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6 Troubleshooting

Many individuals, when presented with a problem on their dialup, pick up the phone and immediately call their ISP. Usually, very few communications problems reported to an ISP have anything to do with their modems or software. Yes, periodically a modem will not answer due to a problem, but most modem problems are on the calling end.

A common misconception among computer users is that the telephone company and long-distance switching network is a "perfect system." The advent of error-correcting, high-speed modems which mask line-noise and push the data transfer tolerance window to the limit, has given us a false sense of belief that everything should work at top speed every time.

Not so! There is that age old problem known as LINE NOISE. Line noise may or may not be apparent to your "naked ear", however your modem can and will react to this annoying phenomenon. Some caller complaints which can be attributed to line noise are:

"The system was very slow in responding tonight."
"What I typed didn't show up on the screen until two words later."
"I'm was only getting #### CPS, what's the matter with your system?
"When I downloaded 156K of the 175K file you dropped carrier on me."

These problems in data signal transmission and recognition can take many forms. The callers quoted above complained of slow system response, buffered typing response and slow transfers. They were certain it was a problem on the ISPs system. What was the reason? Best guess is a lousy connection most likely with excessive line noise.

If you have line noise or any other problem with a modem connecting to your ISP, you should have your equipment or telephone line checked.

What can you do when you experience modem-related difficulty?

1. Remove all other connections to your telephone line (answering machine, fax, extension phones).
2. Use a short, shielded serial cable.
3. Increase your modem's "time-to-drop-carrier" by adding S10=20 to your setup or initialization string.
4. Have your line checked by the phone company.

Additional information:

To have your line checked by the telephone company, call the local business office. Provide your name and telephone number. Tell the telephone company representative you have noise on your telephone line. Some telephone company representatives may not be helpful, claiming that they have no obligation to provide line quality sufficient for modem communications. Here is some information which might prove to be very helpful.


AS SET BY THE FCC, your local telephone company is required by the FCC to provide a minimum level of quality on the lines it maintains for your use. Many lines do not meet these standards, and so are a source of considerable noise in the transfer of data between computers.

The telephone company may tell you there is usually only one option offered: line conditioning (at $170/line!!). This is an extra cost item that they would like to sell you. However, if their line to your computer meets the minimum standards, there is no reason you cannot have error free transfers if an error correcting protocol is used. If the telephone company seems reluctant to test your line and assures you that it meets the minimum requirements, request that they perform POTS Data Testing and provide you with the results. Let them understand that if they do not comply, you will refer the matter to the FCC. They will NOT be happy!

The POTS Data Testing involves the following tests:

1) Frequency sweep 300 to 3000 Hz

2) Roll off 500 to 2500 Hz with -2 to +8 range
Request 300 to 3000 Hz with -3 to +12 range
1000 Hz tone loss -16, 1% in either direction

3) Signal to noise ratio 24 Db level

4) White noise C message Dbrnco

5) Envelope delay measurement

6) Phase jitter not to exceed 10%

7) Impulse noise test minimum 15 minute count

Don't worry if you do not understand what all these tests mean, the telephone company knows. These standards are all available from the FCC upon request. When the lines provided to you meet these minimums, you will experience reliable, accurate, and noise free data transfers.

The Hardware Setup:

Your modem is a modulator-demodulator device. It converts digital signals (bits) sent by your computer's serial port to analog signals which can be transmitted over telephone networks. Purchase the highest quality modem you can afford. If you have an internal modem, you don't have to worry about why motherboard and serial card manufacturers are still designing equipment that won't support high-speed modems. If you have an external modem, then your computer serial port if like most, came to you with a slow UART chip which would work fine with 300, 1200, 2400 and 9600 baud modems. Almost every computer shipped (even today when everyone knows better) has this old slow chip. If you have a fast (V.32/V.32bis/HST/V.34/V.42/V.90) modem, and are experiencing dropped characters, CRC errors and aborted downloads, upgrade your serial port to a buffered UART (16550AFN). Look for 82x50B, 16x450 part numbers on the large 40-pin chips on the serial card to find out if your UART is the out-of-date device.

