This document discusses using Bluetooth Low Energy (BLE) beacons to sense proximity between phones and beacons. It notes that BLE beacons were introduced in Bluetooth 4.0 and are supported on Android 4.3 and iOS 7. It provides an example of how beacon signal strength can be used to determine if Phone A is in close, medium, or far proximity to a beacon compared to Phone B. The document promotes a nervousnet app for navigation at the 31c3 conference and provides a link for more information.