i am asking for a way to send notification... instant notification...
for example, if you use any mail provider, like outlook mail, once a new mail arrives, you get a notification on the top left of the screen [pop up kind notification] ...something similar to that.
I'd usually say that messages is what you want, but in this case...
From what you're describing, I'm thinking 'database'. Which you should already be using for tickets, comments and such.
So all you have to do is query your db for things that concern the logged in user, and display them in a view (I'd use an unmovable modal popup).
If you need to mark things as read/unread, you'll also need to store that information in your database.
I'm not entirely sure what would be the best way to trigger a refresh of the notifications on the front end.
My bet is, a message sent when a new ticket is added, when an assignment changes, etc, that says the db has been updated. You could pass the username/id of the concerned users, so only them need to trigger a refresh.