The Software Setup:

Most of you use the Dial-Up Networking provided as part of Windows or the MAC operating system. These normally install correctly and thus work fine. Some of you use older versions of the operating systems and thus require additional software to make connections to us. Generally, all communications with the ISP have these common factors for connection:

Dialing parameters: 8 DataBits, N (no) Parity, 1 - Stop Bits

Maximum Speed: set for highest recommended by manufacturer (generally 57600 for 14,400 modems, 115200 all faster modems). Special note here: Make sure your com port is set up to match this otherwise this is a bottleneck.


Generally scripting is not required for PPP connections.

Flow Control: Software (XON/XOFF) for older modems (14.4 Kbps and lower), Hardware (CTS/RTS) for high-speed modems (above 14.4 Kbps),


If you have the call-waiting service on your telephone line, you MUST disable it before each modem call or you risk having your connection interrupted. Call waiting is disabled with Ameritech if you dial "*70" before the telephone number. For GTE it is disabled if you dial '70# before the telephone number. The place for this is in the "dialing prefix" in your telecommunications software. If that is not available in your software, you can add this information in your dialing directory.


Believe it or not, rainy days and ice storms can affect telecommunications.


Feedback received on this FAQ entry:
  • Dialup at my Dad's house averages around 8 to 12kbps. I have exhausted and eliminated all possible solutions so I called and made appointment with CenturyLink to come out and specifically asked them to check the phone lines and perform a POTS test. While setting up the appointment, the operator informed me there was a problem on their end. Anyway, the tech picked up a handset and hearing a dial tone said line was fine. He also said he was not obligated to perform any tests as CenturyLink was not obligated to meet any minimum standards and suggested upgrading to DSL (additional $60/monthh). I filed a complaint with the FCC who confirmed that CenturyLink was not required to meet any minimum standards. Apparently quality standards were part of the de-regulation package. I do appreciate the post here. Thank you.

    2014-05-07 18:09:13

by CJPC See Profile edited by TravisB See Profile
last modified: 2006-07-24 23:55:33

The speed of your connection involves many different factors. Not only does it depend on the server into which you have dialed, but also on the speed of any computer from which you are receiving data. If the computer storing the web site you are visiting is slow or heavily loaded, then data transfer to your home PC will be slowed down accordingly. If you find that a site is extremely slow, try again at a less busy time or find an alternate site. If you are having trouble with all web sites being slow, there could also be a configuration problem on your computer.

The telephone lines have a large effect on your dial-up connection -- length, load coils, resistive crosses and taps and other things that do not affect voice quality enough to require repair will adversely affect data speeds. When an analog modem connects that horrid screech/squawk noise is the two modems talking to each other testing the quality of the connection and what maximum speed they can handle over that connection.

by DeiselCat See Profile edited by TravisB See Profile
last modified: 2006-07-24 23:55:39

If your dial-up networking (DUNS) CONNECT shows a speed of 57,600 (57.6k), 115,200 (115.2k), or 38,400 (38.4k), you are getting the port speed between your computer and modem, not the modem CONNECT rate.

With Windows 95/98/NT, a file with the .inf extension is used when you install your modem to define how Windows and Windows Programs interact with the modem.

For DUNS to report the correct speed in all cases, every possible CONNECT message must be defined in the .inf file.


by redxii See Profile edited by TravisB See Profile
last modified: 2006-07-24 23:55:44

To check for line noise, pick up the phone and dial 1. You should hear nothing. If you hear a noise, go to the phone box outside of your house. Connect a phone to that box and do the same test to check for noise. If there is noise, it is a problem with the phone company. If there is no noise, turn all appliances off inside. Then do the test to check for line noise. If there is no noise, turn one appliance on, check for noise, then turn on another if there is no noise. When there is noise after turning on a certain appliance, try to put that appliance away from phone lines.

by NickD See Profile edited by TravisB See Profile
last modified: 2006-07-24 23:55:49

Telephone line requirements vs modem speed.

300bps (Bell 103 & ITU-T V.21 protocol)
2400 to 12,000bps (ITU-T V.32, V.32bis and V.34 protocols)

Connections limited to this range indicates severe channel impairment. Usually noise and bandwidth restrictions cause connect speeds this low. Long cable runs of over 5 or 6 miles can cause it, as can very old analog carrier systems.

