Does something like a wireless-lan receiver and switch in exist?
We have a wireless lan at home and i would like to play my ps2-games online. But the ps2 is in the cellar so there is no network cable down there...
I think I am kinda confused at the question. Any router can be used as a switch.
So you have WiFi set up at your house and you want to hook the PS2 into the router. If the router is WiFi, can you take it to the basement where the PS2 is, hook the PS2 through Cat5E, and run the WiFi throughout the house from there?
If this is a signal loss issue (once the router is in the basement), you can try to turn the antennaes horizontally to allow for horizontal polarization and see if that helps. If not, you can get a WiFi amplifier (they're usually around $15-20 for what you would need it for... and that would be overkill) that hooks up to your antenna connector. TO give you an idea of the amp's ability, we were getting zero signal running on a 19dbi parabolic, trying to hit our aunt .75 miles away. We hooked up this amp and she gets almost full signal. They are within legal transmission limits, so no worries there. I believe they are 250mW. You can get the 500mW flavor, if you need it. But I think the 250 would be overkill for you.
You would need some sort of proprietary "Playstation 2 Wireless Network Adapter."
Nothing exists, that I know of. You might be out of luck.
Your other option would to have a computer in the basement hooked up to the wireless, then have it act as a gateway to share the connection with the PS2... Do you have a computer in the basement that is hooked up to the wireless?
You can get a normal wireless router to act as an access point, connect that your primary router via wireless. use an rj45 to ps2 to access point and bada bing your done, relatively easy!