IT351 - Mobile & Wireless Computing
Tutorial_8
1. If Bluetooth is a commercial success, what are remaining reasons for the use
of infra red (IR) transmission for WPAN?
2. Compare IEEE 802.11 and Bluetooth with regard to their ad-hoc capabilities.
Where is the focus of these technologies?
3. In a Bluetooth network each node is either a master or a slave node. Master
nodes coordinate slaves and can talk to all those nodes. Slaves can only talk to
the master node. Does the hidden terminal problem occur in Bluetooth?
Explain your answer.
4. Compare the offered QoS (delay guarantees) in IEEE802.11 in ad hoc mode
and in Bluetooth networks. What advantages does an additional infrastructure
offer? How is QoS provided in Bluetooth?
5. In what situations can collisions occur in Bluetooth? Distinguish between
collisions on PHY and MAC layer.
6. Does Bluetooth support roaming between different piconets?
7. Bluetooth supports two types of links between a master and a slave. What are
they and what is each one used for?
8. A Bluetooth device can be in two piconets at the same time. In this case which
hopping sequence it should follow?
9. Is there any reason why one Bluetooth device can not be the master of two
piconets at the same time?
10. Explain why a service discovery protocol is needed in a Bluetooth network
11. What are the power saving mechanisms in Bluetooth systems?
Homework
1. As a WPAN technology, describe the main advantages of Bluetooth in
comparison to infra red (IR).
2. What are the differences between a master device in a Bluetooth network and
an access point in IEEE802.11 network?
Answers
1)
One reason for infrared is still cost – IR devices are very cheap and
very simple to integrate. Another advantage is the simple protection from
eavesdropping. Attackers can much more easily
tap
Bluetooth
communication, incautious users even let their Bluetooth devices open for
public access (simply scan for Bluetooth devices at public devices - many are
detectable). IR communication is much more secure as the devices have to
face each other (directed IR).
2)
Both standards offer ad-hoc functionality, although only Bluetooth was
designed with the focus on ad-hoc networking. 802.11 heavily relies on an
access point for many functions (e.g., power control, frequency selection, QoS
in polling mode, access control etc.). Bluetooth on the other hand implements
all functions in all nodes enabling all devices to set up a network.
3)
No. in Bluetooth there are no hidden terminals as the master controls
all visible slaves. If a terminal does not see the master it cannot participate in
communication. If this terminal sends anyway it will not interfere as this
terminal then acts as master with a different hopping sequence
Bluetooth controls access to the shared medium via a Master/Slave
relationship between connected nodes. A related problem occurs if nodes
occur in different piconets and can interfere with each other. Similarly, Wi-Fi
uses the same spectrum as Bluetooth so both must cope with interference.
4)
802.11 does not offer QoS in the ad-hoc mode as it needs the access
point for polling. Bluetooth always works ad-hoc, well, a master controls up to
seven slaves and, thus, forms an ad-hoc access point. Bluetooth therefore can
offer QoS in its ad-hoc mode. QoS in Bluetooth is provided by periodic
polling through the master. This guarantees certain data rates and access
latencies. After a master has been found, Bluetooth can give hard guarantees
for SCO connections. 802.11 can give guarantees if no contention phase is
allowed (polling only). As soon as there is a contention phase, the system
cannot guarantee access latencies.
5)
During polling, there are no collisions on the MAC layers of
Bluetooth as the master controls the medium. Collisions on the PHY layer
may occur in Bluetooth only if another piconet randomly jumps to the same
frequency at the same time. This will destroy data for this time-slot.
6)
No, Bluetooth does not support roaming at all. Nodes changing
piconets have to resynchronise to the new piconet, there is no signalling
between masters for roaming nodes.
7)
Two different kinds of physical links:
Synchronous Connection oriented (SCO) – for audio
Asynchronous ConnectionLess (ACL) – transmission of data
8)
In this case the device should jump back and forth between the two
hopping sequences of the two piconets.
9)
Yes, a single device cannot be master in two piconets at the same time
as the unique MAC address of the master defines the hopping sequence of the
piconet. In this case the two piconets will follow the same sequence so all data
will be corrupted.
10)
Bluetooth nodes should implement the core protocols only but the
other protocols are not mandatory. So a service discovery protocol is needed
to discover the services/ protocols available on the nearby nodes/piconets
11)
Bluetooth systems save power offering several low power modes as
they are typically battery operated: sniff, hold, park and standby.
Negative effects of power saving are the increased latency for spontaneous
transmissions – the devices have to wake-up first.