WANs
it is the 1st WAN tech, it uses a Modem connected to the PSTN to carry data.
https://github.com/QueenieCplusplus/DSL#architectur
Public Switched Telephone Network
Internation GW Exchange Center ---- Primary Center ---- Toll Center ---- End Office
Some Router provides built-in features of DDR, Dial-Up on Demand Routing Capability. DDR enables networking equipments to dynamically dial up a connectio to a remote WAN DES.