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MOB TME - Other aspects of wireless
The layer transport - Bluetooth - Wifi Security
1. TCP and wireless environments
1. Remind the operation principle of the algorithm of TCP congestion by explaining
what correspond to the mechanisms following:
– “Slow start”
– “Congestion avoidance”
– “Fast retransmit” and “fast recovery”.
2. What will the performances of TCP be affected in by the use of wifi connections?
3. What does the use of the mechanism of fast retransmit improve the performances of
TCP in wireless environments in?
4. Another series of proposals consists of cutting the TCP connection into two between
the wire network and the wireless link? For this approach in particular developed in
Indirect-TCP (I-TCP), the access point is then seen like the mobile node for the
corresponding and conversely node. The proxy discharges all the packets emitted by
CN and sends them to the mobile node in another TCP connection.
a) What does this proposal allow improving the performances of TCP in?
b) What can le non maintenance of the semantic from beginning to ending of the TCP
connection represent a disadvantage in?
5. Another approach proposed in the SNOOP protocol, sets up a proxy in the foreign
agent. Snoop retransmits the packets lost in direction of MN and filters the settlements
duplicated in direction of the CN. For this intention, it observes the packets entering in
the interface before passing them to the IP level. The retransmissions are effected when
it detects two settlements duplicated for each packet that it saw and records in its
buffer.
a) If on the level of layer 2 (IEEE 802.11), a frame is lost, it will be retransmitted. It
may be thus that the protocol of layer 2 transmits twice the same packet. If this
packet contains a segment of TCP settlement, which will be the impact of this on
layer 4?
b) What is the goal of this mechanism?

Bluetooth devices will be integrated into typical mobile devices and rely on battery
power. This requires small, low power chips which can be built into handheld devices.
2. How many hosts can one have in 1 piconet?
Each piconet has exactly one master and up to seven simultaneous slaves. The reason
for the upper limit of eight active devices, is the 3-bit address used in Bluetooth.
3. The access technique being of polling type, show that a station has guaranteed minimal
flow.
Polling protocols are centralized. For a polling protocol to be applied, a central entity
(Base Station, BS) is assigned responsibility for polling the stations within the
network. If the BS decides that a specific station grants permission to transmit, it polls
this station, meaning that it sends to the station a small control frame notifying it that it
can transmit one or more frames. After the transmission of this station, the BS proceeds
to poll the other stations of the network. If a station is polled but has no traffic to
transmit, it notifies the BS of this fact, the procedure continues and the next station is
polled.
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4. Knowing the access technologies in piconets, can there be collisions?
Bluetooth applies FH-CDMA for separation of piconets. In an average sense, all
piconets can share the total of 80 MHz bandwidth available. Adding more piconets
leads to a grace-ful performance degradation of a single piconet because more and
more collisions may occur. A collision occurs if two or more piconets use the same
carrier frequency at the same time. This will probably happen as the hopping sequences
are not coordinated.
5. Why a Bluetooth network can coexist on the band of the 2,4 GHz with a Wifi network?
Like IEEE 802.11b, Bluetooth operates in the 2.4 GHz ISM band. However, MAC,
physical layer and the offered services are completely different.
6. Explain what the jump of frequency makes difficult malevolent listening in.
Packets may be 1, 3 or 5 slots long but in all cases the master transmit will begin in
even slots and the slave transmit in odd slots.
7. Does the flow of a Bluetooth network appear sufficient to you for the transport of a

b) Represent then the sequence of frequencies used on the successive time
intervals when the master station has a packet of a duration of three time
intervals to send and the slave stations S1 and S2 are sending a packet of a
duration of a time interval each one.
c) Represent finally the sequence of frequencies used on the successive time
intervals when the slave station S1 has a packet of duration of five time
intervals to emit.
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d) In your opinion, why is the following packet not systematically sent on f
k+1
?
10. Why would a network use a code error corrector instead of a code detector with
retransmission? Give two reasons.
11. Bluetooth equipment can be in two cells at the same time. Is there reason so that
equipment can not play the role of master in the two cells at the same time?
12. Bluetooth supports two types of connection between a master and a slave. What are
they and what is their use?
13. The beacon frames in alternative 802.11 of the modulation technique by dispersion of
spectrum with jump frequency, have a field containing the maintenance time (dwell
time). Does the analog indication frames in Bluetooth contain also this value? Explain
your answer.