Hardware Reference
In-Depth Information
Specification Supplement in the
Specification Adopted Documents page
specifies the
fields that the GATT server must insert in the payload of an advertising packet to make
a particular service's data available to scanners.
As shown in
Table 4-7
, to be able to broadcast service data, a GATT server must include
two different fields in the advertising packet's Service Data section.
Table 4-7. Service Data AD Type
Field
Length in bytes
Description
UUID
2, 4, or 16
The actual UUID identifying the data
Service Data
Variable
The data associated with the service identified by the UUID
The contents of the Service Data field can correspond to the complete or partial value
of a particular characteristic or descriptor within the corresponding service. It is up to
each profile specification to define which, because only the profile has sufficient knowl‐
edge about the data to decide which pieces of information are the most relevant to be
broadcasted.
Features
GATT features are strictly defined procedures that allow GATT-based data exchanges
to take place. They are all based on the different operations that ATT provides (intro‐
duced in
“ATT operations” on page 26
).
To a certain extent, most of the features listed in this chapter are exposed in one way or
another in most GATT APIs. GATT server APIs add the ability to populate the actual
server with attributes, but that is heavily implementation dependant and beyond the
scope of this chapter.
Exchange MTU
This succinct two-packet procedure allows each ATT peer to let the other end know
about the maximum transmission unit (MTU, or effectively maximum packet length)
it can hold in its buffers and can therefore accept.
This procedure is used only whenever either the client or the server (or both) can handle
MTUs longer than the default
ATT_MTU
of 23 bytes (see
“Logical Link Control and
Adaptation Protocol (L2CAP)” on page 25
) and wants to inform the other end that it
can send packets longer than the default values that the specification requires. L2CAP
will then fragment these bigger packets into small Link Layers packets and recombine
them from small Link Layers packets.
