Xref: utzoo comp.dcom.fax:140 comp.protocols.appletalk:5351
Path: utzoo!utgpu!watserv1!watmath!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!wuarchive!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.fax,comp.protocols.appletalk
Subject: Re: Macintosh FAX on a network
Message-ID: <3788.27c24fdc@hayes.uucp>
Date: 20 Feb 91 10:30:52 GMT
References: <1991Feb19.015202.2758@ohsu.edu> <1991Feb19.184801.27124@PacBell.COM>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 49

In article <1991Feb19.184801.27124@PacBell.COM>, perl@PacBell.COM
(Richard Perlman) writes: 

> In article <1991Feb19.015202.2758@ohsu.edu> bj@ohsu470.ohsu.edu
> (Bill Jackson) writes: 
>>FAX/Data modem such as Prometheus Ultima on a Shiva Telebridge...
>>Is there some smart HW/SW combination that will work here?
>>
>>This seems like a highly desirable thing to do, so either somebody has done it
>>or it is impossible!
> 
> Assuming you mean to use the Telebridge as a NetSerial to connect the
> serial Fax modem to the AppleTalk LAN... It is "highly desirable" and
> also won't work.  FAX transmission is synchronous (and continous) there
> is no handshaking for flow control.  If the fax transmission is
> interupted, as can happen on a network, the connection breaks.

Your are correct that FAX transmission is synchronous -- ON THE 
PHONE LINE.  All existing fax modems, however, connect to the PC or 
Mac _asynchronously_, _with flow control_.  The data rate is run at 
higher than the line rate, usually 19200bps async, to allow for the 
stripping of start and stop bits.  Flow control is used to prevent 
the PC from overrunning the modem's transmit buffer.  If the 
delivery of data from the PC to the modem is delayed (either because 
of slow software on the PC, interrupt latency or multitasking 
delays, or even network loading), the modem can insert fill bits 
(zeros) before any end-of-line sequence in the fax transmission to 
allow time for the PC to catch up again (all fax modems I'm familiar 
with do this); you can send up to five seconds of fill bits without 
disturbing the transmission or violating Group 3 specs.  Reception 
is a bit more problematical -- if the PC flows off the modem for too 
long, you WILL lose data since no fax modem has an infinite receive 
buffer, but they do general have sufficient buffer to handle an 
interruption of many seconds (since the data is only coming in at a 
rate of about 1K per second), and since the interface is running at 
usually at least twice the speed of the phone line, the PC can catch 
up quickly.

I think it is quite possible to use a fax modem through an async
server across a network.  Async-to-sync conversion, buffering, flow 
control, running at higher than the line speed, and the ability to 
use fill bits make it much easier than it might appear at first 
glance.

-- 
Toby Nixon, Principal Engineer    | Voice   +1-404-840-9200  Telex 151243420
Hayes Microcomputer Products Inc. | Fax     +1-404-447-0178  CIS   70271,404
P.O. Box 105203                   | UUCP uunet!hayes!tnixon  AT&T    !tnixon
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