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.These devices lookand function like access points (APs), but they are not tetheredto the wired network, as are APs.EPs function as repeaters byboosting signal strength to extend the range of the network byrelaying signals from a client to an AP or another EP.Another component of wireless LANs is the directionalantenna.If a wireless LAN in one building must be connectedto a wireless LAN in another building a mile away, one solu-tion might be to install a directional antenna on the twobuildings each antenna targeting the other and connectedto its own wired network via an access point (Figure W-13). 450 WIRELESS LANSSwitch or HubFigure W-11 Multiple access points extend wireless coverage and enableroaming.Switch or HubFigure W-12 Use of an extension point to extend the reach of a wirelessnetwork. WIRELESS LANS 451Switch or Switch orHub HubWireless LinkDirectional AntennaBuilding A Building BFigure W-13 Adirectional antenna can be used to interconnect wirelessLANs in different buildings.Wireless LAN StandardsThere are several wireless LAN standards, each suited for aparticular environment: IEEE 802.11a and 802.11b, HomeRF,and Bluetooth.For the residential and office environments, the IEEE802.11a offers a data transfer rate of up to 11 Mbps at a rangeof up to 300 feet from the base station.It operates in the 2.4-GHz band and transmits via the direct-sequence spread-spec-trum method.Multiple base stations can be linked to increasethat distance as needed, with support for multiple clients peraccess point.IEEE 802.11a specifies the 5-GHz frequencyband, offering a data transfer rate of up to 54 Mbps.The HomeRF 2.0 Standard draws from IEEE 802.11b andDigital Enhanced Cordless Telecommunication (DECT), a pop-ular standard for portable phones worldwide.Operating in the2.4-GHz band, HomeRF was designed from the ground up forthe home market for both voice and data.It offers throughputrates comparable to IEEE 802.11b and supports the samekinds of terminal devices in both point-to-point and multipointconfigurations.HomeRF transmits at up to 10 Mbps over a 452 WIRELESS LANSrange of about 150 feet from the base station, which makes itsuitable for the average home.HomeRF transmits usingspread-spectrum frequency hopping; that is, it hops aroundconstantly within its prescribed bandwidth.When it encoun-ters interference, like a microwave oven or an adjacent wire-less LAN, it adapts by moving to another frequency.The key advantage that HomeRF has over IEEE 802.11bin the home environment is its superior ability to adapt tointerference from devices like portable phones andmicrowaves.As a frequency hopper, it coexists well withother frequency-hopping devices that proliferate in thehome.Another advantage of HomeRF is that it continuouslyreserves a chunk of bandwidth via isochronous channels for voice services.Speech quality is high; there is no clippingwhile the protocol deals with interference.The IEEE 802.11b Standard does not include frequencyhopping.In response to interference, IEEE 802.11b simplyretransmits or waits for the higher-level TCP/IP protocol tosort out signal from noise.This works well for data but canresult in voice transmissions sounding choppy.Voice and dataare treated the same way, converting voice into data packetsbut offering no priority to voice.This results in unacceptablevoice quality.Another problem with IEEE 802.11b is that itsWired Equivalent Protocol (WEP) encryption, designed tosafeguard privacy, has had problems living up to its claim.Bluetooth also operates in the 2.4-GHz band but was notcreated originally to support wireless LANs; it was intended asa replacement for cable between desktop computers, peripher-als, and handheld devices.Operating at the comparativelyslow rate of 30 to 400 kbps across a range of only 30 feet,Bluetooth supports piconets that link laptops, PDAs, mobilephones, and other portable devices on an as-needed basis
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