In Xcode, Xcode -> Open Developer Tool -> More Developer Tools
After enter Apple Developer Center, download the Hardware IO Tools
After double click "Network Link Conditioner.prefPane", Network Link Conditioner will added to System Preferences.
Click on "Network Link Conditioner", in Profile, it have 3G, DSL,Edge,Wifi.
You can also define your own profile, click on "Manage Profiles..."
After test, must turn off "Network Link Conditioner".