QUALITY OF SERVICE AND TELECOMMUNICATION IMPAIRMENTS
distortion is flattest.
It is for this reason that so many data modems use 1700 or 1800 Hz
for the characteristic tone frequency, which is modulated by the data. A data modem is
a device that takes the raw electrical baseband data signal and makes it compatible for
transmission over the voice channel.
This brings up an important point. Phase distortion (or EDD) has little effect on speech
communications over the telecommunications network. However, regarding data transmis-
sion, phase distortion is the greatest bottleneck for data rate (i.e., the number of bits per
second that a channel can support). It has probably more effect on limiting data rate than
any other parameter (Refs. 3 and 4).
. Noise, in its broadest definition, consists of any undesired signal in
a communication circuit. The subject of noise and noise reduction is probably the most
important single consideration in transmission engineering. It is the major limiting factor
in overall system performance. For our discussion in this text, noise is broken down into
1. Thermal noise
2. Intermodulation noise
3. Impulse noise
. Thermal noise occurs in all transmission media and all com-
munication equipment, including passive devices such as waveguide. It arises from random
electron motion and is characterized by a uniform distribution of energy over the frequency
spectrum with a Gaussian distribution of levels.
Gaussian distribution tells us that there is statistical randomness. For those of you
who have studied statistics, this means that there is a "normal" distribution with standard
deviations. Because of this, we can develop a mathematical relationship to calculate noise
levels given certain key parameters.
Every equipment element and the transmission medium itself contribute thermal noise
to a communication system if the temperature of that element or medium is above absolute
zero on the Kelvin temperature scale. Thermal noise is the factor that sets the lower limit
of sensitivity of a receiving system and is often expressed as a temperature, usually given
in units referred to absolute zero. These units are called kelvins (not degrees).
Thermal noise is a general term referring to noise based on thermal agitations of
electrons. The term "white noise" refers to the average uniform spectral distribution of
noise energy with respect to frequency. Thermal noise is directly proportional to bandwidth
and noise temperature.
We turn to the work of the Austrian scientist, Ludwig Boltzmann, who did landmark
research on the random motion of electrons. From Boltzmann's constant, named for
Ludwig Boltzmann, we can write a relationship for the thermal noise level (
) in 1 Hz
of bandwidth at absolute zero (kelvin scale) or
= -228.6 dBW per Hz of bandwidth for a perfect receiver at absolute zero. (3.3)
"Flattest" means that there is little change in value. The line is flat, not sloping.