Physical Layer

Chapter 6PhysicalLayerDistinguish between analog and digital data.Distinguish between analog and digital signals.Understand the concept of bandwidth and the relationship between bandwidth and data transmission speed.Understand digital-to-digital, digital-to-analog, and analog-to-digital encoding.After reading this chapter, the reader should be able to:OBJECTIVESUnderstand multiplexing and the difference between a linkand a channel.DIGITALANDANALOG6.1Figure 6-1Digital and analog entitiesFigure 6-2Digital dataFigure 6-3Analog dataFigure 6-4Digital signalFigure 6-5Bit and bit intervalTechnical Focus: Units of Bit Rate1 bps1 kbps = 1000 bps1 Mbps = 1,000,000 bps1 Gbps = 1,000,000,000 bps1 Tbps = 1,000,000,000,000 bpsFigure 6-6A sine waveFigure 6-7AmplitudeFigure 6-8Period and frequencyTechnical Focus: Units of Frequency1 Hz1 kHz = 1000 Hz1 MHz = 1,000,000 Hz1 GHz = 1,000,000,000 Hz1 THz = 1,000,000,000,000 HzTechnical Focus: Frequency and ChangeThe concept of frequency is similar to the concept of change. If a signal (or data) is changing rapidly, its frequency is higher. If it changes slowly, its frequency is lower. When a signal changes 10 times per second, its frequency is 10 Hz; when a signal changes 1000 times per second, its frequency is 1000 Hz.Figure 6-9PhaseZero frequency and infinite frequencyFigure 6-10Phase describes the position of a waveform relative to other waveforms.Note:Business Focus: Two Familiar SignalsA familiar signal in our daily lives is the electrical energy we use at home and at work. The signal we receive from the power company has an amplitude of 120 V and a frequency of 60 Hz (a simple analog signal). Another signal familiar to us is the power we get from a battery. It is an analog signal with an amplitude of 6 V (or 12 or 24) and a frequency of zero. Business Focus: The Bandwidth of Telephone LinesThe conventional line that connects a home or business to the telephone office has a bandwidth of 4 kHz. These lines were designed for carrying human voice, which normally has a bandwidth in this range. Human voice has a frequency that is normally between 0 and 4 kHz. The telephone lines are perfect for this purpose. However, if we try to send a digital signal, we are in trouble. A digital signal needs a very high bandwidth (theoretically infinite); it cannot be sent using these lines. We must either improve the quality of these lines or change our digital signal to a complex signal that needs only 4 kHz. TRANSFORMINGDATATO SIGNALS6.2Transforming data to signalsFigure 6-11Digital-to-digital encodingFigure 6-12A digital signal has a much higher bandwidth than an analog signal. There is a need for a better media to send a digital signal. Note:Most LANs use digital-to-digital encoding because the data stored in the computers are digital and the cable connecting them is capable of carrying digital signals.Note:Digital encodingmethodsFigure 6-13Technical Focus: Average Values in Digital SignalsWith one exception, all of the signals in Figure 16.3 have an average value of zero (the positive and negative values cancel each other in the long run). The first signal, unipolar, has a positive average value. This average value, sometimes called the residual value, cannot pass through some devices (such as a transformer). In this case, the receiver receives a signal that can be totally different from the one sent and results in an erroneous interpretation of data. Technical Focus: Synchronization in Digital SignalsTo correctly interpret the signals received from the sender, the receiver’s bit intervals must correspond exactly to the sender’s bit intervals. If the receiver clock is faster or slower, the bit intervals are not matched and the receiver will interpret the signals differently than the sender intended. A self-synchronizing digital signal includes timing information in the data beingtransmitted. This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end ofthe bit interval. If the receiver’s clock is out of synchronization, these alerting points can reset the clock. Digital-to-analog modulationFigure 6-14ASKFigure 6-15FSKFigure 6-16PSKFigure 6-17Technical Focus: Understanding Bit Rate and Baud RateA transportation analogy can clarify the concept of bauds and bits. A baud is analogous to a car; a bit is analogous to a passenger. A car can carry one or more passengers. If 1000 cars go from onepoint to another each carrying only one passenger (the driver), then 1000 passengers are transported. However, if each car carries four passengers (car pooling), then 4000 passengers aretransported. Note that the number of cars, not the number of passengers, determines the traffic and, therefore, the need for wider highways. Similarly, the number of bauds determines the required bandwidth, not the number of bits. Technical Focus: Capacity of a ChannelWe often need to know the capacity of a channel; that is, how fast can we send data over a specific medium? The answer was given by Shannon. Shannon proved that the number of bits that we can send through a channel depends on two factors: the bandwidth of the channel and the noise in the channel. Shannon came up with the following formula: C = B log2 (1 + signal-to-noise ratio)C is the capacity in bits per second; B is the bandwidth.Analog-to-digital conversionFigure 6-18PCMFigure 6-19Technical Focus: Sampling Rate and Nyquist TheoremAs you can see from the preceding figures, the accuracy of anydigital reproduction of an analog signal depends on the numberof samples taken. So the question is, how many samples aresufficient? This question was answered by Nyquist. His theorem states that the sampling rate must be at least twice the highest frequency of the original signal to ensure the accurate reproductionof the original analog signal. So if we want to sample a telephonevoice with a maximum frequency of 4000 Hz, we need a samplingrate of 8000 samples per second.TRANSMISSIONMODES6.3Data transmissionFigure 6-20Parallel transmissionFigure 6-21Serial transmissionFigure 6-22In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte.Note:Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same.Note:Asynchronous transmissionFigure 6-23In synchronous transmission, we send bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits.Note:Figure 6-24Synchronous transmissionLINECONFIGURATION6.4Line configuration defines the attachment of communication devices to a link.Note:Figure 6-25Point-to-point line configurationFigure 6-26Multipoint line configurationDUPLEXITY6.5Half-duplex modeFigure 6-27Full-duplex modeFigure 6-28MULTIPLEXING:SHARING THE MEDIA6.6Multiplexing versus no multiplexingFigure 6-29Categories of multiplexingFigure 6-30FDMFigure 6-31FDM can only be used with analog signals.Note:Technical Focus: Use of FDM in Telephone SystemsAT&T uses a hierarchical system to multiplex analog lines: Prisms in WDM multiplexing and demultiplexingFigure 6-32TDMFigure 6-33TDM can be used only with digital signals. Note:Synchronous TDMFigure 6-34Technical Focus: Use of TDM in Telephone SystemsAT&T uses a hierarchical system to multiplex digital lines: Asynchronous TDMFigure 6-35Multiplexing and inverse multiplexingFigure 6-36Technical Focus: Use of TDM in ATM NetworksAsynchronous TDM is used today in the ATM network, a wide area network that we discuss in Chapter 11. ATM is acell network; the packets traveling through the networkare small packets called cells.