Both 80211.B and 80211.G operate in the 2.4 Ghz band. 80211.A operates near 5 Ghz.
None of them use 900Mhz. Microwaves and 2.4 Ghz cordless phones can interfere with the signals of 80211.B and 80211.G. 80211.A is not as prone to interference but is more line of sight.
