Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!van-bc!sl
From: sl@van-bc.UUCP (Stuart Lynne)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Fax over tcp/ip
Message-ID: <30@van-bc.UUCP>
Date: 14 Oct 89 07:07:39 GMT
References: <8910131208.AA18264@iapetus.rice.edu> <2078@brazos.Rice.edu>
Reply-To: sl@van-bc.UUCP (Stuart Lynne)
Organization: Wimsey Associates
Lines: 32

In article <2078@brazos.Rice.edu> hd@rice.edu (Hubert D.) writes:
>I suggest using the CCITT group 4 document transmition standard instead
>of the CCITT group 3 specification which is used over the phone lines.  The
>group 4 spec uses ONE compression table for the entire file; group 3
>uses a compression table for EVERY line.  The internetwork is a much
>more reliable medium than the telephone lines.  So, a protocol which
>suits the medium should be used

The problem is not so much that you want to choose *ONE* bit encoding spec,
but that you are probably going to have to support several.

There is a basic tradeoff to make. If we choose one standard (eg G3 or G4)
then you will have to convert whatever you have to that standard. This
involves CPU cycles. If you allow multiple standards (ie G3 and G4) then all
sites will have to support multiple formats, but you won't have to convert
from what you have to a standard format if the other end does understand
what you already have. This may involve extra communications bandwidth but
may conserve CPU cycles at each end.

I would suspect that if a fax transfer standard is implemented it should 
involve a handshake at the beginning where each end tells the other what 
it understands. Then if you have a format which the other end doesn't 
understand you know you have to convert before sending otherwise you just 
send it.

We store fax images in TIFF files and support G3, packbits, Lempel-Ziv and
will be doing G4 shortly. Right now we're leaning towards Lempel-Ziv. It 
gives similiar compression ratio's to G3 and is much faster to do (ie less 
CPU cycles). For colour images it's better than G3.

-- 
Stuart.Lynne@wimsey.bc.ca ubc-cs!van-bc!sl 604-937-7532(voice) 604-939-4768(fax)