Hardware Reference
In-Depth Information
Value
The characteristic properties for this characteristic are read only, the charac‐
teristic value handle is
0x002C
, and the characteristic value UUID is the UUID
for the Body Sensor Location (
0x2A38
).
Handle
0x002C
This attribute contains the characteristic value (see
“Characteristic value at‐
tribute” on page 61
), in this case the body sensor location. These are the charac‐
teristic value attribute's fields:
UUID
The same UUID present in the last two bytes of the characteristics definition's
attribute value.
Permissions
This attribute's value is read only: a client can only check where the sensor is
located, but not modify its location (that is up to the server).
Value
The actual body sensor location (fictitiously represented as “finger” for clarity).
Several tools exist to discover and display the different services on a server in a format
similar to
Figure 4-2
, which can be useful during application development.
Chapter 6
has more information on these tools.
Advanced Attribute Concepts
This section introduces additional concepts related to working with attributes that are
also worth mentioning because their understanding is frequently required for many
types of BLE applications.
Attribute Caching
“Attributes” on page 53
discusses how attribute handles allow a client to individually
address all available attributes on a server. Discovering the list of available handles and
the contents of their respective attributes can be a time-consuming (and power-
consuming) process detailed further in
“Service and Characteristic Discovery” on page
69
. But for now, this section describes what a client can do to avoid performing the
discovery procedure every time it reconnects to a server, and under what circumstances
it can do so.
Servers typically tend to maintain a stable set of attributes, and their basic structure does
not, in most cases, change over the lifetime of a server device. But the implementation
imposes no rigid restrictions in this regard, and a server is indeed free to completely
overhaul its attributes and even replace them with a radically new set at any time
