According to Wojciechowski and Xiong, a context model consists of three layers: infrastructure layer, service adaption layer, and user interface layer. The infrastructure layer includes context sensors’ description. The context systems work as subsystems of AAL, providing methods and operating the sensors in the infrastructure layer. The user interface layer corresponds to specific services for specific users. The three-layer context model also works for mobile devices. The major difference is in the infrastructure layer, as sensors in the AAL are independent devices, such as water and gas leakage detectors, while in mobile phones the sensors are combined in a single device, such as iPhone.