Attribute and Data Hierarchy

The GATT establishes a hierarchy to organize attributes. They are organized as a GATT Server Profile and are grouped into:

• Services, containing:
  • Characteristics, containing:
    • Attributes (Declaration, Value, Descriptor (Optional))

Service

A collection of data and associated behaviors to accomplish a particular function is called the Service Definition (i.e. a collection of characteristic attributes). Primary services are discovered by a GATT Client via a "GATT Primary Service Discovery Procedure".

Services are classified as Public or Private

• Public
  • Defined by the Bluetooth SIG (16-bit UUID)
• Private
  • Vendor-defined (128-bit UUID)

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Characteristics

As depicted above, characteristics are essentially containers for user data. They contain a minimum of two attributes:

• Characteristic Declaration Attribute
  • Metadata for the Value Attribute
• Characteristic Value Attribute
  • Contains the data value itself

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A Characteristic can optionally contain Descriptor Attributes, described below.

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Characteristic Descriptors

These are special attributes used to further expand the metadata contained in the Declaration Attribute. Common Descriptor Attributes defined by GATT include:

• Extended Properties
  • Contains additional declaration property bits
• Characteristic User Description
  • Contains a user-readable description for the characteristic in which it is placed
• Client Characteristic Configuration Descriptor (CCCD)
  • A switch, enabling/disabling server-initiated updates

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Client Characteristic Configuration Descriptor (CCCD)

This is a very commonly used descriptor.

The client-server nature of GATT data communications has a drawback, in that the client must typically poll for updates of a characteristic on a server.

The CCCD provides a mechanism to enable server-initiated updates where the server can asynchronously push updated characteristic values to the client without the client having to poll for it.

There are two types of server-initiated updates:

• Notifications (client doesn't acknowledge receipt of the update)
• Indications (client does acknowledge receipt of the update; the server cannot send further indications until received)

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SIG-Approved GATT UUIDs

The Declaration, Value, and Descriptor Attribute Types have pre-defined UUIDS provided by the Bluetooth SIG. The following link provides useful resources (see GATT Heart Rate Service below).

• Standard GATT Services, Attribute Types, Characteristic Descriptors, Characteristic Types

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GATT Profile Example

The following diagram depicts the GATT Heart Rate Service.

Notes:

• Service contains eight attribute data structures.
• Service contains three characteristics.
• "Heart Rate Measurement" characteristic contains three attributes, including a Descriptor Attribute (CCCD Type), set for Notifications.
  • This is a server-initiated update characteristic - the value will be sent to the client whenever the value attribute is updated.

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GATT Profile Example (BM70)

Server profiles are created and depicted in vendor-specific ways. For example, the following depicts the GATT Heart Rate Service being incorporated into the BM70 radio via the BLEDK3 UI Configuration Utility:

Note that the Attribute Handle values are automatically assigned by this tool (starting at 0x8000).

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