Controlling Base Station Sub-system

Description

The Controlling Base Station Sub-system (CBSS) is a fundamental logical entity defined within the 3GPP specifications for the GSM/EDGE Radio Access Network (GERAN) and its interaction with the UMTS Terrestrial Radio Access Network (UTRAN). It operates within the framework of the Iur-g interface, which is the inter-BSS interface standardized to enable direct communication and resource coordination between Base Station Sub-systems. Architecturally, the CBSS is not a separate physical node but a designated functional role assumed by one BSS in a relationship with one or more other BSSs, which are termed Controlled BSSs. This hierarchical control relationship is established to manage procedures that require coordination, such as handovers and certain radio resource management tasks.

In operation, the CBSS acts as the controlling point for specific UE connections that involve resources spanning multiple BSSs. When a User Equipment (UE) is in a scenario requiring resources from or mobility to a Controlled BSS, the CBSS retains control of the connection's core signaling (like the Radio Resource Control connection). It manages the allocation of radio resources in the Controlled BSS by sending control messages over the Iur-g interface. This includes commanding the setup, modification, and release of radio bearers in the Controlled BSS's cells. The CBSS also handles the mobility management for the UE within the domain of the Controlled BSSs, making handover decisions and executing the necessary signaling to transfer the UE's context and traffic.

The key components involved in the CBSS functionality are the BSC (Base Station Controller) software functions that implement the Iur-g interface protocol stack, as defined in 3GPP TS 25.423 and TS 43.130. These protocols carry the control plane messages for Radio Network Subsystem Application Part (RNSAP) adapted for the GERAN context. The CBSS's role is critical for features like Direct Transfer handovers, where a call is handed over directly between two BSSs without involving the core network (MSC) for the radio path switch, thereby reducing handover latency and core network signaling load. It enables more efficient use of radio resources and improved user experience by allowing faster, network-controlled handovers between different BSS coverage areas.

Purpose & Motivation

The CBSS concept was introduced to address the limitations of earlier GSM architectures where handovers and inter-BSS coordination were heavily reliant on the core network Mobile Switching Centre (MSC). In pre-Rel-8 architectures, an inter-BSS handover required the MSC to be the central anchor, receiving measurement reports, making handover decisions, and establishing a new circuit-switched path to the target BSS. This process, known as a basic handover, incurred significant latency and signaling overhead on the core network, which could impact service quality, especially for delay-sensitive services like voice.

The creation of the Iur-g interface and the CBSS role, standardized from 3GPP Release 8 onwards, was motivated by the need to offload this control functionality from the core network to the RAN. By allowing BSSs to communicate directly and enabling one BSS (the CBSS) to control resources in another, handovers could be executed faster and more efficiently. This approach, often called Direct Transfer or Network Controlled handover, reduces handover interruption time and minimizes MSC involvement. It was a step towards a flatter, more intelligent RAN architecture, improving overall network performance and scalability as data traffic and mobility demands increased. It also facilitated better integration and mobility management between GERAN and UTRAN networks.