Tuesday, January 18, 2011

Introduction to 802.11 Standard

     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 subcarriers 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 24 Mbps data rates are mandatory. Four of the subcarriers are pilot subcarriers 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 subcarriers 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 bandwidth with 64 possible subcarrier frequency slots).

    Also in the 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 ivided 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.


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