Subject: [Fwd: PPP in HDLC-like framing (fwd)]

From: "N.Suresh" <[email-masked]>
Date: Thu, 05 Apr 2001 10:55:42 +0530

In ISO/IEC 3309:1993 about HDLC frame structure is is seen in section
4.7.1 that " The FCS  shall be transmitted to the line commencing with the
coefficient of the highest term". This means that the msb of the least
significant octet of the CRC is sent out on the line first.

However, in RFC 1662  it is mentioned in section 5.5 that "All octets are
transmitted
least significant bit first".

Does the term "All octets" include the CRC octets also? If yes, then RFC 1662
specifies a framing structure different from HDLC.

Please clarify.

regards

--
-----------------***************************************----------------
                                N.SURESH
                            Research Engineer,
                  Centre For Development of Telematics,
                           71/1,Sneha Complex,
                            Bangalore-560052
-----------------***************************************----------------

Subject: PPP in HDLC-like framing (fwd)
From: Roopa C <[email-masked]>
To: [email-masked]
Date: Thu, 5 Apr 2001 09:58:51 +0500 (GMT+0500)

---------- Forwarded message ----------
Date: Thu, 5 Apr 2001 09:48:23 +0500 (GMT+0500)
From: Roopa C <[email-masked]>
To: [email-masked]
Subject: PPP in HDLC-like framing

  In ISO/IEC 3309:1993 about HDLC frame structure is is seen in section
4.7.1 that " The FCS  shall be transmitted to the line commencing with the
coefficient of the highest term".In RFC 1662 about 'PPP in HDLC like
framing' it is seen that "The FCS is transmitted least significant octet
first, which contains the coefficient of the highest term" ( section
3.1).In the same RFC in section 5.5 about Bit-stuffed framing transmission
considerations it is given that "All octets are transmitted least
significant bit first". 

  So what shall be the transmission strategy for FCS bits? Shall they be
sent msb first as in ISO/IEC 3309 or does it mean that they shall be sent
lsb first? Please explain.

---

From: James Carlson <[email-masked]>
Date: Thu, 5 Apr 2001 09:06:39 -0400 (EDT)

N.Suresh writes:
> Does the term "All octets" include the CRC octets also?

Yes.

> If yes, then RFC 1662

No, it does not.  PPP uses HDLC framing.

> Please clarify.

Look more carefully at the FCS polynomial in RFC 1662:

   /*
    * The FCS-16 generator polynomial: x**0 + x**5 + x**12 + x**16.
    */
   #define P       0x8408

The CRC calculation is reversed so that the highest-order term of the
residue shows up in the LSB position.

Most media are LSB-first.  The only case that's broken is PPP over
SONET/SDH, where the SONET/SDH specifications require that the bits be
sent in MSB-first ordering, but PPP specifies the same FCS calculation
(as though the bits were transmitted LSB-first).

If you're doing it in hardware, this is an important nit to consider.
It makes the usual shift register approach wrong.

-- 
James Carlson, Internet Engineering       <[email-masked]>
SUN Microsystems / 1 Network Drive         71.234W   Vox +1 781 442 2084
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