Path: cayman!psinntp!psinntp!news.mentorg.com!sun_jimh!gam
   From: gam@sun_jimh.mentorg.com (Gary Merrick)
   Newsgroups: comp.sys.mac.comm
   Keywords: v.42bis
   Date: 1 Dec 92 23:33:35 GMT
   Sender: gam@sun_jimh (Gary Merrick)
   Organization: mentor
   Lines: 24
   Nntp-Posting-Host: sun_jimh.mentorg.com

   Do standard ARA modem scripts take advantage of data compression
   on modems that support it?

   For instance, I am using a modem that supports V.32 and V.42bis,
   which potentially allows a maximum throughput of 38.4 Kbps.
   (This would be *really* nice for file transfers!)
   The manual that came with the modem states that in order to take
   full advantage of data compression, you should set the DTE interface
   speed to four times the speed of the modem link. I would like to know
   if ARA and/or the modem script will do this automagically.

   If not, how can this be accomplished? Is this something that can be
   done by writing your own modem script? 

   I tried connecting to an ARA server with the same modem setup,
   and saw the message "Communicating at 9600 bps." Either this
   message is reflecting the actual baud rate between the modems
   (which is correct), or the rate of the total throughput is only
   9600 bps.

    ____________________________________________________________
   | Gary Merrick                System/Network Admin from Hell |
   | gam@sje.mentorg.com    alt email: gary_merrick@mentorg.com |
   |____________________________________________________________|

 What follows is a list of answers to technical FAQs about ARA. Hope it helps.

Does ARA use the error correction and compression features of the better
modems?

ARA turns off error correction and compression and uses MNP-4
error correction and BTLZ (V.42bis) data compression layered on top of
it done on the CPU that runs the protocol. MNP-4 is also known
as the V.42 alternate procedure for you folks with the APDA published
ARA external reference specification handy.

Do I still need to set my line speed at 4X the maximum bps rate for an ARA
link?

No, the actual speed of your serial port for an ARA link need only be equal
to or the next speed up (ie 19,200 for a 14.4K V.32bis link). This is because
the data is being sent and received from the serial port in compressed form.
the only reason for the higher speeds is if you were gulping data to feed
a modem based compression engine. Not the case here. FYI, the communicating
at message in your ARA connection reflects the bps rate of the connection and
NOT the actual data throughput rate. This rate obviously could fluctuate wildly
due to line conditions and data compressibility.

Why should I pay for a good modem if ARA doesn't use the expensive features?

There is much more to a modem than the error correcting and compression
software. This, in fact, is probably the least important contributor to the
cost of the higher end modems versus the lower cost modems. If you have
worked with ARA you will notice a marked difference in the bit error rate
and throughput depending on the type of modem you use. I will not publicly
endorse or bash any particular modem vendor but it does matter.

Why did Apple do compression and correction in the CPU?

Apple ships an inexpensive 2400 bps modem. This modem has no error correction
or compression associated with it. Also, the DDP code in your macintosh,
despite what you might read in Inside Appletalk really wants to see an error
free link. Thus, Apple could not afford to allow errors from the cheap modem
to creep up to the DDP layer and were apparently unwilling to let someone
else put a "real" modem in the powerbooks by default for them. PPP was not
as developed a standard as it is today also.  So, anyone who wants to do
an ARA implementation better be prepared to have enough CPU cycles per
connection to do the compression and error correction and have 20K dollars
to shell out to British Telecom to license the V.42bis compression algorithm
that is required to make ARA work at all. Any talk about how much more
"efficient" it is to let the CPU do the work instead of the modem is wrong.
Much talk has also been bandied about about how Apple guarantees that the
connection is error free even from serial cable errors and modems do not have
this advantage. Well, my friends, the error rate due to cable problems is
infinitesimal compared to the problems caused by POTS. Also, the ARA protocol
requires packets to be checksummed (even before the DDP layer gets them!!!)
so this is a non-issue. In fact, MNP checks data with a CRC calculation, ARA
then checksums it, then it gets passed to DDP which might checksum it again.
If you think this is really dumb, you are not alone.

Why not just allow me to run the compression and correction in the modem
as an option and tell ARA to NOT do it in the CPU?

Ahh, that would be the life, eh? Unfortunately, ARA uses V.42bis compression
on top of the V.42 alternate procedure for its connections. This is a
marriage of protocols NOT supported by most modems. Most modems with V.42bis
protocols run V.42bis compression on top of LAPM. LAPM is the V.42 "standard"
error correction procedure and the only one allowed in the V.42bis standard.
So, while it might work for some modems, it would not be general, and in
any case, there is no way to get the client to not use the CPU for this so
it is all a mute point.
--
*******************************************************************************

 Ken Siegel                                           Internet: ken@cayman.com
 Cayman Systems, Inc                                     Phone: 617/494-1999
 26 Landsdowne Street                                      FAX: 617/494-9270
 Cambridge, MA 02139                                 AppleLink: D0523
Newsgroups: comp.sys.mac.comm,connect.audit