 channel. The bandwidth of binary data will depend on the shape of the waveform and the order of the 1 and 0 binary bits. This chapter starts with the basics of a typical data transmission system, including the classification and important characteristics of data. Issues relating to the loss of data and its retrieval are dealt with in subsequent subsections. Recent trends are finally covered in the last section. Data Transmission Basics A basic data transmission system consists of a transmitter unit, a transmission medium (also known as a transmission channel), and a receiver unit (see Figure 2.1). The channel or trans- mission medium is a physical path by which a message travels from the transmitter to the receiver. Whenever an input message signal is to be transmitted, a processing subsystem is needed for adaptation of the data according to the transmission link characteristics. The input signal is known as the baseband signal, and the process of making the signal suitable for transmission is called modulation. As the signal travels through the transmission path, it is affected by transmission impairments such as the noise, interference, and distortion. Noise is undesired random and unpredictable signals added to the transmitted signal, interference is contamination by extraneous signals, and distortion is waveform perturbation caused by the imperfect channel response. Unlike noise and interference, distortion disappears when there is no signal. The message needs to be extracted (recovered) from the received signal. This process is referred to as demodulation. Figure 2.1 illustrates transmission in one direction only, from one point to another point. This kind of transmission is known as simplex. If facilities are provided to transmit in the return direction also, then both stations can transmit, either one at a time or simultaneously. Half-duplex transmission refers to the case where only one transmitter can send data at a time. If the transmission is simultaneous from both blocks, it is known as full-duplex. 