We have included the discussion of media in Chapter 7 Chapter 8 discusses

We have included the discussion of media in chapter 7

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cal layer. We have included the discussion of media in Chapter 7. Chapter 8 discusses switching, a topic that can be related to several layers. We have, however, discussed
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48 PART 2 PHYSICAL LAYER only circuit switching, which is mostly a physical-layer issue. To show an application of circuit switching, we have introduced the telephone network and several topics related to this network. The main purpose of most networks today is to access the Internet. Chapter 9 introduces several technologies that allow the user to access the Internet.
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49 CHAPTER 3 CHAPTER 3 Signals One of the major concerns of the physical layer lies in moving data in the form of elec- tromagnetic signals across a transmission medium. Whether you are collecting numeri- cal statistics from another computer, sending animated pictures from a design workstation, or causing a bell to ring at a distant control center, you are working with the transmission of data across network connections. Generally, the data usable to a person or application are not in a form that can be trans- mitted over a network. For example, you cannot roll up a photograph, insert it into a wire, and transmit it across town. You can, however, transmit an encoded description of the pho- tograph. Instead of sending the actual photograph, you can use an encoder to create a stream of 1s and 0s that tells the receiving device how to reconstruct the image of the photograph. But even 1s and 0s cannot be sent as such across network links. They must be fur- ther converted to a form that transmission media can accept. Transmission media work by conducting energy along a physical path. So a data stream of 1s and 0s must be turned into energy in the form of electromagnetic signals. 3.1 ANALOG AND DIGITAL Both data and the signals that represent them can take either analog or digital form. Analog and Digital Data Data can be analog or digital. An example of analog data is the human voice. When someone speaks, an analog wave is created in the air. This can be captured by a micro- phone and converted to an analog signal or sampled later and converted to a digital signal. An example of digital data is data stored in the memory of a computer in the form of 0s and 1s. It can be converted to a digital signal when it is transferred from one position to another inside or outside the computer or modulated into an analog signal and then sent through a transmission medium to another computer. Analog and Digital Signals Like the data they represent, signals can be either analog or digital. An analog signal has infinitely many levels of intensity over a period of time. As the wave moves from value A To be transmitted, data must be transformed to electromagnetic signals.
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50 CHAPTER 3 SIGNALS to value B, it passes through and includes an infinite number of values along its path. A digital signal, on the other hand, can have only a limited number of defined values, often as simple as 1 and 0.
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