IEEE Communications Magazine • September 1997116IEEE 802.11Wireless Local Area Networks0163-6804/97/$10.00 © 1997 IEEEireless computing is a rapidly emerging technologyproviding users with network connectivity withoutbeing tethered off of a wired network. Wireless local area net-works (WLANs), like their wired counterparts, are beingdeveloped to provide high bandwidth to users in a limitedgeographical area. WLANs are being studied as an alternativeto the high installation and maintenance costs incurred by tra-ditional additions, deletions, and changes experienced inwired LAN infrastructures. Physical and environmental neces-sity is another driving factor in favor of WLANs. Typically,new building architectures are planned with network connec-tivity factored into the building requirements. However, usersinhabiting existing buildings may find it infeasible to retrofitexisting structures for wired network access. Examples ofstructures that are very difficult to wire include concretebuildings, trading floors, manufacturing facilities, warehouses,and historical buildings. Lastly, the operational environmentmay not accommodate a wired network, or the network maybe temporary and operational for a very short time, makingthe installation of a wired network impractical. Exampleswhere this is true include ad hoc networking needs such asconference registration centers, campus classrooms, emergen-cy relief centers, and tactical military environments.Ideally, users of wireless networks will want the same ser-vices and capabilities that they have commonly come to expectwith wired networks. However, to meet these objectives, thewireless community faces certain challenges and constraintsthat are not imposed on their wired counterparts.Frequency Allocation — Operation of a wireless networkrequires that all users operate on a common frequency band.Frequency bands for particular uses must typically be approvedand licensed in each country, which is a time-consuming pro-cess due to the high demand for available radio spectrum.Interference and Reliability — Interference in wireless com-munications can be caused by simultaneous transmissions (i.e.,collisions) by two or more sources sharing the same frequencyband. Collisions are typically the result of multiple stations wait-ing for the channel to become idle and then beginning transmis-sion at the same time. Collisions are also caused by the “hiddenterminal” problem, where a station, believing the channel isidle, begins transmission without successfully detecting thepresence of a transmission already in progress. Interference isalso caused by multipath fading, which is characterized by ran-dom amplitude and phase fluctuations at the receiver. Thereliability of the communications channel is typically mea-sured by the average bit error rate (BER). For packetizedvoice, packet loss rates on the order of 10–2are generallyacceptable; for uncoded data, a BER of 10–5is regarded asacceptable. Automatic repeat request (ARQ) and forwarderror correction (FEC) are used to increase reliability.Security — In a wired network, the transmission medium canbe physically secured, and access to the network is easily con-trolled. A wireless network is more difficult to secure, sincethe transmission medium is open to anyone within the geo-graphical range of a transmitter. Data privacy is usuallyaccomplished over a radio medium using encryption. Whileencryption of wireless traffic can be achieved, it is usually atthe expense of increased cost and decreased performance.Power Consumption — Typically, devices connected to awired network are powered by the local 110 V commercial powerprovided in a building. Wireless devices, however, are meant tobe portable and/or mobile, and are typically battery powered.Therefore, devices must be designed to be very energy-effi-Brian P. Crow, The MITRE CorporationIndra Widjaja, Fujitsu Network CommunicationsJeong Geun Kim, University of ArizonaPrescott T. Sakai, Cypress SemiconductorWABSTRACTThe draft IEEE 802.11 Wireless Local Area Network (WLAN) specification is approaching completion. In this article, the IEEE 802.11protocol is explained, with particular emphasis on the medium access control sublayer. Performance results are provided for packetizeddata and a combination of packetized data and voice over the WLAN. Our performance investigation reveals that an IEEE 802.11network may be able to carry traffic with time-bounded requirements using the point coordination function. However, our findingssuggest that packetized voice traffic must be handled in conjunction with an echo canceler.The views and opinions expressed in this article are those of the authorsand do not reflect MITRE’s or Fujitsu Network Communications’ currentposition.IEEE Communications Magazine • September 1997117cient, resulting in “sleep” modes andlow-power displays, causing users tomake cost versus performance andcost versus capability trade-offs.Human Safety — Research is ongo-ing to determine whether radio fre-quency (RF) transmissions from radioand cellular phones are linked tohuman illness. Networks should bedesigned to minimize the powertransmitted by network devices. Forinfrared (IR) WLAN systems, optical transmitters must bedesigned to prevent vision impairment.Mobility — Unlike wired terminals, which are static whenoperating on the network, one of the primary advantages ofwireless terminals is freedom of mobility. Therefore, systemdesigns must accommodate handoff between transmissionboundaries and route traffic to mobile users.Throughput — The capacity of WLANs should ideallyapproach that of their wired counterparts. However, due tophysical limitations and limited available bandwidth, WLANsare currently targeted to operate at data rates between 1–20Mb/s. To support multiple transmissions simultaneously,spread spectrum techniques are frequently employed.Currently, there are two emerging WLAN standards: theEuropean Telecommunications Standards Institute (ETSI)High-Performance European Radio LAN (HIPERLAN) andthe IEEE 802.11 WLAN. Both draft standards cover the phys-ical layer and medium access control (MAC) sublayer of theopen systems interconnection (OSI) seven-layer referencemodel. The HIPERLAN committee has identified the5.15–5.30 GHz and 17.1–17.2 GHz bands for transmission.The 5 GHz band has been ratified for HIPERLAN use by theConference of European Postal and TelecommunicationsAdministrations (CEPT). Data rates up to
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