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wlan


WIIRELESS LANSAwireless local area network (WLAN) is a data communicationssystem implemented as an extension—or as an alternative—to a wired LAN. Using a variety of technologiesincluding narrowband radio, spread spectrum, and infrared,wireless LANs transmit and receive data through the air,minimizing the need for wired connections.

Applications

Wireless LANs have become popular in a number of verticalmarkets, including health care, retail, manufacturing, andwarehousing. These industries have profited from the productivitygains of using handheld terminals and notebook computersto transmit real-time information to centralized hostsfor processing. Wireless LANs allow users to go where wirescannot always go. Specific uses of wireless LANs include Hospital staff members can become more productive whenusing handheld or notebook computers with a wireless LANcapability to deliver patient information, regardless of theirlocation. Consulting or accounting audit teams, small workgroups, ortemporary office staff can use wireless LANs to quickly setup for ad-hoc projects and become immediately productive. Network managers in dynamic enterprise environmentscan minimize the overhead cost of moves, adds, andchanges with wireless LANs, since the need to install orextend wiring is eliminated. Warehouse workers can use wireless LANs to exchangeinformation with central databases, thereby increasingproductivity. Branch office workers can minimize setup requirementsby installing preconfigured wireless LANs. Wireless LANs are an alternative to cabling multiple computersin the home.While the initial investment required for wireless LANhardware can be higher than the cost of conventional LANhardware, overall installation expenses and life-cycle costscan be significantly lower. Long-term cost savings are greatestin dynamic environments requiring frequent moves,adds, and changes. Wireless LANs can be configured in avariety of topologies to meet the needs of specific applicationsand installations. They can grow by adding accesspoints and extension points to accommodate virtually anynumber of users.

Technologies

There are several technologies to choose from when selectinga wireless LAN solution, each with advantages and limitations.Most wireless LANs use spread spectrum, a widebandradio frequency technique developed by the military for usein reliable, secure, mission-critical communications systems.To achieve these advantages, the signal is spread outover the available bandwidth and resembles backgroundnoise that is virtually immune from interception.There are two types of spread-spectrum radio: frequencyhopping and direct sequence. Frequency-hopping spreadspectrum (FHSS) uses a narrowband carrier that changesfrequency in a pattern known only to the transmitter andreceiver. Properly synchronized, the net effect is to maintaina single logical channel. To an unintended receiver, FHSSappears to be short-duration impulse noise.Direct-sequence spread spectrum (DSSS) generates aredundant bit pattern for each bit to be transmitted andrequires more bandwidth for implementation. This bit pattern,called a “chip” (or “chipping code”), is used by thereceiver to recover the original signal. Even if one or morebits in the chip are damaged during transmission, statisticaltechniques embedded in the radio can recover the originaldata without the need for retransmission. To an unintendedreceiver, DSSS appears as low-power wideband noise.Another technology used for wireless LANs is infrared (IR),which uses very high frequencies that are just below visiblelight in the electromagnetic spectrum. Like light, IR cannotpenetrate opaque objects—o reach the target system, thewaves carrying data are sent in either directed (line-of-sight)or diffuse (reflected) fashion. Inexpensive directed systemsprovide very limited range of not more than 3 feet and typicallyare used for personal area networks but occasionally are usedin specific wireless LAN applications. High-performancedirected IR is impractical for mobile users and therefore isused only to implement fixed subnetworks. Diffuse IR wirelessLAN systems do not require line-of-sight transmission, butcells are limited to individual rooms. As with spread-spectrumLANs, IR LANs can be extended by connecting the wirelessaccess points to a conventional wired LAN.





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  1. Admin

    OFDM technology The speed of 802.11a is achieved through OFDM modulation (Offset Frequency Division Multiplexing). OFDM is an impressive engineering solution to the combined problems of high speed data transmission and indoor propagation characteristics at 5 GHz. The high speed data is divided into a series of 52 lower data-rate subcarriers, distributed over 20 MHz of spectrum. Interference between adjacent subcarriers is minimized by making them orthogonal (90-degree relative phase).

    The main propagation effect is multipath. At 5 GHz, in an indoor environment, the delay spread ranges from 50 ns in a home or office, to 300 ns in a warehouse or factory floor. To overcome multipath effects, the OFDM signal includes an 800 ns guard interval to avoid interference between an arriving new symbol and a reflected version of the previous symbol. To help deal with errors due to direct and reflected versions of the same symbol, training sequences are included in the data burst. With these training sequences, signal processing circuitry can automatically compensate for the time difference between several direct and reflected images of the signal. Every technique used to compensate for signal degradation adds overhead to the transmitted data, reducing the theoretical maximum data rate. 802.11a OFDM’s multiple subcarriers permit data rates that exceed what is possible in a single channel at 5 GHz. The cost is increased spectrum occupancy and complex signal processing. The spectrum issue was addressed when the 5 GHz band was re-regulated to permit unlicensed wireless systems such as these. The signal processing issue is handled with the power of DSP, which is readily available in standard and custom integrated circuit solutions.

    Hardware issues The issues with hardware design for 802.11a WLAN equipment can be summed up in three words: frequency, complexity and cost. 5 GHz receiver, transmitter and antenna technology is in the traditional microwave region, where past technology has been labor intensive, produced in relatively small quantities, and expensive. Fortunately, the continued evolution to higher frequencies has helped wireless product manufacturers quickly learn how to make highly integrated and mass-reproducible components that operate these frequencies.

    The complexity issue, as noted above, is handled nicely by DSP technology, which has advanced in both the capability of the devices and in the understanding of signal processing mathematics. Cost of production has also experienced a significant step forward in its evolution. Components and finished consumer products have benefitted from better overall project management techniques, but more companies must adopt these methods to assure their success in this market. A future report will take a closer look at some of the management techniques that enable new technologies to be developed quickly and economically.






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