1EE228a - Lecture 4 - Spring 2006WiFi OperationsScribed by Xiaoyi Tang (xiaoyi@eecs)AbstractThis lecture covers the fundamentals of IEEE standards for wireless LAN, with emphasis on the physical layer andMAC.I. ReferencesReferences for this lecture include the following.• 802.11 Wireless Networks: The Definitive Guide, M. Gast, O’Reilly 2002. Many drawings used in this lecture are fromhere.• IEEE Std 802.11, 1999 Edition• IEEE Std 802.11b-1999• IEEE Std 802.11a-1999• IEEE 802.11e-2005II. Overview of StandardsFig. 1 shows the IEEE 802 standards and their positions in the OSI model. As shown, 802.11 MAC is common to all802.11 Physical Layer (PHY) standards. 802.11 PHY is split into Physical Layer Convergence Procedure (PLCP) andPhysical Medium Dependent (PMD) sublayers.Fig. 1. IEEE 802 Standards & OSI Model.There are several related standards.• Bluetooth is originally intended for interconnecting computing, communication and entertainment devices .• HIPERLAN is an European standard for wireless LANs.• IEEE 802.16 is for broadband wireless MAN. 802.16-2004 addresses needs of fixed broadband wireless access replacingfibers, cables, DSL and etc. 802.16-2005, approved in Dec 2005, provides broadband access at vehicular speed.Table I lists key 802.11 standards and their spectrum usage. The 2.4-2.5 GHz spectrum is referred to as S-BandTABLE IKey specifications of 802.11 standardsKey Standards Max Rate Spectrum (U.S.) Year802.11 2 Mbps 2.4 GHz 1997802.11a 54 Mbps 5 GHz 1999802.11b 11 Mbps 2.4 GHz 1999802.11g 54 Mbps 2.4 GHz 2003802.11e N/A N/A 20052Industrial, Scientific, and Medical (ISM). Microwave ovens and some cordless phones operate in the same band. 802.11auses Unlicensed National Information Infrastructure bands, including 5.15-5.25 GHz, 5.25-5.35 GHz, and 5.725-5.825GHz. 802.11e provides QoS enhancements at the MAC layer, so there is no rate or spectrum specification. 802.11nhas a draft specification approved in Jan 2006. It’s based on multiple-input multiple-output (MIMO) technology whichemploys multiple antennas to increase throughput up to 600 Mbps. The MIMO technique embedded in 802.11n for whichmulti-path wireless diversity is provided can be introductorily understood by the originated paper, S. M. Alamouti, “Asimple transmit diversity technique for wireless communications”, IEEE Journal on Selected Areas in Communications,Vol.16, No.8, pp. 1451-1458, October 1998.III. BasicsBasic service set (BSS) is a set of mobile stations. There are two types of BSS depending on how stations communicate.Independent BSS In an independent BSS (also called ad-hoc BSS), stations communicate with each other directlywithout hops in a peer-to-peer fashion. It’s possible that two stations in the same BSS cannot talk due to range limit.Infrastructure BSS In an infrastructure BSS, stations communicate via a component named Acc es s Point (AP), whichis the centralized coordinator and could be the bottleneck. All packets must be sent twice, between sender and AP, andbetween AP and receiver.Fig. 2 is an illustration of the two types of BSS.Fig. 2. Independent BSS and Infrastructure BSS.Extended service set (ESS) is a set of infrastructure BSS. APs in an ESS communicate via distribution system (DS)to keep track of stations within an ESS and forward packets. Inter Access Point protocol (IAPP) is standardized by802.11f-2003. Fig. 3 shows a diagram of an ESS.Fig. 3. Diagram of an extended service set.Network services provided in a WLAN include the followings.Distribution Communication through the DS.Integration Integration with other WLAN or wired LAN.Association A station must be associated with an AP before it can send data through the AP.Reassociation Change association to a new AP. It’s needed when a station moves around an ESS to a new BSS.Disassociation Cancel an association. It can be initiated by an AP for various reasons (e.g. resource constraints) or astation if it leaves the network.Authentication Provide security control to allow only authorized access.Deauthentication Disable a previously authenticated station to access the network.3Privacy Offer data encryption based on the 802.11 Wired Equivalent Privacy (WEP) algorithm.MAC Service Data Unit (MSDU) delivery Provide reliable delivery of data frames the MAC in one station to the MACin one or more other stations.WLAN provides seamless transition b etween two BSSs within an ESS. The IP address of a station doesn’t change inthis case. However, transitions are not supported between ESSs.IV. Physical LayerA. 802.11bThe physical layer of 802.11b is based on High Rate Direct-Sequence Spread Spectrum (HR/DSSS). It uses Comple-mentary Code Keying (CCK) instead of Differential Quadrature Phase Shift Keying (DQPSK) used at lower rates. 4-bit(for 5.5 Mbps) or 8-bit (for 11 Mbps) symbols from MAC layer arrive at 1.375 million symbols per second. Each symbolis encoded using CCK code word, which provides both modulation and error correction. T he code word is from the set{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} where φ1, φ2, φ3, φ4are decided by symbol bits. It uses same channels as by the low rate DS. In US, channels 1-11 (with center frequenciesat 2.412-2.462 GHz and 5 MHz distance) are available. For 11 Mbps, Channels 1, 6, and 11 give maximum number ofchannels with minimum interference. Fig. 4 illustrates the case. Fig. 5 shows the PLCP format.Fig. 4. Spectrum of 802.11bFig. 5. 802.11b long PLCP format. Optional Short PLCP format is offered for better efficiency.B. 802.11aThe physical layer of 802.11a is based on Orthogonal Frequency Division Multiplexing (OFDM). Fundamental OFDMwork was done in 1960s, and a patent was issued in 1970. Basic idea is to use a number of subchannels in parallel forhigher throughput. OFDM is similar to Frequency Division Multiplexing except it does not need guard bands. But itneed guard times to minimize inter-symbol and inter-carrier interference. It relies on “orthogonality” in the frequencydomain. Fig. 6 shows an example of OFDM spectrum.It remains to be seen if 802.11a will be a success because there are still unanswered questions. For example, is denseraccess point deployment needed due to higher path loss? Will higher power need be a hindrance?In U.S., there are 12 channels, each 20 MHz wide. Fig. 7 shows the specifications of the 12 channels. Fig. 8 shows thespectrum layout of 802.11a. Each
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