Chapter 20. 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) while 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.
Attenuation Distortion
Propagation Delay
Envelope Delay Distortion
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. This frequency is 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 of 1004 Hz.
Decibel (db) is a relative unit of measure. 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, 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's 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).
Attenuation occurs because the signal has to pass through many pieces of electronic equipment and transmission media. 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 the Attenuation Distortion that is on phone lines. The Basic channel conditioning is as follows:
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 this is shown as -8db and +2 db (see the graph on the previous page) .
A +3 db attenuation is equal to -3 db in signal amplitude and a +8 db attenuation equates to -8 db in signal amplitude.
Propagation Delay
Signals transmitted down a phone line will take a finite time to reach the end of the line. The delay from the time the signal was transmitted to the time it was received is called Propagation Delay. If the propagation delay was the exact same across the frequency range, then there would be no problem. This would imply that all frequencies from 300 to 3000 Hz have the same amount of delay in reaching their destination over the phone line. They would arrive at the destination at the same time, but delayed by a small amount of propagation delay.
For example, this delay is heard when talking on long distance telephones. In this instance, we have to wait a little longer before we speak (to ensure that the other person hasn't already started to talk). Actually, all phone lines have propagation delay.
If the Propagation Delay is long enough, the modem or communications package may time-out and close the connection. In other words, it may think that the receive end has shut off!
Envelope Delay Distortion
If the Propagation Delay changes with frequency, we would then have the condition where the lower frequencies--such as 300 Hz-- may arrive earlier or later than the higher frequencies--such as 3000 Hz. For voice communication, this would probably not be noticeable. However, for data communication using modems, this could affect the phase of the carrier or the modulation technique that's used to encode the data.
When the Propagation Delay varies across the frequency range, we call this Envelope Delay Distortion. We measure propagation delay in microseconds (us), and the reference is from the worst case to the best case.
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