802.11g was the third modulation standard for Wireless LAN. It works in the 2.4 GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbit/s, or about 19 Mbit/s net throughput (identical to 802.11a core, except for some additional legacy overhead for backward compatibility). 802.11g hardware is fully backwards compatible with 802.11b hardware. Details of making b and g work well together occupied much of the lingering technical process. In an 11g network, however, the presence of a legacy 802.11b participant will significantly reduce the speed of the overall 802.11g network.

The modulation scheme used in 802.11g is orthogonal frequency-division multiplexing (OFDM) copied from
802.11a with data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s, and reverts to CCK (like the 802.11b standard) for 5.5 and 11 Mbit/s and DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s. Even though 802.11g operates in the same frequency band as 802.11b, it can achieve higher data rates because of its heritage to 802.11a.

source - IEEE 802.11g-2003. (2009, June 8). In Wikipedia, The Free Encyclopedia. Retrieved August 15, 2009, from http://en.wikipedia.org/wiki/802.11g