scussed in Chapter 6. 4.1 Basic Concept of a Transmission System In this first section we look at the basic elements present in all transmission systems. We introduce the basic functions of these elements and discuss their roles for the successful transmission of information. 4.1.1 Elements of a Transmission System The main elements of a communication system are shown in Figure 4.1. The transducers, such as a microphone or a TV camera, that we need to convert an original signal to an electrical form are omitted; unwanted disturbances such as electromagnetic interference and noise are included. Note that Introduction to Telecommunications Network Engineering Input Transmitted Received Output signal signal signal signal Transmission Transmitter Receiver Channel Source Destination Noise, distortion, and interference Figure 4.1 Basic concept of transmission system. bidirectional communication requires another system for simultaneous transmission in the opposite direction. 4.1.1.1 Transmitter The transmitter processes the input signal and produces a transmitted signal suitable to the characteristics of a transmission channel. The signal process- ing for transmission often involves encoding and modulation. In the case of optical transmission, the conversion from an electrical signal format to an optical one is carried out in the transmitter. 4.1.1.2 Transmission Channel The transmission channel is an electrical medium that bridges the distance from the source to the destination. It may be a pair of wires, a coaxial cable, a radio path, or an optical fiber. Every channel introduces some amount of transmission loss or attenuation and, therefore, the transmitted power pro- gressively decreases with increasing distance. The signal is also distorted in the transmission channel because of different attenuation at different fre- quencies. Signals usually contain components at many frequencies and if some are attenuated and some are not, the shape of the signal changes. This change is known as distortion. Note that a transmiss