g factor in overall system performance. For our discussion in this text, noise is broken down into four categories: 1. Thermal noise 2. Intermodulation noise 3. Impulse noise 4. Crosstalk 3.3.3.2 Thermal 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 (Pn) in 1 Hz of bandwidth at absolute zero (kelvin scale) or Pn = -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. 3.3 THE THREE BASIC IMPAIRMENTS AND HOW THEY AFFE