Iur Interface (RNC–RNC Interface)

Description

The Iur interface is the standardized logical interface between two Radio Network Controllers (RNCs) in the UMTS Terrestrial Radio Access Network (UTRAN). It is defined in 3GPP TS 25.401 and referenced in the vocabulary specification TS 21.905. Architecturally, Iur connects RNCs within the same UTRAN, allowing a UE to maintain radio links with cells controlled by different RNCs. This is essential for soft handover, where the UE simultaneously communicates with multiple Node Bs that may be under the control of different RNCs.

Functionally, Iur carries two main categories of traffic: control plane signalling and user plane data. The control plane uses the Radio Network Subsystem Application Part (RNSAP) protocol, which manages radio link setup, addition, deletion, and reconfiguration across RNCs. RNSAP also supports mobility procedures such as serving RNC (SRNC) relocation, where the role of the SRNC is transferred from one RNC to another as the UE moves. The user plane carries dedicated channel (DCH) and later HSDPA/HSUPA data frames using the Iur Frame Protocol, which is similar in structure to the Iub Frame Protocol but adapted for inter-RNC transport.

The Iur interface introduces the concept of the Drift RNC (DRNC). When a UE is connected to an RNC that is not its serving RNC, that RNC acts as a DRNC and provides radio resources to the UE on behalf of the SRNC. The SRNC controls the connection and communicates with the DRNC over Iur. This SRNC/DRNC split is a defining feature of UTRAN and allows seamless mobility across RNC boundaries without interrupting the user's session.

Transport-wise, Iur was originally carried over ATM, with AAL2 for user plane and AAL5 for control plane, similar to Iub. From Rel-5 onwards, IP transport became increasingly common, with SCTP for RNSAP signalling and UDP/IP for user plane framing. The logical interface definition does not mandate a specific physical realization; the Iur may be implemented over point-to-point links, leased lines, or packet-switched backhaul. This flexibility is important because RNCs may be geographically separated and interconnected through an operator's transport network.

In terms of evolution, the Iur definition was revised in Rel-6, where the wording changed from "two RNCs" to "two RNC" and from "need not be" to "may not be" a point-to-point link. This is a minor editorial change rather than a functional one. From Rel-6 through Rel-19, the definition remained essentially the same, with only formatting and minor wording variations. The Iur interface remained a stable part of UTRAN throughout its lifecycle, even as HSPA enhancements added new frame types and procedures.

Purpose & Motivation

The Iur interface exists to enable seamless mobility and coordinated radio resource management across RNC boundaries in UMTS. Without Iur, a UE moving between cells controlled by different RNCs would experience a hard handover, which can cause interruption and degradation of service. Iur allows soft handover across RNCs, where the UE maintains simultaneous radio links with multiple Node Bs, improving coverage and reliability at cell edges.

A second motivation is the need for a standardized inter-RNC protocol. Before Iur, inter-RNC coordination was proprietary or limited, which restricted multi-vendor deployment and made it difficult to build large, contiguous UMTS networks. By defining Iur in 3GPP specifications, the industry enabled RNCs from different vendors to interoperate, supporting flexible network planning and vendor diversity.

Finally, Iur supports the SRNC/DRNC architecture, which separates the control anchor from the radio access point. This separation allows the network to keep a stable control anchor while the UE moves across RNCs, reducing signalling to the core network and improving handover performance. The Iur interface is therefore a foundational element of UTRAN mobility and resource management.