15 December 2025

Michel Duits, Norwegian Directorate for Civil Protection (DSB) and Cochair of TCCA’s IWF Working Group; and Sylvain Allard, Senior Director, Connectivity, Capgemini, and lead author of the IWF white paper

As the critical communications sector increasingly adopts broadband networks to deliver critical services, the Interworking Function (IWF) emerges as a necessary driver for a smooth transition from narrowband to broadband systems.

The IWF is a key component of the 3GPP Mission Critical Services (MCX) architecture that enables interoperability between Land Mobile Radio (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.

It is not a straightforward process - successfully integrating LMR and MCX networks involves overcoming several critical challenges. To ensure a smooth and effective deployment, the following areas must be proactively addressed:

Service mapping – LMR systems are frequently customised with specific features and integrations tailored to user requirements. To maintain continuity, these customised services may need to be replicated within the MCX environment.

Performance and scalability – The IWF must be designed to avoid becoming a bottleneck between the LMR and MCX networks. This requires careful planning of the supporting network infrastructure, computing environment, and IWF software architecture to eliminate single points of failure and performance limitations. Furthermore, the solution should be scalable to support future expansion and the incorporation of new services. It should also be noted that the IWF must fulfil equivalent LMR requirements to ensure that the interworking of both technologies delivers a user experience comparable to that of mission-critical LMR networks.

User training and change management – Transitioning to a new communication system can often face resistance. It is vital to ensure that MCX solutions offer the same reliability for essential LMR functions. Clear communication of the advantages of MCX, coupled with comprehensive user training and dedicated support during the transition, will be crucial for driving adoption and user confidence.

Maintaining operational continuity – To minimise disruption during the migration process, a phased rollout strategy should be developed. Gradual implementation of MCX services ensures that mission-critical communications remain uninterrupted while users adjust to the new system.

Security considerations – Ensuring robust security is paramount in any push-to-talk environment, especially when interworking between LMR and MCX networks. Trust and protection must be established through a combination of standards-based security protocols, operational safeguards, and cyber defence measures. The IWF interfaces provide mechanisms for secure user and group authentication, as well as protection of signalling and media traffic. Additionally, physical and procedural safeguards must be in place to protect the IWF from becoming a vulnerability or attack vector.

Redundancy & resilience – Redundancy and resilience are proven, intrinsic features of LMR that have been built and tested over time, which need to be taken into account when adding the IWF solution.

Control – Public safety agencies often require a high level of authority over their systems – not only in terms of coverage and capacity, but also in determining who can access the system and who receives priority.

The IWF adheres to internationally recognised 3GPP and ETSI standards, providing a secure, scalable, and vendor-neutral interoperability framework, allowing the selection and deployment of best-of-class solutions, and therefore avoiding dependence on proprietary interworking solutions from the incumbent LMR vendor. Designed to support large-scale deployments, the IWF is suitable for nationwide public safety networks and expansive industrial operations. A growing number of LMR system vendors are adopting the 3GPP-defined IWF standards, promoting multivendor interoperability.

The adoption of MCX is steadily increasing among LMR equipment manufacturers, mobile network providers, and software developers. To support this evolution, ETSI organises a series of MCX Plugtests™ events to evaluate interoperability across multi-vendor environments. These testing campaigns focus on verifying MCX server-client compatibility, integration of MCX Application Servers with 4G/5GCore networks for prioritised and multicast communications, and interworking with LMR systems via the IWF interface.

The 2025 edition of ETSI’s MCX Plugtests confirmed that at least eight MCX vendors have successfully implemented the IWF interface, enabling essential interworking features such as group affiliation, group calls, and private calls. Future ETSI MCX Plugtests are expected to expand these evaluations to include emergency calling and Short Data Services (SDS). In parallel, the Global Certification Forum (GCF) in partnership with TCCA has launched a certification programme for 3GPP-compliant MCX solutions. Plans are underway to extend this initiative to cover certification of server-side MCX components, including IWF functionality, to ensure robust end-to-end interoperability across the ecosystem.

The integration of LMR networks with broadband MCX services via the IWF standard marks a major advancement in mission-critical communications. This convergence harnesses the reliability of LMR with the expanded capabilities of 4G and 5G — enabling seamless voice, real-time data transfer, high-resolution video, and enhanced coverage.

The IWF is explored in detail in a white paper from TCCA’s IWF Working Group – the white paper can be found here https://tcca.info/about-tcca/tcca-resources/whitepapers/