IEEE802.11b LAN use wireless media to transmit information. Since the wireless media is more complex than wired media, the MAC layer access controling mechanism of IEEE802.11b is more complex. The thesis describes the MAC layer access controling mechanism of IEEE802.11b--CSMA/CA(carrier sense multiple access/collision avoid) and two implementation modes,which are DCF(distributed coordination function) controling mode and PCF(point coordination function) controling mode. At the same time, the thesis simply introduces the error controling problem.