NETWORKS

A network is a set of devices (often referred to as nodes) connected by

communication links.

A node can be a computer, printer, or any other device capable of sending

and/or receiving

data generated by other nodes on the network.

“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.

The connection need not be via a copper wire; fiber optics, microwaves,

infrared, and communication satellites can also be used.

Networks come in many sizes, shapes and forms, as we will see later.

They are usually connected together to make larger networks, with the

Internet being the most well-known example of a network of networks.

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).

USES OF COMPUTER NETWORKS

1. Business Applications
to distribute information throughout the company (resource sharing).

sharing physical resources such as printers, and tape backup systems, is

sharing information

client-server model. It is widely used and forms the basis of much network

usage.

communication medium among employees.email (electronic mail),

which employees generally use for a great deal of daily

communication.

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.

Desktop sharing lets remote workers see and interact with a

graphicalcomputer screen

doing business electronically, especially with customers and suppliers. This

new model is called e-commerce (electronic commerce) and it has grown

rapidly in recent years.

2 Home Applications

peer-to-peer communication

person-to-person communication

electronic commerce

entertainment.(game playing,)

3 Mobile Users

Text messaging or texting

Smart phones,

GPS (Global Positioning System)

m-commerce

NFC (Near Field Communication)

4 Social Issues

With the good comes the bad, as this new-found freedom brings with it many

unsolved social, political, and ethical issues.

Social networks, message boards, content sharing sites, and a host of

other applications allow people to share their views with like-minded

individuals. As long as the subjects are restricted to technical topics or

hobbies like gardening, not too many problems will arise.

The trouble comes with topics that people actually care about, like

politics, religion, or sex. Views that are publicly posted may be deeply

offensive to some people. Worse yet, they may not be politically correct.

Furthermore, opinions need not be limited to text; high-resolution color

photographs and video clips areeasily shared over computer networks. Some

people take a live-and-let-live view, but others feel that posting certain

material (e.g., verbal attacks on particular countries or religions,

pornography, etc.) is simply unacceptable and that such content must be

censored. Different countries have different and conflicting laws in this area.

Thus, the debate rages.

Computer networks make it very easy to communicate. They alsomake 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.

Another conflict is centered around government versus citizen’s rights.

A new twist with mobile devices is location privacy. As part of the process

of providing service to your mobile device the network operators learn

where you are at different times of day. This allows them to track your

movements. They may know which nightclub you frequent and which

medical center you visit.

Phishing ATTACK: Phishing is a type of social engineering attack often

usedto steal user data, including login credentials and credit card numbers. It

occurs when an attacker, masquerading as a trusted entity, dupes a victim

into opening an email, instant message, or text message.

BOTNET ATTACK: Botnets can be used to perform distributed denial-of

service attack (DDoS attack), steal data, send spam, and allows the attacker

to access the device and its connection.

The effectiveness of a data communications system depends on four

fundamental characteristics: delivery, accuracy, timeliness, and jitter.

I. Delivery. The system must deliver data to the correct destination. Data

must be received by the intended device or user and only by that device or

user.

2 Accuracy. The system must deliver the data accurately. Data that have

been altered in transmission and left uncorrected are unusable.

3. Timeliness. The system must deliver data in a timely manner. Data

deliveredlate are useless. Inthecase of video and audio, timely delivery

means delivering data as they are produced, in the same order that they are

produced, and without significant delay. This kind of delivery is called real

time transmission.

4. Jitter. Jitter refers to the variation in the packet arrival time. It is the

unevendelay in the delivery of audio or video packets. For example, let us

assume that video packets are sent every 30 ms. If some of the packets
arrive
with 30-ms delay and others with 40-ms delay, an uneven quality in the
video is the result. A data communications system has five components

I. Message. The message is the information (data) to be communicated.

Popular forms of information include text, numbers, pictures, audio, and

video.2 Sender. The sender is the device that sends the data message. It

can be a computer, workstation, telephone handset, video camera, and so

on.

3. Receiver. The receiver is the device that receives the message. It can

be acomputer, workstation, telephone handset, television, and so on.

4. Transmission medium. The transmission medium is the physical path

by which a message travels from sender to receiver. Some examples of

transmission media include twisted-pair wire, coaxial cable, fiber-optic

cable, and radio waves.

5. Protocol. A protocol is a set of rules that govern data communications. It

represents an agreement between the communicating devices. Without a

protocol, two devices may be connected but not communicating, just as a

person speaking French cannot be understood by a person who speaks only

Japanese.

Data Representation

Data Flow
Communication between two devices can be simplex, half-duplex, or full

duplexes shown in Figure.

Simplex In simplex mode, the communication is unidirectional, as on a

one- way street. Only one of the two devices on a link can transmit: the

other can only receive (Figure a). Keyboards and traditional monitors are

examples of simplex devices.

Half-Duplex

In half-duplex mode, each station can both transmit and receive, but not at

the same time. When one device is sending, the other can only receive, and

vice versa (Figure b). Walkie-talkies and CB (citizens band) radios are both

half- duplex systems.

Full-Duplex

In full-duplex, both stations can transmit and receive simultaneously

(Figure c).One common example of full-duplex communication is the

telephone network. When two people are communicating by a telephone

line, both can talk and listen at the same time. The full-duplex mode is

used when communication in both directions is required all the time.

Network Criteria
A network must be able to meet a certain number of criteria. The most

important of these are performance, reliability, and security.

Performance

Performance can be measured in many ways, including transit time and

response time. Transit time is the amount of time required for a message to

travel from one device to another. Response time is the elapsed time
between

an inquiry and a response. The performance of a network depends on several

factors, including the number of users, the type of transmission

medium, the capabilities of the connected hardware, and the efficiency of

the

software.

Performance is often evaluated by two networking metrics: throughput and

delay. We often need more throughput and less delay. However, these two

criteria are often contradictory. If we try to send more data to the network,

we may increase throughput, but we increase the delay because of traffic

congestion in the network.

Reliability: In addition to accuracy of delivery, network reliability is

measured by the frequency of failure, the time it takes a link to recover from

a failure, and the network's robustness in a catastrophe.

Security: Network security issues include protecting data from

unauthorized

access, protecting data from damage and development, and implementing

policies and procedures for recovery from breaches and data losses.

Physical Structures

Before discussing networks, we need to define some network attributes.

Type of Connection

A network is two or more devices connected

through

links. A link is a communications pathway that transfers data from one

device to another.

There are two possible types of connections: point-to-point and

multipoint. Point-to-Point A point-to-point connection provides a

dedicated link between two devices. The entire capacity of the link is

reserved for transmission between those two devices. Most point-to

point connections use an actual length of wire or cable to connect the

two ends, but other options, such as microwave or satellite links, are also

possible

When you change television channels by infrared remote control, you are

establishing a point-to-point connection between the remote control and the

television's control system.

Multipoint A multipoint (also called multi-drop) connection is one in

which more than two specific devices share a single link

In a multipoint environment, the capacity of the channel is shared, either

spatially or temporally. If several devices can use the link simultaneously, it

is a spatially shared connection. If users must take turns, it is timeshared

connection.