Computer Viruses

Some of the main reasons for writing viruses are;

• Hacking.
• To prevent copying software.
• Revenge - Employees may plant "Time Bombs" in programs.
• Fraud - Viruses may allow access to an otherwise secure system.
• Political and terrorist motives.
• Commercial sabotage - Damage the reputation of a competitor.
• Warfare - Incorporate a virus to an enemy's computer.

Viruses can be transmitted from one system to another through various media types such as;

• Floppy disks.
• Tape backups.
• Internet.
• Software.

Some of the symptoms that a computer will show up when infected with a virus are listed below.

• Programs take long time to lead than normal.
• The floppy disk drive or hard drive runs when you are not using it.
• New files keep appearing on the system and you do not know where they come from.
• Strange sound or beeping noises come from the computer or keyboard.
• Strange graphics are displayed on your computer monitor.
• Files have strange names you do not recognize.
• Unable to access the hard drive when booting from the floppy drive.
• Program sizes keep changing.
• Conventional memory is less than it used to be and you cannot explain it.

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Network Protocols

           A network protocol is "an agreement on how to converse". The four most commonly used network protocols are TCP/IP, IPX/SPX, X.25 and SNA. Each protocol performs essentially the same functions, but each protocol is incompatible with other unless there is a special device to translate between them.

TCP/IP Network Addressing

              TCP/IP protocol uses 3 different types of addressing to move data between computers: Physical, Logical and Port addresses. The physical addresses (MAC addresses) are utilized to move data within a single LAN, logical addresses (IP addresses) are used to move data from one LAN to another LAN across the network and the port address is used to transmit data to the intended application.

              MAC addresses burned into hardware. IP address is a 32-bit value. This means that there are fore billion different IP address possibilities. IP addresses may be sent in software and are thus flexible. An IP address contains the address of the device itself as well as the address of the network on which the device is located. Therefore, if a device is moved from one network to a different network, the IP address of the device must be changed. IP addresses are hierarchical addresses like phone numbers and zip codes. They provide a better way to organize computer addresses like social security numbers. 

              The International Network Information Center (InterNIC) assigns to companies blocks of IP addresses based on the size of their networks.

How to Determine Your IP Address?

                                                     Go to the Start menu and select Run... Then type "cmd" in the box and click OK. Then type "ipconfig" in the command prompt and press Enter.

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Backbone Network Components

         A backbone network is a network that connects many networks in a single site. There are many backbone network components.

• Router - A router is a special device that enables the connection of two or more networks so that the computers in different networks can communicate with each other. A router may be a special device, a computer with several network interface cards or a special network module in a computer or other network device. Routers also allow a network to be segmented into smaller logical networks or subnets.

• Bridge - A Bridge is an interface that enables similar networks to communicate.

• Gateway - A Gateway is an interface that enables dissimilar networks to communicate. 

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Data Transmission Types

       In a network data can be transmitted in two different ways, serial transmission and in parallel transmission.

• Serial Data Transmission - Bits are transmitted sequentially, one after the other.

• Parallel Data Transmission - Bits are transmitted through separate channels simultaneously.

     The standard unit of measure used to specify the speed of the data flow through a network is bits per second (bps).

Network Interface Card (NIC)

 A network Interface Card provides the physical connection between the computer and a network cable and enables access to a network. Most modern computers come with NIC cards pre-installed, and ready to be connected to networks.

 Types of Networks

             There are two primary types of networks.

• Local Area Networks (LAN) - LAN is a communication network that serves users within a confined geographical area. Private parties generally own LANs and their effective range is limited.

• Wide Area Network (WAN) - WAN is a communications network that has no geographical limit. A WAN may span hundreds or thousands of miles. In general, a WAN is made up of a number of interconnected LANs. WANs can use either analog (telephone lines) or digital signals or a combination of both. WANs may be privately owned by large organizations or may be public.

• Local Area Networks (LAN) - Local area networks are two types: client server and peer- to-peer. 

            - Client Sever

                                   A Client server LAN consists of devices that provide services called servers, and devices that request services from servers, called clients. Typically servers are the devices that manage shared devices such as printers.

           - Pear-to-Peer Networks 

                                         In a peer-to-peer LAN there is no server, instead all devices communicate directly with each other. Peer-to-Peer networks are also called work groups. In a peer-to-peer network there is no hierarchy among the computers. Typically the number of computers in a peer-to-peer network would be less than 10. All the computers in the network are treated equal and are referred to s peers. Each computer may function as a client and a sever and there is no central administrator to manage the entire network. The users at each computer determine what data on his computer should be shared with the other peers.

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Communication Media

           The communication media is the matter or substance that carries the voice or data. Many different types of transmission media are currently in use. All these different media types can be grouped into two categories: Guided media and Radiated media.

Guided Media

          The guided media are those in which the data flows through physical media.

• Twisted-pair Wire - A twisted-pair wire consists of two insulated copper wires, twisted around each other and covered in another layer of plastic insulation.

• Coaxial Cable - A coaxial cable consists of insulated copper wire wrapped in a metal shield, which is then wrapped in an outer external cover. Often many coaxial cables are  bundled together.

• Fiber-Optical Cable - Consists of hundreds of thin glass wires that transmit pulsation beams of light.

Radiated Media

             And the radiated media are those in which the data is broadcast through the air.

