Ambient assisted living systems are based on sensors and actuators, with a diversity of network protocols and vendors. This commonly leads to the introduction of gateways or middlewares into the technical infrastructure in order to address interoperability issues. The xAAL framework presented in this paper aims to provide interoperability and to redesign such "gateways" into well-defined functional entities communicating with each other via a lightweight message bus over IP. Each entity may have multiple instances, may be shared between several boxes, and may be physically located in any box. Thanks to the distributed architecture of the system, each home automation vendor may peacefully provide its own xAAL box without revealing details of its technology. Also, several applications may be plugged together on the xAAL bus without getting bored with underlying heterogeneity. Moreover, the management of the dynamicity allows sensors or applications to enter and leave the system at any time.