Could you give me some details about DSSS? The original 1997 802.11 wireless LAN standard defined three physical
media: Infrared, Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS).
FHSS divides the 2.4 GHz band into 75 1-MHz channels. Transmitters and receivers hop rapidly around these channels at an-agreed-upon interval to avoid collisions. FHSS operates at a maximum rate of 2 Mbps.
DSSS divides the same 2.4 GHz band into overlapping channels that are each 22 MHz wide. Instead of frequency-hopping, DSSS uses a technique called "chipping". Chipping spreads modulated data across the spectrum in a fashion that makes it possible to tolerate some signal loss. A DSSS radio chipping with the 1997 standard Barker Code modulates data using binary or quadrature phase shift keying, operating at 1 or 2 Mbps, respectively.
In 1999, the 802.11b High Rate standard that is now implemented by all "Wi-Fi" products introduced a more efficient chipping method known as Complementary Code Keying (CCK). CCK increased the top data rate to 11 Mbps. The 1999 standard uses only DSSS, but supports both CCK and Barker code chipping for interoperability with older DSSS radios. It also adds dynamic rate shifting from 11 to 5.5 to 2 to 1 Mbps, letting radios automatically adjust for distance, interference, and other changes in signal strength.
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