• Radio Transmission - Radio transmission use the same basic principle as standard radio transmission. When using radio transmission each device/computer on the network should equipped with a radio transmitter/receiver on a specific frequency that does not interfere with commercial radio stations. The transmitters are very low power and are designed typically to transmit a signal to a very short distance, typically up to 500 feet.

• Infrared Transmission - Infrared transmission uses low-frequency light wave, below the visible spectrum, to transmit data through the air. Infrared transmitters are seldom uses to transmit data to and from portable or handheld computers.

• Microwave Transmission - Microwave is a high-frequency beam with short wave length. Microwave can be transmitted over a direct line-of sight path between any two points. This transmission medium is typically used for long-distance data/voice transmission.

• Satellite Transmission - In satellite transmission, singles are transmitted to a satellite 500 to 22,000 miles in space. One disadvantage of satellite transmission is the delay that occurs in transmission, which is known as the propagation delay.

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Network Topologies

           Networks can be laid out in a number of different ways. The physical layout of a network is called its topology. The basic network topologies are Star, Ring, Bus. These basic topologies can be combined in a variety of ways to build complex hybrid network topologies.

Star Topology

           In a star network all computers and other communications devices are connected to a central point such as a hub, file sever or a host computer.

Ring Topology

            In a ring network all communications devices are connected in a continuous ring. Messages are passed around the ring until they reach the right destination.

Bus Topology

           In a bus network communications devices are connected to a common channel. There is no central computer and the communications devices transmit message to the other devices.

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Data Communications

           Data Communication is the process of moving information from one point to another by means of some transmission system.

Network

          A network is a collection of entities that exchange information or good. Few examples of networks are railway system, nervous system of animals, telephone system.

Communications Network

          A communications network is a system of interconnected computers, and communication devices that can communicate with one another and share resources. At the most elementary level, a computer network consists  of two computers connected with each other by a cable to allow them to share data. A device connected to a network is called a node. A node may be a device such as a computer, a printer, workstation etc.

Advantages of Networks

         Networks offer several advantages over stand along systems. Some of these advantages listed below:

• Sharing of peripheral devices: Expensive resources can be shared by connecting them to the network.

• Sharing of programs and data: Some programs may be expensive, and it may not be possible to purchase multiple copies. Such programs can be shared a network.

• Better communications: Networks can be used to establish communication such as e-mail.

• Access to databases: Users can access numerous databases by making them available on the network.

Common Network Elements

            The different types of components that you can find in a network are:

• Servers: Computers that provide shared resources to the network users.

• Clients: Computers that access shared network resources provided by the users.

• Communication Media: The media through which data is being transmitted.

• Network Operating System: Software that manages the activities of a network.

• Shared resources: Any service or resource made available for use by the members of the network.

• Modem: Used to connect a computer to another computer over phone lines.

• Hub: A hub can be thought of as a junction box, permitting new computers to be connected to a network as easily as plugging a power cord into an electrical socket. Hubs are commonly available in 4, 8, 16 port sizes, enabling anywhere from 4 to 16 network devices to be plugged into a network. There is no need to connect all the ports of a hub to network devices simultaneously. When no cables are plugged in, the signals bypass the unused ports. Some hubs have an additional interface port that connects to another hub, increasing the size of the network.

• Switch: An enhanced version of the hub. Typically a switch can distribute the communication load so all the computers connected to the switch can communicate effectively.

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Application Software

               Application software is designed to perform useful general-purpose tasks. For example, an application program called Solitaire is a single program that lets you play a card game. Application software can be grouped as customized software or as application packages. Customized software is generally designed for a particular customer whereas application packages are "off the shelf" programmes designed for the general public. Common types of application software are:

• Word Processing
• Spreadsheet
• Database Management System
• Presentation Software
• Desktop Publishing and Graphic Design
• Web Design and Development

Device Drivers

               Virtually every hardware component located inside or connected externally to a computer requires associated software component to enable that hardware to communicate and function with the operating system, other applications and other hardware components of the computer. This software component is referred ti as driver software of that hardware component. Until you install the proper software derive for a hardware component that hardware component remains logically isolated from the rest of the components of the computer.



Booting the Computer

            The process of loading the operating system into the computer's main memory from the hard disk, floppy disk or CD is called booting. A programme known as the boot routine or bootstrap loader is in change of the booting process and this boot routine is stored permanently in the computer's electronic circuitry, typical in ROM. Another series of programmes which start up prior to the booting process are the diagnostic routines. These programs test the main memory, the CPU and other parts of the system to make sure that they are running properly.



System Start up Process

            When you switch on a computer it automatically loads a program called the Basic Input / Output System (BIOS). The BIOS is stored on a special chip on the computer's motherboard. As it runs, it performs a number of essential tests on the computer hardware. These tests are called Power On Self Test (POST).



Software Evaluation

              Software evaluation is the process of checking the software for suitability for the required function. the major steps involved in software evaluation are:

• Identity clearly the problem or application for which the software is required.

• Inspect the manual or instructions supplied with the software.

           - Is the manual clearly written? 

           - Does it explain the main functions of the software? 

           - Does it provide examples?

• Input / Output Interfaces

             - Do the input and output of the software meet your requirements?

• Test the system with your own data sets to see its performance.

• Reliability

              - Is the system consistent? 

              - Does it give consistent answers to similar or the same problems?

• User Friedliness

              - User interface is easy to use. 

              - Error message are clear. 

              - Time periods to wait between successive screens are acceptable.

• Adaptability

              - To what extent the original problem be modified without affecting the functionality of the software.

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