From Core to Edge: How the RNC Bridges Radio and Transport Layers

In the increasingly flattened architecture of LTE and 5G, where base stations incorporate control functions, it is easy to forget that in the 3G era, network control was distinct from the physical radio layer. At the heart of this separation sits the Radio Network Controller (RNC)—a logical and physical control node that links the core network, transport network, and the radio access layer into one coherent system.

For operators still managing legacy 3G networks or integrating multi-technology environments, understanding how the RNC interconnects these domains is not just academic—it has real implications for operations, resilience, and long-term asset strategy.

The RNC as the Control Hub of the UTRAN

In UMTS networks, the radio access component is known as the UTRAN—the UMTS Terrestrial Radio Access Network. Within this, the RNC functions as the:

• Policy decision-maker for radio resource allocation

• Mobility manager across multiple base stations (Node Bs)

• Coordinator between the UTRAN and the 3G core network (CN)

This role is executed through three key interfaces that span the core-to-radio continuum:

1. Iub – connecting the RNC to Node Bs

2. Iur – linking RNCs to each other

3. Iu-CS and Iu-PS – connecting the RNC to the core for circuit-switched (voice) and packet-switched (data) services

Understanding these interfaces is key to grasping the full architectural role of the RNC.

The Iub Interface: Managing the Radio Layer

The Iub interface connects the RNC to its subordinate Node Bs, enabling the RNC to manage all radio layer functions remotely. This includes:

• Power control

• Channel allocation

• Traffic mapping to physical and logical channels

• Measurement collection from Node B hardware

While the physical transmission medium is typically over ATM, IP, or even dark fibre in some high-capacity urban deployments, the control intelligence lies firmly in the RNC. Unlike in LTE, where the eNodeB integrates this functionality locally, UMTS preserves the hierarchical model, with the RNC acting as the orchestration layer.

The Iub interface also plays a role in QoS enforcement, ensuring that the air interface is dynamically adjusted to meet service requirements and real-time traffic conditions.

The Iur Interface: Facilitating Seamless Mobility

One of the distinguishing features of UMTS is the soft handover, where a mobile device can be connected to multiple Node Bs simultaneously during a transition between cells. This is orchestrated via the Iur interface, which connects peer RNCs.

This interface enables:

• Inter-RNC handover without involving the core

• Load sharing in congested network scenarios

• Measurement exchange to support mobility decision-making

The Iur interface is often overlooked in high-level discussions but is fundamental to the seamless user experience in 3G networks. Operators that bypass or disable Iur due to vendor compatibility or licensing constraints typically see degraded mobility performance and higher handover failure rates.

The Iu Interface: Linking to the Core Network

The RNC’s upward connection is through the Iu interface, which exists in two forms:

• Iu-CS (Circuit Switched) – connects to the Mobile Switching Centre (MSC) for voice traffic

• Iu-PS (Packet Switched) – connects to the Serving GPRS Support Node (SGSN) for data traffic

This dual architecture reflects the transitional nature of 3G networks, where both circuit and packet-switched services coexist. The RNC plays a key role in:

• Session setup and teardown

• QoS mapping between radio and transport layers

• User context management during handover between technologies

Importantly, these interfaces also carry signalling protocols like RANAP (Radio Access Network Application Part), which governs communication between the RNC and core network elements.

Architectural Implications for Operators

The positioning of the RNC between the edge (Node B) and core (MSC/SGSN) means that:

• Latency is a key design factor – especially in high-mobility environments

• Backhaul resilience is critical – since Node Bs are functionally dependent on the RNC

• Geographic location of RNCs influences service performance – particularly in national or transnational networks

Operators that centralised RNC deployments in one or two national locations may face challenges with latency-sensitive applications. Others, who implemented regional RNC nodes, benefit from reduced delay and local resilience—but at the cost of higher capex and operational complexity.

Managing the Transport Network: IP, ATM, and Hybrid Models

Although modern networks are overwhelmingly IP-based, many live RNC installations were originally designed for ATM (Asynchronous Transfer Mode). This impacts:

• Transport protocol choices

• QoS mechanisms

• Interface multiplexing strategies

Some operators have migrated their RNCs onto IP/MPLS transport, often using IP RAN adaptation layers. However, this hybridisation creates complexity in maintenance, monitoring, and integration—particularly when interfacing with IP-native LTE and 5G equipment.

Virtual RNCs and Cloud-RAN Integration: A Transitional Approach

As operators virtualise their network functions, the RNC is increasingly implemented as a vRNC—a virtual instance running on general-purpose servers within a data centre or edge cloud.

Benefits include:

• Reduced hardware footprint

• Flexible scaling based on traffic patterns

• Centralised software lifecycle management

Yet, deploying a virtual RNC still requires robust edge transport and reliable synchronisation between radio and core elements. Operators pursuing Cloud-RAN strategies must decide whether to retire, virtualise, or rehost existing RNC functions within their new architecture.

Strategic Questions for Leadership Teams

For operators maintaining RNC-based infrastructure or planning transitions, key board-level questions include:

• Are our RNC nodes optimally located for latency and resilience?

• What is our exposure to vendor-specific transport protocols (e.g. ATM)?

• Can we migrate RNC functions to virtual platforms without disrupting core services?

• How much value is still being extracted from legacy RNC-capable spectrum?

• What is our cost-benefit timeline for full retirement versus hybrid operation?

These are not simply technical queries—they influence investment decisions, spectrum utilisation strategies, and customer experience outcomes.

Conclusion: Bridging the Divide

The RNC was engineered to perform a delicate balancing act—between legacy circuit-switched and emerging packet-switched services, between geographically distributed radio base stations and centralised core elements, and between control logic and physical infrastructure.

Its bridging function remains essential wherever 3G services are still required. As telcos evolve their networks, recognising the architectural, operational, and strategic role of the RNC will remain vital to managing risk, optimising investment, and maintaining service continuity.