MOB Subject 6 - IEEE 802.11 standards Wifi operation
Master of Computer Science 1 - MOB Mobile Internet and SurroundingFladenmuller Baey - ”IEEE 802.11standards” 1/8MOB Subject 6 - IEEE 802.11 standardsWifi operation1. Generality1. One wishes to install a wireless local network in a closed environment (office). In order tocover all the desired zone, it proves necessary to install several Wifi access points (802.11b).One wishes that whatever the place or the users is, they do not have to configure differentlytheir Wifi card? How is that possible?A set of basic services (BSS) is the set form a acces point and cover stations in its area. A systemDistribution (Distribution System-DS) can connect several BSS between them to form a set ofservices outstretched (Extended Service Set -ESS).2. Would you advise to use the same frequency channel or to choose different for each accesspoint? Why?By using frequency channels different, we reduce risks interference and we increases thebandwidth.3. What is the interest to use the RTS/CTS mechanism?The aim is to avoid the problem of hidden terminals.If two wireless terminals do not see, they are likely to issue in the same time and I do notknow that there is a collision. This is the hidden terminal problem. This problem was solved bythe exchange of packets RTS / CTS (Request To Send / Clear To Send), which gives the controlsentence a base station, which normally sees all the terminals that are in communicationher.A map will emit sends an RTS message to request authorization the base station. The applicationis parameter by a time value that indicates given the quantity of e sueh send. The base stationwith a message repond CTS parameter with time value. During the time indicated PERIOD,only the map that has authorization may be issued. In principle, all cards see the message CTS.The use of RTS / CTS signaling induced overload. If there is no terminal or hidden messages tosend are of small size, this overhead is not profitable. 802.11b provides a threshold for theimplementation of mechanically RTS / CTS. If message size does not pass of the threshold, thesender tries to transmit directly without seeking authorization from RTS.4. In 802.11 standards, the flow of MAC frames is determined partly by its type. The controlframes such that RTS and CTS do not use systematically the maximum flow. Why?For reasons of compatibility with earlier versions, the flow of frames control should be themaximum flow rates authorized by the standards used by the Connected Equipment.5. What makes it possible to increase the flows between the different families of protocol802.11?The data modulation: BPSK to 802.11, CCK (Complementary Code Keying) for 802.11b andOFDM for 802.11a and 802.11g. Master of Computer Science 1 - MOB Mobile Internet and SurroundingFladenmuller Baey - ”IEEE 802.11standards” 2/86. An access point 802.11g sends its physical frame of coded data in OFDM.a) What does that imply for a node provided of an 802.11b card which would be in the samezone?It can detect the data transmitted 802.11g. These data are then collected as interference. Theproblem of hidden terminal arises.b) Propose a solution which allows solving this problem.Email RTS / CTS flow rates understandable by the nodes so that 802.11 they know that atransmission will take place between different points. This solution has been adopted bystandards bodies.7. Does one can make communicate together equipments provided of transmission cards802.11a and 802.11b?No. This is not the same frequency band and not the same encoding: 5 GHz + OFDM for802.11a, 2.4 GHz + CCK for 802.11b.8. The same question with 802.11a and 802.11g?Not the same frequency band 5 GHz for 802.11a, 2.4 GHz for 802.11g. Nevertheless, in thelonger term, the IEEE 802.11g is under elaboration an important step towards the realization ofdual-band 2.4 GHz / 5 GHz. OFDM already being required to operate at 5 GHz, implementing802.11g in a dual-band equipment does not add complexity "hardware" additional productfinished.9. Propose 4 application domains of wireless network?1. Extending a network (LAN): integration of a wireless LAN and a wired local network in orderto extend the coverage of local network;2. Interconnection of local networks: two networks interconnect local radio;3. Realization of an ad hoc network, ie a network without a central server, installed temporarilyto cope with immediate needs. Realization of a network of sensors or a mesh network.4. Setting up a temporary network: lounge, conference .10. Which characteristics appear important to you at the time of the specification phase of a newwireless local network? The flow: The MAC protocol must use the wireless medium more efficentlypossible so as to maximize the capacity. The number of users g Maple: the wireless local area networks can be broughtto manage hundreds of nodes spread over several cells. The interconnection with the local network of the heart: in most cases, the inter-connection with terminals of the network is required. In local networks withoutWireless infrastructure has, this task is easily accomplished thanks to modulescontrol that connect the two types of local networks. It may also be a need ome of mobileusers and wireless network ad-hocs. The coverage area of a cell: the diameter of a typical cell of a networkWLAN varies between 100 and 300 m. Master of Computer Science 1 - MOB Mobile Internet and SurroundingFladenmuller Baey - ”IEEE 802.11standards” 