Information Technology Reference
In-Depth Information
seven. This restriction is not so strong if we consider the purposes and
features of a small personal network. In its active mode, the unit actively
participate in data exchange. During the master-to-slave slots, the unit
listens to the channel: if packets contain its address, it keeps on listening
until all packets are received and answers to the master in subsequent
slots, otherwise it stops the listening until the next master transmission.
•
Sniff mode
. The slave listens to the channel with a lower rate, and it keeps
on listening only if packets contain its address, otherwise it switches to
sleep mode. The sniff mode is one of the three energy saving modes, the
one that allow for the smaller energy saving.
•
Hold mode
. A slave can request to be in this mode or can be forced by the
master. There is no data exchange while in hold mode, even if the device
keeps its AMA. The only active process is the internal clock, thus
allowing for a good energy saving.
•
Park mode
. The parked units have an 8-bit address (the PMA, parked
member address), so there can be up to 256 units in this mode in a
piconet. These units hold the master address and the clock, even if they do
not participate in the piconet activities. Parked units periodically listen to
a special channel (the
beacon
channel) for possible wake-up messages
from the master. In this mode, the unit duty cycle is at the minimum, thus
allowing for the higher energy saving.
Bluetooth devices can exchange both raw data and audio data, so there
are two types of links can be established between master and slave: the
synchronous connection-oriented
(SCO) and the
asynchronous connection-
less
(ACL).
The SCO link is a symmetric, point-to-point link between the master
and a specific slave. The SCO link reserves slots and can therefore be
considered as a circuit-switched connection between the master and the
slave. The SCO link typically supports time-bounded information like
voice. The master can support up to three SCO links to the same slave or to
different slaves. A slave can support up to three SCO links from the same
master or two SCO links if the links originate from different masters. SCO
packets are never retransmitted. In this case a hand-shaking phase is
needed, in which master and slave agree on synchronisation and on audio
packet format to be used.
In the slots not reserved for SCO links, the master can exchange packets
with any slave on a per-slot basis. The ACL link provides a packet-switched
connection between the master and all active slaves participating in the
piconet. Both asynchronous and isochronous services are supported. Between
a master and a slave, only a single ACL link can exist. For most ACL
packets, packet retransmission is applied to assure data integrity.
