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Computer Networks, Andrew S.Tanenbaum 5th PDF 下载
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1.1 USES OF COMPUTER NETWORKS
Before we start to examine the technical issues in detail, it is worth devoting
some time to pointing out why people are interested in computer networks and
what they can be used for. After all, if nobody were interested in computer net-
works, few of them would be built. We will start with traditional uses at com-
panies, then move on to home networking and recent developments regarding
mobile users, and finish with social issues.
1.1.1 Business Applications
Most companies have a substantial number of computers. For example, a
company may have a computer for each worker and use them to design products,
write brochures, and do the payroll. Initially, some of these computers may have
worked in isolation from the others, but at some point, management may have
decided to connect them to be able to distribute information throughout the com-
pany.
Put in slightly more general form, the issue here is resource sharing. The
goal is to make all programs, equipment, and especially data available to anyone
on the network without regard to the physical location of the resource or the user.
An obvious and widespread example is having a group of office workers share a
common printer. None of the individuals really needs a private printer, and a
high-volume networked printer is often cheaper, faster, and easier to maintain
than a large collection of individual printers.
However, probably even more important than sharing physical resources such
as printers, and tape backup systems, is sharing information. Companies small
and large are vitally dependent on computerized information. Most companies
have customer records, product information, inventories, financial statements, tax
information, and much more online. If all of its computers suddenly went down, a
bank could not last more than five minutes. A modern manufacturing plant, with
a computer-controlled assembly line, would not last even 5 seconds. Even a small
travel agency or three-person law firm is now highly dependent on computer net-
works for allowing employees to access relevant information and documents
instantly.
For smaller companies, all the computers are likely to be in a single office or
perhaps a single building, but for larger ones, the computers and employees may
be scattered over dozens of offices and plants in many countries. Nevertheless, a
sales person in New York might sometimes need access to a product inventory
4 INTRODUCTION CHAP. 1
database in Singapore. Networks called VPNs (Virtual Private Networks) may
be used to join the individual networks at different sites into one extended net-
work. In other words, the mere fact that a user happens to be 15,000 km away
from his data should not prevent him from using the data as though they were
local. This goal may be summarized by saying that it is an attempt to end the
‘‘tyranny of geography.’’
In the simplest of te
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