Initial Registration

Description

Initial registration is a fundamental mobility management procedure in cellular networks, particularly in the IP Multimedia Subsystem (IMS) and 5G System (5GS). It refers to the process initiated by a User Equipment (UE) to register with the network when it has no valid registration, either because it is powering on, has moved to a new network, or has been deregistered. The procedure establishes the UE's context in the network, authenticates the subscriber, and sets up the necessary bearers or PDU sessions for communication. In 3GPP specifications, initial registration is defined for both IMS (as per 24.229) and 5G core (as per 23.501), with the latter being the focus in recent releases.

In the 5G System, initial registration is part of the Registration Management (RM) state model. The UE transitions from RM-DEREGISTERED to RM-REGISTERED state through this procedure. The UE initiates the registration by sending a Registration Request message to the Access and Mobility Management Function (AMF) via the Radio Access Network (RAN). The request includes parameters such as the UE's identity (SUCI or 5G-GUTI), requested network slice selection assistance information (NSSAI), and UE capabilities. The AMF then performs authentication and security procedures, potentially involving the Unified Data Management (UDM) and Authentication Server Function (AUSF). Upon successful authentication, the AMF registers the UE with the UDM and may select a Session Management Function (SMF) for PDU session establishment.

The initial registration procedure involves multiple network functions and interfaces. The AMF acts as the main anchor, handling mobility management and forwarding session management messages. The UDM stores subscription data and provides it to the AMF. The Policy Control Function (PCF) may provide policy rules. The procedure also includes the exchange of UE parameters such as the 5G-GUTI, which is allocated by the AMF for future registrations. In IMS, initial registration involves the UE sending a SIP REGISTER request to the Proxy Call Session Control Function (P-CSCF), which forwards it to the Interrogating CSCF (I-CSCF) and Serving CSCF (S-CSCF). The S-CSCF authenticates the user and stores the registration state.

The initial registration procedure is critical for enabling services. It ensures that the network knows the UE's location, capabilities, and subscription profile, allowing it to route incoming calls, data, and other services. It also establishes security contexts and QoS flows. Without a successful initial registration, the UE cannot access network services. The procedure is designed to be robust, with retransmissions and fallback mechanisms in case of failures. In 5G, the procedure supports network slicing, allowing the UE to register to multiple slices simultaneously.

Over the releases, the initial registration procedure has evolved significantly. In early IMS releases (Rel-6 to Rel-13), the focus was on SIP-based registration for voice and multimedia services. With the introduction of 5G in Rel-15, the procedure was redefined for the new core network architecture, introducing concepts like RM states, network slicing, and enhanced security. Rel-17 brought a wording change to align with the RM state model, emphasizing the UE's state transition. Subsequent releases continued to refine the procedure for new features like non-3GPP access and satellite integration.

Purpose & Motivation

Initial registration exists to solve the problem of how a mobile device announces its presence to the network and gains authorization to use services. Without a registration procedure, the network would not know which devices are active, where they are located, or what services they are entitled to. In early cellular systems, registration was implicit or handled differently, but as networks became more complex with IP-based services and roaming, a standardized initial registration became essential. It allows the network to authenticate the user, set up security, and establish the necessary context for service delivery. This is particularly important for IMS, where users must register to make and receive SIP-based calls.

In 5G, the initial registration procedure addresses additional challenges such as network slicing, where a UE may need to register to multiple logical networks simultaneously. It also supports diverse access types, including 3GPP and non-3GPP access. The procedure ensures that the UE and network agree on capabilities, security parameters, and mobility policies. It is the foundation for all subsequent interactions, including session establishment and handover. The evolution from IMS-specific registration to the 5G RM state model reflects the convergence of voice, data, and IoT services onto a common core.

The motivation for standardizing initial registration was to enable interoperability and global roaming. Before 3GPP specifications, different vendors implemented proprietary registration mechanisms, which hindered roaming. The 3GPP defined procedures ensure that any compliant UE can register with any compliant network, regardless of vendor. This has been crucial for the success of mobile communications. As networks move towards 6G and beyond, initial registration will continue to evolve to support new requirements like ultra-reliable low-latency communications and massive IoT.