The IEEE 802.11g-2003 standard defines several modulation types:
- ERP-DSSS refers to PHYs using Direct Sequence Spread Spectrum (DSSS) modulation. DSSS was defined in the original IEEE 802.11-1999 standard to support data rates of 1 and 2 Mbps.
- ERP-CCK refers to PHYs using Complementary Code Keying (CCK) modulation. CCK was added in the IEEE 802.11b-1999 supplement to support data rates of 5.5 and 11 Mbps.
- ERP-PBCC refers to PHYs using extended rate Packet Binary Convolutional Coding (PBCC) modulation. PBCC was added as an option in the IEEE 802.11b-1999 supplement to support data rates of 5 and 11 Mbps. In 802.11g, ERP-PBCC option also supports data rates of 22 and 33 Mbps.
- ERP-OFDM refers to PHYs using Orthogonal Frequency Division Multiplexing (OFDM) modulation. OFDM was added in the IEEE 802.11a-1999 supplement. In 802.11g, ERP-OFDM supports data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.
- DSSS-OFDM refers to PHYs using hybrid DSSS-OFDM modulation. DSSS-OFDM was added in the IEEE 802.11g-2003 standard and is an optional mode that does NOT use the Extended Rate PHY (ERP) protection mechanism. Instead, DSSS-OFDM combines the DSSS preamble and header with the OFDM payload, supporting rates similar to ERP-OFDM.
Which modulation type will used depends on the capabilities of your 802.11g AP and stations, their configuration, need to coexist with 802.11b, and environmental conditions. There's a nice comparative chart of these modulation types in this Texas Instruments 802.11g white paper (PDF).
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