HSPEED.ZIP

High Speed Transfers, Line Noise, Etc.

What follows is not intended to be a technical discussion on high speed
modems, but tries to answer some commonly asked questions.  The new user
and the expert alike may ask why file transfers are slow above 9600 baud.

Line noise over common telephone wires has been a problem since the first
modems made their connection.  For years it was said that 2400 bps was as
fast as normal telephone lines could transmit data and that we would never
see faster speeds due to the low band-width and high noise levels.  Modem
technology has advanced over the years, but line noise is not a thing of
the past.  Unfortunately, it is still with us.  These new modems are not
magic either.  They manage to hide the line noise and some are able to
filter it out, but it is still there.

If line noise occurs during the connection process, where your modem and
the host modem perform their hand-shaking sequence looking for common
ground, then it is very likely that the two modems will agree to a slower
speed to avoid having problems during the remainder of the call.  In fact,
if line noise occurs during the call the modems may decide to shift down at
that time as well.  When line noise occurs during a file transfer between
two modems which have established an error correction session, the only
evidence of this line noise may very well be slow file transfers and
nothing else.  Many modems give no other indications this extra work is
being required, and may eventually give up and just drop the carrier
completely.

If you find that downloading ASCII files produces excellent file transfers
rates but downloading Zip files reduces the rate, then very likely you have
turned on an option known as "data compression."
It is like trying to use PKZip to zip a Zip file.  In its attempt to shrink
the file, it actually causes the file to expand in size and this slows down
the file transfer.

For high speed file transfers to work you generally want to use some type
of flow control.  If your modem or software is not set up correctly, it
possible that flow control problems will result in errors.  When data is
lost due to flow control problems there is only one solution and that is to
send the data again.  This results in slowing down your file transfer.

Choosing the right file transfer protocol is important and Xmoden is not
always appropriate at high speeds.  Zmodem has the ability to re-send the
last block of data while other protocols, such as Ymodem/G, have no way to
request that data be resent, so the transfer is aborted.  Ymodem/G is one
of the fastest, but Zmodem is more dependable.

And if all that is not enough, it seems like certain times of the year
introduce more line noise on the phone lines than others.  Some causes of
line noise are wet ground conditions, which introduces electrical changes
in the phone company's lines.  Increased traffic on the telephone lines
leads to increased noise.  Some of the worst conditions can occur on local
calls, as local circuits seldom receive the maintenance attention given to
long distance circuits.

So what can you do about line noise?  You can contact your local phone
company.  Sometimes the phone company does find a correctable problem.
Using high quality software and equipment can hide some of the problems.
Or you could move to France where fiber optic telecommunications is a
reality.

Courtesy Board of Trade BBS (813) 862-4772