A modern mobile device can incorporate functions of multiple conventional electronic devices, including a personal computer, a cellular phone, a radio transceiver, and/or a wireless (e.g., WiFi or GPS) transceiver. For example, a modern mobile phone or personal data assistant (PDA) can provide telephony services, as well as hosting user applications for organizing contacts, playing music, providing email communications, browsing the Internet, and so on. In some mobile devices, a positioning system integrated with, or coupled to, the mobile device also provides opportunities for applications to provide location-based services (e.g., map services, location-based search refinements).
A mobile device that provides both telephony services and application services typically includes a baseband subsystem and an application subsystem. The baseband subsystem includes one or more baseband processors and supports a baseband operating system that operates in a constant active mode. The baseband subsystem can generate real-time responses to external events, and is often reserved for the telephony functions of the mobile device. The application subsystem includes one or more application processors and an application operating system. User applications are typically installed and executed within the application subsystem of the mobile device. The application subsystem can switch between an active mode and a dormant mode. In general, the application subsystem consumes more power than the baseband subsystem when in its active mode. Switching the application subsystem into the dormant mode after an extended idle period can help conserve battery power of the mobile device.