Mandrake Linux on dell laptop with PrismII 2.5 USB 802.11b port cannotjoin the ad-hoc LAN.

Guys,

I have a small wireless network with two XP machines, and two linux
laptops. All with the same type of PrismII 2.5 USB 802.11b port. I have
prism2_usb in the Mandrake 2.4.21 kernel. I was able to sniff the
wireless LAN which was set to use channel 11 to communicate with each
others as the following:
# wlanctl-ng wlan0 lnxreq_wlansniff enable=true channel=11

However, if I simply did
# wlanctl-ng wlan0 lnxreq_ifstate=enable
# wlanctl-ng wlan0 lnxreq_autojoin ssid=name authtype=opensystem
# ifconfig wlan0 ip

the linux machine won't join the network, on which those two XP machines
had been talking to each other happily. On the XP sites, the wlan was
set to authtype=Open and no WEP.

If I did:
# iwlist channel

it shown that the linux machine was using another channel rather than
11. I couldn't get it on channel 11.

I wonder what the problem was.

thanks very much

steve