GATT Characteristic

What Is a GATT Characteristic?

A ATT">gatt-characteristic/" class="glossary-term-link" data-term="GATT Characteristic" data-definition="Data value within a BLE service with R/W/N properties." data-category="GATT & ATT">GATT Characteristic is the fundamental data unit inside a BLE service. It represents a single value -- such as a temperature reading, device name, or control point command -- together with metadata that describes how the value can be accessed and observed.

Characteristic Structure

Each characteristic is composed of three or more attributes in the ATT database:

1. Characteristic Declaration (UUID 0x2803) -- contains the properties bitmask, the value handle, and the characteristic UUID.
2. Characteristic Value -- the actual data payload, addressed by the value handle.
3. Descriptors (optional) -- additional metadata such as the CCCD, Characteristic User Description, or Characteristic Presentation Format.

Properties Bitmask

The declaration includes an 8-bit properties field that advertises what operations the characteristic supports:

SIG-Defined vs. Custom Characteristics

The Bluetooth SIG maintains a registry of standardized 16-bit characteristic UUIDs. Heart Rate Measurement (0x2A37), Battery Level (0x2A19), and Temperature Measurement (0x2A1C) are common examples. Custom characteristics use full 128-bit UUIDs and can carry arbitrary payloads up to the negotiated MTU minus the ATT header.

Data Flow

For read-heavy use cases (e.g., configuration values), the client issues ATT Read Requests. For streaming sensor data, the server typically uses notifications or indications, which requires the client to first enable them by writing to the CCCD. Write operations can be acknowledged (Write Request) or fire-and-forget (Write Command), depending on reliability requirements.

Design Tips

Keep characteristic payloads compact. If a sensor produces multiple related values (X, Y, Z axes), consider packing them into a single characteristic rather than creating three separate ones, because each additional characteristic adds service-discovery overhead and consumes ATT handles. Document the byte layout clearly so that client developers can parse the raw payload without ambiguity.