Data communications refers to the transmission of this digital data between two or more computers and a computer network

or data network is a telecommunications network that allows computers to exchange data.

Components of data communication

 Message - It is the information to be communicated. Popular forms of information include text, pictures, audio, video etc.
 Sender - It is the device which sends the data messages. It can be a computer, workstation, telephone handset etc.
 Receiver - It is the device which receives the data messages. It can be a computer, workstation, telephone handset etc.
 Transmission Medium - It is the physical path by which a message travels from sender to receiver. Some examples include
twisted-pair wire, coaxial cable, radio waves etc.
 Protocol - It is a set of rules that governs the data communications. It represents an agreement between the communicating
devices. Without a protocol, two devices may be connected but not communicating.

Transmission Media

The means through which data is transformed from one place to another is called transmission or communication media.

Wired transmission media

1. Coaxial cable
Coaxial cable gets its name because it contains two conductors that are parallel to each other. The center conductor in the
cable is usually copper. The copper can be either a solid wire or stranded martial. Outside this central Conductor is a non-
conductive material. It is usually white, plastic material used to separate the inner Conductor form the outer Conductor. The
other Conductor is a fine mesh made from Copper.

2. Fiber optic cable Fiber optic cable uses electrical signals to transmit data. It uses light. In fiber optic cable light only moves
in one direction for two way communication to take place a second connection must be made between the two devices. It is
actually two stands of cable. Each stand is responsible for one direction of communication. A laser at one device sends pulse
of light through this cable to other device. These pulses translated into “1’s” and “0’s” at the other end.

3. Twisted Pair Cable It is light weight, easy to install, inexpensive and support many different types of network. It also
supports the speed of 100 mps.Twisted pair cabling is made of pairs of solid or stranded copper twisted along each other.

Wireless media

a. Radio signal Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer
than infrared light. Radio waves have frequencies as high as 300 GHz to as low as 3 kHz, though some definitions describe
waves above 1 or 3 GHz as microwaves, or include waves of any lower frequency.
b. The microwave band is well suited for wireless transmission of signals having large bandwidth. This portion of the RF
electromagnetic radiation spectrum encompasses many thousands of megahertz.
c. Satellites communicate by using radio waves to send signals to the antennas on the Earth. The antennas then capture those
signals and process the information coming from those signals
d. Infra-red is a wavelength just greater than that of the red end of the visible light spectrum but less than that of microwaves.
Infrared radiation has a wavelength from about 800 nm to 1 mm, and is emitted particularly by heated objects
e. Bluetooth is a standard for the short-range wireless interconnection of mobile phones, computers, and other electronic
devices.
f. Wifi is a name of a popular wireless networking technology that uses radio waves to provide wireless high-speed Internet and
network connections. A common misconception is that the term Wi-Fi is short for "wireless fidelity,"

Data networking devices

 Modem– translate analog to digital signal and vice versa

 Hub – a common connection point for devices in a network that contain several port
 Switch – is a small hardware device that joins multiple computer together within one local area network.

A Network Topology is the arrangement with which computer systems or network devices are connected to each other. Topologies
may define both physical and logical aspect of the network. Both logical and physical topologies could be same or different in a same
network.

 Bus topology, all devices share single communication line or cable. Bus topology may have problem while multiple hosts
sending data at the same time.
 Star topology are connected to a central device, known as hub device, using a point-to-point connection. That is, there exists
a point to point connection between hosts and hub.
 Ring topology, each host machine connects to exactly two other machines, creating a circular network structure. When one
host tries to communicate or send message to a host which is not adjacent to it, the data travels through all intermediate hosts.
 Hierarchical Topology/tree, this is the most common form of network topology in use presently. This topology imitates as
extended Star topology and inherits properties of Bus topology.
 Hybrid is a network structure whose design contains more than one topology that inherits merits and demerits of all the
incorporating topologies.
Network security
All security threats are intentional i.e. they occur only if intentionally triggered. Security threats can be divided into the following
categories:
 Interruption
Interruption is a security threat in which availability of resources is attacked. For example, a user is unable to access its
web-server or the web-server is hijacked.
 Privacy-Breach
In this threat, the privacy of a user is compromised. Someone, who is not the authorized person is accessing or
intercepting data sent or received by the original authenticated user.
 Integrity
This type of threat includes any alteration or modification in the original context of communication. The attacker
intercepts and receives the data sent by the sender and the attacker then either modifies or generates false data and sends
to the receiver. The receiver receives the data assuming that it is being sent by the original Sender.
 Authenticity
This threat occurs when an attacker or a security violator poses as a genuine person and accesses the resources or
communicates with other genuine users.

