Information Technology Reference
In-Depth Information
Figure 3: Master-slave setup in a piconet.
2.1
Piconets
To communicate with each another, Bluetooth units must be a part of small
networks, called
piconets
, in which one unit plays the
master
role and the
others act as
slaves
(Figure 3) [2].
The difference between master and slave is only at a logical level and it is
tightly coupled with the piconet existence: when the piconet is loosened,
master and slave roles are cancelled consequently. This means that there is no
difference among Bluetooth devices at the physical level: all of them can act
as both slave and master.
By definition, the master is the device that starts a piconet with other
devices, which in turn become all slaves. Figure 3(a) shows a point-to-point
connection between a master and exactly one slave, whereas Figure 3(b)
shows a point-to-multipoint connection between a master and more than one
slave. When a Bluetooth device establish a piconet (thus becoming a master),
it sends its own parameters to other devices allowing them for the
synchronisation.
All communications in a piconet can take place only between the master
and one slave, no direct slave-to-slave communication is possible. The media
access is managed by the master in a centralized way, by means of a polling
scheme, in order to avoid collisions. A slave is authorized (and forced) to
transmit in a given time slot only when enabled by the master with a suitable
data packet sent during the preceding slot. Communications in a piconet take
always place in both directions: when the master sends a packet to a slave,
there is always an answer packet sent by the slave to the master.
A slave can show four operation modes:
•
Active mode
. The slave has a unique 3-bit address (the AMA, active
member address). The '000' address is always assigned to the master and
it is used for message broadcast to active members within the piconet.
The number of active slave members of a piconet is therefore limited to
