The usual problems:
1. Radio interference.
2. MAC filtering in the router.
3. WEP key mismatch.
To troubleshoot, start by disabling *all* security.

On 28 Jul 2006 06:09:50 -0700, "moepmoep" <moepmoep77@yahoo.de> wrote in
<1154092190.394429.296040@i3g2000cwc.googlegroups. com>:

>I have a very strange problem with my WLAN-Router (Sinus 1054
> DSL). So far I only used it with LAN, which is working just fine. I
>just occasionally turned on the WLan when I had visitors. Then the
>problem occured: The network was visible (and strong) but it was not
>possible to connect (it did not give the laptop an IP address). This
>happend with a number of notebooks under both linux and windows. All of
>those visitors were computer geeks and checked the configuration of the
>router, but everything was fine (different configurations were tried
>(with, without encryption; with, without MAC-Filter), but nothing
>worked.)
>
>Now the strange thing happens: With one of these friends it suddenly
>worked the next morning: I did not change the configuration of the
>router nor did my friend change his settings. With another friend it
>worked after changing the channel.
>
>This kind of gives me the idea that maybe the WLan has some kind of
>weird problem with another WLan, since that could explain the above
>phenomenon. Anyway I never really tried to solve the problem, since I
>did not have a laptop to use the WLAN for, until last week.
>Now the same problem happens again, but switching the channel does not
>work this time. Also the configuration of the notebook is fine, because
>I can log on to another (someones :-)) network, which is much weaker.
>
>I have absolutely no idea what else I could try - also it does not
>sound like a hardware problem, because occasionally it does work. I am
>also fairly certain that the configuration of the router is fine, since
>I had independently different people (all of whom are computer experts)
>check it because of the problem and noone could find anything wrong.
>
>I would greatly appreciate any suggestions.
>
>Best, moepmoep

--
Best regards, FAQ for Wireless Internet: <http://Wireless.wikia.com>
John Navas FAQ for Wi-Fi: <http://wireless.wikia.com/wiki/Wi-Fi>
Wi-Fi How To: <http://wireless.wikia.com/wiki/Wi-Fi_HowTo>
Fixes to Wi-Fi Problems: <http://wireless.wikia.com/wiki/Wi-Fi_Fixes>