Serving Radio Network Subsystem

Description

The Serving Radio Network Subsystem (SRNS) is a logical grouping within the UMTS Terrestrial Radio Access Network (UTRAN) that encompasses all the radio network entities responsible for the connection of a specific User Equipment (UE) at a given time. Its core component is the Serving Radio Network Controller (SRNC), which holds the control plane connection (RRC) and manages the radio resources for the UE. The SRNS also includes the one or more Node Bs (base stations) that are currently involved in the radio communication with the UE, such as those in the active set for a soft handover.

Functionally, the SRNS is the UTRAN side of the Radio Access Bearer (RAB), which is the service provided by the CN to the UE for transporting user data. The SRNS is responsible for establishing, maintaining, and releasing these RABs as requested by the Core Network. It maps the QoS parameters of a RAB to the appropriate transport and radio resources across the Iub and Uu interfaces. The SRNS manages the mobility of the UE within its coverage area, executing handovers and updating the active set of cells.

A key architectural aspect is the SRNS relocation procedure. When a UE moves such that another RNC becomes more appropriate to serve as the SRNC (e.g., for efficiency or because the Iur interface is not available), the SRNS role is transferred from the old SRNC to the new RNC. This procedure involves transferring the entire UE context, including security keys and radio bearer configurations, to the new RNC, which then becomes the new SRNC and establishes a direct Iu connection to the Core Network. The concept of the SRNS, therefore, defines a clear 'anchor point' in UTRAN for each UE, simplifying core network routing and mobility management.

Purpose & Motivation

The SRNS concept was introduced to provide a clear and manageable abstraction for the radio access network's role in serving a mobile user. In the complex, decentralized architecture of UTRAN with potentially multiple RNCs and widespread soft handover, it was necessary to define a single, responsible subsystem for each UE from the perspective of the Core Network. This solved the problem of the Core Network needing to communicate with a potentially changing set of RNCs as the user moved; instead, it communicates only with the current SRNS (via the SRNC).

It addressed the limitations of earlier cellular systems where the control entity (like the BSC in GSM) was statically tied to a set of base stations. The SRNS is a dynamic assignment that can follow the user, enabled by the Iur interface and SRNS relocation procedures. This was essential for supporting seamless mobility and service continuity for packet-switched and circuit-switched services in 3G, especially for real-time applications.

Furthermore, the SRNS model cleanly separates the concerns of radio resource control (handled within the SRNS) from core network routing and service control. It provides a stable endpoint for the Iu interface, allowing the core network to treat the radio access as a service-providing subsystem, which later facilitated the evolution towards flatter architectures in LTE, where many SRNC functions were distributed to the base stations (eNBs).