The physical layer is the infrastructure required to gather location data.
This is composed of smart phones, Wi-Fi router, 3G data network, and optical network.
The smart phone works as an RTLS agent with built-in GPS and WiFi modules and has the customized app installed on it.
Its function is to receive signals, including GPS signals and WiFi signals, and to upload these digitalized data to the server at
fixed time intervals and to calculate current locations using the trilateration algorithm [38].