about scan and probe request

[belle.shi at 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 for advance!!
belle