f impulse noise are natural, such as from lightning. However, man- made impulse noise is ever-increasing, such as that from automobile ignitions and power lines. Electromechanical space division switches were acute sources. In most countries, this type of exchange is obsolete and that source of impulse noise has disappeared. For our discussion of data transmission, two types of noise are considered: (1) random (or Gaussian) noise and (2) impulse noise. Random noise measured with a typical trans- mission measuring set appears to have a relatively constant value. However, the instan- taneous value of the noise fluctuates over a wide range of amplitude levels. If the instantaneous noise voltage is of the same magnitude as the received signal, the receiving detection equipment may yield an improper interpretation of the received signal and an error or errors will occur. Thus we need some way of predicting the behavior of data transmission in the presence of noise. Random noise or white noise has a Gaussian dis- tribution and is considered representative of the noise encountered on a communications channel, which, for this discussion is the voice channel. 9 From the probability distribution curve of Gaussian noise shown in Figure 10.12, we can make some statistical predictions. It may be noted from this curve that the probability of occurrence of noise peaks that 9The definition of the term Gaussian distribution is beyond the scope of this book. It essentially means to us that this type of noise is well-characterized, and we will use the results of this characterization such as found in Figure 10.12. 10.9 DIGITAL TRANSMISSION ON AN ANALOG CHANNEL Bit error probability (parts/105) Figure 10.12 Probability of bit error in Gaussian (random) noise, Nyquist bandwidth, and binary polar transmission. The reader should note that this is for a baseband signal. Reference 1 (IEEE) defines Gaussian noise as "noise characterized by a wide frequency range with regard to the desired signal of (the) communication channel, statistical