Cross layer design in wireless sensor networks

What is cross layer design, what is its requirements and types of cross layer design

-~-~~-~~~-~~-~-
Please watch: “(112) what is unicorn startup @Let’s Crack UPSC CSE @Tech Guru99 @Drishti IAS ”
https://www.youtube.com/watch?v=rkfleScLNmo
-~-~~-~~~-~~-~- .