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Recall from 3 that Ethernet uses Carrier Sense Multiple Access/Collision Detection (CSMA/CD) to control access to a wire WLANs use a similar mechanism called Carrier Sense, Multiple Access/Collision Avoidance (CSMA/CA) Unlike Ethernet, it is impossible to detect collisions in a wireless medium In a WLAN, a device cannot simultaneously send or receive and thus cannot detect a collision: it can only do one or the other To avoid collisions, a device will use Ready-to-Send (RTS) and Clear-to-Send (CTS) signals When a device is ready to transmit, it first senses the airwaves for a current signal If there is none, it generates an RTS signal, indicating that data is about to send It then sends its data and finishes by sending a CTS signal, indicating that another wireless device can now transmit WLAN devices also have another similarity to Ethernet LANs: they use an access point to connect devices together logically The access point serves a function similar to that of a hub in Ethernet One problem with a WLAN implementation, like Ethernet, is that it is a shared medium: the more devices in the topology, the less throughput each device individually gets because they have to share the finite amount of bandwidth and because of the increased collisions that can occur when devices simultaneously sense the airwaves and send an RTS signal Another problem with WLANs is security Unlike Ethernet, anyone with a compatible device can sniff the airwaves and eavesdrop on communications; at least with Ethernet, this would require someone to have access to the Ethernet
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cable the traffic was traversing The most common method of dealing with this problem is to encrypt the wireless traffic
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Radio frequencies (RF) are generated by antennas that propagate the waves into the air As they are being propagated, various factors can reflect or deflect the signal, affecting its quality and strength Antennas fall under two different categories: directional and omni-directional Directional antennas are commonly used in point-to-point configurations (connecting two distant buildings), and sometimes point-to-multipoint (connecting two WLANs) An example of a directional antenna is a Yagi antenna: this antenna allows you to adjust the direction and focus of the signal to intensify your range/reach Omni-directional antennas are used in point-to-multipoint configurations, where they distribute the wireless signal to other computers or devices in your WLAN An access point would use an omni-directional antenna These antennas can also be used for point-to-point connections, but they lack the distance that directional antennas supply Three main factors influence signal distortion:
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Objects that absorb the RF waves, such as walls, ceilings, and
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Objects that disperse the RF waves, such as rough plaster on a wall, carpet on the floor, or drop-down ceiling tiles Objects that reflect the RF waves, such as metal and glass
Reflection
Common objects that will cause RF distortion problems include walls, ceilings, oors, glass windows, metal
objects such as desks and le cabinets, and many others
WLAN Standards
CERTIFICATION OBJECTIVE 502
WLAN Standards
Typically, government agencies control the use of RF bands For example, in the United States, the Federal Communications Commission (FCC) regulates wireless transmissions, which includes new transmission methods, frequencies, and modulations; in Europe, the European Telecommunications Standards Institute (ETSI) is the regulator However, one international agency, the International Telecommunication Union-Radio Communication Sector (ITU-R) is responsible for managing the radio frequency (RF) spectrum and satellite orbits for wireless communications: its main purpose is to provide for cooperation and coexistence of standards and implementations across country boundaries Two standards bodies are primarily responsible for implementing WLANs: the Institute of Electrical and Electronic Engineers (IEEE) and the Wi-Fi Alliance IEEE defines the mechanical process of how WLANs are implemented in the 80211 standards so that vendors can create compatible products The Wi-Fi Alliance basically certifies companies by ensuring that their products follow the 80211 standards, thus allowing customers to buy WLAN products from different vendors without having to be concerned about any compatibility issues Cisco is a founding member of the Wi-Fi Alliance You can find a list of certified vendor products at wwwwi-ficom
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