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Line noise explained
*********************************
LINE NOISE EXPLAINED
Thanks to Robert K. Ricketts
*********************************
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 EVERYTHING 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 radiation 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 Ma Bell 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.
--end--
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