1IEEE 802.11 Wireless LANsReferencesStandardsBasicsPhysical Layer802.11b802.11aMACFraming DetailsManagementPCFQoS (802.11e)SecurityTake Away PointsTOC – 802.11References802.11 Wireless Networks: The Definitive Guide, M. Gast, O’Reilly, 2002*ANSI/IEEE Std 802.11, 1999 EditionANSI/IEEE Std 802.11b-1999ANSI/IEEE Std 802.11a-1999*Most drawings used in the lectures are from this bookTOC – 802.11 – ReferencesIEEE 802 Standards & OSI ModelObserve 802.11 MAC is common to all 802.11 Physical Layer (PHY) standards802.11 PHY is split into Physical Layer Convergence Procedure (PLCP) and Physical Medium Dependent (PMD) sublayersTOC – 802.11 – StandardsRelated Standards BluetoothOriginally intended for interconnecting computing and communication devices HIPERLANEuropean standard for Wireless LANs IEEE 802.16 Broadband WirelessAddresses needs of fixed broadband wireless access replacing fibers, cables, etc.TOC – 802.11 – Standards802.11 Standards and Spectrum2003199919991997Year2.4 GHz11 Mbps802.11b2.4 GHz54 Mbps802.11g5 GHz54 Mbps802.11a2.4 GHz2 Mbps802.11Spectrum (U.S.)Max RateKey Standards2.4 – 2.5 GHz for all above except 802.11a (referred to as C-Band Industrial, Scientific, and Medical (ISM))Microwave ovens and some cordless phones operate in the same band802.11a uses Unlicensed National Information Infrastructure bands5.15 – 5.25 GHz5.25 – 5.35 GHz5.725 – 5.825 GHzTOC – 802.11 – StandardsBasic Service Sets (BSSs)Independent BSSs are also referred to as Ad Hoc BSSsObserve that the AP in an Infrastructure BSS is the centralized coordinator and could be a bottleneckTOC – 802.11 – Basics2Extended Service Set (ESS)BSSs in an ESS communicate via Distribution SystemA DS has to keep track of stations within an ESSInter Access Point protocol (IAPP) is not yet standardizedTOC – 802.11 – BasicsNetwork ServicesDistributionIntegrationAssociationReassociationDisassociationAuthenticationDeauthenticationPrivacyMAC Service Data Unit (MSDU) deliveryTOC – 802.11 – BasicsSeamless TransitionSeamless transition between two BSSs within an ESSBetween ESSs, transitions are not supportedTOC – 802.11 – Basics802.11b: HR/DSSS*PHYUse Complementary Code Keying (CCK) instead of Differential Quadrature Phase Shift Keying (DQPSK) used at lower rates4-bit (for 5.5 Mbps) or 8-bit (for 11 Mbps) symbols form MAC layer arrive at 1.375 million symbols per secondEach symbol is encoded using CCK code word{ej(φ1+φ2+φ3+φ4), ej(φ1+φ3+φ4), ej(φ1+φ2+φ4), −ej(φ1+φ4), ej(φ1+φ2+φ3), ej(φ1+φ3), −ej(φ1+φ2), ejφ1}φ1, φ2, φ3, and φ4 are decided by symbol bits*High Rate Direct-Sequence Spread SpectrumTOC – 802.11 – Physical Layer – 802.11b802.11b: HR/DSSS PHY - 2 Uses same channels as by the low rate DS In US, channels 1-11 (with center frequencies at 2.412 –2.462 GHz and 5 MHz distance) are available For 11 Mbps, Channels 1, 6, and 11 give maximum number of channels with minimum interferenceTOC – 802.11 – Physical Layer – 802.11b802.11b: HR/DSSS PHY - 3 Long PLCP format Optional Short PLCP format is offered for better efficiencyTOC – 802.11 – Physical Layer – 802.11b3802.11a: 5 GHz OFDM PHYFundamental Orthogonal Frequency Division Multiplexing (OFDM) work was done in 1960s, and a patent was issued in 1970Basic idea is to use number of subchannels in parallel for higher throughputWill 802.11a be a success?Denser Access Point deployment needed due to higher path loss?Will higher power need be a hindrance?TOC – 802.11 – Physical Layer – 802.11a802.11a: 5 GHz OFDM PHY - 2 OFDM is similar to Frequency Division Multiplexing except it does not need guard bandsBut need guard times to minimize inter-symbol and inter-carrier interference Relies on “orthogonality” in frequency domainTOC – 802.11 – Physical Layer – 802.11a802.11a: 5 GHz OFDM PHY - 3In U.S., there are 12 channels, each 20 MHz wideSpectrum layoutTOC – 802.11 – Physical Layer – 802.11a802.11a: 5 GHz OFDM PHY - 4Each channel is divided into 52 subchannels: 48 are used for dataPLCP Protocol Data Unit (PPDU) formatPHY uses rate of 250K symbols per secondEach symbol uses all 48 channelsConvolution code is used by all subchannelsTOC – 802.11 – Physical Layer – 802.11aMAC: Access ModesMAC Access Modes:Distributed Coordination Function (DCF) Based on Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)Point Coordination Function (PCF) Restricted to Infrastructure BSSs Not widely implemented Access Point polls stations for medium accessTOC – 802.11 – MACMain Ideas of MAC: CSMA/CA Interframe Spacing (IFS)Short IFS: For atomic exchangesPCF IFS: For prioritized PCF accessDCF IFS: For Normal DCF accessExtended IFS: For access after error Medium AccessTOC – 802.11 – MAC4Main Ideas of MAC: CSMA/CA - 2 If medium is idle for DIFS interval after a correctly received frame and backoff time has expired, transmission can begin immediately If previous frame contained errors, medium must be free for EIFS If medium is busy, access is deferred until medium is idle for DIFS and exponential backoff Backoff counter is decremented by one if a time slot is determined to be idle Unicast data must be acknowledged as part of an atomic exchangeTOC – 802.11 – MACInterframe SpacingInterframe Spacing values are physical layer dependentSIFS and Slot_Time are explicitly specified, and the others are derivedPIFS = SIFS + Slot_TimeDIFS = SIFS + 2·Slot_TimeEIFS = SIFS + DIFS + (Ack_Time @ 1 Mbps)For 802.11a and 802.11bSIFS is 16 µs and 10 µs, respectivelySlot_Time is 9 µs and 20 µs, respectivelyTOC – 802.11 – MACContention WindowBackoff is performed for R slots: R is randomly chosen integer in the interval [0, CW]CWmin CW CWmaxCWmin= 31 slots and CWmax= 1023 slots (for 802.11b)Up to CWmax, CW = (CWmin+ 1)·2n– 1, where n = 0, 1, 2, … is (re)transmission numberTOC – 802.11 – MACError Recovery Each frame is associated with a retry counter based on frame size as compared to RTS/CTS thresholdShort retry counterLong retry counter Fragments are given a maximum lifetime by MAC before discarding themTOC – 802.11 – MACWLAN ProblemsHidden Terminal and Exposed
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