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The following repeats information from the Modems and Modem Initialization Strings page.

This is followed by Telephone Line Testing (this page).

Phone Line Quality and Line Noise

Factors involved in phone line quality include: Obviously some of these things are beyond the user's control, short of obtaining a T-1 connection. Some things can be controlled, however. Most people won't need to do anything (except maybe the Slick 50 56-KFlex Oil Treatment). If you are getting more-or-less consistent connections, you are probably okay. If your connect speed is 2400 below the maximum connect speed, consider it an enhancement to stability. (If you start with a 2400 baud modem and connect at 2400 below that, it will be very stable.)

You may find that the signal levels are not high enough to generate a maximum speed connection. That's usually not a major disadvantage, provided that the signal is clean. My own connection was at a consistent low level but also consistently clean. I'll take that over fast and dirty any day, and besides, think how much harder it is for my dog to eavesdrop on my conversations!

Telephone Line Testing

It is possible to test line quality with some modems.

The test results themselves won't help, because the telephone company is only obligated to provide "voice grade" service on ordinary telephone lines (sometimes called POT for 'plain old telephone line'). It is possible to use the data to back up a complaint on voice quality issues. In such a case, you would need to qualify the data by clearly stating that this is a problem with "voice quality," and that the modem data is there only to substantiate the "voice quality" problem.

Most common problems involve static and high frequency dropoff. I often have a difficult time getting people to hear the number 'six' in conversation. My phone lines show a frequency degradation at the high end, and I suspect that if my line had better high frequency response, others would hear me say 'six'.

To test line noise with the modem, find a known good connection, such as an internet provider with ISDN lines, digital lines, or 56K capable lines. Some of these tests require that you escape to command mode with the escape code. The default escape code is +++ (S2 set to ascii 43). You reconnect with ATO (the letter O).

Rockwell Chipsets
AT&V1
AT&L - only during a session while in command mode (escape with +++; reconnect with ATO)
AT&Q - only during a session while in command mode (escape with +++; reconnect with ATO)
USR modems
ATY11
3Com (USR) Line Test
1-847-262-6000, and log on as "Line Test." They give you a nice graphic representation of your signal response. A 3Com website describes it.

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Stan Protigal (email)     http://www.scn.org/~bk269/


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