I am trying to expand my knowledge of networking beyond the basics. I have started reading about PPP, and how it is used in DSL modems with PPPoE and PPPoA.
My first impression of PPP was "well that seems pretty similar to Ethernet". They are both data link layer protocols. They both have fields to identify the encapsulated protocol (e.g. IP). They both have related protocols to assign IP addresses (DHCP and NCP). So my first question was "so what's the point of PPP, why not just use Ethernet?". The answer to that was fairly straightforward - Ethernet is not supported over a wide range of media like serial lines, and is a fairly specific technology to LAN's using CAT5 or similar.
HOWEVER - then I was reading about PPPoE, and the obvious thought was "well if we are doing something over Ethernet, then Ethernet must be available and in use, so why not just use it?". In other words, PPPoE seems to be encapsulating one data-link layer protocol in another very similar protocol. Why do IP-inside-PPP-inside-Ethernet when we could just be doing IP-inside-Ethernet, and use DHCP rather than NCP to assign the IP address to the home router?