Introduction to Data Communications
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25. 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:

25a. 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.

Introduction to Data Communications
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