Wednesday, June 20, 2007

Detail discription of 802.11a stardards and technology

The IEEE 802.11a is an Orthogonal Frequency Division Multiplexing (OFDM) system very similar to Asymmetrical Digital Subscriber Loop (ADSL) Discrete Multi Tone (DMT) modems sending several sub-carriers in parallel using the Inverse Fast Fourier Transform (IFFT), and receiving those sub-carriers using the Fast Fourier Transform (FFT). The OFDM of the 802.11a system provides a Wireless LAN with data payload communication capabilities of 6, 9, 12, 18, 24, 36, 48 and54 Mbps. The support of transmitting and receiving at data rates of 6, 12, and 24 Mbps is mandatory in the standard. The802.11a system uses 52 sub-carriers that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16Quadrature Amplitude Modulation (QAM), or 64 QAM. Forward Error Correction (FEC) coding (convolutional coding) is used with a coding rate of 1/2, 2/3, or 3/4.In 802.11a the transmission medium is wireless and the operating frequency band is 5 GHz.

The OFDM PHY layer consists of two protocol functions: first a PHY convergence function, which adapts the capabilities of the Physical Medium Dependent (PMD) system to the PHY service. This function is supported by the Physical Layer Convergence Procedure (PLCP), which defines a method of mapping the IEEE 802.11 PHY Sublayer Service Data Units (PSDU) into a framing format suitable for sending and receiving user data and management information between two or more stations using the associated PMD system. Second a PMD system whose function defines the characteristics and method of transmitting and receiving data through a wireless medium between two or more stations, each using the OFDM system.

802.11a utilizes 300 MHz of bandwidth in the 5 GHz Unlicensed National Information Infrastructure (U-NII) band. Though the lower 200 MHz is physically contiguous, the FCC has divided the total 300 MHz into three distinct 100 MHz domains, each with a different legal maximum power output. The "low" band operates from 5.15 - 5.25 GHz, and has a maximum of 50 mW. The "middle" band is located from 5.25 - 5.35 GHz, with a maximum of 250 mW. The "high" band utilizes5.725 - 5.825 GHz, with a maximum of 1 W. Because of the high power output, devices transmitting in the high band will tend to be building-to-building products. The low and medium bands are more suited to in-building wireless products. One requirement specific to the low band is that all devices must use integrated antennas.

Different regions of the world have allocated different amounts of spectrum, so geographic location will deter-mine how much of the 5 GHz band is available. In the United States, the FCC has allocated all 3 bands for unlicensed transmissions. In Europe, however, only the low and middle bands are free. Though 802.11a is not yet certifiable in Europe, efforts are currently underway between IEEE and the European Telecommunications Standards Institute (ETSI)to rectify this. In Japan, only the low band may be used. This will result in more contention for signal, but will still allow for very high performance.

The frequency range used currently for most enterprise-class unlicensed transmissions, including 802.11b, is the 2.4GHz Industrial, Scientific & Medical (ISM) band. This highly populated band offers only 83 MHz of spectrum for all wireless traffic, including cordless phones, building-to-building transmissions, and microwave ovens. In comparison, the 300 MHz offered in the U-NII band represents a nearly four-fold increase in spectrum; all the more impressive when considering there is limited wireless traffic in the band today.

802.11a represents the next generation of enterprise-class wireless LAN technology, with many advantages over current options. At speeds of 54 Mbps and greater, it is faster than any other unlicensed solution. 802.11a and 802.11b both have a similar range, but 802.11a provides higher speed throughout the entire coverage area. The 5 GHz band in which it operates is not highly populated, so there is less congestion to cause interference or signal contention. And, the 8 non overlapping channels allow for a highly scalable and flexible installation. 802.11a is the most reliable and efficient medium by which to accommodate high-bandwidth applications for numerous users.

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