The more I get into this book the more it becomes confussing. I think his explantaions for things could go a bit deeper.

In one part he says:

"Each layer performs N-functions. The functions are the different things the layer does. Therefore, the functions of the transport layer are the different tasks that the layer provides. For most purposes in this guide, functions and entities mean the same thing."

But what does the "N" stand for? Why is it called "N-Functions", why not "Z-Functions"?