Modify Txpower

[roby]

Hi, 

 I have build a small network, based on WPA and 802.1x authentication 
sistem. In this network there are two access point (Hostap driver-Based) 
connected with a cabled DS to an Access Server. At this time I need to 
reduce the transmission power for simulate the handover, but when I try to 
do it i receive these message: 

#########################################
$iwconfig wlan0 txpower 10
Error for wireless request "Set Txpower" (8B26):
  SET failed on device wlan0; Operation not supported.
######################################### 

Anyome now why? 

Thk a lot.
Roby.