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Networks and Telecommunications Design and Operation second edition - Clark M.P.

Clark M.P. Networks and Telecommunications Design and Operation second edition - Wiley & sons , 1991. - 958 p.
ISBN 0-47197346-7
Download (direct link): networksandtelecommunications1991.pdf
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By the mid-1970s all this began to change, and very rapidly. Cheap semiconductors heralded the appearance of the microcomputer, which when packaged with the newly developed floppy diskette systems, opened a new era of cheap and widespread computing activity. Personal computers {PCs) began to appear on almost every manager’s
178_________________________________________________DATA NETWORK PRINCIPLES AND PROTOCOLS
desk, and many even invaded peoples’ homes. Suddenly, computing was within reach of the masses, and the creation and storage of computer data was easy, cheap and fast.
All sorts of individuals began to prepare their own isolated databases and to write computer programmes for small scale applications, but these individuals soon recognized the need to share information and to pass data between different computers. This could be done by transferring floppy diskettes from one machine to another, but as time went on that method on its own proved inadequate. There was a growing demand for more geographically widespread, rapid, voluminous data transfer. More recently, the demands of distributed processing computer networks comprising clients and servers have created a boom in demand for data networks.
A very simple computer or data network consists of a computer linked to a piece of peripheral equipment, such as a printer. The link is necessary so that the data in the computer’s memory can be reproduced on paper. The problem is that a ‘wires-only’ direct connection of this nature is only suitable for very short connections, typically up to about 20 metres. Beyond this range, some sort of line driver telecommunications technique must be used. A number of techniques are discussed here. A long distance point-to-point connection may be made using modems. A slightly more complex computer network might connect a number of computer terminals in outlying buildings back to a host (mainframe computer) in a specialized data centre. Another network might be a Local Area Network (or LAN), used in an office to interconnect a number of desktop computing devices, laser printers, data storage devices (e.g. file servers), etc. More complex computer networks might interconnect a number of large mainframe computers in the major financial centres of the world, and provide dealers with ‘up-to-the-minute’ market information.
The basic principles of transmission, as set out in the early chapters of this book, apply equally to data which are communicated around computer networks. So circuit-switched networks or simple point-to-point lines may also be used for data communication. Data communication, however, makes more demands on its underlying network than a voice or analogue signal service, and additional measures are needed for coding the data in preparation for transmission, and in controlling the flow of data during transmission. Computers do not have the same inherent ‘discipline’ to prevent them talking two at a time. For this reason special protocols are used in data communication to make quite sure that information passing between computers is correct, complete and properly understood.
As we learned in Chapter 4, data are normally held in a computer or computer storage medium in a binary code format, as a string of digits with either value ‘0’ or value ‘Ă. A series of such binary digits can be used to represent alphanumeric characters (e.g. ASCII code), or graphical images, such as those transmitted by facsimile machines, video or multimedia signals.
In Chapter 5 we went on to discuss the principles of digital transmission, and found that it was ideal for the conveyance of binary data. Digital transmission has become the
backbone of both private and public networks. However, despite the increasing availability and ideal suitability of digital transmission for data communication, it is unfortunately not always available. In circumstances where it is not, digitally-oriented computer information must instead be translated into a form suitable for transmission across an analogue network. This translation is carried out by a piece of equipment called a modulator/demodulator, or modem for short. Modems transmit data by imposing the binary (or digital) data stream onto an audio frequency carrier signal. The process is very similar to that used in the frequency division multiplexing of voice channels described in Chapter 3.
Figure 9.1 illustrates two possible configurations for data communication between two computers using either a digital or an analogue transmission link. The configurations look very similar, comprising the computers themselves (these are specific examples of data terminal equipment (DTE)); sandwiched between them in each case is a line and a pair of data circuit terminating equipments (DCE).
A digital DCE (Figure 9.1(a)) connects the customer’s digital DTE to a digital transmission line, perhaps provided by the public telecommunications operator (PTO). The DCE provides several network functions. In the transmit direction, it regenerates the digital signal provided by the DTE and converts it into a standardized format, level and line code suitable for transmission on the digital line. More complex DCEs may also interpret the signals sent by the DTE to the network to indicate the address desired
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