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1、Dr.Muid MuftiID TechnologiesIslamabad,Introduction to IEEE 802.11 Protocol,What 802.11?,802.11 is an IEEE standard for MAC and Physical Layer for Wireless Local Area Network(WLAN).,Why Standard?,Multi Vender inter operabilityProtects customer investmentEconomies of scale,Why not Wireless Ethernet?,E
2、thernet is simple,widely used,cheap.ButCollision DetectionNot possible in wirelessWould require a full duplex radioReceiver sensitivityCarrier SensesHidden StationsMobilityPower Save,802.11 Versions,802.11-1997Maximum data rate:2Mbps2.4Ghz bandIndoor Range:20 metersOutdoor Range:100 metersWide range
3、 of Physical layers802.11a-1999Maximum data rate:54Mbps5.1-5.8Ghz bandIndoor range:35 metersOutdoor range:120 meters54 Mbps,802.11 Versions,802.11b-1999Maximum data rate:11Mbps2.4Ghz bandIndoor Range:38 metersOutdoor Range:140 meters11 Mbps802.11g-2003The current industry adopted specificationMaximu
4、m data rate:54Mbps2.4Ghz band(backwards compatible with 802.11b)Indoor range:38 metersOutdoor range:140 meters 54 Mbps,Operating Modes,Adhoc NeworkInfrastructure Network,Adhoc Network,Infrastructure Network,What is WiFi?,A trademark of the Wi-Fi AllianceFounded in 1999 as WECA(Wireless Ethernet Comp
5、atibility Alliance).More than 300 companiesWiFi certification warrants interoperability between different wireless devicesEnsures correct implementation IEEE 802.11Tests the wireless components to their own terms of reference,Basic Service Set(BSS),Extended Service Set(ESS),Portal,Distribution Syste
6、m,Services,Station services:authentication,de-authentication,privacy,delivery of dataDistribution ServicesassociationdisassociationreassociationdistributionIntegration,Medium Access Control,MAC,Medium Access Control,FunctionalityReliable data deliveryFairly control access Protection of dataDeals wit
7、hNoisy and unreliable mediumFrame exchange protocol-ACKHidden Node Problem RTS/CTSParticipation of all stationsReaction to every frame,Coordination Functions,Distributed Coordination Function(DCF)Medium access is contended by all the members of the network Point Coordination Function(PCF)Access Poin
8、t is solely responsible for medium access,MAC Mechanism,Retry CountersShort retry counterLong retry counterLifetime timerBasic Access MechanismCSMA/CABinary exponential back-offNAV Network Allocation VectorTiming IntervalsSIFS,Slot Time,PIFS,DIFS,EIFS,CSMA/CA,Physical Carrier SenseVirtual Carrier Se
9、nseNetwork Allocation Vector(NAV),DCF Operation,PCF Operation,Poll eliminates contentionPC Point CoordinatorPolling ListOver DCFPIFSCFP Contention Free PeriodAlternate with DCFPeriodic Beacon contains length of CFPCF-Poll Contention Free PollNAV prevents during CFPCF-End resets NAV,Frame Types,Proto
10、col VersionFrame Type and Sub TypeTo DS and From DSMore Fragments Retry Power Management More Data WEPOrder,FC,Duration/ID,Address 1,Address 2,Address 3,SequenceControl,Address4,DATA,FCS,22666260-23124 bytes,NAV informationOrShort Id for PS-Poll,BSSID BSS IdentifierTA-Transmitter RA-ReceiverSA-Sourc
11、eDA-Destination,IEEE 48 bit addressIndividual/Group Universal/Local46 bit address,MSDUSequence NumberFragment Number,CCIT CRC-32 Polynomial,Upper layer data2048 byte max256 upper layer header,Frame Subtypes,RTSCTSACKPS-PollCF-End&CF-End ACK,DataData+CF-ACKData+CF-PollData+CF-ACK+CF-PollNull Function
12、CF-ACK(nodata)CF-Poll(nodata)CF-ACK+CF+Poll,BeaconProbe Request&ResponseAuthenticationDeauthenticationAssociation Request&ResponseReassociation Request&ResponseDisassociationAnnouncement Traffic Indication Message(ATIM),CONTROL,DATA,MANAGEMENT,Other MAC Operations,FragmentationSequence control field
13、In burstMedium is reservedNAV is updated by ACK,PrivacyWEP bit set when encrypted.Only the frame body.Medium is reservedNAV is updated by ACKSymmetric variable key,MAC Management,Interference by users that have no concept of data communication.Ex:MicrowaveInterference by other WLANsSecurity of dataM
14、obilityPower Management,Authentication,AuthenticationProve identity to another station.Open system authenticationShared key authenticationA sendsB responds with a textA encrypt and send backB decrypts and returns an authentication management frame.May authenticate any number of station.,Security Pro
