Line Noise Overview

When To Suspect Line Noise

With data calls, line noise can cause numerous problems. In general, the faster you communicate, the better your phone lines need to be. When you get up to 28.8 and 33.6 bps, it is critical that you have clear lines from end to end. Line noise can cause the following problems when trying to make high speed data connections:

  •     No connection.
  •     Connection, but only "garbage" characters appear on screen.
  •     Connection, but no communication.
  •     Connection, then disconnection for no apparent reason.
  •     Connection, but modem speed drops.

How To Check Line Noise
Connect to a remote computer or BBS using a terminal emulation program such as Telix, ZTerm, or Hyper Terminal.

Type: +++ (OK, should appear on your screen)
Type: AT%Q (press the enter key) (you should see a number on your screen ranging from 000 and 255)
Type: A/ (a few times, repeatedly) (you should see numbers each time you press the enter key)
Type ATO (the letter O) to return from the command mode

Any numbers over 30, you have considerable line noise.
* Note: The above modem commands are not supported by all modem manufacturers.


Where Does the Problem Originate?
Line noise can be introduced at the remote site, by the phone lines, or at your local site. Before making any changes, try calling a few different remote modems, to see if you have the same line noise problems in each instance.

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. Your local telephone company is required by the FCC to provide a minimum level of quality on the lines it maintains for your use. Contact your local telephone company and ask them to perform a line qualification test if you suspect this may be the problem.

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 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.


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Last Updated: Monday, 11-January-99 10:01:14 PDT