Drift Radio Network Controller

Description

The Drift Radio Network Controller (DRNC) is a fundamental concept in the UMTS Radio Access Network (UTRAN) architecture, specifically within the control plane for mobility management. It is an RNC that is currently controlling the cells (via Node Bs) that a User Equipment (UE) is using, but it is not the RNC that holds the Radio Resource Control (RRC) connection for that UE. The RNC that does hold the RRC connection is termed the Serving RNC (SRNC). The DRNC's primary role emerges during inter-RNC soft handover, where a UE is simultaneously connected to cells belonging to different RNCs. In this scenario, the SRNC remains the sole point of control for the UE's RRC connection, but the DRNC manages the radio resources of its own cells and acts as a routing agent.

Architecturally, the DRNC sits between the SRNC and the Node Bs under its control. The Iur interface connects the SRNC and the DRNC. User plane data and control signaling for the UE are transported over this Iur interface. The DRNC does not perform ciphering or integrity protection for the UE; these functions remain with the SRNC. Instead, the DRNC is responsible for the allocation and management of radio resources (like channelization codes and scrambling codes) on its own cells, executing commands received from the SRNC via the Radio Network Subsystem Application Part (RNSAP) protocol over the Iur.

From a data flow perspective, in the downlink, the SRNC sends the user data to the DRNC via the Iur interface. The DRNC then forwards this data to the appropriate Node B(s) under its control via the Iub interface. In the uplink, the process is reversed: the DRNC receives data from its Node Bs, combines it if the UE is in softer handover within the DRNC's cells, and then forwards it to the SRNC. This relaying function is transparent to the UE. The DRNC's role is critical for enabling seamless mobility and maintaining call quality during handovers between RNCs without requiring the SRNC role to be transferred, which is a more complex procedure known as SRNC relocation.

Purpose & Motivation

The DRNC concept was introduced in 3GPP Release 99 to solve the fundamental problem of seamless mobility and radio resource management in a distributed, multi-RNC UMTS network. Prior to UMTS, GSM networks used Base Station Controllers (BSCs) that typically managed contiguous geographic areas, and handovers between BSCs were hard handovers with a break-before-make characteristic. UMTS, with its WCDMA air interface, enabled soft handover, where a UE can be connected to multiple cells simultaneously for macro-diversity gain. This capability needed to extend across administrative boundaries controlled by different RNCs.

The DRNC mechanism allows the network to leverage the benefits of soft handover (improved coverage, reduced call drop, lower required transmit power) even when a UE moves into an area controlled by an RNC different from its SRNC. Without the DRNC role and the standardized Iur interface, such inter-RNC soft handover would be impossible, forcing either a hard handover (degrading performance) or an immediate and potentially unnecessary SRNC relocation (increasing signaling load and complexity). The DRNC thus enables a more flexible and efficient network architecture where the SRNC can be anchored while the UE's radio connection utilizes the best available resources from multiple RNC domains, optimizing both mobility performance and network resource utilization.