How does internet works

Terms

 Client – a computer that request data or services

 Server – a central computer that is supplying data or services
 Network Access point (NAP)- later on called as Internet Service Provider (ISP), a routing computer at a point on the internet
where several connections came together
 Internet Backbone – a high speed, high capacity transmission lines that use the newest communication technology to transmit
data across the internet
 Protocol – set of rules that govern the access of data
 TCP/IP – Transmission Controlled Protocol / Internet Protocol, is used to determine how sending devices indicate it has
finished sending a message, and how a receiving device indicate that it has received the message
 Packets – fixed length blocks of data transmission. It allows the data to split up and its parts sent by separate routes
 Internet Protocol – uniquely identifies each computer on the network
 Dynamic IP Address- address that changes each time you connect to the internet
 Static IP Address – remains constant each time a person logs on to the internet

History of Internet

World Wide Web

 Browser – a software that enable the user to find and access various parts of the web
 Web site – a unique web address
 Web page – a document on the web that includes text and images
 Home page – first page that contains link to other pages

Uniform Resource Locator(URL) is consist of

a. Web protocol
b. Domain name
c. Directory on the server
d. File

EMAIL

An email address is made up of

 A user’s account name

 @ sign separator
 Domain name

Six domain name

1. gov
2. edu
3. com
4. net
5. mil
6. org
blind carbon copy(bcc)- sending to a group, the recipients of the message is unknown to others.

Carbon copy(CC)- all the recipients of the message are listed.

SPAM – contain unsolicited mails, unwanted mails

Computer Virus – a block of program which attachés itself to or otherwise replaces another program in order to reproduce itself
without the knowledge of the user.

Why do people create a virus?

 Some people think its funny to create mischief

 Some people take revenge
 Some people are intellectuals who want the challenge of trying to create something that replicate itself
 Profit and fame

History of a virus

Traditional viruses are created because of several factors

 Viruses are use to lock a personal computer that was used by the experts
 Viruses are use for computer bulleting boards, specially download files that led to Trojan Horse Viruses
 Infections of floppy disk.
Fred Cohen – writer of the first computer virus, the main purpose of the virus is for computer security

Brain- the first computer virus that escape to the laboratory and has been detected and treated after a year

Jerusalem Virus- first destructive virus that was written

Virus Mischief

 Print nasty messages

 Erase files and document
 Computer Stops functioning properly
 It clogs your computer
Payload - damage done by a virus

Propagation Trick

 Trojan Horse – pleasant seeming program that secretly contains a virus

 Time Bomb – a virus that damage your computer immediately, they delay the processes until the entire system is infected
Viruses can be transmitted by

 Booting a PC from an infected medium

 Executing an infected program
 Opening an infected file
Common routes for virus infiltration

 Floppy or other media that the user exchange

 Email Attachments
 Pirated software
 Shareware
Virus Symptoms

 Slower operation
 Decrease memory
 Lighting up for no apparent reason
7 kinds of viruses

1. file virus - secretly attached itself to an innocent program

2. boot sector virus - a virus that hides in a boot sector

3. multi-partite virus
 stealth – make special effort to hide itself to an anti virus
 polymorphic – change its appearance each time it infect a file
 armored – protect itself against snit virus disassemble
4. macro virus – it hides in little program embedded in a applications
5. email virus- e-mail attachments
6. denial of service attack virus – attacks programs that slow down the operations
7. hoax – emails that contain scary rumors and warn to pass the message to your friends
Anti Viruses Measures

 preparation - making backups of all software

 prevention- creating user awareness, implementing rules, using disc authorization or placing a quarantine
 detection – using anti viruses and disinfect virus
 containment - involves identifying and isolating the infect items
 recovery- involves disinfecting and removing infected items and recovering or replacing corrupted data

DATABASE MANAGEMENT SYSTEM

Database Management System
A software system that enables users to define, create, and maintain the database and that provides controlled access to this database.
Example of Database System
 Purchases from the supermarket
 Purchases using your credit card
 Booking a holiday at the travel agents
 Using the local library
 Renting a video
 Using the Internet
Database is a Shared collection of logically related data (and a description of this data), designed to meet the information needs of an
organization.
Database Application program is a software program that interacts with the database by issuing an appropriate request (typically an
SQL statement) to the DBMS.
Views allows each user to have his or her own view of the databases .
Benefits of views include:
 Provide a level of security;
 Provide a mechanism to customize the appearance of the database;
 Present a consistent, unchanging picture of the structure of the database, even if the underlying database is changed.
Components of DBMS

 Hardware
 Software
 Data
 Procedures
 People
Functions of Database
 Data Storage, Retrieval, and Update.
 A User-Accessible Catalog.
 Transaction Support.
 Concurrency Control Services.
 Recovery Services
 Authorization Services.
 Support for Data Communication.
 Integrity Services.
 Services to Promote Data Independence.
 Utility Services.
Advantage of Database
 Control of data redundancy
 Data consistency
 Sharing of data
 Improved data integrity
 Improved maintenance through data independence.

Disadvantage
 Complexity
 Cost of DBMS
 Cost of conversion
 Performance
 Higher impact of a failure