37CHAPTER 3Data TransmissionIn this chapter we discuss the common issues related to the transmission of datathrough a local area network. Transmission can be either digital or analog. We first dis-cuss digital and analog signals. Digitaltransmission includes the processes of line cod-ing (transforming binary information into a digital signal) and sampling (transformingan analog signal into a digital signal). Analogtransmission includes modulation of dig-ital and analog signals. Multiplexing is then discussed as a means to divide a physicallink into several digital or analog channels. Finally, we discuss some theoretical aspectsof data transmission.3.1ANALOG AND DIGITAL SIGNALSTo transmit data from one device to another through a local area network, we needto represent data as signals that can be carried by a medium. There are two types ofsignals: digital and analog.Digital SignalsA digital signalcan only have a limited number of values; it is discrete. The transitionfrom one value to another is instantaneous (see Figure 3.1).Figure 3.1A digital signalValueTime
38CHAPTER 3DATA TRANSMISSIONBit Interval and Bit RateTwo terms are often used in connection with digital signals: bit rate and bit interval (orbit period). The bit intervalis the time required to send one single bit. The bit rateis thenumber of bit intervals per second. This means that the bit rate is the number of bits sentin 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 viceversa:BitRate 1 / (BitInterval) BitInterval 1 / (BitRate)Analog SignalsAn analog signalcan have an infinite number of values; it is continuous. The transitionfrom 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 3.2Bit rate and bit intervalFigure 3.3An analog signalAmplitude1 s58 bit intervalsbit rate58 bpsTime10110001Bit intervalTime
SECTION 3.1ANALOG AND DIGITAL SIGNALS39A Simple Periodic Signal: Sine WaveThe sine waveis the most fundamental form of a periodic analog signal. Figure 3.4shows a sine wave. Each cycle consists of a single arc above the time axis followed bya 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 amplitudeof a signal is the value of the signal at anypoint on the wave. It is equal to the vertical distance from a given point on thewave form to the horizontal axis. The maximum amplitude of a sine wave is equalto the highest value it reaches on the vertical axis (see Figure 3.5).
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