14.4Kbps (ITU-T V.32bis and V.34 protocols)
This is the minimum speed that a V.34 modem should be able to connect at *if* the telephone line is just able to meet specs. If this speed cannot be obtained either the modem is defective or the line is out of spec in some way.

16.8Kbps (ITU-T V.34 protocol)
This is the minimum speed that most V.34 modems will actually be able to connect at on a minimally specified line. That means if the modem _can_ get this speed, the line *must* be within specs.

This is also the upper limit for connections through various types of digital carrier systems that use ADPCM or 32K bits per channel instead of standard 64Kbps PCM encoding. Due to lower sampling rate and fewer bits per sample more quantization noise is generated by the analog-to-digital/digital-to-analog conversions in these systems, thus reducing the Signal to Noise Ration (SNR) limiting speed.

19.2Kbps to 26.4Kbps (ITU-T V.34 protocol)
Speed in this range generally indicates a bandwidth limited channel. Some older carrier systems and some digital system that packetize and compress data are limited to 19.2 or 21.6Kbps connections. Long cable runs are also reasons for connect rates at 26.4K or below.

26.4Kbps, perhaps with occasional 24K and 28.8K (ITU v.34 protocol)
Usually indicates a SLC that has what is called a "universal" interface to the Telco switch. Such an interface adds an extra DA/AD conversion, which prohibits a V.90/92 connection, and also adds at least 3 dB of quantization noise and a small amount of bandwidth restriction, all of which combine to usually disallow 28.8K connections. Note that for modems which can measure Signal-to-Noise Ratios, 37 dB is the *best* that can be obtained on a connection through any form of 64Kbps/channel PCM digital carrier. The specification for a voice grade telephone line is only 24 dB.

28.8Kbps to 33.6Kbps (ITU-T V.34 protocol)
Indicates the line is so close to perfect that it would be difficult for the Telco to actually measure any change that would improve speed. This means SNR is better than about 32 dB, the bandwidth of the channels being used is at maximum, and the cable must not be very long. 33.6Kbps simply means the line is about as good as it can get.

28.0Kbps to 53.3Kbps (ITU-T V.92 & V.90 protocols)
Means there are no sharp bends in your cable, no goblins camped in any splice boxes, and you've been blessed by a supernatural entity. These connections amount to pure FM (Freaking Magic, if you will). If you breathe funny tomorrow your speed may drop significantly! Brand new, perfect line cards may or may not work. Cables that look good might work, or might not etcetera. It all depends on parameters that have nothing to do with a voice grade telephone line. If you cannot get a V.90/92 connection the Telco is not obligated to change anything to fix it.

The minimum requirements to get a V.90/92 connection are a virtually noise free local loop (modems which measure SNR for v.90 connections will show values from 45 dB for lower speeds all the way up to 55-60 dB with higher speed connections), plus a digital connection to the ISP which has exactly one Analog-to-Digital conversion (the codec in the line card for your line at the Telco switch) between you and the ISP.

V.90/92 modems preferentially connect in V.90 mode if they cannot they automatically fallback to V.34.

High-end connection speed of V.34 and low end of V.90/92 overlap. To determine if the modem is in V.90/92 mode external modems typically include a mode indicator. For modems without this feature connect speed can be used to determine mode. If the modem reports 28,000, 29,333, 30,667, 32,000, 33,333, 34,667, 36,000 or higher it is in V.90/92 mode. If the report is 33,600, 31,200, 28,800 or lower it is in V.34 mode.


Feedback received on this FAQ entry:
  • Hi , recently had DSL installed and have issues with dropped internet light on A90-7500-18-07 modem. Have purchased 7mbps and routinely get 5mbps on "speed tests" . Local telco says the SNR is too low [6.5] "would like to see something like 8.0 or higher. All the numbers above and on other sites talke about SNR being in the number 30 and up range. Are we talking about the same SNR as it doesn't relate that a 6.5 SNR will deliver 5mbps and minimum requirements are in the 30s. Local telco has set my gateway to 5. 4 from 7.0 and now the internet stays up but at the loss of 1.5 meg in download speed on 'speedtest" . Any explanation would be greatly appreciated.

