802.11b power management

I need some help in a problem I am having with Intersil PRISM based PCMCIA
card driver.

I have disabled power save mode in the driver and also disabled XP wireless
networking so my utility will configure the card.

What I see is that about every 60 seconds I see a 802.11b packet sent with
Power management bit set and this packet also contains 4 bytes of NULL data.
I do not know the source of this packet.

1) Why is this packet sent? Is it related to PnP power management or 802.11b
power management?
2) How is this sent?
3) Can it be disabled?

Thanks,

...Ram