Wireless Future - Telenor

tween the slaves, or the slaves set up a piconet
of their own (in which one of the slaves now
becomes the master). The master decides which
slave gets access to the piconet channel by addressing
it. After having received something from
the master, the addressed slave can return information
in the following time slot. To give a
slave a possibility to initiate communication,
the master must poll the slave periodically with
a maximal interval that was agreed upon when
the slave joined the piconet.
When several piconets exist simultaneously in
range of each other, they constitute a scatternet.
From a system point of view, the aggregate
throughput is increased by creating many piconets
that occasionally interfere with each other,
rather than having one or a few large piconets
where interference between piconets is non-existent
or very small. A unit (slave or master) can
participate in more than one piconet at a time
by utilising time sharing between the different
piconets (note that by definition, a unit can only
be master in one of the piconet(s) it belongs to).
The process of sharing time between piconets is
referred to as interpiconet scheduling. Similarly,
interpiconet communication occurs when information
flows between two piconets. Three sample
topologies are depicted in Figure 4.
Each Bluetooth device obtains a unique, factory
pre-set, 48 bit address, denoted BD_ADDR.
This address is used when setting up a connection
to a device. This procedure is denoted paging.
If, however, a device does not know which
other units are in range, it must find out this first
through the inquiry procedure. Bluetooth devices
that receive an inquiry message respond with
their BD_ADDR and some timing information.
Using this information, the inquirer can then
page an arbitrary device that has responded.
When the connection is made, the piconet master
will give the slave a three bit active member
address, AM_ADDR, that is used for subsequent
addressing within the piconet. The three address
bits restrict the number of active slaves to seven,
since the all-zero AM_ADDR is reserved for
broadcast messages.
Slave A
Master B
Telektronikk 1.2001
Slave A
Master B
a) b)
Slave C
2.4 GHz
Bluetooth Radio
Host
Link Manager
and I/O
A Bluetooth packet consists of a 72 bit access
code, a 54 bit packet header, followed by the
payload data. The access code serves two purposes:
it is used to fine-tune the receiver in order
to remove DC-offset, and the receiver can time
synchronise to align time slots with the transmitter.
These operations are necessary due to relative
drift between the crystals in the involved
units. The packet header contains information
about the recipient and what packet type the payload
is. Information used by the ARQ protocol
is also included there.
In the Bluetooth standard, different packet types
have been defined. Packets that are typically
used for data traffic contain cyclic redundancy
check (CRC) bits to check the payload for
errors. The payload length is typically 240 bits.
If conditions are favourable (i.e. when the number
of bit errors due to noise is relatively small)
longer packets can be used with a payload length
up to 2745 bits. The short packets can be sent
within one time slot. Longer packets can occupy
several (up to 5) time slots. Since the frequency
is not changed during the transmission of a
packet, the effective hop rate decreases when
long packets are used. Packets typically used for
Slave A
Master B
c)
Slave C
Slave Y
Master X
Baseband
Link Controller
Figure 3 Functional blocks in
the Bluetooth system
Figure 4 Different topologies
for Bluetooth piconets;
a) single slave operation,
b) multi-slave operation, and
c) scatternet with interpiconet
communication
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