Tanenbaum, A. S. & Wetherall, D. J. (2011). Computer Network. 5 th edition. Prentice
Hall: Boston.
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Chapter 1: Introduction (pages 1- 88)
Throughout the book we will use the term ‘‘computer network’’ to mean a collection of
autonomous computers interconnected by a single technology. Two computers are said
to be interconnected if they are able to exchange information. There is considerable
confusion in the literature between a computer network and a distributed system. The
key distinction is that in a distributed system, a collection of independent computers
appears to its users as a single coherent system. Usually, it has a single model or
paradigm that it presents to the users. Often a layer of software on top of the operating
system, called middleware, is responsible for implementing this model. A well-known
example of a distributed system is the World Wide Web. It runs on top of the Internet
and presents a model in which everything looks like a document (Web page).
(Tannenbaum, p.2)
USES OF COMPUTER NETWORKS
1.Business Applications: Most companies have a substantial number of computers.
The Goal (resource sharing) 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. (Tannenbaum, p.3)
,Another goal of setting up a computer network has to do with people rather than
information or even computers. A computer network can provide a powerful
communication medium among employees. Virtually every company that has two or
more computers now has email (electronic mail), which employees generally use for a
great deal of daily communication. In fact, a common gripe around the water cooler is
how much email everyone has to deal with, much of it quite meaningless because
bosses have discovered that they can send the same (often content-free) message to
all their subordinates at the push of a button.
Telephone calls between employees may be carried by the computer network instead of
by the phone company. This technology is called IP telephony or Voice over IP (VoIP)
when Internet technology is used. The microphone and speaker at each end may
belong to a VoIP-enabled phone or the employee’s computer. Companies find this a
wonderful way to save on their telephone bills. (Tannenbaum, p.5)
2.Home Applications
Internet access provides home users with connectivity to remote computers. As with
companies, home users can access information, communicate with other people, and
buy products and services with e-commerce. The main benefit now comes from
connecting outside of the home. (Tannenbaum, p.8-9)
3.Mobile Users
Mobile computers, such as laptop and handheld computers, are one of the fastest-
growing segments of the computer industry. Their sales have already overtaken those
of desktop computers. Connectivity to the Internet enables many of these mobile uses.
Since having a wired connection is impossible in cars, boats, and airplanes, there is a
lot of interest in wireless networks. Cellular networks operated by the telephone
companies
are one familiar kind of wireless network that blankets us with coverage for mobile
phones. Wireless hotspots based on the 802.11 standard are another kind of wireless
network for mobile computers. They have sprung up everywhere that people go,
resulting in a patchwork of coverage at cafes, hotels, airports, schools, trains and
planes. Anyone with a laptop computer and a wireless modem can just turn on their
computer on and be connected to the Internet through the hotspot, as though the
computer were plugged into a wired network. (Tannenbaum, p.11)
,4.Social Issues
Computer networks make it very easy to communicate. They also make it easy for the
people who run the network to snoop on the traffic. This sets up conflicts over issues
such as employee rights versus employer rights. Many people read and write email at
work. Many employers have claimed the right to read and possibly censor employee
messages, including messages sent from a home computer outside working hours. Not
all employees agree with this, especially the latter part. (Tannenbaum, p.15)
Another conflict is centered around government versus citizen’s rights. The FBI has
installed systems at many Internet service providers to snoop on all incoming and
outgoing email for nuggets of interest. Of course, the government does not have a
monopoly on threatening people’s privacy. The private sector does its bit too by
profiling users. For example, small files called cookies that Web browsers store on
users’ computers allow companies to track users’ activities in cyberspace and may also
allow credit card numbers, social security numbers, and other confidential information to
leak all over the Internet (Berghel, 2001). Companies that provide Web-based services
may maintain large amounts of personal information about their users that allows them
to study user activities directly. For example, Google can read your email and show you
advertisements based on your interests if you use its email service, Gmail.
Other information is frequently unwanted. Electronic junk mail (spam) has become a
part of life because spammers have collected millions of email addresses and would-be
marketers can cheaply send computer-generated messages to them. The resulting
flood of spam rivals the flow messages from real people. Fortunately, filtering software
is able to read and discard the spam generated by other computers, with lesser or
greater degrees of success. Still other content is intended for criminal behavior. Web
pages and email messages containing active content (basically, programs or macros
that execute on the receiver’s machine) can contain viruses that take over your
computer. They might be used to steal your bank account passwords, or to have your
computer send spam as part of a botnet or pool of compromised machines. Phishing
messages masquerade as originating from a trustworthy party, for example, your bank,
to try to trick you into revealing sensitive information, for example, credit card numbers.
Identity theft is becoming a serious problem as thieves collect enough information about
a victim to obtain credit cards and other documents in the victim’s name. It can be
difficult to prevent computers from impersonating people on the Internet. This problem
, has led to the development of CAPTCHAs, in which a computer asks a person to solve
a short recognition task, for example, typing in the letters shown in a distorted image, to
show that they are human (von Ahn, 2001). This process is a variation on the famous
Turing test in which a person asks questions over a network to judge whether the entity
responding is human. (Tannenbaum, p.16)
NETWORK HARDWARE
Broadly speaking, there are two types of transmission technology that are in widespread
use: broadcast links and point-to-point links. Point-to-point links connect individual
pairs of machines. To go from the source to the destination on a network made up of
point-to-point links, short messages, called packets in certain contexts, may have to
first visit one or more intermediate machines. Often multiple routes, of different lengths,
are possible, so finding good ones is important in point-to-point networks. Point-to-point
transmission with exactly one sender and exactly one receiver is sometimes called
unicasting. (Tannenbaum, p.17)
In contrast, on a broadcast network, the communication channel is shared by all the
machines on the network; packets sent by any machine are received by all the others.
An address field within each packet specifies the intended recipient. Upon receiving a
packet, a machine checks the address field. If the packet is intended for the receiving
machine, that machine processes the packet; if the packet is intended for some other
machine, it is just ignored.
PANs (Personal Area Networks) let devices communicate over the range of a person.
A common example is a wireless network that connects a computer with its peripherals.
Almost every computer has an attached monitor, keyboard, mouse, and printer. Without
using wireless, this connection must be done with cables. So many new users have a
hard time finding the right cables and plugging them into the right little holes (even
though they are usually color coded) that most computer vendors offer the option of
sending a technician to the user’s home to do it. To help these users, some companies
got together to design a short-range wireless network called Bluetooth to connect
these components without wires.
