Subject: [rohc] SIP dictionary and SigComp - question

From: "Vimal Vikrant Vardhan M" <[email-masked]>
Date: Wed, 13 Nov 2002 11:55:31 +0530

Hello,

	I'd like to know in what way a compression algorithm implementation would
change when it is going to use a SIP dictionary? Considering something like
LZ77/LZSS as mentioned in the SigComp user guide, would the implementation
of the algorithm change to a large extent if it needs to use the SIP
dictionary? In that case are any such implementations of the compression
algorithms available, i.e., which use a SIP dictionary?
	There are sample compressed messages given in the Sigcomp UG. Can a UDVM
which implements all the instructions that one comes across in the bytecode,
say of LZSS, as specified in the UG, decompress the message without the need
for the SIP dictionary? If it can, then how will using the SIP dictionary
change the situation and also the compressed message given in the UG?

Any help in this regard would be greatly appreciated.

Thanx and regards
Vimal

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From: Carsten Bormann <[email-masked]>
Date: Wed, 13 Nov 2002 15:53:55 +0100

Hi Vimal,

> dictionary?

The compression algorithm would make use of elements of the dictionary 
in addition to previously encoded text.
One very simple mechanism is to pre-prend the dictionary (more 
precisely: its text part) to the text to be compressed.
You don't actually send the dictionary through the compressor, of 
course, but the actual text can then be encoded with backreferences 
into the dictionary.

As always in SigComp, decisions by the compressor are local decisions, 
in the sense that it can do what it wants as long as it sends 
appropriate code to the decompressing UDVM to regenerate the 
uncompressed text.

> change the situation and also the compressed message given in the UG?

If the compressor decides to use the dictionary, it will send UDVM code 
that references it.
This code would then not work without the dictionary, precisely because 
the UDVM implements the referencing instruction.

Gruesse, Carsten

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From: "Vimal Vikrant Vardhan M" <[email-masked]>
Date: Wed, 20 Nov 2002 14:48:22 +0530

Thanx Carsten for the info.
What's not clear and I'd like to know is can the SIP dictionary be used (not
just the text part) with DEFLATE algorithm.
I'm not able to figure out how to use the length/offset table part of the
SIP dictionary in DEFLATE as it just refers to string/substrings using the
backward distance/length pairs.
Thanx and Rgds,
Vimal

-----Original Message-----
From: Carsten Bormann [mailto:[email-masked]]
Sent: Wednesday, November 13, 2002 8:24 PM
To: Vimal Vikrant Vardhan M
Cc: Carsten Bormann; [email-masked]
Subject: Re: [rohc] SIP dictionary and SigComp - question

Hi Vimal,

Gruesse, Carsten

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From: Carsten Bormann <[email-masked]>
Date: Wed, 20 Nov 2002 15:25:51 +0100

Vimal,

there is no use for the length/offset table in LZ77 style algorithms 
like deflate.
They are meant to be used with LZ78 style algorithms like LZW.
For LZ77, you don't even need to load the length/offset table -- that's 
why the draft gives offset/length information (for use in 
STATE-REFERENCE) for loading the string table only.

Gruesse, Carsten