[ubuntu] HELP: ubuntu 13.04 can connect to a private wifi but cannot connect to a public wifi

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http://ubuntuforums.org – I installed ubuntu 13.04 on my laptop(lenovo ideapad y450a). After installation, I tried to connect to a public wifi but failed. However, I can connect to my private wifi. the wireless network card type is Intel Corporation PRO/Wireless 5100 AGN. Another laptop(thinkpad x41s) with ubuntu 13.04 is OK when connecting to a public wifi. so, what should I do? (Hardware)