11 IS NOT THE ONLY WIRELESS NETWORKING TECHNOLOGY AVAILABLE, BUT IT IS CERTAINLY THE MOSTPOPULAR AND MUST BE UNDERSTOOD IN ORDER TO GAIN A SOLID BACKGROUND FOR WORKING WITHWIRELESS NETWORKING USING RECENT VERSIONS OF WINDOWS
802.11 is not the only wireless networking technology available, but it is certainly the mostpopular and must be understood in order to gain a solid background for working withwireless networking using recent versions of Windows:Windows 2000 Professional, Server,and Advanced Server;Windows XP Home and Professional; and most important,WindowsServer 2003.The process of connecting to a wireless network is often transparent to users and, fromtheir perspective, is no different from connecting to a copper- or fiber-based Ethernet net-work, with the exception that no wires are involved.With Windows XP and WindowsServer 2003, which boast automatic configuration and seamless roaming from one wirelessnetwork to another, the ease with which users can connect to wireless networks furtherbelies the complexity of the technology involved and differences between wired and wire-less networks.Furthermore, because the experience of using a wireless network is identical to that ofusing a wired Ethernet network, there is a tendency to treat both kinds of networks asthough they were the same; however, they are quite different from one another, and anunderstanding of those differences is critical to providing an informed and effective imple-mentation of a secure wireless network.
Communication in a Wireless Network
Wireless networks, like their wired counterparts, rely on the manipulation of electricalcharges to enable communication between devices. Changes or oscillations in signalstrength from 0 to a maximum value (amplitude) and the rate of those oscillations (frequency)allow the encoding and decoding of information.When two devices understand the method(s) used to encode and decode informationcontained in the changes to the electrical properties of the communications medium, theycan communicate with each other. A network adapter is able to decode the changes in theelectrical current it senses on the wire and convert them to meaningful information (bits)that it can subsequently send to higher levels for processing. Likewise, a network adaptorcan encode information (bits) by manipulating the properties of the electrical current fortransmission on the communications medium (the cable, in the case of wired networks).Radio Frequency Communications
The obvious and primary difference between wired and wireless networks is that wirelessnetworks use radio waves to transmit their data across an intermediate medium, instead ofpushing electrons through a wired connection. Radio waves are created by applying alter-nating current (AC)to an antenna to produce an electromagnetic (EM)field.Devices use theresulting radio frequency (RF)field for broadcast and reception.In the case of wireless networks, the medium for communications is the EM field, theregion of space that is influenced by the electromagnetic radiation. (Unlike audio waves,radio waves do not require a medium such as air or water to propagate.) As with wired net-works, amplitude decreases with distance, resulting in the degradation of signal strength andthe ability to communicate. However, the EM field is also dispersed according to the prop-erties of the transmitting antenna, not tightly bound, as is the case with communication ona wire.The area over which the radio waves propagate from an electromagnetic source isknown as the Fresnel zone.Like the waves created by throwing a rock into a pool of water, radio waves are affectedby the presence of obstructions and can be reflected, refracted, diffracted, or scattered,depending on the properties of the obstruction and its interaction with the radio waves.Reflected radio waves can be a source of interference on wireless networks.The interfer-ence created by bounced radio waves is called multipath interference.When radio waves are reflected, additional wave fronts are created.These different wavefronts can arrive at the receiver at different times and be in phase or out of phase with themain signal.When the peak of a wave is added to another wave (in phase), the wave is ampli-fied.When the peak of a wave meets a trough (out of phase), the wave is effectively cancelled.Multipath interference can be the source of problems that are difficult to troubleshoot.In planning for a wireless network, administrators should consider the presence of commonsources of multipath interference.These sources include metal doors, metal roofs, water,metal vertical blinds, or any other source that is highly reflective of radio waves. Antennascould help compensate for the effects of multipath interference, but these have to be care-fully chosen. In fact, many wireless access points (APs) have two antennas for precisely thispurpose because a single omnidirectional antenna might not be of any use in curbing thiskind of interference.Another source of signal loss is the presence of obstacles.Whereas radio waves cantravel through physical objects, they will be degraded according to the properties of theobject they travel through. A window, for example, is fairly transparent to radio waves, but itcould reduce the effective range of a wireless network between 50 and 70 percent,depending on the presence and nature of coatings on the glass. A solid core wall can reducethe effective range of a wireless network up to 90 percent or greater.EM fields are also prone to interference and signal degradation by the presence of otherEM fields. In particular, 802.11 wireless networks are prone to the interference produced bycordless phones, microwave ovens, and a wide range of devices that use the same unlicensedIndustrial, Scientific, and Medical (ISM) or Unlicensed National Information Infrastructure(UNII) bands.To mitigate the effects of interference from these devices and other sources of electro-magnetic interference, RF-based wireless networks employ spread-spectrum technologies.Spread-spectrum provides a way to “share” bandwidth with other devices that are operatingin the same frequency range. Rather than operating on a single, dedicated frequency, as isthe case with radio and television broadcasts, wireless networks use a “spectrum” of fre-quencies for communication.Spread-Spectrum Technology
The concept of spread-spectrum communication was first conceived by Hollywood actressHedy Lamarr and composer George Antheil in 1940 as a method to secure military com-munications from jamming and eavesdropping during World War II. Spread-spectrumdefines methods for wireless devices to simultaneously use a number of narrowband fre-quencies over a range of frequencies for communication.The narrowband frequencies used between devices change according to a random-appearing but defined pattern, allowing the use of individual frequencies to contain parts ofthe transmission. Someone listening to a transmission using spread-spectrum would hearonly noise, unless their device “understood” in advance what frequencies were used for thetransmission and could synchronize with them.Two methods to synchronize wireless devices are:■
Frequency-hopping spread-spectrum (FHSS)■
Direct-sequence spread-spectrum (DSSS)