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Unformatted text preview: 37 CHAPTER 3 Data Transmission In this chapter we discuss the common issues related to the transmission of data through a local area network. Transmission can be either digital or analog. We first dis- cuss digital and analog signals. Digital transmission includes the processes of line cod- ing (transforming binary information into a digital signal) and sampling (transforming an analog signal into a digital signal). Analog transmission includes modulation of dig- ital and analog signals. Multiplexing is then discussed as a means to divide a physical link into several digital or analog channels. Finally, we discuss some theoretical aspects of data transmission. 3.1 ANALOG AND DIGITAL SIGNALS To transmit data from one device to another through a local area network, we need to represent data as signals that can be carried by a medium. There are two types of signals: digital and analog. Digital Signals A digital signal can only have a limited number of values; it is discrete. The transition from one value to another is instantaneous (see Figure 3.1). Figure 3.1 A digital signal Value Time 38 CHAPTER 3 DATA TRANSMISSION Bit Interval and Bit Rate Two terms are often used in connection with digital signals: bit rate and bit interval (or bit period). The bit interval is the time required to send one single bit. The bit rate is the number of bit intervals per second. This means that the bit rate is the number of bits sent in one second, usually expressed in bits per second (bps) (see Figure 3.2). The following formulas can be used to convert a bit interval into bit rate and vice versa: BitRate 1 / (BitInterval) BitInterval 1 / (BitRate) Analog Signals An analog signal can have an infinite number of values; it is continuous. The transition from one value to another is smooth (see Figure 3.3). An analog signal can be periodic, which means that the signal has a repeated pat- tern, or it can be aperiodic, which means that there is no repeating pattern. Figure 3.2 Bit rate and bit interval Figure 3.3 An analog signal Amplitude 1 s 5 8 bit intervals bit rate 5 8 bps Time 1 1 1 1 Bit interval Time SECTION 3.1 ANALOG AND DIGITAL SIGNALS 39 A Simple Periodic Signal: Sine Wave The sine wave is the most fundamental form of a periodic analog signal. Figure 3.4 shows a sine wave. Each cycle consists of a single arc above the time axis followed by a single arc below it. Sine waves can be fully described by three characteristics: ampli- tude, period or frequency, and phase . Amplitude. On a graph, the amplitude of a signal is the value of the signal at any point on the wave. It is equal to the vertical distance from a given point on the wave form to the horizontal axis. The maximum amplitude of a sine wave is equal to the highest value it reaches on the vertical axis (see Figure 3.5). ...
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