Product Specs

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Some other related attributes are read-only (such as a Characteristic User

Description attribute), some control the behavior of the characteristic (such as

the Client Characteristic Configuration attribute which is used to enable notify

or indicate operations).

Every attribute has a UUID. These may be either 16 bits (e.g. “0x180A“) or 128 bits

(e.g. “00001530-1212-EFDE-1523-785FEABCD123 “). All 16-bit UUIDs are

defined by the Bluetooth SIG and are known as adopted UUIDs. All 128-bit UUIDs

are custom and may be used for any purpose without approval from the Bluetooth

SIG. Two very common 16-bit UUIDs that you will see are 0x2901, the Characteristic

User Description attribute and 0x2902, the Client Characteristic Configuration

attribute (to enable either “notify” or “indicate” on a characteristic).

One important note is that some attribute UUIDs do not technically need to be unique.

Their handles are always unique, but the UUIDs occasionally overlap. For example,

every Client Characteristic Configuration attribute has a UUID of 0x2902, even though

there may be a dozen of them in a single GATT database.

1.1.5 Advertising and Connections

The Generic Access Profile (GAP) specifically describes behaviors and procedures for

device discovery, connection establishment, security, authentication, and service

discovery, and this along with performing a single defining role. In essence, a

Bluetooth device may incorporate either initiating or accepting procedures, and the

peer device must support the corresponding functionality. One of the most important

things to understand with Bluetooth low energy is how two devices first find one

another, work out what they can do with one another, and how they can find and

connect with one another repeatedly. This is really what GAP defines.

There are four GAP roles defined for a Bluetooth low energy device:

 Broadcaster

 Observer

 Peripheral

 Central

A broadcaster is a device that sends advertising packets. Typically, this is used to

broadcast some data from a service to other devices that happen to be in an observer

role. A broadcast must have a transmitter but does not need a receiver. A broadcast-

only device, therefore, only needs a transmitter.

An observer is a device that scans for broadcasters and reports this information to an

application. An observer must have a receiver; it can also optionally have a transmitter.

A peripheral is a device that advertises by using connectable advertising packets. As

such, it becomes a slave once connected. A peripheral needs to have both a transmitter

and a receiver. The E-RL 2 has adopted the role of peripheral device and as such is

sending advertising packets while not connected.