IMS User Equipment

Description

IMS User Equipment (IMS UE) is defined by 3GPP as user equipment used for IMS media communications over access networks. The definition, found in 3GPP TS 33.328 and TS 33.828, specifies the security requirements and UICC presence rules for such equipment. The presence of the UICC in this equipment is optional when the equipment does not support any 3GPP access technology. However, in the case where the user equipment is used for IMS media communications over any 3GPP access network, the IMS User Equipment shall contain a UICC.

The IMS UE is the endpoint device that terminates IMS media and signaling. It typically includes an ISIM application on the UICC for IMS authentication, or may use USIM-based authentication in certain configurations. The UICC provides the secure credentials and key material needed for IMS AKA authentication, ensuring that the subscriber identity is protected and that the device can securely register with the IMS core. When the device accesses IMS over a non-3GPP access network (e.g., Wi-Fi), the UICC may be optional if the device does not support any 3GPP access technology, but for 3GPP access (LTE, 5G NR), the UICC is mandatory.

Security for the IMS UE is covered extensively in TS 33.328, which defines the security architecture for IMS media plane, and TS 33.828, which covers security aspects of IMS media plane for 5G. These specifications address key management, media protection, and authentication procedures. The IMS UE must support the security mechanisms required by the IMS core, including IPsec for SIP signaling protection and SRTP for media protection, depending on operator policy and network configuration.

The IMS UE concept is important because it clarifies the security boundary and the role of the UICC in IMS access. It ensures that devices accessing IMS over 3GPP networks have a secure, operator-issued credential, which is essential for authentication, authorization, and charging. It also allows for flexibility in non-3GPP access scenarios, where devices without 3GPP access capabilities may still use IMS services under certain conditions.

The definition of IMS UE in TS 21.905 (or equivalent vocabulary) underwent a notable wording change in Rel-12. Prior to Rel-12, the definition stated that UICC presence is optional when the equipment does not support any 3GPP access technology, and that for 3GPP access the IMS UE shall contain a UICC. In Rel-12, the wording was simplified to state that use of such equipment for IMS media communications over any 3GPP access network shall require presence of a UICC. This change removed the explicit optionality clause for non-3GPP-only devices, making the requirement more direct. From Rel-12 through Rel-19, the definition remained unchanged.

Purpose & Motivation

The IMS User Equipment definition exists to establish clear security and identity requirements for devices that access IMS services. In an all-IP environment, ensuring that only authorized and properly authenticated devices can access IMS is critical for network security, service integrity, and billing accuracy. The UICC serves as the hardware root of trust, holding the credentials needed for IMS AKA authentication and enabling secure key agreement.

Before this definition was formalized, the requirements for UICC presence in IMS devices were less explicit, leading to potential ambiguity for devices that could access IMS over both 3GPP and non-3GPP networks. The definition clarifies that for any 3GPP access, a UICC is mandatory, which aligns with the security architecture of 3GPP networks and ensures a consistent authentication framework. For non-3GPP-only devices, the optional UICC allowance provides flexibility, recognizing that such devices may use other authentication mechanisms.

The Rel-12 wording change further streamlined the requirement, removing the conditional optionality language and making the UICC requirement for 3GPP access more direct. This reflects the maturation of IMS security specifications and the need for unambiguous compliance statements. The IMS UE definition thus supports interoperability, regulatory compliance, and secure service delivery across diverse access technologies.