Subject: [rohc] sigcomp - meaning of the F bit in the input_bit_order register

From: Cristian CONSTANTIN <[email-masked]>
Date: Wed, 13 Apr 2005 12:48:07 +0200

hi!

please help me to grasp the meaning of the F/H bits :-) 
in the SigComp rfc, at 8.2.  Requesting Additional Compressed Data there
is this statement:

  "When an INPUT instruction requests n bits of compressed data, it
   interprets the received bits as an integer between 0 and 2^n - 1.
   The F-bit and the H-bit specify whether the bits in these integers
   are considered to arrive in MSB to LSB order (bit set to 0) or in LSB
   to MSB order (bit set to 1)."

how should this be interpreted?

1. same as for the P bit; that is the bytes themselves are always in 
network byte order but the bits in the bytes are in LSB to MSB order when
F=1 and the other way around for F=0.

2. the bits in the _(short) integer_ are:
   
a. LSB to MSB for F=1. that would mean that the component bytes are
   also swapped which would more or less contradict another statement in
   8.2:
   "Note that the input_bit_order register cannot change the order in
   which the bytes themselves are passed to the INPUT instructions
   (bytes are always passed in the same order as they occur in the
   SigComp message)."
 
b. MSB to LSB for F=0. 

now, if 1 is the right interpretation, then wouldn't P=1 && F=1 cancel 
each other's effect?

thank you for your attention.
bye now!
cristian
-- 
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---

From: Carsten Bormann <[email-masked]>
Date: Wed, 13 Apr 2005 13:16:21 +0200

> now, if 1 is the right interpretation, then wouldn't P=1 && F=1 cancel

No, because P is about the way the bits are taken out of the input 
bytes and F/H are about the way they are assembled to binary numbers.

If you are only ever assembling 8-bit numbers, they do cancel out.

For 16-bit numbers, as you notice, the combination of P and F/H gives 
you a way to do the two different byte orders (which does NOT 
contradict the fact that bytes keep their order when being parsed into 
bits).

For fractional-byte numbers (which tend to straddle input byte 
boundaries), it's best to draw a picture :-)

Gruesse, Carsten

---

From: [email-masked]
Date: Mon, 16 May 2005 16:57:12 +0530

Hi,

Please read in-line

Cristian CONSTANTIN <[email-masked]> 
Sent by: [email-masked]
04/13/2005 04:18 PM

To
[email-masked]
cc

Subject
[rohc] sigcomp - meaning of the F bit in the input_bit_order    register

hi!

<Partha> Yes, F-Bit is same as P-Bit for the INPUT-BITS instruction 
</Partha>

 
<Partha>  The input_bit_order register talks only about the order of flow 
of bits within a byte.
          It has nothing to do with the order of flow of bytes </Partha>
 
b. MSB to LSB for F=0. 

<Partha>  Yes, your understanding of the above point is correct. </Partha>

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