There is a new wireless stack starting in the mainline kernel since 2.6.22 called mac80211. As newer versions of the kernel get released more and more wireless devices are being supported by it. It has the huge advantage of being included in the kernel itself. The mac80211 stack has features such as software MAC (media access controller), hostapd, WEP, WPA, WME, a “link-layer bridging module,” and a QoS (quality of service) implementation. Of specific interest to aircrack-ng is native monitor mode and injection support.
The legacy drivers use the ieee80211 or net80211 stacks. And quite often there is one stack per wireless device. Depending on the driver, it does not provide native monitor mode or injection support.