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Phone Line Help

Sources of ADSL Telephone Line Interference

BRIDGE-TAPS & HALF-TAPS

These are "extra" wires connected to the ADSL provisioned telephone wire pair. This problem sometimes shows up as radio stations noise or buzz-type noises being heard in the background during calls in some cases. ADSL modems that use complex signaling techniques can be greatly affected by these "extra" wires. Telecommunication carriers can rectify this problem if it is on the telephone service supply side of the boundary connector to the end user site.

LOADS (Step-Up Transformers)

These are commonly used to increase the volume of voices on phone lines that run over long distances (ie.cause the frequencies to be shifted up or down from their original values). They cause distortion of the rated 300hz to 3000hz voice-bandpass and also effecting the ADSL connection.

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 exchange and the local site.
Return Loss
This is a measurement of the signal loss on the line. This is affected by the line itself and the devices connected to it. It is basically a measurement of the impedance match between the exchange and the line and the total number of devices.
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

Quantization noise
Digital links have quanitization noise which is inherent in analog/digital converters (ADC/DAC) (eg. ADSL modems A/D converter).
Thermal Noise
This is naturally occurring noise on occuring carrier loops (ie. local site to exchange typical thermal noise of 22 dBrn) sourced from normal electrical decay and energy transformation.
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 (roughly 50Hz) 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.). In Australia such things are covered by the Australian Communications Authority.

COMMON TESTS TO ASK YOUR PHONE COMPANY ABOUT

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 dBm) 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 +3dBm to -5dBm range of the value of the center frequency (a DATA grade line should be closer to 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. -45dBm is a reasonable level, -60dBm is not acceptable.
MOLDS test
Modem Online Diagnostic System test. This is a test which Telstra can perform for you. You ring Telstra (Residential 132203) and book an appointment - Telstra will then call you back with a phone number for your modem to dial. After sending test data through to your computer for approximately two minutes at speeds of up to 33.6kbps, their computer will tell you the maximum speeds your modem reached, the amount of times it had to re-request information, signal attenuation etc.

COMMON PROBLEMS, REPAIRS AND TESTS TO ADDRESS YOURSELF

Try the modem/fax with ALL other devices removed from the line.
This tests for two common problems:
1. Noise
  From devices like cordless, cellular and "neon-display" phones most commonly, but also can be sourced by any telephone line device such as answering and fax 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 display only take power when the handset is picked up) will lower the power and signal levels on the line negatively affecting the signal to noise ratio.
Check the condition of the phone wiring in your home

"High Twist" or "Data Grade" ("twisted pair") 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.
Insulation should be in good condition.
Wiring runs should be kept out of direct sunlight and weathering
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!)

STANDARDS COMPLIANCE

In Australia, all devices connected to the public telephony network must be approved. This will be indicated by either:

AUSTEL

Australian Communications Authority.

C Tick

ADSL CPE Equipment Regsiter
This is an Interoperability Register showing that certain manufacturers of ADSL equipment: filters and modems, have passed all tests for the IP1149 Standard-telecomms network end to end access.