In this lesson, we will learn both HDLC and PPP Protocols one by one. Now, let’s go to each of them and learn the basics of these WAN protocols.

HDLC (High-Level Data Link Control)

HDLC (High-Level Data Link Control) is a Layer 2 WAN Encapsulation Protocol that we use it on Syncronous data links. It is the simplest WAN Protocol that can connect your remote offices over leased lines.

HDLC has two versions. One of them is the standard one and the other is the Cisco proprietary version. The frame of standard version and Cisco proprietary version is similar. Only in Cisco proprietary HDLC, there is one additional proprietary field. Below, you can check both of the frames:


Cisco HDLC is the default enabled WAN protocol of Cisco for Point-to-Point WAN links. And we can use Cisco HDLC only between Cisco devices. Other vendor devices can not use Cisco HDLC.

Lastly, there is no Authentication mechanims in HDLC. So, security is a concern for this WAN protocol.

PPP (Point to Point Protocol)

PPP (Point to Point Protocol) is also a WAN Encapsulation Protocol that is based on HDLC but we can say that it is the enhanced version of HDLC. There are many additional features in PPP if we compare with HDLC.

PPP Authentication

PPP supports two Authentication Protocols. These Authentication Protocols are:

• PAP (Password Authentication Protocol)
• CHAP (Challenge Handshake Authentication Protocol)

PAP (Password Authentication Protocol) is the simplest Authentication method. It uses 2-way handshake. Both end send the passwords in “clear text” in this method. And passwords are exchanged only at the beginning.


CHAP (Challenge Handshake Authentication Protocol) is the more complex Authenticaion method. CHAP uses 3-way handshake and with this mechanims it checks the remote node periodically. CHAP uses MD5 hash. One end sends “Hash” to other node and the other node also sends a hash. If the hashes are same, then the communication starts.

Back to: HCNA > WAN Protocols
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