I have a wlan0 interface configured on my machine and I want to change some of it's parameters via the 'iwconfig' command, but whenever I try to do so I get this error msg:

linux:~# iwconfig wlan0 frag 2346
Error for wireless request "Set Fragmentation Threshold" (8B24) :
    SET failed on device wlan0 ; Operation not supported.
I'm running debian:
chupa:~# uname -a
Linux chupa #2 PREEMPT Mon May 8 13:47:03 EDT 2006 i686 GNU/Linux
The wireless card I'm using is a D-Link 520+ (DWL-520+)

Anyone have an idea what's causing this?

thanks in advance,