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To access the contents, click the chapter and section titles.
Wireless Networking Handbook
Wireless Network InterfaceComputers process information in digital form, with low direct current (DC) voltages representing data ones and zeros. These signals are optimum for transmission within the computer, not for transporting data through wired or wireless media. A wireless network interface couples the digital signal from the end-user appliance to the wireless medium, which is air, to enable an efficient transfer of data between sender and receiver. This process includes the modulation and amplification of the digital signal to a form acceptable for propagation to the receiving location. Modulation is the process of translating the baseband digital signal to a suitable analog form. This process is very similar to the common telephone modem, which converts a computers digital data into an analog form within the 4 KHz limitation of the telephone circuit. The wireless modulator translates the digital signal to a frequency that propagates well through the atmosphere. Wireless networks employ modulation by using radio waves and infrared light. Amplification raises the amplitude of the signal so it will propagate a greater distance. Without amplification, you would have a difficult time talking to a crowd of 1,500 people outside in an open area. But, add amplification, such as a PA system, and everyone can hear you.
The wireless network interface also manages the use of the air through the operation of a communications protocol. For synchronization, wireless networks employ a carrier sense protocol similar to the common ethernet standard. This protocol enables a group of wireless computers to share the same frequency and space. As an analogy, consider a room of people engaged in a single conversation in which each person can hear if someone speaks. This represents a fully connected bus topology (where everyone communicates using the same frequency and space) that ethernet and wireless networks, especially wireless LANs, utilize. To avoid having two people speak at the same time, you should wait until the other person has finish-ed talking. Also, no one should speak unless the room is silent. This simple protocol ensures only one person speaks at a time, offering a shared use of the communications medium. Wireless networks use carrier sense protocols and operate in a similar fashion, except the communications are by way of radio signals or infrared light. Figure 1.6 illustrates the generic carrier sense protocol.
Wireless networks handle error control by having each station check incoming data for altered bits. If the destination station does not detect errors, it sends an acknowledgment back to the source station. If the station detects errors, the data link protocol ensures that the source station resends the packet. To continue the analogy, consider two people talking to each other outside. If one person is speaking and a disruption occurs, such as a plane flying overhead, the dialog might become distorted. As a result, the listener asks the speaker to repeat a phrase or two. The wireless network interface generally takes the shape of a wireless NIC or an external modem that facilitates the modulator and communications protocols. These components interface with the user appliance via a computer bus, such as ISA (Industry Standard Architecture) or PCMCIA (Personal Computer Memory Card International Association). The ISA bus comes standard in most desktop PCs. Many portable computers have PCMCIA slots that accept credit card-sized NICs. PCMCIA specifies three interface sizes, Type I (3.3 millimeters), Type II (5.0 millimeters), and Type III (10.5 millimeters). Some companies also produce wireless components that connect to the computer via the RS-232 serial port. The interface between the users appliance and NIC also includes a software driver that couples the clients application or NOS software to the card. Several de facto driver standards exist, including ODI (Open Data-Link Interface) and NDIS. AntennaThe antenna radiates the modulated signal through the air so that the destination can receive it. Antennas come in many shapes and sizes and have the following specific electrical characteristics:
The propagation pattern of an antenna defines its coverage. A truly omnidirectional antenna transmits its power in all directions, whereas a directional antenna concentrates most of its power in one direction. Figure 1.7 illustrates the differences. Radiation power is the output of the radio transmitter. Most wireless network devices operate at less than 5 watts of power.
A directional antenna has more gain (degree of amplification) than the omnidirectional type and is capable of propagating the modulated signal farther because it focuses the power in a single direction. The amount of gain depends on the directivity of the antenna. An omnidirectional antenna has a gain equal to one; that is, it doesnt focus the power in any particular direction. A directional antenna, however, is considered to add gain (amplification) to the signal in certain directions. Consider the example of watering your lawn with a garden hose. Attach a circular sprayer to the end of the hose and turn the water on. The water pressure is divided among many directions, and the resulting water spray will reach seven or eight feet. If a more directive nozzle is attached to the hose, the spray might reach twenty feet because the device concentrates the water pressure in one direction. Similarly, the combination of transmit power and gain of an antenna defines the distance the signal will propagate. Long-distance transmissions require higher power and directive radiation patterns. With wireless networks, these signals are relatively low power, typically one watt or less. Most wireless LANs and WANs utilize omnidirectional antennas, and wireless MANs use antennas that are more directive.
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