From the version 1.8.0 we support Pusher for sending real time notifications for new orders and call waiter option. To setup the plugin follow these steps:
Visit Pusher official site. Create new account or login with existing one.
After login you will see the screen for option to create Channels or Beams. Select Channels and create your first channel that you will use in your website.
Click on your channel that you have created and after that select App Keys. The credentials will be auto generated.
Now lets add these credentials in your Settings page. Select Plugins option.