Introduction

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The goal of this website is to explain, in students language, the protocols PPP and HDLC, both provide service of the data link layer (layer 2) of the OSI model.

This website combines PPP and HDLC, specifies each protocol components, compares them, explains how PPP utilizes the HDLC format and includes a nice quiz for the “finalist” student.

In general, both PPP and HDLC are Point-to-Point (P2P) protocols.

P2P protocol is a protocol for communication between two computers using a serial interface (as shown in the following diagram):

or connecting networks at separate physical locations by using modems and telephone lines (as shown in the following diagram):

High-Level Data Link Control (HDLC) is a data link layer protocol developed by the International Organization for Standardization (ISO). At the beginning it was a part of the X.25 protocol stack. Later on, it was modified by IBM to become the SDLC protocol. Nowadays, HDLC is the basis for the synchronous PPP (point to point protocol). PPP was designed much later than the original HDLC, therefore the PPP includes many additional features.

The Point-to-Point Protocol (PPP) originally emerged as an encapsulation protocol for transporting IP traffic over point-to-point links, thus it is sometimes considered a member of the TCP/IP suite of protocols. PPP is commonly used for connection over synchronous and asynchronous circuits, and was designed to work with several network layer protocols, such as IP, IPX and AppleTalk.

PPP is usually preferred over the earlier de facto standard Serial Line Internet Protocol (SLIP), since it can handle synchronous as well as asynchronous communication, and share a line with other users because it has error detection that SLIP lacks.

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