wireless
Bluetooth Scatternets
A Cost-Effective Solution for Communication
The most popular application of Bluetooth is communication between
two directly paired devices. Here, we explore a less popular but powerful
application of Bluetooth which can help extend the range of Bluetooth
communication and provide a free infrastructure for communication
Scatternets: A set of
piconets

Ahzam Ali
B
luetooth is a low-power, lowcost and short-range wireless
technology. It was originally
developed by Ericsson for short-range
communication between personal
devices, e.g., data transfer between
a cellphone and a computer, or communication with a printer. Bluetooth
devices use the unlicensed ISM band
at 2.4 GHz.
Nowadays most of the mobile devices, including mobile phones, MP3
players, digital cameras, video cameras, PDAs, laptop PCs, tablet PCs and
so forth, are equipped with Bluetooth.
So you can exchange data between
these devices without requiring the
use of wires. Bluetooth is considered
to be a high-potential technology for
providing wireless communication in
a home-networking environment.
Bluetooth is an evolving technology. It has gone through three (standardised) revisions of the approved
standard by Bluetooth Special Interest
Group (SIG), the latest one being Bluetooth 2.1 EDR (enhanced data rate) in
early part of 2008. The latest standard
supports enhanced usability, i.e., pairing of devices by means of near-field
communication (NFC), enhanced
security and broader range of device
profiles.
When a number of Bluetooth devices communicate to each other in the
same vicinity, there is a high level of
interference. To combat interference,
Bluetooth technology applies a fast
frequency-hopping scheme which hops
over 79 channels 1600 times per second.
For devices to communicate to each
other using Bluetooth they need to be
paired with each other to have synchronised frequency-hopping sequence.
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When there is a collection of devices
paired with each other, it forms a
small personal area network called
‘piconet.’ A piconet consists of a master and at most seven active slaves.
Each piconet has its own hopping sequence and the master and all slaves
share the same channel. In a piconet,
the master and slave devices transmit packets in even and odd slots,
respectively.
Two or more piconets connected
to each other by means of a device
(called ‘bridge’) participating in both
the piconets, form a scatternet. The role
of the bridge is to transmit data across
piconets. Fig. 1 shows a scatternet
structure in which devices ‘a’ and ‘c’
play the master role and device ‘b’ acts
as the bridge.
The scatternet formation has not
been formally defined in the Bluetooth
SIG specifications. As a result, numerous protocols have been proposed.
While modeling ad-hoc networking,
in general, is complex, the additional
restrictions imposed by the Bluetooth
specifications—such as low cost of
the device, low power consumption
and network resilience while using
piconets that have a maximum of
seven active nodes—have created a
c
P1
a
b
P2
MASTER
SLAVE
BRIDGE
Fig. 1: Scatternet structure
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significant challenge.
Formation of scatternets
There has been a comprehensive study
to propose an efficient formation of connected scatternet. One important problem that exists with scatternet as well
as piconet formation is to keep track of
the device that comes in and goes out of
the network—since these are low-power
handheld devices with limited communication range over Bluetooth. A device
connects to another device at random,
according to their 48-bit Bluetooth addresses and clocks, which control the
hopping behaviour in inquiry or inquiry
scan states. Most protocols seek to reduce the scatternet formation time and
form fast routing algorithms.
It turns out that a good scatternet
formation protocol should:
1. Be fully distributed and rely on
local information
2. Generate connected scatternets
3. Be resilient to the disconnection
of nodes anywhere in the scatternet
4. Provide multiple routes for robustness and be self-healing
5. Limit the number of bridges
6. Limit the number of roles a device can assume
7. Be aware of device resources
While forming a scatternet, keep
in mind that you are dealing with, in
general, small, energy- and processing-power-starved devices. The following points should be taken care of
while developing scatternet formation
protocols:
1. Minimise scatternet construction
time
2. Minimise the amount of control
data transmitted
3. Minimise the number of hops
required for communication between
devices, in order to improve response
times
4. Minimise power consumption.
Master-and-bridge mode of operation
requires more power than slave mode
5. Minimise the number of roles
assigned to nodes
6. Minimise the number of piconets
to provide faster routing and reduced
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Fig. 2: Showing efficient network after role
switching
Fig. 3: Unnecessary bridge elimination
packet collisions
7. Maximise the amount of data
throughput
Given a scatternet, you can evaluate
its performance based on the number
of piconets, the number of nodes per
piconet, the number of bridge nodes,
the number of roles per node, average
traffic delay, throughput and maximum traffic delay.
Numerous models have been
proposed for constructing Bluetooth
scatternets, some of which may require
modifications to the existing Bluetooth
specifications.
Role switching
Role switching enables two devices
to exchange roles very rapidly, rather
than reconnecting by executing the
time-consuming inquiry and inquiry
scan processes. The role switching
operation involves fewer slots than the
inquiry/inquiry scan and page/page
scan operations in switching the roles
of the devices.
There are three major types of role
switching operations:
1. Combining. A situation where a
single node works both as slave in one
piconet and master in another.
2. Splitting. A piconet with a large
number of devices divided in two
piconets with smaller number of devices.
3. Take-over. When the existing
mast or a bridge is about to move out
of the scatternet, it transfers its responsibilities to another device in the
scatternet.
There is a switching delay (called
‘guard time’) introduced by bridges
while they try to transmit packets
across the piconets. Eliminating the
unnecessary bridges from the network will improve the performance of the scatternet. There exist
various protocols and algorithms
to optimise the roles that a device
assumes in a scatternet.
We see in the following examples how effective role assignment
can reduce the hop count while
routing a packet, help reduce
the number of bridges in the network
and result in an efficient scatternet
structure.
