Roles

• At Link layer level there is the master - slave relationship
• At GAP layer level there is the central - peripheral relationship
• At GATT layer level there is the client - server relationship

Then, you have the roles of a GATT Server and a GATT Client. Normally, the Server is the device that contains data, that the Client can read.
However, there is no connection between these roles. Even though it is most common for a Peripheral to be a Server and a Central to be a Client, it is perfectly possible to have a Peripheral that is only a Client, or a Central that is both a Server and a Client.

So, a wareable or sensor is a peripheral (GAP) and a server (GATT), a phone or tablet is a central (GAP) and a client (GATT).

Broadcasting

When using connectionless broadcasting, a BLE device sends data out to any scanning device or receiver that is within acceptable listening range. Essentially, this mechanism allows a BLE device to send data out one-way to anyone or anything that is capable of picking up the transmission.
Broadcasting defines two separate roles:

• Broadcaster: Sends non-connectable advertising packets periodically to anyone willing to receive them.
• Observer: Repeatedly scans the pre-set frequencies to receive any non-connectable advertising packets.

Broadcasting is the only way for a device to transmit data to more than one peer at a time. These broadcasted data are sent out by using the advertising features of BLE. It is possible to advertise sensor data using this communication methode, so to read e.g. a temperature sensor it is not necessary to connect to that sensor.

Connecting

For bi-directional data transmission in BLE a connection needs to be present. A connection in BLE is nothing more than an established, periodical exchange of data at certain specific points in time (connection events) between the two BLE peers involved in it. Typically, the data are exchanged only between the two BLE connection peers, and no other device is involved. Connections define two separate roles:

• Central (master): Repeatedly scans the pre-set BLE frequencies for connectable advertising packets and, when suitable, initiates a connection. Once the connection is established, the central manages the timing and initiates the periodical data exchanges.
• Peripheral (slave): A device that sends connectable advertising packets periodically and accepts incoming connections. Once in an active connection, the peripheral follows the central’s timing and exchanges data regularly with it.

Connections provide the ability to organize the data with much finer-grained control over each field or property through the use of additional protocol layers and, more specifically, the Generic Attribute Profile (GATT). Data are organized around units called services and characteristics.

Generic Profiles

Generic profiles are defined by the Bluetooth specification and two of them are fundamental as they ensure the interoperability between BLE devices from different vendors:

• Generic Access Profile (GAP): Specifies the usage model of the lower-level radio protocols to define roles, procedures, and modes that allow devices to broadcast data, discover devices, establish connections, manage connections, and negotiate security levels; GAP is essentially, the uppermost control layer of BLE. This profile is mandatory for all BLE devices, and all must comply with it.
• Generic Attribute Profile (GATT): Addresses data exchanges in BLE and specifies the basic data model and procedures to allow devices to discover, read, write, and push data elements between them. It is basically, the topmost data layer of BLE.

GAP and GATT are so fundamental to BLE that they are often used as the base for the provision of application programming interfaces (APIs) that act as the entry point for the application to interact with the protocol stack.

GATT side of the stack

Attribute Protocol (ATT) defines the protocol of transferring the attribute data. This includes GATT related functionality such as Write Request, Write Response, Notification, Read Response. In short, GATT defines and creates appropriate attributes for a given application and ATT creates outgoing and parses incoming packets that defines the action in the GATT layer.

Generic Attribute Profile (GATT) is the layer developers will often interface with the most and accustomed to. GATT defines the protocol of transferring data between two Bluetooth Low Energy devices. Application data is uniquely identified through a small entity known as an Attribute. Group of Attributes form Characteristics which adds additional properties such as permission and rules of interaction for a unique set of data. A group of Characteristics form a Large entity known as Service which adds a larger blueprint for a given feature or functionality. For Example, Battery Service includes a Battery Level Characteristic which includes the battery level of a given device. [1]

The Generic Attribute Profile (GATT)

In other words, it is a set of rules describing how to bundle, present and transfer data using BLE. Read the Bluetooth Core Specification v4.2 for more information [8]. It is only 2772 pages.

Use-Case-Specific Profiles

Use-case-specific profiles are usually limited to GATT-based profiles. Largely all of these profiles use the procedures and operating models of the GATT profile as a base building block for all further extensions. However, in version 4.1 of the specification, Logical Link Control and Adaptation Protocol (L2CAP) connection-oriented channels have been introduced, which indicates that GATT-less profiles are also possible.

On top of the GATT profile we have pre-defined GATT-based profiles.
GATT-based profiles:
Adopted Bluetooth Profiles, Services, Protocols and Transports [2]

• BAS = Battery Service
• HID = HID Service
• FMP = Find Me Profile
• HTP = Health Thermometer Profile
• UDS = User Data Service

On-air operations and properties

Most on-air operations happen by using the handle, since this uniquely identifies each attribute.
Use of the characteristic is dependent on its properties. [3]

Characteristic properties include:
- Write
- Write without response
- Read
- Notify
- Indicate

More properties are defined in the Bluetooth specification, but these are the most commonly used.

• Write and Write without response

Write and Write without response allow the GATT Client to write a value to a characteristic in a GATT Server.
The difference between them is that Write without response happens without any application level acknowledgment or response.

• Read

The Read property makes it possible for the GATT Client to read the value of a characteristic in a GATT Server.

• Notify and Indicate

Notify and Indicate allow a GATT Server to make the GATT Client aware of changes to a characteristic. The difference between Notify and Indicate is that Indicate has application level acknowledgment, while Notify does not.

• Notification don’t need acknowledged, so they are faster. Hence, server does not know if the message reach to the client. [9]
• Indication need acknowledged to communicated. The client sent a confirmation message back to the server, this way server knows that message reached the client. One interesting thing defined by the ATT protocol is that a Server can’t send two consecutive indications if the confirmation was not received. In other words, you have to wait for the confirmation of each indication in order to send the next indication.

GAP side of the stack

The Generic Access Profile (GAP) layer is responsible for the overall connection functionality
GAP specifies four roles that a device can adopt in a BLE network:

• Broadcaster: The device is advertising with specific data, letting any initiating devices know for example that it is a connectable device. This advertisement contains the device address and optionally additional data such as the device name.
• Observer: The scanning device, upon receiving the advertisement, sends a “scan request” to the advertiser. The advertiser responds with a “scan response”. This is the process of device discovery, after which the scanning device is aware of the presence of the advertising device, and knows that it is possible to establish a connection with it.
• Central: when initiating a connection, the central must specify a peer device address to connect to. If an Advertisement is received matching the peer device’s address, the central device will then send out a request to establish a connection (link) with the advertising device having the particular connection parameters.
• Peripheral: once a connection is established, the device will function as a slave if it was the advertiser and as master if it was the initiator.

Footnotes