Obviously, one of the most important factors that your app needs to be able to understand is the distance of mobile devices from the beacon itself. We'll cover this in depth in Chapter 4, Ranging Beacons – Hunting for Treasure, when we'll build a treasure hunt app that relies on the range of the beacon. For now, it's important to know that there's one extra piece of information that is broadcasted by iBeacons.
iBeacons broadcast one extra byte of data known as measured power. Measured power is a value representing the received strength signal indication (RSSI) value at a distance of 1 meter. RSSI is measured in dBm and indicates the measurable signal strength of the beacon, which decreases with distance. This RSSI value varies across the vendor beacon implementations and so isn't very reliable to determine the distance. This is where measured power comes into play.
When iBeacon vendors are configuring their hardware iBeacons, it's important that they configure the measured power broadcast value accurately because the Core Location framework uses this value to determine the distance of the beacon and converts it into a distance that you, as a developer, can utilize.