In Fig. 2, the original network was
formed with ‘c’ as a bridge node and ‘a’
and ‘b’ as the masters on two different
piconets p1 and p2. Using role switching, with the help of ‘c,’ improves the
structure of the scatternet (now formed
as a ‘piconet’), shortens the routing
path and eliminates the bridge delay
in the network.
Fig. 3 shows another example of
better role switching operation that
results in an improved network structure.
Scatternet applications
By now you might be wondering what
possible use these scatternets could be
put to. Well, there are papers proposing
voice communication between two mobile devices connected over a Bluetooth
scatternet, up to a distance of 100 metres
in an indoor environment. One of the
popular applications developed by Nokia, called ‘sensor,’ works on somewhat
similar principles. It lets users communicate to each other, bypassing the service
provider’s network, over Bluetooth.
Sensor networks are another example where the Bluetooth scatternets
can be used. Since sensor networks are
generally ad-hoc, Bluetooth scatternet
can be used for communication of
sensors with each other and the base.
This will eliminate the requirement of
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developing a special-purpose protocol
for sensor networks.
Problems to be addressed
The Bluetooth scatternet formation
devices are generally small, running on
batteries, with low-powered CPU and
low memory. At the same time, these are
highly mobile and may frequently move
in and out of the network. This leads us
to think about:
1. Topology and size of scatternets.
What is the optimal number of piconets
in the scatternet and how many nodes
are there? The number of bridge nodes
increases (proportionally) with the
increasing number of piconets. The increases, in turn, incur significant overhead: loss of one or more time slots to
readjust clocking when shifting from
one piconet to another, more traffic
than non-bridge nodes and increased
battery power consumption to perform
these tasks.
What is the optimal number of
piconets in which a bridge should participate? The more the piconets with
which a bridge communicates, the less
the time during which the bridge will
be available for dealing with traffic
from/to each individual piconet, thus
increasing the likelihood of bottlenecks,
at the bridge itself.
2. Formation delay and resilience.
How much time is required to construct, optimise and maintain the scatternet? The ad-hoc, dynamic nature of
a Bluetooth network requires constant
modifications to the scatternet topology to support devices that join and
leave. How to handle the nodes that
join or leave the scatternets?
3. Polling and scheduling. In what
order will the slaves be polled by the
master? Does the scatternet topology
impact the ability to handle scheduled
communications from slaves in sniff or
hold state?
4. Routing. Does the scatternet topology allow efficient routing of messages?
Work in the area
There are a number of papers discussing the effective routing protocols
for Bluetooth scatternets. Also, there
are quite a few papers which discuss
the point of organising the scatternet
in an efficient fashion, dealing with
disparate aspects of scatternet optimisation. A proposal in ‘Bluetooth
Scatternet Formation for Supporting
Device Mobility,’ by Chorng-Horng
Yang and Yi- Sheng Chen, deals with
mobility support in a Bluetooth scatternet network, but relies on a Bluetooth backbone network consisting of
devices that are not mobile. Another
work done on similar lines is ‘Tracking
the Optimal Configuration of a Bluetooth Scatternet’ by Csaba Kiss Kalló
and Carla-Fabiana Chiasserini, which
deals with similar problem of redefining the role of nodes in scatternets but
concentrates on improving the energy
efficiency of the nodes. Also, there are
numerous proposals for routing protocols including MANET (mobile ad-hoc
network) protocols.
Future in Bluetooth
The new Bluetooth specification, called
the Bluetooth 2.1, is already published.
It is fully backward-compatible with
1.1. The specification includes many
new security and usability enhancements. The most interesting of these
is incorporation of the near-field
communication (NFC) device, which
helps in pairing of two Bluetooth devices equipped with NFC chip by just
bringing them close to each other. For
example, a headset can be paired with
a Bluetooth 2.1 phone having NFC
chip just by bringing the two devices
close to each other. Another example
is automatic uploading of photos from
a mobile phone to a digital picture
frame just by bringing the phone close
to the frame. Features like this and
other security enhancements will help
Bluetooth gain user confidence.
A lot of work is in progress in
the area of Bluetooth, to enhance and
widen the application of Bluetooth
protocol. In the future, we may expect
features like:
1. Broadcast channels. These will
enable Bluetooth information points
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that broadcast information to all Bluetooth devices in the vicinity. So users
will be able to pull relevant information from the system.
2. Topology manger. It will enable
better configuration of piconets and
even scatternets.
3. Alternate media access physical
layer. It will enable use of some other
wireless technology for actual data
transfer at higher rate while maintaining the initial pairing on standard
Bluetooth.
4. QoS improvements. These will
enable audio and video data transmission at a higher quality.
Nokia and Bluetooth SIG have announced that Wibree (Nokia’s low power wireless communication protocol) will
be a part of the Bluetooth specification
as an ultra-low-power Bluetooth technology, to be used in caller ID watches,
sports sensors (used for monitoring the
wearer’s heart rate during exercise) as
well as medical devices. A medical devices profile and associated protocols are
being developed by the Medical Devices
Working Group (MDWG).
Bluetooth 3.0. The next version of
Bluetooth after v2.1, code-named Seattle, is proposed to adopt ultra-wideband (UWB) radio technology. This
will allow Bluetooth use over UWB
radio, enabling very fast data transfers
of up to 480 Mbps, while taking the advantage of very low-power idle modes
of Bluetooth.
Extending the scope
Even though work on Bluetooth scatternet is going on for about a decade
now, it has failed to generate interest
amongst general consumer electronics
goods manufacturers. Bluetooth scatternets find most of their use in sensor
networks. When there is a limitation
on setting up the communication infrastructure, Bluetooth scatternet could
turn out to be a cost-effective solution.
In future, as the processing power of
electronic devices grows and they become more energy-efficient, Bluetooth
might see its way into already evolving
Bluetooth standards. 
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