292 THE MEDIUM ACCESS CONTROL SUBLAYER CHAP. 44.4 WIRELESS LANSAlthough Ethernet is widely used, it is about to get some competition. Wire-less LANs are increasingly popular, and more and more office buildings, airports,and other public places are being outfitted with them. Wireless LANs can operatein one of two configurations, as we saw in Fig. 1-35: with a base station andwithout a base station. Consequently, the 802.11 LAN standard takes this intoaccount and makes provision for both arrangements, as we will see shortly.We gave some background information on 802.11 in Sec. 1.5.4. Now is thetime to take a closer look at the technology. In the following sections we willlook at the protocol stack, physical layer radio transmission techniques, MACsublayer protocol, frame structure, and services. For more information about802.11, see (Crow et al., 1997; Geier, 2002; Heegard et al., 2001; Kapp, 2002;O’Hara and Petrick, 1999; and Severance, 1999). To hear the truth from themouth of the horse, consult the published 802.11 standard itself.4.4.1 The 802.11 Protocol StackThe protocols used by all the 802 variants, including Ethernet, have a certaincommonality of structure. A partial view of the 802.11 protocol stack is given inFig. 4-25. The physical layer corresponds to the OSI physical layer fairly well,but the data link layer in all the 802 protocols is split into two or more sublayers.In 802.11, the MAC (Medium Access Control) sublayer determines how the chan-nel is allocated, that is, who gets to transmit next. Above it is the LLC (LogicalLink Control) sublayer, whose job it is to hide the differences between the dif-ferent 802 variants and make them indistinguishable as far as the network layer isconcerned. We studied the LLC when examining Ethernet earlier in this chapterand will not repeat that material here.The 1997 802.11 standard specifies three transmission techniques allowed inthe physical layer. The infrared method uses much the same technology as televi-sion remote controls do. The other two use short-range radio, using techniquescalled FHSS and DSSS. Both of these use a part of the spectrum that does notrequire licensing (the 2.4-GHz ISM band). Radio-controlled garage door openersalso use this piece of the spectrum, so your notebook computer may find itself incompetition with your garage door. Cordless telephones and microwave ovens al-so use this band. All of these techniques operate at 1 or 2 Mbps and at lowenough power that they do not conflict too much. In 1999, two new techniqueswere introduced to achieve higher bandwidth. These are called OFDM and HR-DSSS. They operate at up to 54 Mbps and 11 Mbps, respectively. In 2001, asecond OFDM modulation was introduced, but in a different frequency band fromthe first one. Now we will examine each of them briefly. Technically, these be-long to the physical layer and should have been examined in Chapter 2, but sincethey are so closely tied to LANs in general and the 802.11 MAC sublayer, weSEC. 4.4 WIRELESS LANS 293802.11Infrared802.11FHSS802.11DSSS802.11aOFDM802.11bHR-DSSSMACsublayerPhysicallayerData linklayerUpperlayersLogical link control802.11gOFDMFigure 4-25. Part of the 802.11 protocol stack.treat them here instead.4.4.2 The 802.11 Physical LayerEach of the five permitted transmission techniques makes it possible to send aMAC frame from one station to another. They differ, however, in the technologyused and speeds achievable. A detailed discussion of these technologies is farbeyond the scope of this book, but a few words on each one, along with some ofthe key words, may provide interested readers with terms to search for on theInternet or elsewhere for more information.The infrared option uses diffused (i.e., not line of sight) transmission at 0.85or 0.95 microns. Two speeds are permitted: 1 Mbps and 2 Mbps. At 1 Mbps, anencoding scheme is used in which a group of 4 bits is encoded as a 16-bit code-word containing fifteen 0s and a single 1, using what is called Gray code. Thiscode has the property that a small error in time synchronization leads to only asingle bit error in the output. At 2 Mbps, the encoding takes 2 bits and produces a4-bit codeword, also with only a single 1, that is one of 0001, 0010, 0100, or 1000.Infrared signals cannot penetrate walls, so cells in different rooms are well iso-lated from each other. Nevertheless, due to the low bandwidth (and the fact thatsunlight swamps infrared signals), this is not a popular option.FHSS (Frequency Hopping Spread Spectrum) uses 79 channels, each 1-MHz wide, starting at the low end of the 2.4-GHz ISM band. A pseudorandom294 THE MEDIUM ACCESS CONTROL SUBLAYER CHAP. 4number generator is used to produce the sequence of frequencies hopped to. Aslong as all stations use the same seed to the pseudorandom number generator andstay synchronized in time, they will hop to the same frequencies simultaneously.The amount of time spent at each frequency, the dwell time, is an adjustableparameter, but must be less than 400 msec. FHSS’ randomization provides a fairway to allocate spectrum in the unregulated ISM band. It also provides a modicumof security since an intruder who does not know the hopping sequence or dwelltime cannot eavesdrop on transmissions. Over longer distances, multipath fadingcan be an issue, and FHSS offers good resistance to it. It is also relatively insensi-tive to radio interference, which makes it popular for building-to-building links.Its main disadvantage is its low bandwidth.The third modulation method, DSSS (Direct Sequence Spread Spectrum), isalso restricted to 1 or 2 Mbps. The scheme used has some similarities to theCDMA system we examined in Sec. 2.6.2, but differs in other ways. Each bit istransmitted as 11 chips, using what is called a Barker sequence. It uses phaseshift modulation at 1 Mbaud, transmitting 1 bit per baud when operating at 1Mbps and 2 bits per baud when operating at 2 Mbps. For years, the FCC requiredall wireless communications equipment operating in the ISM bands in the U.S. touse spread spectrum, but in May 2002, that rule was dropped as new technologiesemerged.The first of the high-speed wireless LANs, 802.11a, uses OFDM (Orthogo-nal Frequency Division Multiplexing) to deliver up to 54 Mbps in the wider 5-GHz ISM band. As the term FDM suggests, different frequencies are used—52 ofthem, 48 for data and 4 for synchronization—not unlike ADSL. Since transmis-sions are present on multiple frequencies at the same time, this
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