The IEEE 802.11g standard is an evolution of 802.11b however it will work at 22Mb and has some new modulations as well.
I'd hesitate to call 11g 'fully backwards compatible' (with 802.11b). There are four modulations in the 802.11g draft:
PBCC @ 22Mbps. This is the "TI" approach (note that PBCC has been in the 802.11b spec since its ratification in 1999, but just try to find an implementation.
- "OFDM-CCK" @ 22Mbps. This is the Intersil approach.
- "OFDM" with the (1Mbps) 802.11 CCK preamble
- "OFDM" with an OFDM preamble.
Only the first three are 'backwards compatible' (will allow an 802.11b network to inter-operate, at least in terms of not interfering). The fourth will knock any existing 802.11b network off the air, due to the greater spectral density of OFDM .vs CCK (or PBCC, see #1).
I have seen proposals to let #4 run with RTS/CTS before every packet. You do the math an see what your throughput will be.
The fourth is what gets all the attention, of course, since its the only one that can compete with 802.11a's technology and resultant marketing. Note how it will obliterate 11b in the process (see previous message on why current 802.11b gear will be replaced anyway).