Introduction to Data Communications
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17. Telephone Line Characteristics

Telephone lines are not perfect devices due to their analog nature. The quality of the telephone line determines the rate that modulated data can be transferred. Good noise free lines allow faster transfer rates such as 14.4 kbps, poor quality lines require the data transfer rate to be stepped down to 9600 bps or less. Phone lines have several measurable characteristics that determine the quality of the line:

  1. Attenuation Distortion
  2. Propagation Delay
  3. Envelope Delay Distortion

17a. Attenuation Distortion

Attenuation Distortion is the change in amplitude of the transmitted signal over the Voice Band. It is the frequency response curve of the Voice Band.

Attenuation versus Frequency

To measure Attenuation Distortion, the phone line has a test frequency transmitted from 0 - 4 kHz into the line at a standard amplitude of 0 db. The loss of signal or attenuation is measured at the receiving end and compared to a standard reference frequency: 1004 Hz.

db is short for decibel which is a relative unit of measure (similar to a unit like a dozen). It is a log unit and a +3 db gain will indicate an amplitude of 2x the reference. It is a logarithmic ratio between input voltage and output voltage. It is calculated by the following formula:

db =10 x log (Vout/Vin)

The resulting information is graphed on an Attenuation vs. Frequency chart. Attenuation is a loss of signal amplitude - the receive signal is a smaller amplitude than the transmitted signal. It is indicated by a positive db. It is also possible to have a signal appear at the receiving end with a larger amplitude than when it started - this is indicated by negative db.

The attenuation is due to the many pieces of electronic equipment and transmission media that the signal has to pass through, some can amplify the signal (make it a larger amplitude) and some may attenuate the signal (make it smaller).

There are maximum and minimum acceptable limits for Attenuation Distortion for phone lines. The Basic channel conditioning is:

Frequency RangeLoss (db)
500 - 2500-2 to +8
300 - 3000-3 to +12

The above Loss is a range of acceptable values for the frequency range. In the Basic Channelling Conditioning, it is acceptable to have a loss in signal in the frequency range of 500-2500 Hz of "8 db loss to -2 db loss" referenced to the amplitude at 1 kHz. Note that on the graph on the previous page that this is shown as -8db and +2 db.

+3 db attenuation is equal to -3 db in signal amplitude and +8 db attenuation equates to -8 db in signal amplitude.

Introduction to Data Communications
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Copyright Eugene Blanchard Jan 1998, January 2007