Description
The Drift Base Station Subsystem (DBSS) is a fundamental architectural concept within the 3GPP UMTS Radio Access Network (UTRAN), defined in technical specification 25.423. It operates within the context of the Iur interface, which connects two Radio Network Controllers (RNCs). In this architecture, when a User Equipment (UE) is involved in an inter-RNC handover or is utilizing resources from an RNC different from its controlling RNC, the involved RNCs take on specific roles: the Serving RNC (SRNC) and the Drift RNC (DRNC). The DBSS is the collective term for the network elements (primarily the DRNC and the Node Bs it controls) that provide radio resources to the UE but are not the primary point of control for that UE's connection to the core network.
The DBSS works by establishing a logical connection between the SRNC and the DRNC over the Iur interface. The SRNC maintains the connection to the Core Network (CN) via the Iu interface and handles higher-layer protocols like Radio Resource Control (RRC). The DRNC, as part of the DBSS, provides additional radio resources (like cells) to the UE. User and control plane data are tunneled between the SRNC and the DRNC. The DRNC is responsible for the allocation and management of radio resources (like channelization codes) in its cells and performs lower-layer scheduling and combining/decombining for macro-diversity if the UE is in soft handover between Node Bs belonging to different RNCs.
Key components of the DBSS include the Drift RNC (DRNC) itself and the set of Node B base stations under its control that are being used by the UE. The DRNC contains the necessary functionality for Iur interface management, resource allocation for drifting UEs, and the splitting/combining of data streams for the radio interface. The role of the DBSS is to extend the service area and capacity available to a UE without requiring a handover of the core network connection point (the SRNC). This allows for more flexible resource utilization and efficient support for mobility, especially in dense or overlapping network deployments.
From a protocol perspective, the DBSS participates in specific Iur signaling procedures. The SRNC controls the connection via Radio Network Subsystem Application Part (RNSAP) signaling to the DRNC. The DRNC acts largely on commands from the SRNC regarding radio link setup, addition, and deletion for the UE. User data is transported between the RNCs using the Iur data transport bearers, which are typically based on ATM or IP, depending on the network implementation. The DBSS concept is integral to the UTRAN's distributed architecture, enabling features like inter-RNC soft handover and inter-RNC hard handover, which are essential for seamless mobility and service continuity.
Purpose & Motivation
The DBSS concept was created to address the limitations of a rigid, single-RNC controlled architecture in UMTS networks. In early cellular designs, a mobile terminal was typically served by a single base station controller. However, as networks grew denser and user mobility patterns became more complex, a mechanism was needed to allow a user to utilize radio resources from multiple, geographically separated controller domains without incurring the signaling overhead and potential service interruption of constantly relocating the core network anchor point (the MSC or SGSN connection). The DBSS architecture solves this by introducing a hierarchical control model.
It specifically solves the problem of efficient resource utilization and seamless mobility across administrative boundaries within the RAN. Without the DBSS/Drift RNC concept, any time a UE moved into a cell controlled by a different RNC, a full SRNC relocation procedure would be required, involving the core network. This is a complex procedure that can impact call quality. The DBSS model allows the UE to 'drift' into the coverage area of another RNC, using its resources while maintaining its original signaling connection (RRC connection) and core network path through the original Serving RNC. This minimizes core network signaling, reduces handover delay, and enables advanced features like inter-RNC macro-diversity combining.
Historically, this was a significant evolution from GSM's BSS architecture, where the Base Station Controller (BSC) had a more direct and exclusive relationship with its served MS. The 3GPP UTRAN design, with its separation of SRNC and DRNC roles, provided a more flexible and scalable framework for managing radio resources in a packet-oriented network supporting high-speed data and sophisticated handover types. The DBSS is thus a cornerstone of UTRAN's capability to support soft handover across Node Bs connected to different RNCs, a key feature for improving coverage and quality at cell boundaries.
Key Features
- Enables inter-RNC mobility without SRNC relocation
- Supports inter-RNC soft and hard handover procedures
- Allows resource sharing across RNC administrative boundaries
- Minimizes core network signaling for user mobility
- Facilitates macro-diversity combining from Node Bs under different RNCs
- Uses Iur interface for control (RNSAP) and user data tunneling between RNCs
Evolution Across Releases
Introduced as part of the foundational UTRAN architecture in the 3GPP standards. Defined the Drift RNC (DRNC) role and the Drift BSS concept for managing UEs that utilize resources from an RNC other than their Serving RNC. Established the Iur interface procedures and RNSAP signaling for radio link setup, addition, and deletion under DBSS control, enabling basic inter-RNC mobility and handover support.
Defining Specifications
| Specification | Title |
|---|---|
| TS 25.423 | 3GPP TS 25.423 |