What is "line noise"?
Line noise is static on the phone line. Cellular and cordless phones have a lot of line noise; it can be heard as static in the background of the conversation. For data transmissions, particularly at high speeds (really, anything over 300 bps), line noise can be a major problem. High speed data transmissions are much more sensitive to noise in the lines, and can be affected by static that you cannot hear in the line.
Line noise can be caused by several factors. Electro-magnetic interference is often caused by wiring in and around the connection from the modem to the wall jack. It may exist in the phone line being used, or in the jack being connected to. It may be a loose screw on a connector block somewhere between you and the exchange or water in one of the Telco cable ducts around worn wires. Or it may be anywhere in the wiring within the house, apartment complex, or hotel. The age of the building and its wiring plays a major factor.
Static will get read as data and cause packets to be lost and resent. At best, it limits the connection speed to something like 26,400. At worst, it will not allow you to connect at all. Generally, though, it is something in between and can be very frustrating.
How to Test for Line Noise
Errors to Expect (top)
The following DUN errors can all be symptoms of line noise or
trunking issues.
Error 645: Internal Authentication Error
Error 678: No Answer
Error 679: No Carrier
Error 691: Authentication Failure
Error 718: PPP Timeout
Error 731: Protocol Not Configured
Instant disconnect
Disconnect at variable times
Each of these errors could be something else and should be checked first. The thing we're looking for is inconsistency. If the you are getting a 691 every time, it's most likely a mistyped user ID or password. If you are getting a 691, a 679, then a 731, then connect and get dropped, it's a line noise issue.
Check Line Quality (top)
Listening to a phone line can give you an idea of the noise in the line. In order for you to hear the line clarity, you need to get rid of the dial tone. Plug a handset into the jack, lift it off-hook and press 1. This will give you about 30 seconds of silence to listen for noise. However, this will not always be helpful.
If you are able to connect, but cannot connect at the speeds you expect with the protocols installed, line noise is often the problem. If you get a connection speeds greater than 33.6 and less than 50K, like around 40K, you should check for line noise.
There are tests that can be taken over the internet and by calling into test facilities of many modem manufacturers. USR's Line Test at http://www.3com.com/56k/need4_56k/linetest.html is the best known. However, this will not give you an indication of the line quality between your home and the access location of your ISP. By following this test you can get an idea of whether the quality of the phone line, between your home and your ISP access point, is conducive to making a high speed connection to your ISP. This test will work for many 56k modems.
Open up Terminal in Windows 3.1x or Hyperterminal in Windows 95/98/NT. At the terminal window at a cursor prompt, type ATZ and hit the Enter key, even if you don't see anything appear on the screen. It should respond with OK. Now type ATDT####### (where ####### is the ISP access number) and hit Enter.
The modem should dial and connect to your ISP. You will either see a "Login" or "Password" prompt. If the prompt has mixed signals (i.e. "Log#BJDx_in"), there is noise on the line. If you get nothing but garbled text streaming across the screen, you definitely have a line noise issue. Type +++ but DO NOT HIT ENTER. The modem should respond OK after a second.
Type AT&E1&V1 and hit Enter. The modem should respond with a list of information related to the connection. Look for a listed as Line Quality and write it down.
For Rockwell modems and others, type AT%L%Q. The Line Level (%L) is in -dBm and should be less than 24. The Quality Monitor level (%Q) of 15 or higher mean there are real line problems with the member's Telco circuit. They will need to contact the Telco.
For USR modems, type ATI6 and look for "Blers" (Bit Link Error RateS) value. It should be less than 3, ideally 0. Anything higher and the member has a line noise issue. Type ATH and Enter to disconnect the modem from Mindspring. Repeat steps 3 through 6 AT LEAST 5 TIMES. This will give you an average value for the Line Quality between your home and your ISP. Line Quality values greater than 25 indicate that the modem is sensing excessive line noise. When this is the case, there is usually not much that can be done to decrease the noise. You may want to contact the phone company to determine whether they may be able to enhance the line quality.
Check Local Trunking (top)
(This is for ISP and company techs only). Dial the POP yourself
from a test machine. If you connect, the POP is fine. If you get a Ring No
Answer, Fast Busy, or Operator Message (e.g. "All circuits are
busy..."), check the NOC page and Bad POP it. Have the member dial another
POP long distance. If he connects, it is not a problem with the wiring in his
location. Have the member dial their local POP with a long-distance prefixe
(e.g. 10-10-220). If he connect, it is (almost) assuredly a trunking issue
between the local CLEC's.
Solutions
Reduce Local Line Noise (top)
By local line noise, I mean the line noise generated by devices and lines the member has setup in conjunction with the modem. Remove other phones, fax machines, answering machines, caller ID boxes, other computers, line splitters, and surge protectors from the line. Connect the modem directly to the wall jack. Clear away all other power sources and wires around the phone line and jack. Try changing the connection wire. Try connecting to another jack in the building. If practical, try another location. If you can dial long distance and the problem disappears, then your equipment is fine.
Modem Settings (top)
There are some modem settings you can adjust to help deal with line noise. Lower the modem port speed and/or the FIFO buffers. Enable software compression. Enable error control with data compression (WinModems will have problems with this). Set flow control to hardware. Set S10=50 or higher in Extra Settings. This is the amount of time in tenths of a second that the modem will wait before dropping the carrier if it receives noise.
Well, who do I talk to now? (top)
Only after fully teching it, and eliminating a matter of factors, either it's a bad modem, a bad line, or a trunking issue. If you think it's a bad POP or a trunking issue, call your ISP. If the issue is a local line noise issue or a modem problem, contact your modem manufacturer, computer retailer, or their local telephone company for service.
Please note that telephone companies (Teleco) only have to support voice quality phone lines and may not help you with the line noise. Your option then is to look into DSL, Cable Modems, or other high bandwidth connections. Most of these bypass many of the line noise issues, but not always.
This page was last updated on Sunday, December 03, 2000 .
If you have any comments, suggestions or questions Email: [email protected] or [email protected].