802.11 – Step Back Review

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Direct sequence spread spectrum (DSSS) PHY specification for the 2.4 GHz band designated for ISM applications

The DSSS system provides a wireless LAN with both a 1 Mbit/s and a 2 Mbit/s data payload communication
capability. According to the FCC regulations, the DSSS system shall provide a processing gain of at least
10 dB.


This shall be accomplished by chipping the baseband signal at 11 MHz with an 11-chip PN code. The
DSSS system uses baseband modulations of differential binary phase shift keying (DBPSK) and differential quadrature phase shift keying (DQPSK) to provide the 1 Mbit/s and 2 Mbit/s data rates, respectively.


15.2 DSSS PLCP sublayer

This clause provides a convergence procedure in which MPDUs are converted to and from PPDUs. During
transmission, the MPDU shall be prepended with a PLCP Preamble and Header to create the PPDU.

The entire PLCP Preamble and Header shall be transmitted using the 1 Mbit/s DBPSK modulation


802.11b

802.11b radio devices accomplish this feat by using a different spreading/coding technique called Complementary Code Keying (CCK) and modulation methods using the phase properties of the RF signal. 802.11 devices used a spreading technique called the Barker code.

The end result is that 802.11b radio devices support data rates of 1, 2, 5.5, and 11 Mbps. 802.11b systems are backward compatible with the 802.11 DSSS data rates of 1 Mbps and 2 Mbps. The transmission data rates of 5.5 Mbps and 11 Mbps are known as HR-DSSS


802.11a

802.11a radio cards operating in the 5 GHz UNII bands are classified as clause 17 devices. As defined by the 802.11a amendment, these devices are required to support data rates of 6, 12, and 24 Mbps with a maximum of 54 Mbps.

With the use of a spread spectrum technology called Orthogonal Frequency Division Multiplexing (OFDM), data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps are supported in most manufacturers’ radio cards. It should be noted that an 802.11a radio does not have to support all these rates, and one vendor may have an implementation of data rates that is not compatible with another vendor.

The UNII bands are as follows…

  • UNII-1 (lower) is 5.15–5.25 GHz.
  • UNII-2 (middle) is 5.25–5.35 GHz.
  • UNII-2 Extended is 5.47–5.725 GHz.
  • UNII-3 (upper) is 5.725–5.825 GHz.

The IEEE defines 802.11g cards as clause 19 devices, which transmit in the 2.4 GHz to 2.4835 GHz ISM frequency band. Clause 19 defines a technology called Extended Rate Physical (ERP). All aspects of the 802.11g ratified amendment can now be found in clause 19 of the 802.11-2007 standard.

The mandatory PHYs are ERP-OFDM and ERP-DSSS/CCK. To achieve the higher data rates, a PHY technology called Extended Rate Physical OFDM (ERP-OFDM) is mandated.

Data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps are possible using this technology, although once again the IEEE requires only the data rates of 6, 12, and 24 Mbps.

To maintain backward compatibility with 802.11 (DSSS only) and 802.11b networks, a PHY technology called Extended Rate Physical DSSS (ERP-DSSS/CCK) is used with support for the data rates of 1, 2, 5.5, and 11 Mbps.

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