new security solution technologies. Web Links Authentication and Authorisation for WLAN using 802.1X http://www.surfnet.nl/innovatie/wlan/
Content 3.3 Wireless Media 3.3.5 The radio wave and microwave spectrums Computers send data signals electronically. Radio transmitters convert these electrical signals to radio waves. Changing electric currents in the antenna of a transmitter generates the radio waves. These radio waves radiate out in straight lines from the antenna. However, radio waves attenuate as they move out from the transmitting antenna. In a WLAN, a radio signal measured at a distance of just 10 meters (30 feet) from the transmitting antenna would be only 1/100th of its original strength. Like light, radio waves can be absorbed by some materials and reflected by others. When passing from one material, like air, into another material, like a plaster wall, radio waves are refracted. Radio waves are also scattered and absorbed by water droplets in the air. These qualities of radio waves are important to remember when a WLAN is being planned for a building or for a campus. The process of evaluating a location for the installation of a WLAN is called making a Site Survey. Because radio signals weaken as they travel away from the transmitter, the receiver must also be equipped with an antenna. When radio waves hit the antenna of a receiver, weak electric currents are generated in that antenna. These electric currents, caused by the received radio waves, are equal to the currents that originally generated the radio waves in the antenna of the transmitter. The receiver amplifies the strength of these weak electrical signals. In a transmitter, the electrical (data) signals from a computer or a LAN are not sent directly into the antenna of the transmitter. Rather, these data signals are used to alter a second, strong signal called the carrier signal. The process of altering the carrier signal that will enter the antenna of the transmitter is called modulation. There are three basic ways in which a radio carrier signal can be modulated. For example, Amplitude Modulated (AM) radio stations modulate the height (amplitude) of the carrier signal. Frequency Modulated (FM) radio stations modulate the frequency of the carrier signal as determined by the electrical signal from the microphone. In WLANs, a third type of modulation called phase modulation is used to superimpose the data signal onto the carrier signal that is broadcast by the transmitter. In this type of modulation, the data bits in the electrical signal change the phase of the carrier signal. A receiver demodulates the carrier signal that arrives from its antenna. The receiver interprets the phase changes of the carrier signal and reconstructs from it the original electrical data signal. Interactive Media Activity Point and Click: Electromagnetic Fields and Polarization This activity includes a series of animated, graphical explanations of EM fields and polarization. Interactive Media Activity Interactivity: Electromagnetic Spectrum This activity allows you to scroll through the EM spectrum by moving your mouse to the left or right over the graphic. Web Links The Electromagnetic (RF) Spectrum http://www.sss-mag.com/spectrum.html
Content 3.3 Wireless Media 3.3.6 Signals and noise on a WLAN On a wired Ethernet network, it is usually a simple process to diagnose the cause of interference. When using RF technology many kinds of interference must be taken into consideration. Narrowband is the opposite of spread spectrum technology. As the name implies narrowband does not affect the entire frequency spectrum of the wireless signal. One solution to a narrowband interference problem could be simply changing the channel that the AP is using. Actually diagnosing the cause of narrowband interference can be a costly and time-consuming experience. To identify the source requires a spectrum analyzer and even a low cost model is relatively expensive. All band interference affects the entire spectrum range. Bluetooth™ technologies hops across the entire 2.4 GHz many times per second and can cause significant interference on an 802.11b network. It is not uncommon to see signs in facilities that use wireless networks requesting that all Bluetooth™ devices be shut down before entering. In homes and offices, a device that is often overlooked as causing interference is the standard microwave oven. Leakage from a microwave of as little as one watt into the RF spectrum can cause major network disruption. Wireless phones operating in the 2.4GHZ spectrum can also cause network disorder. Generally the RF signal will not be affected by even the most extreme weather conditions. However, fog or very high moisture conditions can and do affect wireless networks. Lightning can also charge the atmosphere and alter the path of a transmitted signal. The first and most obvious source of a signal problem is the transmitting station and antenna type. A higher output station will transmit the signal further and a parabolic dish antenna that concentrates the signal will increase the transmission range. In a SOHO environment most access points will utilize twin omnidirectional antennae that transmit the signal in all directions thereby reducing the range of communication.
Content 3.3 Wireless Media 3.3.7 Wireless security As previously discussed in this chapter, wireless security can be difficult to achieve. Where wireless networks exist there is little security. This has been a problem from the earliest days of WLANs. Currently, many administrators are weak in implementing effective security practices. A number of new security solutions and protocols, such as Virtual Private Networking (VPN) and Extensible Authentication Protocol (EAP) are emerging. With EAP, the access point does not provide authentication to the client, but passes the duties to a more sophisticated device, possibly a dedicated server, designed for that purpose. Using an integrated server VPN technology creates a tunnel on top of an existing protocol such as IP. This is a Layer 3 connection as opposed to the Layer 2 connection between the AP and the sending node. VPN technology effectively closes the wireless network since an unrestricted WLAN will automatically forward traffic between nodes that appear to be on the same wireless network. WLANs often extend outside the perimeter of the home or office in which they are installed and without security intruders may infiltrate the network with little effort. Conversely it takes minimal effort on the part of the network administrator to provide low-level security to the WLAN. Web Links The