UTRAN Radio Network Temporary Identifier

Description

The UTRAN Radio Network Temporary Identifier (U-RNTI) is a fundamental temporary identifier used in 3GPP UMTS networks. It serves as a unique address for a User Equipment (UE) within the scope of the UTRAN, specifically from the perspective of the Serving Radio Network Controller (SRNC). The U-RNTI is constructed from two main components: the Serving RNC Identity (SRNC-ID) and the Serving RNC Radio Network Temporary Identity (S-RNTI). The SRNC-ID uniquely identifies the SRNC within the network, while the S-RNTI is a unique value assigned by that SRNC to the UE. This combination ensures global uniqueness across the network.

The U-RNTI is assigned to the UE when it enters the CELL_DCH or CELL_FACH state, indicating an active connection with dedicated or shared channel resources. It is used extensively in signaling messages between the UE and the UTRAN over the air interface. For instance, during radio resource control (RRC) connection establishment, reconfiguration, handover, and cell update procedures, messages are addressed using the U-RNTI to identify the target UE. The identifier is included in the headers of RRC protocol data units.

A key architectural role of the U-RNTI is to enable efficient UE identification without relying on permanent identifiers like the IMSI, thus enhancing user privacy over the radio interface. It also allows the network to manage mobility. During an SRNC relocation procedure (a type of handover where the serving RNC changes), the U-RNTI may be reallocated by the new SRNC. The U-RNTI remains valid as long as the UE has an RRC connection with the UTRAN and is released when the RRC connection is released. Its design is integral to the UTRAN's control plane, ensuring reliable and addressed delivery of RRC messages.

Purpose & Motivation

The U-RNTI was created to fulfill the need for a secure, efficient, and temporary method of identifying a UE within the radio access network's signaling procedures. Using a permanent identifier like the International Mobile Subscriber Identity (IMSI) over the air interface for frequent signaling would pose significant security and privacy risks, as it could be intercepted and tracked. The U-RNTI solves this by providing a temporary alias that is meaningless outside the specific network context and session.

Its purpose extends to facilitating robust mobility management. In the complex hierarchy of UMTS cells and RNCs, the U-RNTI provides a consistent handle for the SRNC to manage a UE's connection as it moves. It is essential for procedures like hard handover and SRNC relocation, where the network needs to unequivocally identify which UE connection is being transferred. The creation of the U-RNTI, standardized from Release 4 onwards, was a key part of defining a mature and scalable RRC protocol for UMTS, separating the concerns of core network subscriber identity from RAN-level connection identity.

Key Features

• Globally unique temporary identifier for a UE within UTRAN
• Composed of a Serving RNC Identity (SRNC-ID) and an S-RNTI
• Used for addressing RRC signaling messages over the air interface
• Assigned during RRC connection establishment in CELL_DCH/FACH states
• Enhances privacy by avoiding transmission of permanent UE identities over the radio
• Essential for mobility procedures like handover and SRNC relocation

Evolution Across Releases

Defining Specifications