Point to Point Protocol, 

• Point-to-Point Protocol (PPP) is a data link layer (layer 2) computer networking protocol that allows direct communication between two routers without the use of a host or any other networking infrastructure. It can offer data compression, transmission encryption, and loop connection authentication.

• PPP is used over a variety of physical networks, including fibre optic links like SONET, phone lines, serial cables, trunk lines, mobile phones, specialised radio links, and ISDN. Internet service providers (ISPs) use PPP for user dial-up access to the Internet because IP packets can't be transmitted over a modem line on their own without a data link protocol that can pinpoint where the transmitted frame starts and ends.