ight intensity at one point in an ana- log television image. One example of an analog message is the voice current on a conventional subscriber telephone line as illustrated in Figure 3.5. In Section 2.2 we explained how the current is produced. Because the information resides in a time-varying waveform, an analog communication system should deliver this waveform with a specific degree of fidelity. Because the strength of signals may vary in a range from 30 to 100 dB, depending on the application, the analog systems should have good line- arity from the weakest signal to 1,000 to 10,000 million times stronger signal values. 3.4.3.3 Digital Message A digital message is an ordered sequence of symbols selected from a finite set of discrete elements. Examples of digital messages are the letters printed on this page or the keys you press at a computer keyboard. When you press a key at your computer keyboard, each key stroke represents a digital message that is then encoded into a set of bits for binary transmission. Because the information resides in discrete symbols, a digital commu- nication system should deliver these symbols with a specified degree of accu- racy in a specified amount of time. The main concern in the system design is that symbols remain unchanged, which is the final requirement for transmis- sion accuracy. Introduction to Telecommunications Network Engineering Continuously variable current Telephone Analog signal, exchange voice Analog subscriber Digital signal interface Binary data Modem Telephone network Analog voice band signal, which carries digital values (each corresponds to a set of bits from binary data) Digital signal Digital signal, Binary data line encoded binary data ISDN or data network Network terminal Figure 3.5 Examples of messages. We need modems for the transmission of digital messages over analog channels. The modems receive a message from the terminal in the form of binary data and send it as an analog waveform to the speech channel as shown in Figure 3.5. Current modems do not 