I am trying to understand the relation and difference between multiplexing and multiple access. 

Since [multiplexing appears in physical layer, and multiple access in data link layer (or MAC sublayer)](https://www.quora.com/What-is-the-difference-between-multiple-access-and-multiplexing-like-in-FDMA-and-FDM), what is the difference between physical layer and data link layer, which can help to explain the difference between multiplexing and multiple access? 
I don't completely understand the Quora replies there, and I am not sure if they are correct 100%. But they may point out the right direction to understand it.

[Another link](http://www.techexams.net/forums/off-topic/74579-difference-between-multiplexing-multiple-access-methods.html) also says:

> 
> Two types of combining signals are:
> 
> 
> * Multiplexing - combining signals from the same sources
> * Multiple-access - combining signals from multiple sources
> 
> 
> 

Do the signals in multiplexing come from the same source, or can the signals also come from multiple sources?

Thanks.