Tuesday, June 19, 2007

Introduction to 802.11a standards

It is a Very High-Speed, Highly Scalable Wireless LAN Standard.

The Institute of Electrical and Electronics Engineers [IEEE] has developed 802.11a, a new specification that represents the next generation of enterprise-class wireless LANs. Among the advantages it has over current technologies are greater scalability, better interference immunity, and significantly higher speed, up to 54 Mbps and beyond, which simultaneously allows for higher bandwidth applications and more users. This paper provides an overview, in basic terms, of how the 802.11a specification works, and its corresponding benefits.

IEEE 802.11 standard specifies a 2.4 GHz operating frequency with data rates of 1 and 2 Mbps using either Direct Sequence Spread Spectrum (DSSS) or Frequency Hopping Spread Spectrum (FHSS). The IEEE 802.11a standard specifies an OFDM physical layer (PHY) that splits an information signal across 52 separate sub-carriers to provide transmission of data at a rate of 6, 9, 12, 18, 24, 36, 48, or 54 Mbps. In the 802.11a IEEE standard the 6, 12, and 24Mbps data rates are mandatory. Four of the sub-carriers are pilot sub-carriers that the system uses as a reference to disregard frequency or phase shifts of the signal during transmission.

In the 802.11a standard, a pseudo binary sequence is sent through the pilot sub-channels to prevent the generation of spectral lines. In the 802.11a, the remaining 48 sub-carriers provide separate wireless pathways for sending the information in a parallel fashion. The resulting subcarrier frequency spacing in the IEEE 802.11a standard is 0.3125 MHz(for a 20 MHz with 64 possible subcarrier frequency slots).

802.11a standard, the primary purpose of the OFDM PHY is to transmit Media Access Control (MAC) Protocol Data Units (MPDUs) as directed by the 802.11 MAC layer. The OFDM PHY of the 802.11a standard is divided into two elements: the Physical Layer Convergence Protocol (PLCP) and the Physical Medium Dependent (PMD) sublayers. The MAC layer of 802.11a standard communicates with the PLCP via specific primitives through a PHY service access point. When the MAC layer instructs, the PLCP prepares MPDUs for transmission. The PLCP also delivers incoming frames from the wireless medium to the MAC layer. The PLCP sublayer minimizes the dependence of the MAC layer on the PMD sublayer by mapping MPDUs into a frame format suitable for transmission by the PMD.

Under the direction of the PLCP, the PMD provides actual transmission and reception of PHY entities between two stations through the wireless medium. To provide this service, the PMD interfaces directly with the air medium and provides modulation and demodulation of the frame transmissions. The PLCP and PMD communicate using service primitives to govern the transmission and reception functions.With 802.11a OFDM modulation, the binary serial signal is divided into groups (symbols) of one, two, four, or six bits, depending on the data rate chosen, and converted into complex numbers representing applicable constellation points. Ifa data rate of 24 Mbps is chosen, for example, then the PLCP maps the data bits to a 16QAM constellation. After mapping, the PLCP normalizes the complex numbers in the 802.11a standard to achieve the same average power for all mappings. The PLCP assigns each symbol, having duration of 4 microseconds, to a particular subcarrier. An Inverse Fast Fourier transform (IFFT) combines the sub-carriers before transmission.

As with other 802.11 based PHYs, in the 802.11a the PLCP implements a clear channel assessment protocol by reporting a medium busy or clear to the MAC layer via a primitive through the service access point. The MAC layer uses this information to determine whether to issue instructions to actually transmit an MPDU. The 802.11a standard requires receivers to have a minimum sensitivity ranging from -82 to -65 dBm, depending on the chosen data rate.

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