    2011-01-16 06:58:56

by tschmidt See Profile edited by TravisB See Profile
last modified: 2006-07-24 23:55:57

Common DUN error codes can be found at ModemSite

by redxii See Profile edited by TravisB See Profile
last modified: 2006-07-24 23:56:02

Call waiting is enabled on the phone line that you are using for your modem. The call waiting beep cuts off the signal long enough that the modem thinks that the connection has been dropped. Before following through the steps to disable it, you have to determine the call waiting code for your area, the most common code is *70 . Three other codes that are used at times are 1170 or 70# There are a few ways to check, one way is to pick up your phone, and try each code, if after dialing the code you get a dial tone again, then that code works. You can also call the operator or check the phone book for the appropriate code.

How to Disable Call Waiting

Windows 95/98/98SE/ME Users:

    •Double Click on My Computer

    •If you use WinMe, double-click on Control Panel

    •Double Click on Dial Up Networking

    •Double Click on the connection you use to connect to the internet

    •You should see a button that says Dial Properties, click on it

    •In the window that pops up, you should see a section that says To disable
    call waiting, dial with a check box in front of it, check that box, and the
    click on the drop down, and choose the option for your area, if it isn't there,
    you can type it in there manually, followed by a comma (the comma causes a
    two-second pause before dialing, which is necessary in most areas).

    •Click on OK, and then the computer will be setup to dial the call waiting
    disable code for that location (as set in the computer), and you can either
    click on Connect to connect to the internet or cancel if you don't.

For Windows 2000 Users:

    •Click on the Start button, go to Settings, and click on Network and Dial-Up

    •Double Click on the connection you use to connect to the internet

    •You should see a button that says Dialing Rules, click on it

    •Click on the Edit button

    •In the window that pops up, you should see a section that says To disable
    call waiting, dial: with a check box in front of it, check that box, and the
    click on the drop down, and choose the option for your area, if it isn't there,
    you can type it in there manually, followed by a comma (the comma causes a
    two-second pause before dialing, which is necessary in most areas).

    •Click on OK, OK again, and then the computer will be setup to dial the call
    waiting disable code for that location (as set in the computer), and you can
    either click on Dial to connect to the internet or cancel if you don't.

Windows 95/98/98SE/ME problems? Try these steps:

    •Double click on My Computer.

    •Double click on Dial Up Networking.

    •Locate the connection that you use to access the Internet. Place your mouse
    cursor over the connection icon and right click on your mouse. Select Properties.

    •Locate the area where the number you dial is located. Insert *70, in front
    of the number, or the code that will work for you to disable your call waiting.
    If you do not know the code, look in your telephone book. Now the number on
    mine would read *70,757-2051. If you call a 1-800 number, you will need to
    enter the entire number in that box and remove the area code from the left-hand
    side box. For example you would call *70,1800-555-1212.
    Click "OK" and
    the next time you dial in, your call waiting will be disabled!

If you use any other operating system, then the steps necessary will vary, but you need to setup the computer to dial the appropriate code and a comma before it dials the number.


by NickD See Profile edited by TravisB See Profile
last modified: 2006-07-24 23:56:09

Using Windows 95/98/98SE
1. Double-click My Computer on your desktop. You should see an icon that says Control Panel.
2. Double-click Control Panel.
3. Double-click Add/Remove Programs.
4. Click the Windows Setup tab.
5. Double-click the Communications component.
6. Uncheck the Dial-Up Networking component.
7. Click OK until you are back at your desktop.
8. Reboot your computer.

1. Insert your Windows 95/98 CD. If it AutoPlays, close the window it opens.
2. Follow steps 1 - 5 above. On step 6, check the Dial-Up Networking component.

To reinstall RAS in NT:
You must either have the Windows CD or CAB files in order to perform this.

1. From the Control Panel open up Network and then go to the Services tab. Select Remote Access Service and click Remove. Select Close and reboot. When the computer is back up go back to this screen and click Add.
2. Select Remote Access Service and then OK.
3. Select the modem and then OK.
4. Click on Continue
5. Click Close and then reboot.
6. Make a new connection.

by TravisB See Profile
last modified: 2006-07-24 23:56:15