INTRODUCTION Public Safety and Disaster Recovery (PSDR) organizations rely extensively on Professional Mobile Radio (PMR) communication systems to conduct their daily tactical and emergency oper- ations. Presently, many of these communication systems are based on Terrestrial Trunked Radio (TETRA) [1, 2] or Project 25 [3, 4] specifica- tions and provide a mission-critical communica- tions environment as the one depicted in Fig. 1 (for illustration purposes, we depict a system based on TETRA technology). This environment is essentially a wide-area mobile radio network tailored to support the unique requirements of PSDR agencies, such as law enforcement, ambu- lance services, civil emergency management/dis- aster recovery, fire services, coast guard services, search and rescue services, government adminis- tration, and so forth. The TETRA Switching and Management Infrastructure (SwMI) is the core component that comprises the necessary networking, switch- ing, management, and service-provision elements of the system. It also provides digital narrowband radio services to a wide geographical area by means of a plurality of TETRA BSs, deployed in strategic locations according to the overall radio coverage, traffic, and availability requirements. The TETRA specifications define several types of air interface, including the most commonly used voice plus data (V+D) air interface [1], the packet data optimized (PDO) air interface [5], and the direct mode operation (DMO) air inter- face [6] (Fig. 1). The latter forms the means of enabling a fundamental mode of ad hoc net- working, in which the radio users communicate directly to each other on a peer-to-peer basis without the need of the TETRA infrastructure. This is particularly important in emergency situ- ations that arise in locations outside the V+D coverage area. The V+D air interface operates in trunking mode, that is, it allocates and releas- es the available radio resources dynamically and on demand basis. This way, the available radio spectrum can be efficiently shared across many different groups of users, or even across many different PSDR agencies. As shown in Fig. 1, the SwMI also provides the essential interface components for linking the PSDR command and control centers with the portable, vehicular, and airborne public safe- ty users on the go, and for interoperating with conventional analogue (legacy) PMR networks, telephone networks, public or private data net- works, and even with other TETRA SwMIs oper- ated and administered by other organizations. Several IP-based data services are typically used in practice today, the most common being the Automatic Vehicular Location (AVL) service and the remote database query. In terms of services, the TETRA specifica- tions developed by the European Telecommuni- cations Standards Institute (ETSI) define a vast range of voice, data, and supplementary ser- vices, the most notable of which include dis- patch services, group and private voice services, security services, telephone interconnect ser- vices, short data and status messaging, and IEEE Communications Magazine • January 2006 38 0163-6804/06/$20.00 © 2006 IEEE PUBLIC SAFETY Apostolis K. Salkintzis, Motorola ABSTRACT This article contributes to the evolution of public safety communication systems by specify- ing a novel solution for integrating WLAN and Terrestrial Trunked Radio (TETRA) networks. The specified solution allows TETRA terminals to interface to the TETRA Switching and Man- agement Infrastructure (SwMI) over a broad- band WLAN radio access network, instead of the conventional narrowband TETRA radio net- work. These terminals are fully interoperable with conventional TETRA terminals and can employ all TETRA services, including group calls, short data messaging, packet data, and so forth. In addition, however, such terminals can support a range of brand new capabilities enabled by the WLAN, such as broadband data services, true concurrent voice and data services, simultaneous reception of many group calls, reduced call setup and voice transmission delays, improved voice quality, and so forth. The speci- fied solution is solely based on IP multicast and Voice-over-IP (VoIP) technologies and thus fits ideally to the all-IP architecture being intro- duced by the MESA project for the next genera- tion of public safety and disaster relief communication systems. Evolving Public Safety Communication Systems by Integrating WLAN and TETRA Networks