The fundamental communication technique used in Bluetooth Low Energy (BLE) is client-server architecture. This mode of communication has been used in the computing industry for over half a century and is proven to be very successful over the years. The term client-server is used in its critical definition:

• Client: A device that asks for the data from the server
• Server: A device that serves the data to the client

It is a distributed architecture that divides the responsibilities between a receiver and a provider, in other words, a client and a server. Technically, client and server reside on different machines but it is completely possible that single-machine hosts both. Bluetooth Low Energy works on the same concept of data exchange. There is a BLE GATT client which asks for the data from the BLE GATT server.

Keeping GATT in mind, we will now discuss the concept of Peripheral-Central communication in BLE. Peripheral is a device that advertises itself and let other devices (centrals) know about their existence. Central, upon hearing the notification initiates the connection request. These terms are defined solely on the basis of the connection request and advertisement and they are not related to the client and server roles of the BLE communication. Although there is no connection in these terminologies, peripherals mostly serve as servers and centrals as clients. These roles are easily exchangeable based on the type of application.

A non-connecting variant of Peripheral and Central are called Broadcaster and Observer, respectively. Just like central and peripheral, these terminologies are also defined by Bluetooth Low Energy Core Specifications and often used in the literature. Broadcaster advertises itself by sending advertisement messages while observer hears the advertisement and receives the scanned data. These are non-connecting roles that means that the broadcaster and observer will never connect to each other for further data exchange.

In order to put things in black and white, refer to the following diagram: