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PPP and HDLC comparison


There are lot of difference between PPP and HDLC which arises lot of questions. The following are some genric diffrences between HDLC and PPP are:


1.It is vendor specific.

2.It doesn’t support authentication.

3.It supports synchronous network only.

4.It doesn’t support multiplexing.


1.It is a standard protocol.

2.It supports authentication, like: PAP and CHAP.

3.It supports synchronous as well as asynchronous network.

4.But it supports multiplexing, like: multilinks.

I came to know one more thing about these two protocols few days back that PPP is conection oriented but HDLC is not. But how when they both belong to IP and which has no relation with establishment of connection. This all is done by TCP.

And if HDLC doesn't support asynchronous network then how can we use it for ISDN as an encapsulation.

3 Replies 3


Connection oriented and Connectionless are not just something that is applicable to protocols in the Transport or Network layer. PPP is connection oriented, because its capable of error detection, uses sequence numbers

and acknowledgments to provide reliable transmission.

You should understand that PPP is only a slight modification of HDLC. All it does is add a protocol field to HDLC frame format. Thus PPP is a standard that essentially permits HDLC to run over async lines- only that it guarantees reliable transmission

HDLC support over ISDN is there in Cisco routers, for many reasons. It simplifies the configuration. IOS uses by default HDLC encap on all serial and async lines, unless you change it to PPP.


Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus

thanks for the reply shanky.

in case of tcp it establishes the connection but when there is udp and ppp combination, which protocols establishes the connection.

And how the hdlc is connectionless?

This all is confusing me.

I was only usint TCP as a comparison to explain behaviour how PPP works reliably at layer 2. PPP uses sequence numbers, like TCP does, at layer 2. So no matter what the higher layer protocol is (TCP/UDP) it will use its own sequence numbers to find out if the payload was reliably transported across the link. HDLC does not do this.

Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus
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