There are different types of connections available for the BLE device. A BLE connected item may have up to 4 different functions:
The “Broadcaster” shall be used as a server. Thus, its purpose is to transfer data to a device on a regular basis, but it does not support any incoming connection.
The “Observer” : In a second step, the device may only monitor and read the data sent by a “broadcaster”. In such a case, the object is not able to send any connection to the server.
The “Central” usually consists of a smartphone or tablet. This device provides two different types of connection: either in advertising mode or in connected mode. It is leading the overall process as it triggers data transfer.
The “Peripheral” device allows connections and data transfer with the ¨Central¨ on a periodical basis. This system´s goal is to ensure universal data transmission by using the standard process, so that other devices also may read and understand the data.
Also have a look on below picture to have some more clarity about the above concept.