Android 5.1 Lollipop

Android 5.1 Lollipop is the second latest version of Android. It was first revealed on the Android One site, then officially announced on the Android Blog. Its predecessor is Android 5.0 Lollipop, and its successor is Android 6.0 Marshmallow. Android 5.1 latest release on April 21 2015 as android 5.1.1.

New Features
Android 5.1 includes a couple new features, including:
 * Native multi-SIM support
 * HD voice calling
 * Ability to join Wi-fi and Bluetooth networks from the quick settings menu
 * Nexus 6 and Nexus 9 receive device protection (a type of "kill switch")