Installing Apple's Network Link Conditioner Tool
It's in an additional download. Use this menu item:
Xcode > Open Developer Tool > More Developer Tools...
and get "Hardware IO Tools for Xcode".
For Xcode 8+, get "Additional Tools for Xcode [version]".
Double-click on a .prefPane file to install. If you already have an older .prefPane installed, you'll need to remove it from /Library/PreferencePanes.
You can also install any of the Hardware IO Tools without installing XCode itself. Simply visit Apple's Download Center and search for "Hardware IO".
Update on the answer December 2019 Xcode 11.1.2
Apple has moved Network Link Conditioner Tool to additional tools for Xcode
Go to the below link
https://developer.apple.com/download/more/?q=Additional%20Tools
Install the dmg file, select hardware from installer
select Network Link conditioner prefpane