tailieunhanh - Mạng và viễn thông P9

Data Network Principles and Protocols We have described the binary form in which data are held by computer systems, and how such data are conveyed over digital line transmission systems, but we shall need to know more than this before we can design the sort ofdevices which can communicatesensibly with one another in somethingequivalent to human we shalldiscuss indetailthe networks required, the and so-called conveyance of data between computer systems, the ‘protocols’ they will need to ensure that they are communicating properly | Networks and Telecommunications Design and Operation Second Edition. Martin P. Clark Copyright 1991 1997 John Wiley Sons Ltd ISBNs 0-471-97346-7 Hardback 0-470-84158-3 Electronic _9 Data Network Principles and Protocols We have described the binary form in which data are held by computer systems and how such data are conveyed over digital line transmission systems but we shall need to know more than this before we can design the sort of devices which can communicate sensibly with one another in something equivalent to human conversation. In this chapter we shall discuss in detail the conveyance of data between computer systems the networks required and the so-called protocols they will need to ensure that they are communicating properly. In the second half of the chapter some practical data and computer network topologies and the equipment needed to support them are described. COMPUTER NETWORKS Between 1950 and 1970 computers were large and unwieldy severely limited in their power and capabilities and rather unreliable. Only the larger companies could afford them and they were used for batch-processing scientific business or financial data on a large scale. Data storage in those days was laborious limited in capacity long in preparation not at all easy to manage. Many storage mechanisms . paper tape and punched cards were very labour-intensive they were difficult to store and prone to damage. Even magnetic tape when it appeared had its drawbacks digging out some trivial information buried in the middle of a long tape was a tedious business and the tapes themselves had to be painstakingly protected against data corruption and loss caused by mechanical damage or nearby electrical and magnetic fields. Computing was for specialists. Computer centre staff looked after the hardware while software experts spent long hours improving their computer programmes squeezing every last drop of power out of the computers .