Drift Radio Network Subsystem

Description

The Drift Radio Network Subsystem (DRNS) is a logical grouping within the UMTS Terrestrial Radio Access Network (UTRAN) architecture. It comprises a single Drift Radio Network Controller (DRNC) and all the base stations (Node Bs) that are connected to and controlled by that DRNC. The concept of a DRNS is defined relative to a specific User Equipment (UE). When a UE is in an inter-RNC soft handover state—connected to cells belonging to different RNCs—the RNC that is not the Serving RNC (SRNC) is designated the DRNC for that UE, and its entire controlled domain becomes the DRNS for that UE's connection.

Architecturally, the DRNS is a subset of the UTRAN. It interfaces with the core network via the Iu interface (though typically, the SRNC acts as the sole point of contact with the CN for a given UE). Its most critical external interface is the Iur interface, which connects it to the Serving RNS (SRNS)—the domain of the SRNC. All control signaling and user data pertaining to the UE that traverse between the SRNC and the Node Bs within the DRNS must pass through this Iur link. Internally, the DRNS uses the Iub interface to connect the DRNC to its Node Bs.

The DRNS operates under the command of the SRNC for resources allocated to a specific UE. The SRNC uses the Radio Network Subsystem Application Part (RNSAP) protocol over the Iur interface to request radio link setup, modification, or deletion within the DRNS. The DRNC, as the manager of the DRNS, is responsible for translating these requests into actions on its Iub interfaces, allocating local resources like channelization codes and scrambling codes, and managing the physical radio links. For the user plane, the DRNS acts as a switch: it receives downlink frames from the SRNC, routes them to the correct Node B(s), and combines uplink frames from its Node Bs before forwarding the combined frame to the SRNC. This allows the SRNS to maintain a single, coherent view of the radio connection while utilizing radio resources from multiple RNS domains.

Purpose & Motivation

The DRNS concept was created alongside the DRNC in Release 99 to provide a formal architectural model for managing radio resources outside the serving domain during soft handover. It addresses the need to abstract and manage a set of network elements (an RNC and its Node Bs) as a single, controllable entity from the perspective of the serving controller. Before UMTS, GSM's BSC and its BTSs formed a similar subsystem, but inter-BSC handovers were hard and did not require the sustained, coordinated resource sharing that UMTS soft handover demands.

Defining the DRNS allows the 3GPP specifications to clearly delineate responsibilities and interfaces. It establishes that the SRNC has a peer-to-peer relationship with another complete administrative entity (the DRNS), not just with an individual RNC. This modeling is crucial for specifying procedures like radio link addition, where the SRNC requests a resource from another subsystem. It also simplifies network management and fault domain isolation. The DRNS concept encapsulates all the localized radio resource management functions, allowing the SRNS to focus on end-to-end connection management for the UE. This separation of concerns is key to achieving scalable and robust mobility in a large, multi-vendor UMTS network.