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Your iPhone has several power-related modes, including the following:
Active mode— Your iPhone is turned on and fully functional.
Sleep mode— Your iPhone’s screen is turned off; however, apps continue running in the background and can automatically access the Internet to access data or information. If you leave your iPhone dormant for a predetermined amount of time, to conserve battery life, it automatically enters into Sleep mode. You can adjust this setting within the Settings app. When you wake up the iPhone, the Lock screen always appears.
Power Off— All functionality of your iPhone is turned off. No apps are running in the background, and the device cannot access the web automatically. Likewise, you cannot make or accept phone calls. All incoming calls are redirected to voicemail.
Airplane mode— Your iPhone is fully functional, except that it is not able to access the Internet using a 3G connection or make or receive calls. This also precludes it from accessing your email, allowing you to surf the Web, or sending/receiving text messages, for example. When you place it in Airplane mode, you can manually turn on Wi-Fi functionality to connect to the Internet via a Wi-Fi hotspot. Basically, when in Airplane mode, your iPhone is unable to transmit or receive anything wirelessly via the 3G network. Thus, when it’s in Airplane mode, it can still be used while aboard an airplane. Keep in mind, after putting your iPhone into Airplane mode, you can turn on Wi-Fi connectivity separately from within the Settings app.