MOB TME - Other aspects of wireless The layer transport - Bluetooth - Wifi Security
1MOB TME - Other aspects of wirelessThe layer transport - Bluetooth - Wifi Security1. TCP and wireless environments1. Remind the operation principle of the algorithm of TCP congestion by explainingwhat 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 ofTCP in wireless environments in?4. Another series of proposals consists of cutting the TCP connection into two betweenthe wire network and the wireless link? For this approach in particular developed inIndirect-TCP (I-TCP), the access point is then seen like the mobile node for thecorresponding and conversely node. The proxy discharges all the packets emitted byCN 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 TCPconnection represent a disadvantage in?5. Another approach proposed in the SNOOP protocol, sets up a proxy in the foreignagent. Snoop retransmits the packets lost in direction of MN and filters the settlementsduplicated in direction of the CN. For this intention, it observes the packets entering inthe interface before passing them to the IP level. The retransmissions are effected whenit detects two settlements duplicated for each packet that it saw and records in itsbuffer.a) If on the level of layer 2 (IEEE 802.11), a frame is lost, it will be retransmitted. Itmay be thus that the protocol of layer 2 transmits twice the same packet. If thispacket contains a segment of TCP settlement, which will be the impact of this onlayer 4?b) What is the goal of this mechanism?c) In this approach, the SNOOP agent never discharges itself the TCP segments. Whyin your opinion?2 Other wireless technologies: Bluetooth and 802.11a2.1 BluetoothBluetooth is one of technologies allows setting up a WPAN (Wireless Personal AreaNetwork). Bluetooth temporally divides the data communication channel into slots of fixedsize (625 µs). In nominal operation, a slot receives the transmission of a packet. A packet iscomposed of three parts: an access code, a header and the body. The access code is used likeidentify of piconet. The header of packet describes the packet. It contains information such asthe type of packet and the address of the recipient. The body contains the data to transmit. Theradio layer uses the dispersion of spectrum by jumping of FHSS frequency (Frequency 2Hopping Spread Spectrum). Each packet to transmit is put on different frequency of whichused previously (1600 jumps a second). The jump sequence is pseudo-random and eachdistinct network (piconet) has its own jump sequence and is not synchronized with the nearbypiconet.A piconet is a transitory network being created according to the needs. The management ofthe piconet is centralized and entrusted to a station called master. By extension, all the stationswhich are not master included in a piconet are called slaves.They are the characteristics of the master who define those of the piconet. In particular,each station Bluetooth has an address of the type IEEE 802.3 (48 bits) like an internal clock.The sequence of pseudo-random jump which defines the channel is initialized by using theIEEE 802.3 address of the master. The synchronization within piconet is effected by using theclock parameter of the master.The master manages also the access point to the piconet while allocating each new stationa short address in 3 bits allowing the station to communicate within the piconet. The unitconstituted at least of two piconets interconnected is called scatternet. The figure 1 shows anexample of network topology where a scatternet constituted from two piconets is represented.Fig.1 –Example of network topology.1. Why the Bluetooth range is only of a few meters?Bluetooth devices will be integrated into typical mobile devices and rely on batterypower. 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 reasonfor 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 minimalflow.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 thenetwork. If the BS decides that a specific station grants permission to transmit, it pollsthis station, meaning that it sends to the station a small control frame notifying it that itcan transmit one or more frames. After the transmission of this station, the BS proceedsto poll the other stations of the network. If a station is polled but has no traffic totransmit, it notifies the BS of this fact, the procedure continues and the next station ispolled. 34. Knowing the access technologies in piconets, can there be collisions?Bluetooth applies FH-CDMA for separation of piconets. In an average sense, allpiconets can share the total of 80 MHz bandwidth available. Adding more piconetsleads to a grace-ful performance degradation of a single piconet because more andmore collisions may occur. A collision occurs if two or more piconets use the samecarrier frequency at the same time. This will probably happen as the hopping sequencesare 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 ineven slots and the slave transmit in odd slots.7. Does the flow of a Bluetooth network appear sufficient to you for the transport of avideo flow?In Classic Bluetooth, which is also referred to as basic rate (BR) mode, the modulationis Gaussian frequency-shift keying (GFSK). It can achieve a gross data rate of 1 Mbit/s.In extended data rate (EDR) π/4-DQPSK and 8DPSK are used, giving 2, and 3 Mbit/srespectively, even reached 24 Mbit/s with Bluetooth version 3.0 + HS. So bluetoothprovides a secure way to connect and exchange information between devices such asfaxes, mobile phones, telephones, laptops, personal computers, printers, GlobalPositioning System (GPS) receivers, digital cameras; but with the speed requirementsof the video flow is about 38Mbit/s, they can not meet.8. Which defines the membership of several nodes in the same piconet?A packet is composed of three parts: an access code, a header and the body. The accesscode is used like identify of piconet.9. All the nodes of piconet operate a jump sequence after each sending of packet. Thus, ifa master sends a packet on an interval of time (slot) on the frequency fk, the packet sentimmediately after by the slave terminal will be on the frequency fk+1. The Bluetoothpackets can have duration of 1, 3 or 5 slots of 625 µs each one. If a packet is sent on thefrequency fk, the following will be on frequency fkn, with n duration of the packet in anumber of slots (1, 3 or 5). 4a) Represent with assistance of a diagram the sequence of frequency used on thesuccessive time intervals when three slave stations S1, S2 and S3 are sending apacket of duration of a time interval each one.b) Represent then the sequence of frequencies used on the successive timeintervals when the master station has a packet of a duration of three timeintervals to send and the slave stations S1 and S2 are sending a packet of aduration of a time interval each one.c) Represent finally the sequence of frequencies used on the successive timeintervals when the slave station S1 has a packet of duration of five timeintervals to emit. 5d) In your opinion, why is the following packet not systematically sent on fk+1?10. Why would a network use a code error corrector instead of a code detector withretransmission? Give two reasons.11. Bluetooth equipment can be in two cells at the same time. Is there reason so thatequipment 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 arethey and what is their use?13. The beacon frames in alternative 802.11 of the modulation technique by dispersion ofspectrum with jump frequency, have a field containing the maintenance time (dwelltime). Does the analog indication frames in Bluetooth contain also this value? Explainyour answer. . 1MOB TME - Other aspects of wirelessThe layer transport - Bluetooth - Wifi Security1 . TCP and wireless environments1. Remind the operation principle of. 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