It's a nice thought that every Wi-Fi wireless card will work with every Wi-Fi network. After all, that's what standards are all about. Unfortunately, this isn't always the case -- there's no certification process for Wi-Fi, so anybody can call anything Wi-Fi compliant until proved otherwise.
In locations with a lot of transmitters -- either other Wi-Fi users or things like microwave ovens, TV relays and so on -- badly designed circuits pick up more interference and stop working sooner than good ones. So, your wireless card may work fine at home and not so well in a busy coffee bar. The problems are only going to get worse, as more and more companies start implementing non-standard extensions to their Wi-Fi devices -- early chips that implemented a pre-approval form of the 802.11g 55Mbps standard were particularly bad at working with 802.11b networks, even though they were designed so to do.
Diagnosing problems with incompatible Wi-Fi adaptors is difficult: the only sensible way is by swapping cards and checking against ones that are known to work. You can be sure that hot spot operators will claim that any problem is due to your setup -- "after all, it works with everyone else's" -- even if their choice of hardware is more deviant from the 802.11b standard. Practically speaking, you may have to accept that. If you are going to be relying on one particular hot spot or provider, it may be best to ask them for hardware recommendations before purchasing an adaptor.
For more information on Wi-Fi standards and interoperability check out the following links or click here for the networking toolkit. Find your nearest wireless connection with ZDNet UK's Wi-Fi hot spots map.
A to Z of wireless standards
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