Streaming through 802.11b (WLAN)

[systest]

I want to use VLC to stream mpeg4 video through 802.11b. Following is my step:

1. Connect PC server with VLC (0.8.2/Linux) to a switch with multicast support
2. Use VLC to stream video to a multicast address (UDP)
3. Connect my laptop to the multicast switch, use VLC as client to receive multicast stream, it's OK, can play the stream normally.
4. Add an wireless AP to the multicast switch, at same time, disconnect the wire connection between switch and my laptop.
5. Use wireless card on my laptop to connect to the AP, ping the PC server(VLC streaming), get correct response.
6. Use VLC(on my laptop) as client to receive multicast stream, it can't work, can't access the multicast address, why?

PS: in the same topology, MS's media server/player can work normally, if VLC doesn't has a perfect multicast support?
Pls give me some suggestion!

[dionoea]

please don't double post