The implementation and validation of a 5-GHz wireless LAN modem based on the HIPERLAN/2 standard is presented. In modern wireless communication systems, there is a demand for higher flexibility and more computational efficiency. Therefore the emphasis of this work is on the hardware–software structure of the developed modem and its processes, in order to offer a good balance of these requirements. In order to efficiently design and validate the behaviour of the modem, a behavioural model was developed in UML (Unified Modelling Language) as a part of the overall HIPERLAN/2 system's model. The processes of the modem were implemented in an instruction-set processor and custom hardware, combining the advantages of both software and hardware implementations. The communication between the software and hardware parts of the modem is achieved through a specialised programmable interface unit. The UML-based model of the actual HIPERLAN/2 system is used in order to validate the modem's behaviour, using scenarios from in-field usage (such as transfer of data using FTP or HTTP). Furthermore, the validation of the algorithms implemented within the modem was based on this system model, and performed through the use of a custom-validation framework. This framework produces patterns for the validation of the modem's algorithms, at three different development phases (algorithmic, HDL, FPGA-based prototyping), derived from the simulation of the system model in a consistent and automatic way. Implementation figures and co-simulation results for the developed wireless LAN modem are also given.