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what is the difference between PPP and HDLC encapsulation?

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Hari Reddy

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Jun 2, 2010, 3:23:54 PM6/2/10
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what is the difference between PPP and HDLC encapsulation?

Steinar Haug

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Jun 3, 2010, 3:09:40 AM6/3/10
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> what is the difference between PPP and HDLC encapsulation?

This doesn't really have anything to do with SDH or SONET, so you're
in the wrong group. That said

- HDLC is a bit synchronous framing format and protocol. See

http://en.wikipedia.org/wiki/HDLC

The basic HDLC format doesn't have any protocol field, and thus no way
to demultiplex on a higher level protocol. "Cisco HDLC" (maybe that's
what you meant) is based on the basic HDLC framing format, but adds a
protocol field and some other stuff. Here's some info on "Cisco HDLC",
picked up from Usenet 20 years ago:

http://www.nethelp.no/net/cisco-hdlc.txt

- PPP is a data link protocol *based on* the HDLC framing format. See

http://en.wikipedia.org/wiki/Point-to-Point_Protocol

It's important to be clear on what you mean by HDLC, because both PPP
and "Cisco HDLC" is used for the same purpose (layer 2 encapsulation
on point to point links), and they are both based on the original HDLC
framing format - which is itself based on the IBM SDLC framing format/
protocol.

Steinar Haug, Nethelp consulting, sth...@nethelp.no

Hari Reddy

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Jun 4, 2010, 11:31:13 PM6/4/10
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Dear Steinar Haug,

I hope i am in correct group. I faced one problem recently as follows:

On STM16 level i given loop to customer from my NMS. He is not getting
the loop in his router.But after changing his Encapsulation from HDLC
to PPP, We got the loop. So,my doubt is here. If you know, can you
explain on this which will be greateful to you..

Regards
Hari Reddy

Michelot

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Jun 6, 2010, 3:59:29 PM6/6/10
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Bonsoir Hari,

About your initial question.

You are 2 manners to encapsulate PPP over SDH.

(1) The old manner, before GFP, was through HDLC (C2 byte = 0x16). In
this case, silents between frames are filled by HDLC 0x7E bytes. So
the encapsulation is PPP over HDLC over SDH VC.

In this encapsulation, to avoid 0x7E byte occurences inside the PPP
frame position, a zero is added each times you have 5 x logical 1. So,
the contents of PPP is changed, randomly, during the transport.

(2) The present manner is to encapsulate PPP directly in SDH. This is
done through the GFP adaptation. The silents between PPP frames are
filled with empty GFP frames. The content of PPP is not changed.

Best regards,
Michelot

Hari Reddy

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Jun 7, 2010, 4:23:36 AM6/7/10
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Thanks Michelot.....this information is useful to me...:)

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