So, let's talk about HDLC encapsulation. It is proprietary to each vendor; it was not intended to encapsulate different network layer protocols, so each individual vendor had to put in the header of HDLC their own specific way to identify the network layer protocols.
What does this mean? If you are going to use HDLC, you must have the same vendor on both sides.
So, how do we circumvent this issue, so that we can use whatever vendor we want? Well we use the point-to-point, or PPP, encapsulation open standard protocol. It uses a network control protocol field in layer 2 of the OSI model, to identify the network layer protocol. It also allows for authentication and multilink connections over asynchronous links.
Let's look at PPP ...