Telecommunications companies however offer serial communications links around the globe right now and have done so for many years. When serial links that are part of the public telephone system are used, care must be taken to ensure the authenticity of all communications. To this end PPP incorporates user name and password security. Thus, a router or server receiving a request via PPP where the origin of the request is not secure, would require authentication. Since PPP was designed to do things that were either impossible or unnecessary with Ethernet, users are often confused as to why one would want to use PPP over Ethernet at all.
Streets can serve many access points.
It is easy to get on to and off of the street. Additional access points can be added with little disruption. It is hard to tell how many cars are actually using each street. PPP, on the other hand, would be comparable to a railway. Travel is generally between two well defined points.