So MPLS is one of the many ways to connect two or more distant sites. In other words, it is some kind of a "WAN solution".
But what is the "WAN solution" for MPLS?
Take for example the following basic MPLS diagram. This one is from the book Computer Networking: A Top-Down Approach.
The grey routers are standard routers, while the blue ones are MPLS-capable routers. The black lines are, obviously, the links between the routers.
I understand how MPLS basically works. But how do we connect two distant MPLS routers? (Don't tell me those routers are on the same site.)
T1 and SONET are dead, as one of the top responders here in NESE always say, so I don't think those black lines are T1 nor SONET. DSL and GPON are used in access networks, so I also don't think those are used between such core routers.
What is the WAN solution for MPLS?