dge of the clock signal (see Figure 4.19). At the rising edge of the clock signal the input value is read into the output by the D-type flip-flop. The output value remains the same until the next rising edge of the clock signal. The operating principle of a binary regenerative repeater is presented in Figure 4.19. The regenerated digits that contain no noise are delivered to the destination or via a cable to the next repeater station (in the case of an intermediate repeater). Introduction to Telecommunications Network Engineering Regenerated data Original Amplified and equalized Distorted and without noise transmitted data with noise attenuated line signal Regenerator V ref D-type flip-flop Error rate 10 -5 versus S/N Rising edges of in each the clock signal repeater S/N dB Figure 4.19 Operating principle of a regenerative repeater. If the equalized signal is below threshold Vref at the input of the com- parator, the output is low and a zero is regenerated at the rising edge of the clock signal. If noise is too high, the input of the comparator may be above threshold even though a zero is transmitted. If this occurs at the rising edge of the clock signal, the value 1 is regenerated and an error has occurred. In the same way, high values may be in error if noise reduces the high- amplitude value below the threshold level at an instant of the rising edge of the clock signal. Then 0 is regenerated and an error has occurred. How frequently errors occur depends on the noise level or S/N. If noise is assumed to have a Gaussian amplitude distribution (as thermal noise does), the error rate (bit error probability) follows the shape of the curve, error rate versus S/N, in Figure 4.19. As an example let us assume that we have a channel, for instance cable, that attenuates a signal SO much that the resulting S/N in the repeater is 15 dB. The error rate would then be around 1 X 10-5 according to the curve in Figure 4.19. If we place a new repeater in the middle of the repeater section (in the middle of the cable), attenuatio