I'd tried several USB cards mainly to have headaches with their performance. The typical USB card has a poor antenna (on purpose to limit range) and thats no good where there are multiple signals reflecting around. Ive also had trouble with Knoppix changing wireless network settings or not being able to set them default. The N-adapters are supposed to be better at collision avoidance, but theres only so much it can do without a good antenna. Also, the USB port is slow. Solution - throw the USB adapter under a passing bus and convert a router. Theres a utility called DD-WRT that allows certain routers to be converted to wireless network devices off the LAN port. No more configuration problems, fast to connect, high speed data transfer between PC and network device and many have external antenna ports for directional antennas. It works GREAT and eliminates problems with software on USB ports or having too few USB ports for devices. Drawbacks, a converted router is much larger physically and requires a power adapter. DD-WRT is not exactly simple, and apparently there is a chance of doing whats called "bricking the router" (making it non functional), a little time consuming and depending on whether you have an old router, a little more expensive. BTW, Im using Knoppix 6.4.4 if it matters.
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