WIRELESSUNs
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art technologies in wireireless local area networks emerged from less LAN research. They near disaster in the cover a very broad scope, mid-’90s to become one of including all key layers today’s driving wireless techof the network hierarnologies, delivering high data chy. The first two artirateson unlicenseds p e c ” for clesdealwithphysicallayer both the enterprise and the research. The article “Cohome. Channel Interference The fact that U S . megasCancellation Based on BENNY BlNG CHRISHEEGARD Bo8 HELE tores such as Starhucks and MIMO OFDM Systems” Sears are deployingwireless L 9 N s shows how prevalent the by Luca Giangaspero et al. demonstrates how multiple technologyhasnowbecome. However, there are severalmajor input multiple output (MIMO) systems can be exploited for wireless LAN standards (802.11a, 802.11b, and most recentbroadband wireless indoor applications. Two MIMO ly R02.11g) operating on different frequency hands (2.4 and OFDM-based systems, Wind-Flex and Ubiquitous Antenna, 5 GHz), and it is unclear which technology will eventually are considered. The authors aim to increase the system capacity of both systems using different approaches: prevail, or whether combo solutions involving combined increasing the single link data rate and increasing the numstandards are required. With new wireless personal devices ber of users in the entire system. The next article, “Reduced involving Bluetooth starting to gain traction commercially, Dimension Space-Time Processing for Multi-Antenna thewirelessworld continues to create difficult optionsfor both end users and service providers. Wireless Systems” by Jens Jelitto and Gerhard Fettweis, Wireless LANs provide high-speed, cable-free access priintroduces and evaluates a new space-time processing concept that reduces thc signal dimension space at the receiver marily for indoor environments. A substantial portion of the cost of LAN deployment is in interconnecting end-user by exploiting spatial correlation properties of received signals. Since the antenna signals are treated as independent devices,which many networkingexpertsachowledge can sometimes exceed the cost of computer hardware and software. signal components when they arrive at the receiver, the proA wireless LAN removes the labor and material costs inherposed technique involves no special assumptions concernent in wiring. It also offers the flexibility to reconfigure or ing the structure of the antenna. Furthermore, no feedback information is required at the transmitter. Although the add more nodes to the network without much planning effortand the cost of recabling, therebymaking future upgrades newconcept isstudied in thecontextofsingle transmit andmulinexpensive and easy. The ability to add new mobile comtiple receive antennas, it does not prevent the application of multipletransmit antennasormultiple data streamscommonly putingdevicesquicklyis another main consideration forchoosingawirelessLAN. Thus,theproliferation ofcheaper, smaller, applied in space-time coding schemes, and more powerful portable notebook computers has For radio LANs, sharing of bandwidth is essential fueled tremendous growth in the wireless LANindustry in because radio spectrum is not only expensive but also inherrecent years. ently finite. This is in contrast to wired networks, where A wireless LAN need not transfer purely data traffic. It bandwidth can be increased arbitrarily by adding extra can also support packetized voice transmission. People cables. However, the broadcast nature of the wirelesslinkposes today are spending huge amounts of money, even from a difficult problem for multiple access in that the success of office tooffice, calling on cellularphones. With awirelessLAN a transmission is no longer independent of other transmisinfrastructure, it costs them much less than it does using sion. To make a transmission successful, interference must cellular phones or any other equipment. A more compelling he avoided 0; at least controlled. Otherwise, multiple transuse ofwireless LANs is in overcoming the inherent limitamissions may lead to collisions and corrupted signals. A tions of wireless WANs. Current 3G wireless data rates go multipleaccess (or medium access control) protocol is required. up to 2 Mhis only with restricted mobility, whereas wireless to resolve these accesscontentionsamong nodes and transform LANs offer data rates of up to 54 Mbis and operate on unliabroadcast wireless LAN into alogicalpoint-to-point network. censedfrequencyhandsThis has ledtosome technologistspreIn “Adequacybetween MultimediaApplicationRequirements dicting that eventually we are more likely t o see dense and Wireless Protocols Features,” the authors describe and urban broadband wireless LANs that are linked together compare the medium access control mechanisms in the into one network than widespread use of high-powered IEEE 802.11, Bluetooth, HomeRF, and HIPERLAN stanWAN handsets cramming many bits into expensive and nardards. Inordertosupportmultimediaapplications,the authors row slices of radio spectrum. suggest service differentiation and trafficclasses tobe provided The articles in this special issue focus on the state-of-theby the underlying wireless network. To achieve these goals,
