The Interworking Function: ensuring critical networks work together

Operators of land mobile radio (LMR) systems can derive substantial benefits from the early adoption of Mission Critical Services (MCX) built on open 3GPP standards. Interworking LMR networks with 4G/5G-based MCX solutions requires leveraging the standard-defined Interworking Function (IWF). The aim of this integration is to strengthen communication capabilities, achieve seamless interoperability, and enable a gradual adoption of broadband technologies.

Depending on the specific architecture of the LMR system, interworking may be implemented natively through integrated software components in the LMR and MCX systems or via an external IWF. However, several key challenges must be addressed to ensure successful integration between LMR and MCX systems, including service mapping, system performance, scalability, user training and acceptance, operational continuity, and security concerns.

Land mobile radio networks

LMR systems are designed for two-way voice communication, primarily using handheld devices, vehicle-mounted units and dispatch consoles. These networks are extensively utilised in sectors such as public safety, transportation, utilities and industrial operations.

Their core functionality centres on group communications facilitated by push-to-talk (PTT) radios, which enable coordinated efforts among large teams. Widely recognised mission-critical LMR standards include TETRA and APCO P25. Other systems, such as Tetrapol and Digital Mobile Radio (DMR), follow more proprietary protocols, despite being based on published standards. Although these frameworks define the air interface, interoperability with external systems is typically implemented through vendor-specific gateways.

Mission Critical Services

MCX refers to mission-critical push-to-talk, data and video services delivered over 4G and 5G broadband mobile networks. It is defined by 3GPP, the standardisation body responsible for guiding the evolution and ensuring multi-vendor interoperability of cellular network standards, including 3G, 4G and 5G, and which is already working on 6G. MCX services leverage the broadband capabilities and open interfaces of 3GPP networks to enable public safety voice, data and video services. MCX integrates network and application layers to provide features such as priority access, group communications via multicast, and end-to-end interoperability across devices, servers and gateways.

MCX services can be deployed on public, private or hybrid 4G/5G infrastructures. Public deployments managed by mobile network operators (MNOs) support multiple organisations under defined service level agreements (SLAs), addressing both business- and mission-critical needs; for example, AT&T operates FirstNet in the United States. Private deployments can serve localised areas such as airports, underground railways and industrial sites, and several public safety agencies are targeting the deployment of a nationwide private network for their MCX services, which is complemented by public MNO networks to provide additional capacity and redundancy; one such notable deployment is SafeNet in South Korea.

The Interworking Function

The IWF is a key component of the 3GPP MCX architecture that enables interoperability between LMR systems and MCX services. It provides a standardised approach for facilitating seamless communication across both domains, ensuring compatibility in voice, data and signalling services. To achieve interoperability, an IWF is required, either as an embedded software component or as a standalone function/entity. This integration may be implemented by the LMR equipment manufacturer, a third-party developer or the network operator.

The objective of standardising interworking for MCX is to achieve seamless interoperability between broadband MCX platforms — defined by 3GPP for LTE and 5G — and existing narrowband LMR networks. This standardisation is essential for enabling a smooth and reliable transition from narrowband to broadband solutions. It allows public safety agencies, emergency responders and other mission-critical users to maintain secure, dependable and efficient communication across mixed technological environments.

By establishing a consistent and open framework — exemplified by the IWF as specified in 3GPP standards — the objective is to ensure continuity of key services such as voice and data, while optimising cost-efficiency and aligning with the evolving requirements of mission-critical communication.

Possible architectures

The integration of LMR and MCX networks can be realised through several architectural models (Figure 1). These configurations depict LMR user devices (eg, handheld radios, vehicle-mounted units) on the left and MCX user devices (eg, smartphones, tablets, ruggedised terminals) on the right.

Figure 1. For a larger image, click here.

• Native IWF integration: The first approach incorporates IWF support directly within the LMR network infrastructure. This method removes the need for an external IWF. However, native IWF integration is generally offered by the LMR manufacturer as a licensed capability and may require additional dedicated hardware.
• ISI/ISSI interface-based architecture: The second method leverages standardised interfaces on the LMR side to enable interworking. Interfaces such as the TETRA Inter-System Interface (ISI) and the APCO P25 Inter-RF Subsystem Interface (ISSI) are particularly suitable. These interfaces facilitate key communication services such as PTT group calls, private calls, and data messaging, and connect to the MCX domain via an IWF.
• Proprietary dispatcher interface (API-based) architecture: The third approach utilises a vendor-specific dispatcher interface or API. Many LMR vendors provide APIs that allow third-party developers to create customised dispatch solutions. These applications can emulate group calls, private communications and data messaging, thereby enabling interworking through an IWF.
   

The interworking capabilities supported in each architecture depend heavily on the underlying interface and the functionality of the IWF. Generally, native IWF integration provides the most comprehensive feature set, followed by standardised interfaces like ISI and, lastly, proprietary dispatcher interfaces.

Market segments and applications

MCX are primarily targeted at the public safety and emergency response sectors, but are also increasingly relevant across various industries that demand secure, reliable and prioritised communications. Developed on open 3GPP standards for LTE and 5G, MCX services are specifically designed to meet the rigorous demands of mission-critical operations.

The primary market includes agencies and organisations responsible for maintaining public safety, responding to emergencies and managing crisis situations. MCX solutions are deployed to replace or augment traditional LMR systems such as TETRA and P25 by offering enhanced capabilities including high-speed data, live video streaming and real-time situational awareness.

Augment or transition

The IWF plays a pivotal role in enabling hybrid communication environments that combine LMR systems with broadband solutions. By bridging these technologies, organisations can either augment existing LMR infrastructure or transition gradually towards fully broadband-based MCX. The various use cases outlined throughout this article clearly demonstrate the value of interworking during all phases of integration and migration. IWF is positioned to be a key enabler in the broader adoption of broadband communications across public safety, industrial sectors and critical infrastructure domains, where hybrid LMR–MCX solutions will serve as both a complement and a stepping stone to full migration.

Understanding the benefits and challenges of IWF-based interworking is essential when selecting the appropriate MCX and IWF architecture. While IWF introduces powerful capabilities, several key considerations must be addressed, including bandwidth constraints, latency and reliability, and cost and implementation complexity.

Organisations are encouraged to prioritise open, standards-based approaches — such as those defined by 3GPP — over proprietary solutions, as these enhance interoperability and support broader ecosystem collaboration. Open standards help ensure compatibility across devices and vendors, which is particularly valuable for critical communications. They also provide a foundation for robust security practices and can contribute to more predictable and potentially optimised total cost of ownership, depending on deployment context and scale.

The IWF and its implementations are explored in detail in the white paper from TCCA’s IWF Working Group, available at https://tcca.info/about-tcca/tcca-resources/whitepapers/.

*Harald Ludwig is Chair of the TCCA Technical Forum.

^Sylvain Allard is Senior Director Connectivity at Capgemini.

Image credit: iStock.com/Chalabala