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 a lot 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 suppressors 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 (especially 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 clarify 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 occurred sometime during the files' past.