802.11 连接过程

 <--1.client sends probe request via RF. --->
---> 2.APs(a/b) send probe response via RF.<--
client evaluates probe response and selects best
AP(a)
<--- 3.client sends authentication request to
  selected AP(a) via RF.
---> 4.AP(a) confirms authentication via RF packet.
<--- 5.client sends association request to selected
  AP(a) via RF packet
---> 6.AP(a) confirms association request via RF
  packet and registers client.