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Line Noise and the Problems it Can Cause with File Transfers
Many people have left messages on my bulletin board asking me why there
are so many 'garbage' characters on their screens and why file transfers are
riddled with errors. These garbage characters are really line noise and can
be introduced in many different places. One of the more common and familiar
introduction points of line noise is in the telephone company's system and
even here there are several ways noise is introduced. A signal is routed
through multiple stations before it eventually makes it to the other end and
some of these stations aren't exactly new. Older areas may have older, less
sophisticated equipment that is more apt to be affected by ambient noise.
This is one reason some people continue to have noise problems even after
hanging up and calling back multiple times. Also, a given physical connection
at one of these junctions may not be up to snuff. If your particular bout of
line noise is solved by hanging up and calling back, then it's probable that
you were previously connected through an intermittent or 'dirty' connection.
Some of these trunk lines (large, multi-caller 'pipes') may pass through an
area that has alot of ambient RFI (Radio Frequency Interference) present
although this is not usually the case.
Another common noise introduction point is in your home. Most residential
homes have televisions, radios, microwave ovens, VCR's, and if you are reading
this, a micro-computer. All these devices radiate radio waves that can (and
often do) get into the phone lines and cause noise. Electric motors and
mechanical dimmer controls can introduce noise into the electrical wiring in
your house and cause problems. If your line noise problem does not go away
after repeated hanging up and calling back, then you may be suffering from one
of these household problems. If you are suffering from this problem, you can
take steps to eliminate it. First of all, turn off EVERTHING except the
fridge (If it IS the fridge, then you're SOL. Can't live life with your ice
box unplugged) and see if the noise persists. If it goes away, then start
turning things back on, checking the computer each time until you see the
noise start up again. It may be that a single device is not bugging you but
several devices plotting together to annoy you. This elimination tournament
may take awhile.
Another area to check is your wiring at the computer. Use noise
supressors on your power connections to both the PC and the modem (if
external). Use a shielded RS-232 cable to connect your modem to the PC.
Ribbon cables (especailly long runs of it) are great antennas and will cause
problems. Re-route the RS-232 cable so it does not run next to the PC power
supply or any other transformer. Many 'clone' monitors do not have internal
metal shielding and can radiate lots of noise. Make sure the cable does not
run near the monitor. If you are particularly adventuresome, you can line the
interior of the monitor with foil and ground it with a ribbon grounding strap.
Be VERY CAREFUL if you attempt this. Monitors generate THOUSANDS of volts of
electricity and can knock you clear into next week. You'd best NOT attempt
this unless you are experienced in electronics. If you live near a freeway or
highway, then interference from CB radio can present a problem. Many
interstate truckers have 100+ watts of power (illegally) on their CB rigs and
frequently have sloppy amplifiers that can emit spurious radition all over the
radio spectrum.
And now a little discussion about the modem itself. First of all, I'd
like to clerify a commonly misused term - BAUD. The term "Baud" is actually a
man's name - J.M.E. Baudot (Pronounced: Baw-doe) a French Telegraphy expert.
1,200 and 2,400 Baud is NOT the same as 1,200 and 2,400 BPS (Bits Per Second).
The usage of "Baud" to describe line speed in terms of data through-put is
incorrect. 1,200 and 2,400 BPS modems both operate at 600 Baud. Basically,
without getting to technical, a Baud is a "blip" of information. 1,200 BPS
modems use four states per blip (or Baud) and 2,400 BPS modems use sixteen
states per blip. If you want more information on what Baud and BPS mean and a
full explanation of how data is actually represented and transferred by the
modem, please refer to PC Magazine Volume 6, Number 9 (May 12, 1987).
Modems operating at 2,400 BPS are much more intolerant of line noise than
are modems operating at 1,200 BPS. Conversely, modems capable of 2,400 BPS
operate better at 1,200 BPS than do 1,200 BPS only modems. If you are being
hopelessly attacked by noise at 2,400 BPS, trying calling back at 1,200 BPS.
It's very possible that the noise will be greatly reduced or disappear
altogether. I know, you didn't buy a 2,400 BPS modem just to retard it to
1,200 BPS. The brand of the modem plays a part in the immunity to line noise.
Some modems can digest more noise (lower signal-to-noise radio) than others.
PC Magazine (same issue mentioned above) ran a test on 87 different modems.
You might check the results to see how your modem ranks. Most 2,400 BPS
modems operating at 1,200 BPS have approximately -8 to -10 db error threshold
while the same modem has about -16 to -20 db threshold operating at 2,400 BPS.
For this reason, line quality is much more critical at 2,400 BPS operation.
Additionally, a friend of mine who runs a bulletin board from their
office has been plagued with line noise problems at 2,400 BPS but very little
noise at 1,200 BPS. The culprit is the office's centralized telephone system.
Many office buildings have a given number of trunks that actually enter the
building while there may be many, many more extension within the building.
These types of telephone systems have their own controllers and line
assignment devices and are frequently not as high in quality as a hard-wired
Mabell line. The acceptable signal-to-noise ratio in some of these
inter-office phone controllers are lower than necessary for reliable 2,400 BPS
operation but not too low for 1,200 BPS.
If you gets transmission errors while downloading or uploading a file,
don't fret it. The Xmodem (or whatever protocol) incorporates an error
checking/correction mechanism that automatically detects and corrects any
errors that may occur during transmission. The very fact that Xmodem reported
the error in the first place means that he caught it and corrected it. The
only errors you have to worry about are the ones that Xmodem does NOT report
Any reported error has already be corrected. Xmodem, especially the CRC
flavored one, is a very reliable file transfer protocol. Even if you got 100
errors during transmission, chances are still pretty slim that the file got
corrupted. Occasionally, a file will be corrupted after transfer, but many
times this may be due to a bad ARCing of the file or perhaps a disk error that
may have occured sometime during the files' past.
I hope this text helped explain some facts about modems, line noise, and
file transfers. If you have other, more specific questions, concerning modems
or communications in general, leave a C)omment to the SYSOP on the PC
Consultant. I'll try to answer them.
the PC Consultant
SYSOP: Robert K. Ricketts
P.O. BOX 42086
Houston, TX 77242-2086
Node 1 (713)270-7408 - Free and open to all.
Node 2 (713)270-8129 - Contributors only.