Differential Binary Phase-shift Keying (DBPSK)
DBPSK, sometimes referred to as BPSK, is the modulation method used for 1-Mbps communication in the original IEEE 802.11 specification.
Quadrature Phase-shift Keying (QPSK)
QPSK is the modulation method used for 2-Mbps communication in the original IEEE 802.11 specification.
Complementary Code Keying (CCK)
CCK is the modulation method used for 5.5-Mbps and 11-Mbps communication in the IEEE 802.11b specification.
Orthogonal Frequency-division Multiplexing (OFDM)
OFDM is the modulation and frequency spreading method specified in IEEE 802.11a and 802.11g for communication at 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. It works by modulating multiple carrier frequencies simultaneously; all other 802.11x modulation methods use only one carrier at a time. Depending on the desired signaling rate, modulation may be done by BPSK, QPSK, or quadrature amplitude modulation (QAM).
The U.S. FCC originally required all devices operating in the 2.4-GHz band to use either frequency hopping or direct-sequence spread spectrum (FHSS or DSSS) technology as a means of avoiding interference. Individual OFDM products began receiving FCC approval in 2001, and in May 2003 the FCC changed its rules to allow the use of OFDM in the 2.4-GHz band.
CCK-OFDM
CCK-OFDM (formerly written as CCK/OFDM) is an optional modulation scheme in IEEE 802.11g. It uses CCK for the first portion of a packet -- the preamble and header -- and OFDM for the payload. The CCK preamble and header tell 802.11b devices operating in the same area that a transmission is coming and how long it will last; those devices then hold off their own transmissions while the payload is delivered. CCK-OFDM as described in 802.11g allows speeds up to 22 Mbps.
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