several registration states, and the Not Registered State indicates that the identity is not currently registered with the IMS network and has no Serving-CSCF (S-CSCF) assigned to it. This means that the user is not reachable for IMS services, and any terminating session attempts (e.g., incoming calls) will fail or be redirected based on operator policy.

The registration state is maintained by the Home Subscriber Server (HSS) and is queried by IMS nodes such as the Interrogating-CSCF (I-CSCF) during session setup. When a UE registers with IMS, it sends a SIP REGISTER request to the Proxy-CSCF (P-CSCF), which forwards it to the I-CSCF. The I-CSCF queries the HSS to determine the registration state of the Public Identity. If the identity is in the Not Registered State, the HSS may provide the capabilities required for S-CSCF selection, and the I-CSCF will select an S-CSCF to assign to the identity. Once the S-CSCF is assigned and the registration is successful, the state transitions to Registered. Conversely, when the UE deregisters or the registration expires, the state returns to Not Registered, and the S-CSCF assignment is removed.

Key components involved in this state include the HSS, which stores the registration state and S-CSCF assignment; the I-CSCF, which queries the HSS and routes requests; the S-CSCF, which serves as the registrar and session control function; and the P-CSCF, which acts as the entry point for the UE. The Not Registered State is crucial for session establishment: when a terminating session arrives, the I-CSCF checks the state. If the Public Identity is not registered, the I-CSCF may either reject the session, redirect it to voicemail, or attempt to assign an S-CSCF if the identity is allowed to receive sessions without registration (e.g., for certain services). However, typically, the session cannot be delivered to the user.

The state is also used for service authorization and routing. For example, if a Public Identity is not registered, the HSS may indicate that the identity is not available for terminating services, and the I-CSCF will not attempt to route the session to an S-CSCF. This prevents unnecessary signaling and ensures that resources are not wasted. The Not Registered State is distinct from other states such as Registered (where an S-CSCF is assigned and the user is reachable) and Unregistered (where the user is not registered but may still have an S-CSCF assigned for certain services, though this is a different concept).

In summary, the Not Registered State is a key indicator of a user's availability in IMS. It is managed through the HSS and queried by I-CSCF during session setup. Understanding this state is essential for engineers working with IMS, as it directly impacts call delivery, service access, and network resource utilization.

Purpose & Motivation

The Not Registered State exists to provide a clear and standardized way to represent the availability of a Public Identity in IMS. In circuit-switched networks, a user's availability was often tied to the physical connection or location register. In IMS, which is access-agnostic and IP-based, a more dynamic mechanism was needed to track whether a user is registered and reachable. The Not Registered State solves the problem of determining whether a Public Identity can receive terminating sessions. Without this state, the network would not know whether to attempt to route a session to a user, leading to inefficient use of resources and poor user experience.

Historically, IMS was introduced in 3GPP Release 5 to support multimedia services over IP. The registration state concept was defined early on to manage user reachability. The Not Registered State is the default state for a Public Identity that has not been registered or whose registration has expired. It allows the network to quickly identify that the user is not available and to take appropriate actions, such as forwarding to voicemail or returning an error response. This is particularly important for operators to manage network resources and provide consistent service behavior.

The state also supports the separation of registration and session control. By having a distinct Not Registered State, the IMS can handle scenarios where a user is not registered but may still have services configured (e.g., for emergency calls or certain operator services). It also enables the HSS to maintain accurate records and for the I-CSCF to make routing decisions based on the state. Over releases, the definition has remained stable, indicating its fundamental role in IMS architecture.