Contents

This document is a combination of data from engineers at Supra, a telephone call with an engineer who works for AT&T, electronic mail from other people involved with the modem industry, 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. 
 

SOURCES OF NOISE

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 

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. 
SOURCES OF SIGNAL DAMAGE 

Quanitazation noise. Digital links have quanitazation noise which is inherent in A/D converters. 

Thermal Noise. This is naturally occurring noise sourced from normal electrical decay and energy transformation. Local loops, if properly engineered, have a thermal noise of 22 dBrn. But it is not uncommon to find it at higher levels. (22 dBrn in "reference noise" is equal to -68 dBm.) 

Digital Compression. Compressing the signal to get a higher number of voice signals on one carrier. (This is just fine for voice, as the human ear does not notice the small gaps in the signal this creates, but modems can't deal well with gaps in the data received.) 

Power line hum. Low frequency noise conducted into the phone line as it passes power lines. (Also it is possible for power hum to be passed into the phone line from power sources in the phone system itself.) 

Phase jitter. The shifting or bouncing of the carrier signal in relation to time. 

Crosstalk. Noise or sounds from other lines in the phone system. (In very bad conditions this can be loud enough you can hear other lines ringing and even hear other voice or data signals.) 

RFI. (Radio Frequency Interference) High frequency electrical signals conducted into the phone lines. (In very bad cases radio stations or other radio transmissions can be heard.) If you can figure out the source of the signal (like the name of the radio station), then you should contact them in regard to curing this problem. In the US, FCC regulations require the person in charge on the transmission to fix this problem at his expense. (In the case of a radio station, contact the station engineer. He can normally construct a filter to remove the signal from your phone line.)

Common Tests to ask to have done by the phone company:

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. -45dBm is a reasonable level, -60dBm is not acceptable. 

Common Problems, Tests, and Repairs for end users to address:

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, such as FAX or answering machines. 

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. 

"High Twist" or "Data Grade" phone cable, rather than standard "low twist" or non-twisted phone cable, wire is better for high-speed data transfer. 

"Silk" cable (that flat cable that runs between the phone device and the phone jack) should be kept short (under 8 feet). It is better to put in a new phone jack (preferably wired with "High Twist" cable), then to connect the modem via a long "silk" type extension cable. 

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 outdoor 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.