Hardware Reference
In-Depth Information
In practical terms, devices looking for all possible discoverable peers should opt for the
general discovery procedure. Use the limited discovery procedure to find only devices
in limited discovery mode.
Connection Establishment
For a central to initiate a connection establishment with a peripheral, the latter must be
in a
connectable
mode. Similar to discovery, several modes and procedures control the
selection of devices with which to interact, in an organized and standardized fashion.
Connection establishment modes
The differences between the following connection establishment modes reflect a pe‐
ripheral's use of different types of advertising packets (detailed in
“Advertising and
Scanning” on page 19
):
Non-connectable mode
A device in this mode either does not send advertising packets at all or it sends
ADV_NONCONN_IND
or
ADV_SCAN_IND
advertising packets (see
Table 2-1
). In both
cases, the device is, as the mode name implies, not connectable, meaning that no
centrals may establish a connection with it.
Directed connectable mode
A device in this mode sends
ADV_DIRECT_IND
advertising packets (see
Table 2-1
).
When performing directed advertising, a device sends advertising packets at a high
frequency and for a short time, with no user data payload and with a target central
Bluetooth Address. This is provided as a “fast reconnect” mode typically used when
the peripheral has a strong suspicion that the target central is already trying to
initate a connection and wants to establish it as fast as possible. Only the central
whose Bluetooth address matches the one in the advertising packets sent by the
peripheral will receive them.
Undirected connectable mode
A device in this mode sends
ADV_IND
advertising packets (see
Table 2-1
). This is the
standard connectable mode, through which a peripheral makes itself connectable
for a longer period of time and may be trying to connect to a new central or to one
that is already previously known by it.
Both connectable modes implicity require the device to send the advertising packets
with the intent to connect to a central.
Connection establishment procedures
Because a central device has no means to select which advertising packet types it will
receive when scanning with the intent to connect (they will always be of type
ADV_IND
or
ADV_DIRECT_IND
), the distinctions between connection establishment procedures do
