about 802.11 scan and probe request

[belle.shi@yy.com]

dear all,
   is there anyone who read the standard "Part 11: Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY) Specifications" ?
   and i spend two month to read the stardard, and i read a few code of 
wpa_suppliant and
wpa_gui,there is one question: the scan have two mode:active or 
passive,but when i run
the wpa_gui, there is only scan button, in the standard the scan 
request is like
MLME-SCAN.request (
                 BSSType,
                 BSSID,
                 SSID,
                 ScanType,
                 ProbeDelay,
                 ChannelList,
                 MinChannelTime,
                 MaxChannelTime
                 )
but there are only three MAC frame formats:data frame, control frame 
and Management
frame, and there is no scan frame, there is only a Probe Request frame, 
    so i am
stumped, what's the relationship between MLME-SCAN.request and Probe 
Request frame, is
the MLME-SCAN.request and Probe Request frame only in different layer? 
but the items is
also different, the MLME-SCAN.request have:(
BSSType, BSSID, SSID,  ScanType, ProbeDelay,  ChannelList,  
MinChannelTime,   MaxChannelTime), but the  Probe Request frame body 
only have two parts:  SSID
and Supported rates.     and in my view, i think the active scan is to 
send Probe Request
frame, and the passive scan to receive the beacon frame, is it right?
    thanks very much for advance!!
belle