15、blemA rogue APSSID of ESSAnnounce its presence with beaconingA active rogue reach higher layer data if unencrypted.,Authentication Frame Sequence,Station,AP,Authentication Request 1(authentication Type),Authentication Response 1Challenge Text,Authentication Request 2Encrypted packet,Authentication R
16、esponse 2Pass/Fail,Association,AssociationTransparent mobilityAfter authenticationAssociation request to an APAfter established,forward dataTo BSS,if DA is in the BSS.To DS,if DA is outside the BSS.To AP,if DA is in another BSS.To“portal”,if DC is outside the ESS.Portal:transfer point:track mobility
17、.(AP,bridge,or router)transfer 802.1hNew AP after reassociation,communicates with the old AP.,Association Frame Sequence,Station,AP,Association Request,Association Response,Power Management,Independent BSSDistributedData frame handshakeWake up every beacon.Awake a period of ATIM after each beacon.Se
18、nd ACK if receive ATIM frame&awake until the end of next ATIM.Estimate the power saving station,and delay until the next ATIM.Multicast frame:No ACK:optional,OverheadSenderAnnouncement frameBuffer Power consumption in ATIMReceiverAwake for every Beacon and ATIM,Power Management,Infrastructure BSSCen
19、tralized in the AP.Greater power savingMobile Station sleeps for a number of beacon periods.Awake for multicast indicated in DTIM in Beacon.AP buffer,indicate in TIMMobile requests by PS-Poll,Power Save Timing,Beacon,Beacon,Beacon,Beacon,DTIM,DTIM,DTIM,DTIM,DTIM,Power Save Frame Sequence,Station,AP,
20、Data Packet with PS=1,PVBM bit=1,PS-POLL,Data,Sleep Mode,ACK,Hidden Node Problem,RTS CTS Frame Sequence,Station,AP,RTS,CTS,Data,ACK,Synchronization,Timer Synchronization in an Infrastructure BSSBeacon contains TSFStation updates its with the TSF in beacon.Timer Synchronization in an IBSSDistributed.
21、Starter of the BSS send TSF zero and increments.Each Station sends a BeaconStation updates if the TSF is bigger.Small number of stations:the fastest timer value Large number of stations:slower timer value due to collision.Synchronization with Frequency Hopping PHY LayersChanges in a frequency hoppin
22、g PHY layer occurs periodically(the dwell meriod).Change to new channel when the TSF timer value,modulo the dwell period,is zero,Scanning&Joining,ScanningPassive Scanning:only listens for Beacon and get info of the BSS.Power is saved.Active Scanning:transmit and elicit response from APs.If IBSS,last
23、 station that transmitted beacon responds.Time is saved.Joining a BSSSyncronization in TSF and frequency:Adopt PHY parameters:The BSSID:WEP:Beacon Period:DTIM,802.11aOFDM,Physical Layer,OFDM Transmitter,Scrambler,Convolution Encoder,Puncturer,Interleaver,Mapper,IFFT,Cylix Prefix,Pilots,Data,Preamble
24、,Data,Tx Samples,Data,OFDM Receiver,Descrambler,Depuncturer,Deinterleaver,Demapper,Viterbi Decoder,Cylix Prefix,Pilots,Preamble,Data,Rx Samples,Data,Estimation,FFT,Estimation,Definitions IEEE 802.11a,Scrambler,S(x)=x7+x4+1Repeats after 127 bits,Convolutional encoder,Constraint length=7Industry-stand
25、ard generator polynomials,g0=(133)8 and g1=(171)8,Puncturing Coding Rate 3/4,Puncturing Coding Rate 2/3,Interleaving,First permutation i=(NCBPS/16)(k mod 16)+floor(k/16)k=0,1,NCBPS 1Second permutation j=s floor(i/s)+(i+NCBPS floor(16 i/NCBPS)mod s i=0,1,NCBPS 1s=max(NBPSC/2,1),Modulation(Mapping),Mo
26、dulation(Mapping),Pilots Insertion,IFFT,52 Data Subcarriers4 Pilots64 Point IFFT,OFDM Frequency Usage,Cyclic Prefix,Copy,64 pt IFFT out,80 samples OFDM Symbol,IQ Modulation,I,Q,Sin(wct),Cos(wct),Output,Physical Layer,802.11b,FHSS,Frequency Hopping Spread Spectrum1 and 2 MbpsBand 2400-2483.5 MHzGFSK(
27、Gaussian Frequency Shift Keying)Sub-channels of 1 MHzOnly 79 channels of the 83 are usedSlow hopping(2.5 hops per second)3 main sets each with 26 different hopping sequences,CCK,5.5 and 11 MbpsBarker sequences spreading(3 to 13 bits)with very good autocorrelation propertiesReplaced by Complementary
28、Code Keying(CCK)Eight chips,where each chip is a complex QPSK bit-pair at a chip rate of 11Mchip/sIn 5.5 Mbit/s and 11 Mbit/s modes respectively 4 and 8 bits are modulated onto the eight chips of the symbol c0,.,c7,802.11g,Modulation Scheme,Backward compatible with 802.11bOFDM when operating independentlyCCK when operating with 802.11b nodes,Extended Features,RTS to selfFlexible Modulation schemes,Comparison,
