BLE GATT
An important concept to understand with GATT is the server/client relationship.
The peripheral is known as the GATT Server, which holds the ATT (attribute) lookup data, service and characteristic definitions, and the GATT Client (the phone/tablet), which sends requests to this server.
All transactions are started by the master device, the GATT Client, which receives response from the slave device, the GATT Server.
###Services and Characteristics GATT transactions in BLE are based on high-level, nested objects called Profiles, Services and Characteristics, which can be seen in the illustration below:
####Profiles A Profile doesn't actually exist on the BLE peripheral itself, it's simple a pre-defined collection of Services that has been compiled by either the Bluetooth SIG or by the peripheral designers.
####Services Services are used to break data up into logic entities, and contain specific chunks of data called characteristics. A service can have one or more characteristics, and each service distinguishes itself from other services by means of a unique numeric ID called a UUID.
####Characteristics The lowest level concept in GATT transactions is the Characteristic, which encapsulates a single data point (though it may contain an array of related data, such as X/Y/Z values from a 3-axis accelerometer, etc.).