 Signals Carried over the Network 3.4 Analog and Digital Signals and Systems Most of the systems in the modern telecommunications network are digital instead of analog. In this section we look at the fundamental characteristics of analog and digital signals and how they influence the performance and operation of telecommunications systems. 3.4.1 Analog and Digital Signals The difference between analog and digital form is easily understood by look- ing at the two watches in Figure 3.4. A true analog watch has hands that are constantly moving and always show the exact time. A digital watch displays "digits" and the display jumps from second to second and shows only dis- crete values of time. Another example could be the slope of analog voltage where all values of voltage can be measured as shown in Figure 3.4. In "digital slope," only discrete values may be measured. In the example of the figure, we have eight discrete values, 0 to 7, in the digital slope. This does not mean that the digital systems perform worse than analog systems. If we want to improve the accu- racy of the digital system, we just increase the number of steps and, in princi- ple, any voltage level can be represented with the digital system as well. Analog and digital watch Signal value Analog signal 02:05 Signal value Digital signal Slope of analog and digital voltages. Volts Volts (Digital and) binary signal Signal value Figure 3.4 Analog and digital signals. Introduction to Telecommunications Network Engineering A special and very important case of digital signals is a binary signal where only two values, binary digits 0 and 1, are present as illustrated in Figure 3.4. Examples of binary signals are light on and off, voltage versus no voltage, and low current versus high current. Binary signals are used internally in computers and other digital sys- tems to represent any digital signal. For example, we can encode eight voltage levels of the slope in Figure 3.4 into three binary bits and each of these three bit words then represents one of the 