******************************************************************************* * * * Line noise, The Phone Company, and Your Modem or FAXModem * * * ******************************************************************************* In reading the following information on your subscriber line, you must remember that you can only address the type of phone service you pay for and the long- distance carrier your call is routed through. You normally have NO control over conditions that may be present in local office switches or calls placed in your "LOCAL LONG DISTANCE" area. This means that regardless of how much the phone company may do to your line, there will still be places you can't get a good connection to. Also, if the whole problem turns out to be the local switching equipment, you are just plain out of luck (unless you can get the local phone company to replace equipment that runs between tens of thousands to hundreds of thousands of dollars). POTS (Plain Old Telephone Service) This is the type of phone service most people have. This is what it you get: -most 300hz to 3000hz sounds are passed. -You aren't likely to get any more than -10db loss from the local telephone office to your phone jack. (Measured from a 1mW 1khz tone source.) -VOICE use (you should be able to hear another person's voice using normal, FCC approved, telephone devices via this service). (No implied ability to use Modems, Faxes, or other "non-voice" devices over this service.) The legal requirements for voice lines aren't even this good, but this is an average service. DATA-GRADE / ASSURED QUALITY (PAC BELL) / or other "BETTER" service offered. These types of service are offered as an "upgrade" from POTS (often at a much higher installation fee, and an additional monthly service fee). The services supported and/or promised by such upgrades will vary widely from one telephone service provider to the next and are not supported at all in some cases. In general, they will provide a "cleaner" telephone connection that is able to support fax and data communication to a specific carrier rate, and normally have no more than a 5db loss. These services may also provide a specially installed line with no LOADS and no BRIDGE-TAPS. Ideally, these lines will be able to supply a 600 ohm balanced line, and should respond very well when devices (like FCC approved Telephones, Modems, Faxes, etc.) are connected. Items and Conditions that commonly degrade telephone lines: BRIDGE-TAPS & HALF-TAPS These are "extra" wires connected to the wire pair coming to your home from the local telephone office. Often these are the result of normal service and repair procedures that occur over the years. These don't normally create a problem for VOICE use (although they have been traced to things like radio stations or buzz- type noises in some cases). Devices using complex signaling tones and patterns, however, can be greatly affected by these "extra" signals on the phone line. Theses extra wires act like antennas and pick up noises from a wide range of sources. LOADS (Step-Up Transformers) These are commonly used to increase the volume of voices on longer phone lines. They cause distortion of the rated 300hz to 3000hz band pass, and can cause the frequencies to be shifted up or down from their original values. The volume of these frequencies is also not increased equally. In some cases, the line can become poor enough that TOUCH-TONE dialing may not be supported, and only PULSE dialing is possible. LOADS are most commonly used in older, rural settings, but still do exist in urban installations. LOADS also serve a means of introducing high levels of noise onto the line, both by increasing the level of noise on the line along with the voice level and by picking up environmental noise from the area directly around the transformer itself (other transformers, power supplies, high voltage power lines, etc.). Line Loss This is the composite of all of the types of loss on your phone line which include the following: -Pure Loss This natural loss in the phone line caused by the resistance of the wire and connections between the local office and your home. -Return Loss This is a measurement of the signal loss on the line. This is affected by the line itself (see above) and the devices you have connected to it. It is basically a measurement of the impedance match between the local office and the line and the devices you have connected (problems in any part will affect the over-all measurement). -Frequency Distortion and Attenuation. This is basically the loss of volume and shift in frequency on the line. Problems often arise in high-speed data transfers because only part (or in very poor conditions none) of the original signal gets transferred faithfully to the receiving end. In some cases many frequencies have been "reflected" back on to each other further damaging the signal. Common Tests to ask to have done by the phone company: -Frequency Test (also called Frequency "Slope" or Frequency "Twist" test) This test shows both total loss in signal, and the amount of difference in signal loss at different frequencies. A 1mW 1Khz signal is measured for total loss (in db) and then compared to readings taken from signals at 400hz and 2800Hz. For a VOICE line, the high and low frequency values should be within a +3db to -5db range of the value of the center frequency (a DATA grade line should be closer the reading of the center value). In conditions where this problem is quite bad, you will hear the voices as being "tinny" and it may be more difficult to recognize voices over those lines. -Loss and/or Return Loss Test This will test the loss of a signal transmitted from one end of the line to the other. (In the case of a Return Loss Test, it tests the loss of a signal transmitted into the line and reflected back from the other end. -45db is a reasonable level, -60db is not acceptable. Common Tests to do yourself / Common Problems from devices connected: -Try the modem/fax with ALL other devices removed from the line. This tests for two common problems: 1. NOISE -from devices like cordless phone and "neon-display" phones most commonly, but also can be sourced by any telephone line device. 2. Signal Loss -from devices that "pull" power off the phone line. Phones and devices that take power from the line when not actively in use for dialing memory, status lights, and lighted displays (although most lighted displays are only taking power when the handset is picked up) will lower the power level on the line, and will also lower the signal levels being passed on the line. -Check the condition of the phone wiring in your home. Wires should be at least 26 gauge, 22 gauge is better. Insulation should be in good condition. Wiring runs should be kept out of direct sunlight and weathering. (If wiring must be done outside, use phone wire rated for out-door use.) Remove wiring runs no longer in use (this does not mean to rip out any phone outlet you aren't using, but it IS a good idea to remove wiring that has already been disabled or covered up by remodeling -many older homes and rentals have phone lines that go nowhere!) *WARNING* If you find equipment that you don't understand connected to the phone lines in your home, or "extra" wiring to transformers or grounding rods/water lines, do not disturb it without talking to the phone company about it. *** This document will be updated at random points in time, so please check the Supra BBS {503-967-2444} to assure the most current information. This document is a combination of data from engineers at Supra, a telephone call with an engineer who works for AT&T, and my own knowledge and education relating to data communication and transmission lines. Please feel free to post this document in its original and unedited form. Patrick Moore Supra Technical Support SysOp Supra BBS. Feb. 25, 1993