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IEEE Wireless Communications * December 2002
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apriority-based multiple access protoml and the efficientmanagement of different traffic queues are required. The use of the Internet over mobile devices is becoming increasingly pervasive. However, a well-known problem for TCP connections over wireless links is that errors introduced by the wireless channel interfere with the TCP protocol, which operates well on a reliable physical link. Several methods to optimize TCP transmission over 802.11h wireless LANs are discussed in the article "Vertical Optimization of Data Terminals for Mobile Wireless Terminals," including improvements in power consumption, throughput, delay, and jitter. T h e performance improvements require modifications only on the mobile device, therehyallowingimprovedperformance evenwhen amobile terminalroams into different wireless LANs. The free-space wireless link is more susceptible to eavesdropping, fraud, and unauthorized transmission than its wired counterpart. Unauthorized people can tap the radio signal from anywhere within range. If someone sets a mobile terminal within a wireless coverage area to transmit packets endlessly, all other clients are prevented from transmitting, thus bringing the network down. Being an open medium with no precise bounds makes it impractical to apply physical security like in wired networks. Nevertheless, several security mechanisms can be used to prevent unauthorized access of data transmitted over a wireless LAN. The article "Your 802.11 Wireless Network has No Clothes" by William Arbaugh et al., discusses the flaws in the security mechanisms used by most access points supporting the IEEE 802.11 wireless standard. A key management system based on the Dynamic Host Configuration Protocol (DHCP) for mobile terminals aswell as a higher-layer authentication system and the use of a higher-layer transport mechanism (e& IPSec) are proposed andevaluated. The combination of these mechanisms provides a robust interim solution until hardware supporting the new standards is deployed. Being layer2 technologies, wirelessLANsofferlimited Internetroamingcapabilities andglobal user managementfeatures, including billing and'identification features. The final article, "Cellular Access Control and Charging for Mobile Operator Wireless Local Area Networks" by Henry Haverinen, Jouni Mikkonen, and Timo Takamaki, describes a system that efficiently combines wireless LAN access with the widely deployed Global System for Mobile Communicationsicenera1 Packet Radio Service (GSMIGPRS) roaming infrastructure. In addition, the architecture exploits GSM
IEEE Wireless Communications December 2002
authentication, SIM-based user management, and secured billing mechanisms. This gives the cellular operator a major competitive advantage overlntemet serviceproviderswho have neither a large mobile customer base nor a cellular-type roaming service. While service management, user trust, and global roamingare keyfactors injustifyingthearchitecturepresented, we also note the critical role assumed by the Extensible Authentication Protocol (EAP) in ensuring network security. The proposed architecture is currently being commercializedandisgenericenoughto heusedon anyaccessnetwork that supports EAP. . The guest editors are grateful to the authors and reviewers for their hard work in ensuring that'some very fine articles are published in this feature topic. It is hoped that the articles will stimulate more innovative research in this important subject area. ~
BIOGRAPHIES BENNY BING (
[email protected]) is a research faculty member with the School of Electrical and Computer Engineering a t the Georgia Institute of Technology. He has published over 30 technical papers and six books. including Wireless Local Area Networks. which has been adopted by Cisco Syrtemr worldwide. He is a technical consultant to several wireless and networking companies. and organizes the International Conference on Wireless LAN$ and Home Networks ( w . i c w l h n . o r g ) . Recently, he war featured in the MIT Technology Review in a special isme on wired and wirelesstechnologies. His CUP rent research interests include wirelerr LANr, cable networks. optical networks, protocol design. and queuing theory. CHRIS HEEGARD [F] i s currently an independent investor, consultant. and cattle
rancher. Previously he was the chief technology officer for the Texas Initrumenti Wireless and HameNetworkingBurinersUnits. Heco-foundedandsewed
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