MOB TME - Other aspects of wireless The layer transport - Bluetooth - Wifi Security

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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?

c) In this approach, the SNOOP agent never discharges itself the TCP segments. Why

in your opinion?

2 Other wireless technologies: Bluetooth and 802.11a

2.1 Bluetooth

Bluetooth is one of technologies allows setting up a WPAN (Wireless Personal Area

Network). Bluetooth temporally divides the data communication channel into slots of fixed

size (625 µs). In nominal operation, a slot receives the transmission of a packet. A packet is

composed of three parts: an access code, a header and the body. The access code is used like

identify of piconet. The header of packet describes the packet. It contains information such as

the type of packet and the address of the recipient. The body contains the data to transmit. The

radio layer uses the dispersion of spectrum by jumping of FHSS frequency (Frequency