Hardware Reference
In-Depth Information
Table 4-3
shows the include declaration attribute with all its fields.
Table 4-3. Include Declaration attribute
Handle
Type
Permissions
Value
Value length
0xNNNN
UUID
include
Read only
Included service handle, end group handle, Included Service UUID
6, 8, or 20 bytes
Again, the UUID
include
(
0x2802
) is a special SIG-assigned UUID used exclusively in in‐
clude declarations, and the value field in this case contains both the start and end handles
of the include service, as well as its UUID.
Characteristics
You can understand characteristics as containers for user data. They always include at
least two attributes: the
characteristic declaration
(which provides metadata about the
actual user data) and the
characteristic value
(which is a full attribute that contains the
user data in its value field).
Additionally, the characteristic value can be followed by descriptors, which further ex‐
pand on the metadata contained in the characteristic declaration. The declaration, value,
and any descriptors together form the
characteristic definition
, which is the bundle of
attributes that make up a single characteristic.
Table 4-4
shows the structure of the first two attributes of every single characteristic.
Table 4-4. Characteristic declaration and characteristic value attributes
Handle
Type
Permissions
Value
Value length
UUID
characteristic
Read only
Properties, value handle (
0xMMMM
), characteristic UUID
5, 7, or 19 bytes
0xNNNN
0xMMMM
Characteristic UUID
Any
Actual value
Variable
All GATT characteristics are always part of a service, and can therefore always be found
enclosed in one.
Characteristic declaration attribute
Once again, the characteristic declaration attribute's type UUID (
0x2803
) is a standar‐
dized, unique UUID used exclusively to denote the beginning of characteristics. As with
all other declarations (such as service and include), this attribute has read-only per‐
missions, because clients are allowed only to retrieve its value but in no case modify it.
Table 4-5
lists the different items concatenated within the characteristic declaration's
attribute value.
