Different device brands in adhoc mode

Hi,

I was using three at76c503a-based devices (D-Link DWL-120 rev. E) for 
almost two years now.
They are connected to each other using adhoc mode.

Recently I tried to bring in some other WLAN devices (Prism-based 
DWL-122 and Ralink-based RT2500).
Those devices immediately detect our current setup, however data 
transmission is not possible.
I.e. I can "see" the other devices using Kismet or tcpdump, but even 
ping-ing one of the Atmel devices does not work.
Besides WEP-encryption I don't use any MAC-filters or alike.

Are there any known incompatibilities between different brands in adhoc 
mode?

    Florian

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