3/8 The energy consumption and battery life: Mobile Usersuse terminals fed by batteries. Good autonomy is oftenrequired to work over in wireless environments. A pro-MAC protocol which requires a constant scan of the access point or inducesfrequent exchanges with a base station is therefore not appropriate for thisview. The typical implementations of wireless local area networks are inimplements features that can reduce energy consumption whenthe system is not used as standby. The robustness of transmission and security: if not properly designed, a networkWireless LAN will be subject to interference and can easily be listened to. Designa wireless LAN must provide a robust transmission includinga very noisy environment and must provide a certain level of protection againstthe tapping. The wireless local area networks becoming increasingly popular, it islikely that two or more networks functioning in the same area or in an area orinterferences between local networks are possible. Such interference caninterfere with the normal operation of a MAC algorithm and allow the wrong accesshas one of these local networks. The ability to operate without a license: users prefer to purchase and operateequipment for wireless LAN without having to reserve a license on the tapeof frequencies used by the local network. The management of mobility: the MAC protocol as used in wireless LANshould enable mobile devices to connect to another cell (han-do/roaming). The con guration dynamic MAC addressing and managing the local network mustallow the dynamic addition and automates terminals, and their removaldisplacement seamless service to other users.11. Which difference is there between a mono-cellular and multi-cellular wireless localnetwork?Wireless LAN single cell: all terminals are within reach of a similar access point.Wireless LAN multi-cell: several access points are interconnected to work-to a local wired network, each access point manages a number of terminalslocated within reach.12. What are the advantages and the disadvantages of wireless local network using infraredtechnologies?Advantages: The infrared frequency band is very large (virtually unlimited), which ore the possibility of achieving extremely high flow rates. The infrared band is regulated in any country, the beaches of microwave frequencieswave are. The infrared light is reflected diffusely by small colored objects;thus, it is possible to use Refexions ceiling to cover entirely in one room. The infrared light does not penetrate walls and opaque objects. This has two advantages:first, the infrared communication can be more easily Master of Computer Science 1 - MOB Mobile Internet and SurroundingFladenmuller Baey - ”IEEE 802.11standards” 4/8secured against the listening as microwave communications, and secondly,a separate infrared system can be operated in each piece ofbuilding without interference, which allows to build very large local networksinfrared. Another advantage of the infrared: the equipment is relatively inexpensive and simple.Disadvantages: Many equipment an indoor infrared radiation suffer interferenceintense enough from the sunlight and the lighting. These rays appear as ambient noise for aninfrared receiver, which requires the use of a higher power level than would otherwise benecessary and which also limits the scope. Increasing the power emitted is limited by the constraints of safety (eye protection) andreasonable power consumption.13. Which difference is there between an access point and a gate within the meaning of 802.11standards?For the purposes of the standard, a point of access for connecting several wireless local areanetworks 802.11 but does not access a local network of other technology.A portal provides him an interconnection point between a wireless LAN 802.11 a local wirednetwork.In practice, the access points sold in stores are also equipped with a based portal.14. List some services offered by the IEEE 802.11 standard. Authentication: used by the terminals to identify each other. Each terminal proves hisknowledge of a password. Association: establishment of an association between a terminal and an access point.Exchange of information on the possibilities of transmission of access point andterminals. Reassociation: to transfer an association established an access point to another, allowingthe roaming terminal in a BSS to another. Dissociation: hypnotics by a terminal or a point of acc are an association has ended. Aterminal must send this hypnotics before leaving the lattice or extinction of. Desauthentication: This service is invoked when an authentication ends. Distribution: used by the terminals to share them when MAC frames the frame must passthrough the distribution system (Distribution System (DS)) to pass a terminal attached toa set of basic services (Basic Service Set (BSS)) has a terminal attached to another BSS. Integration: enables the transfer of data between a terminal of a local network 802.11 anda terminal of a local network integrated 802.x (MSDU delivery: MAC Service Data Unitdelivery). Security and encryption: avoids reading the contents of messages by other persons as theintended recipient.15. How is the association concept linked to the mobility?The mobility makes reference to the types of transition can be a mobile terminal in anenvironment 802.11 no transition, roaming from one BSS to another within a same ESS roamingin an ESS has a another. Master of Computer Science 1 - MOB Mobile Internet and SurroundingFladenmuller Baey - ”IEEE 802.11standards” 5/8The association is a service that allows a terminal that has made a transition to identify theaccess point in a BSS, which then allow him to participate in exchanges of data with otherterminals.16. In what has the IEEE 802.11a standard a better potential than the IEEE 802.11b or IEEE802.11g standard?The IEEE 802.11a greater potential than other standards in the frequency band of 2.4 GHzbecause it has a much wider bandwidth. In terms of frequency, 802.11a has 8 disjoint bands inthe U.S. (200 MHz) and 13 in Europe, against 3 disjoint bands for 802.11b (83 MHz) and threebands separated for 802.11g (83 MHz) .The IEEE 802.11a, however, difficult to win because its installation is more complex and802.11g makes it an important competition because of its compatibilitywith 802.11b and its equivalent flow rates.17. Why is the effective flow lower than its theoretical flow?On the one hand, the flow varies depending on the quality of the link, the scope and interferencewith the environment. On the other hand, the use of time intervals (Inter Frames Spaces (IFS) inEnglish) used to manage access to waste a lot of time for the actual transmission.18. Wifi telephony terminals are already on sale. Why this solution is not viable in the longterm?In wireless networks, there is no QoS. The voice packets are coming into competition with othertransmissions without having a particular priority. The voice packets are then stronglyretarded.For this to work to the reasonable, must be the only wireless device connects to theaccess point. The IEEE 802.11 is expected to improve this by assigning a high priority tosensitive packet2. Access Method1. Represent graphically, with assistance of a diagram of floods, the algorithm follow-up by atransmitter to send data. Master of Computer Science 1 - MOB Mobile Internet and SurroundingFladenmuller Baey - ”IEEE 802.11standards” 6/8a station wantsto transmit(data)Listening mediumthemedium isfreeWaiting DIFSNoYesthemedium isfreeYesNotransmit (data)receipt ofan ACKNoYesdata transmissionsuccessfullimitretransmissionsdata transmissionfailedYesWait until themedia is freeWaiting DIFSTimeralreadycalculated?load timertimer backoffNodecremented by one timeslotand listens mediathemedium isfreeYestimerexpiresNoYesreceipt ofan ACKNodata transmissionsuccessfulYes Master of Computer Science 1 - MOB Mobile Internet and SurroundingFladenmuller Baey - ”IEEE 802.11standards” 7/82. Supplement the figure 1 by indicating the moments when the frames will be sent accordingto the access method define by standard 802.11. The moments when a terminal wants to emitare marks by an arrow. One will take the duration of each data frame into account as well asthe value of the withdrawal timer specific to each terminal. Is there had collision?See Fig. below.Yes, there was a collision between the issuance of Terminal 2 and the terminal 5.Fig.1 – Emission of data packets according to the DFWMAC-DCF protocol.3. When a terminal wishes to emit but notes that the support is not free, it waits for durationequal to the value of a withdrawal timer before trying new emission. The value of thiswithdrawal timer corresponds with a time slots, take out randomly in an interval includedbetween 0 and W. This interval is called contention window. The value of W variesaccording to the collisions which occur. Initially, W = 7. Each time that one collision occurs,W is double. W is however limited the maximum value of 255. When a transmission effectssuccessfully, W is reinitialized. In your opinion, why has one chosen this proceed?If the window is small, the risk of collision are increased. If it is large, there is less good use ofthe network because the terminals are inactive longer on average without delivering. In case oflow load, it is preferable that W is small.4. Does the CSMA/CA access method appear to you that it is more or less efficacy than theCSMA/CD of the Ethernet network?In CSMA / CD, each terminal shall issue the media is free. The collision detection is immediate.There is no mechanism for payment. The withdrawal mechanism is implemented only in acollision.In CSMA / CA, when a terminal wishes to emit but notes that the media is not free, it expectsoutset for a period equal to the value of a timer for withdrawal before attempting to transmitagain. An additional period is also introduced to the acquittal. This affects the utilization ofsupport and therefore effective throughput of the system but can reduce the risk of collision. Master of Computer Science 1 - MOB Mobile Internet and SurroundingFladenmuller Baey - ”IEEE 802.11standards” 8/8The resolution of the problem of hidden terminal mechanism requires the use of RTS / CTS,which also leads to further delay.The actual throughput of CSMA / CA is lower than that of an Ethernet network.5. RTS/CTS Mechanisma) Does the use of RTS/CTS packets allow avoiding collisions?No.b) What are the exchanges of RTS/CTS packets used for?To avoid the problem of hidden terminal.To reduce the risk of collision because the period of restraint corresponds to the transmission ofshort messages of control rather than data frames. . 1 - MOB Mobile Internet and SurroundingFladenmuller Baey - IEEE 802. 1 1standards 1/ 8MOB Subject 6 - IEEE. otherterminals. 16. In what has the IEEE 802. 11a standard a better potential than the IEEE 802. 11b or IEEE8 02.11g standard?The IEEE 802. 11a greater potential