Could you give me some details about DSSS?
The
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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.
This was first published in August 2002