External Base Station Subsystem Function (Interface ExternalBssFunction)

Description

The Interface ExternalBssFunction is defined within the 3GPP Generic Network Resources NRM as an Information Object Class (IOC) inheriting from GenericNetworkResourcesNRMDefs::ManagedFunction. It models a Base Station Subsystem (BSS) — the GSM/EDGE radio access network component comprising Base Transceiver Stations (BTS) and Base Station Controllers (BSC) — that is external to the managed network, typically belonging to another operator or PLMN. The 'External' prefix distinguishes it from internal managed functions that are under the direct control of the owning operator's OSS.

In the NRM containment hierarchy, ExternalBssFunction is a ManagedFunction, meaning it represents a functional entity that can be managed through the Itf-N interface. Unlike internal BSS functions, an ExternalBssFunction is not directly configurable by the managing OSS; instead, it serves as a representation of an external entity for the purpose of modelling relationships, such as neighbour relations, inter-RAT handover targets, or inter-PLMN roaming agreements. Attributes of ExternalBssFunction may include the PLMN identity, the BSS identifier, and supported capabilities.

The IOC is defined in 3GPP TS 28.656 (GSM/EDGE Radio Access Network NRM IRP) and referenced in TS 32.656 (Configuration Management IRP for GERAN). These specifications are part of the broader 3GPP management framework that standardises how network resources are modelled, configured, and monitored. The Generic Network Resources NRM provides the base classes, while technology-specific NRMs (GERAN, UTRAN, E-UTRAN, NR) extend them with technology-specific IOCs.

ExternalBssFunction plays a crucial role in multi-vendor and multi-operator environments. When an operator's network needs to hand over a call to a neighbouring operator's GSM network, or when coordination is required for interference management across PLMN boundaries, the ExternalBssFunction provides the management representation of that external BSS. This allows the OSS to configure and monitor inter-operator relationships without requiring direct management access to the external network.

The management of ExternalBssFunction instances is performed through standard IRP operations. The Basic CM IRP allows the OSS to create, read, update, and delete instances, while the Bulk CM IRP supports bulk configuration operations. The Notification IRP enables event-driven management, allowing the OSS to receive notifications when the state of an external BSS changes. Through these mechanisms, ExternalBssFunction supports coordinated management across administrative domains.

Purpose & Motivation

ExternalBssFunction was created to address the management challenges of multi-operator and multi-RAT environments. In early GSM deployments, each operator managed its own network independently, and inter-operator coordination was handled through bilateral agreements and manual configuration. As networks became more interconnected and roaming and inter-RAT handover became commonplace, a standardised way to model external network entities in the management plane became necessary.

The IOC solves the problem of representing network elements that are outside the operator's administrative control but still need to be referenced in management operations. For example, when configuring neighbour relations for inter-RAT handover from LTE to GSM, the E-UTRAN cell needs to know about the target GSM cell. If that GSM cell belongs to another operator, it cannot be managed directly, but it can be represented as an ExternalBssFunction, allowing the OSS to configure the relationship without needing management access to the external network.

Additionally, ExternalBssFunction supports regulatory and operational requirements for network sharing and national roaming. In shared network scenarios, multiple operators may share the same radio access network, and the management system must be able to distinguish between internal and external functions. By providing a standardised representation, ExternalBssFunction enables consistent management across different sharing arrangements and simplifies the integration of multi-vendor networks.