What is NAS Protocol and how does it work?

Hello All,
How NAS Protocol works (functionality) in LTE?

You can check this very informative video, it explain it clearly (among other protocols).
This video gives overview of LTE air interface protocol stack with proper representation.
It talks about NAS, RRC, PDCP, RLC, MAC, and Physical layer.
Air interface protocol in a nutshell. Excellent video!