Mac Mania 2/MacMania 2.toast/Demo's/Tools&Utilities/Netwerk⁄Telecom/HighSpeedModemMusings Folder/HighSpeedModemMusings

Internet Message Format | 1994-02-07 | 7.4 KB | [TEXT/ERIC]

Date: Tue, 18 Jan 94 13:00:54 EST
From: [e-mail] ( Daniel J. Hofferth [phone] AGT/8896)
Subject: High Speed Modem Musings


Hi all,

I've now spent a month or so with a new V.32bis (14.4KB) modem that gave
me a variety of little fits until I finally got everything running just
right. I was upgrading from a 2400 baud modem, and I was mentally unpre-
pared for the many complications introduced by the much higher speeds.
The new modems are NOT always plug-n-play replacements for older models.

While quietly researching solutions, I've noticed quite a few postings in
different lists/digests (such as this one) from people who are suffering
from problems similar to those I have puzzled over.  In the hope of help-
ing someone else, I am posting the problems I had (that I've now learned
are fairly common) and solutions that worked for me.

 <After a whole month I should be an expert, eh?  I'm a neophyte, I'm
  the first to admit it.  I've been a professional computer nerd for many
  years, but I know almost zip about high speed modems (as I shamelessly
  prove below).  Please feel free to comment, clarify, etc...>

Think these are FAQ's?  Maybe so, but I couldn't find answers in any one
place.  Interesting thing is, these seem like perfectly natural problems
waiting for unsuspecting slow modem users who are about to move up.

All problems posted below are related to high-speed (HS) communications,
and they occurred while using both CTB and non-CTB applications (Apple's
Communications Tool Box):

1) Trouble establishing a connection.

   I had my modem attached to the wall jack via a long (50 feet) ribbon
   style phone cable... the kind where the wires run parallel to each-
   other in a semi-clear casing <what's the official name?>.  To make
   matters worse, it ran in long straight lengths (around an intermediate
   room), and then into a tangle of power cords for the computer.

   It was a great antenna, but a lousy HS modem extension.  It worked just
   fine for the 2400 baud modem, mind you.  Went to Radio Shack and picked
   up "twisted pair" phone cable (cheap).  Then rerouted it away from the
   power cords.  Bingo - connections made.

   I understand there is also a "high-twist" wire available for data-
   grade communications.  What I bought worked fine for me... my house
   uses the standard twist in the walls... I didn't see any sense in
   going to data grade for just this one length.  They also sell line
   filters to weed out noise - didn't need that either.


2) I could now connect at HS with error correction and flow control,
   and work the user-interface without apparent trouble, but file
   transfers kept causing my modem to hang-up abruptly.

   The Mac's serial port issues the hardware-handshaking (HH) signal on a
   pin that HH-cables also carry to DTR on the modem.  My modem was set
   to hang-up on loss of DTR (AT &D2), when it should have been told to
   ignore DTR (AT &D0).  No more unexpected hang-ups.  For the older
   modem, this was not an issue - it used Xon/Xoff handshaking.


3) No more hang-ups, but high speed uploads and downloads were full of
   packet-errors.  Effective throughput was only about 700-800 cps.

   I knew that I had a HH-cable, software that had HH support enabled,
   and a modem-to-modem connection that was error correcting and flow
   controlled.  But I was still getting sporadic packet errors on
   file transfers.  Clearly HH wasn't working.

   I got out my multi-meter and tested the cable supplied by the modem
   manufacturer.  It WAS a HH-cable, but not wired as suggested by
   Apple... I'm no EE, but it seemed worth swapping out to me.  At my
   local computer store I bought another HH-cable (after being assured
   that I could return it if it didn't work), brought it home and tested
   it with the meter.  This one was almost identical to Apple's version
   except that pin-7 (GPi) on the Mac side wasn't connected.  As I
   gather from other readings, GPi is a special purpose pin that isn't
   used by simple home connectors like me.  I tried the new cable, and
   file transfers were MUCH cleaner, but still not perfect.

   <Clearly not all HH-cables are appropriate for all machines.  Just
    because your modem manufacturer supplied a cable for you, don't
    assume it is correct for your Mac.  BTW, there are a few Macs that
    don't support HH... Mac Classic is one, I forget the others....>


4) File transfers still suffered from packet errors if I switched to
   another application, or simply held the mouse button down on a menu.
   Otherwise, they were going O.K.  What now?!

   Apple claims the priorities for a variety of system related tasks can
   cause the Mac to miss incoming information.  They suggest the following
   guidelines for improving the CPU's attention to the serial port:

      - Limit your Mac-to-modem connection to 19.2KB.

        While the new modems can theoretically achieve throughput of up
        to 56KB, this is almost NEVER really done.  Text file transfers
        can reach 3500 cps (or so), but most file transfering that we do
        is on already compressed files.  Throughput for these is much
        closer to the modem-to-modem implied speed of 1440 cps.  Thus,
        19.2KB is fast enough for general use.  38.4KB may be safe if
        you Mac is fast enough (mine isn't).

      - Don't use 24-bit mode, stick to 32-bit addressing.

        24-bit addressing mode apparently peppers the CPU with extra
        interrupts that clouds its ability to watch over the serial port
        flow control... possibly resulting in missed characters.

      - Don't use virtual memory.

        Same reasoning as above.  The overhead required to manage the
        virtual memory impares the systems ability to keep up with the
        flood of serial port data.

    I was already at 19.2KB between my Mac and modem (having already
    learned that 38.4KB greatly increased my problems), but I was still
    in 24-bit mode with virtual memory on.  I've been pretty religious
    about collecting only 32-bit clean software, and I've got enough real
    RAM to turn VM off without problems, so following these guidelines
    was painless for me.

    And they worked.

I now have rock-solid high speed connections on a slow Mac, and file
transfers that are robust enough to let me switch between applications
without packet errors.

By the way, to those of you who wonder about file transfers that abort
when you try to launch a big application, or impose some other long
pause...  To me this doesn't necessarily imply a handshaking error,
perhaps the computer you were connected to decided it had waited long
enough and simply timed-out?  Short pauses should certainly be handled
without errors, but the longer you go - the more likely it is that the
host gave up.  No?

I read one interesting comment from a modem manufacturer... They noted
that Apple's serial port buffer is painfully small, making the timing
on handshaking a critical and touchy issue.  They noted that as modem
speeds have increased, fortunately so have Mac speeds - thus somewhat
compensating.  Apparently, we slow Mac owners seem to be in the yellow
zone in the war between CPU horsepower and modem data flow.  It doesn't
take much to tip the battle one way or the other.

Sorry for the length of this, but I hope it helps someone else.

   <Mac Classic II, System 7.1, HSU 2.0.1, 4/40+120, Zoom VFX V.32bis>

Dan Hofferth
[e-mail]