Computer Networks SIXTH EDITION By Tanenbaum, Feamster, Wetherall

Computer Networks SIXTH EDITION By Tanenbaum, Feamster, Wetherall
INTRODUCTION
Each of the past three centuries was dominated by a single new technology.
The 18th century was the era of the great mechanical systems accompanying the
Industrial Revolution. The 19th century was the age of the steam engine. During
the 20th century, the key technology was information gathering, processing, and
distribution. Among other developments, we saw the deployment of worldwide
telephone networks, the invention of radio and television, the birth and unprecedented growth of the computer industry, the launching of communication satellites,
and, of course, the Internet. Who knows what miracles the 21st century will bring?
As a result of this rapid technological progress, these areas are rapidly converging in the 21st century, and the differences between collecting, transporting,
storing, and processing information are quickly disappearing. Organizations with
hundreds of offices spread over a wide geographical area routinely expect to be
able to examine the current status of even their most remote outpost at the push of
a button. As our ability to gather, process, and distribute information grows, the
demand for more sophisticated information processing grows even faster.
1.1 USES OF COMPUTER NETWORKS
Although the computing industry is still young compared to other technical
industries such as automobiles and air transportation, computers have made spectacular progress in a short time. During the first two decades of their existence,
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2 INTRODUCTION CHAP. 1
computer systems were highly centralized, usually within a single room. Often,
this room had glass windows, through which visitors could gawk at the great electronic wonder inside. A medium-sized company or university might have had one
or two computers, while large institutions had at most a few dozen. The idea that
within fifty years vastly more powerful computers smaller than postage stamps
would be mass produced by the billions was science fiction.
The convergence of computers and communications has had a profound influence on the organization of computer systems. The once-dominant concept of the
‘‘computer center’’ as a room with a single large computer to which users bring
their work for processing is now obsolete (although data centers holding hundreds
of thousands of Internet servers are common). The old model of a single computer
serving all of the organization’s computational needs has been replaced by one in
which a large number of separate but interconnected computers do the job. These
systems are called computer networks. The design and organization of these networks are the subjects of this book.
Throughout the book, we will use the term ‘‘computer network’’ to mean a collection of interconnected, autonomous computing devices. Two computers are said
to be interconnected if they can exchange information. Interconnection can take
place over a variety of transmission media including copper wire, fiber optic cable,
and radio wav es (e.g., microwave, infrared, communication satellites). Networks
come in many sizes, shapes, and forms, as we will explore throughout the book.
They are usually connected to make larger networks, with the Internet being the
most well-known example of a network of networks.
1.1.1 Access to Information
Access to information comes in many forms. A common method of accessing
information via the Internet is using a Web browser, which allows a user to retrieve
information from various Web sites, including increasingly popular social media
sites. Mobile applications on smartphones now also allow users to access remote
information. Topics include the arts, business, cooking, government, health, history, hobbies, recreation, science, sports, travel, and many others. Fun comes in
too many ways to mention, plus some ways that are better left unmentioned.
News organizations have largely migrated online, with some even ceasing print
operations entirely. Access to information, including the news, is increasingly personalizable. Some online publications even allow you to tell them that you are interested in corrupt politicians, big fires, scandals involving celebrities, and epidemics, but no football, thank you. This trend certainly threatens the employment
of 12-year-old paperboys, but online distribution has allowed the distribution of
news to reach far larger and broader audiences.
Increasingly, news is also being curated by social media platforms, where users
can post and share news content from a variety of sources, and where the news that
any giv en user sees is prioritized and personalized based on both explicit user
SEC. 1.1 USES OF COMPUTER NETWORKS 3
preferences and complex machine learning algorithms that predict user preferences
based on the user’s history. Online publishing and content curation on social
media platforms supports a funding model that depends largely on highly targeted
behavioral advertising, which necessarily implies gathering data about the behavior
of individual users. This information has sometimes been misused.

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