Short Notes of GIT

7/30/2019 Short Notes of GIT

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A floppy disk or a diskette is a round flat removable piece of plastic plate coated with a Ferric Oxide layer, and encased ina protective plastic cover, called the disk jacket. The FerricOxide layer is capable of holding a magnetic field.

capacities range from a few hundred megabytes to many gigabytes. The speed of a hard drive

is measured in terms of average access time, the speed at which the hard drive finds data.Theaverage access time is measured in milliseconds (1/1000 of a second).

Magnetic TapeMagnetic tapes are similar to audiotapes and store data in a serial manner (i.e. data isstoredone after the other on the tape).

Digital Versatile Discs (DVD)

Data HierarchyData in storage can be organized into a hierarchy of categories. Character : a single letter, number or a special character. Field : Typically, a sequence of characters. Record : Collection of related fields File : Collection of related records Databases : Collection of interrelated files.

light-emitting diode (LED) digital signal processing (DSP) Optical character recognition (OCR) visual display unit or VDU. The quality of a computer monitor is based on the following properties:

Resolution: Resolution is the number of pixels, which the screen can display. Apixel is the picture element and refers to the smallest area of the screen that thecomputer can change. The number of colours it can display (dependent on the computer as well) Radiation output.

CRT monitor A cathode-ray tube

Larger monitorswith high resolution are used for specialised applicationsuch as desktop publishing and CAD.

Main differences of LCDs as compared with CRT monitors are :

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Take s very little space Small energy consumption Sharp pictures No flicker or geometric distortion Fairly expensive

Limited viewing angle Slower response time

. Some examples of utility software are Norton Utilities and DownloadAccelerator.

POST= Power on self test BIOS= Basic input output system

Software Evaluation

Software evaluation is the process of checking the software for suitability for the requiredfunction. The major steps involved in software evaluation are: Identify clearly the problem or application for which the software is required. Inspect the manual or instructions supplied with the software.o Is the manual clearly written?o Does it explain the main functions of the software?o Does it provide examples? Input/Output Interfaceso Do the input and output of the software meet your requirements? Test the system with your own data sets to see its performance. Reliabilityo Is the system consistent? - Does it give consistent answers to similar or thesame problems? User Friendlinesso User interface is easy to use.o Error messages are clear.o Time periods to wait between successive screens are acceptable. Adaptability: To what extent the original problem be has to be modified withoutaffecting the functionality of the software.

A device connected to a network is called a node .

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

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Sharing of peripheral devices: Expensive resources can be shared by connecting themto 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 over a network. Better communications: Networks can be used to establish communication such as

email. Access to databases: Users can access numerous databases by making them availableon the network.

Common Network ElementsThe 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:

Switch: An enhanced version of the hub. Typically a switch can distribute thecommunication load so all the computers connected to the switch can communicateeffectively.

The physical layout of a network is

called its Topology. The basic network topologies are star, ring, and bus.

Star TopologyIn a star network all computers and other communications devices are connected to acentral

point such as a hub, file server or a host computer.

Ring TopologyIn 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 TopologyIn a bus network communications devices are connected to a common channel. There isnocentral computer and the communications devices transmit messages to the other devices.

All these different media types can

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be grouped into two categories: guided media and radiated media. The Guided media arethosein which the data flows through physical media and the radiated media are those in whichthedata is broadcast through the air.

Communication Media Guided Media

Twisted-pair wireA twisted-pair wire consists of two insulated copper wires, twisted around each other andcovered in another layer of plastic insulation Coaxial cableA Coaxial cable consists of insulated copper wire wrapped in a metal shield, which isthenwrapped 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

beamsof light.

Radiated Media Radio Transmission: Radio transmission use the same basic principle as standard radioTransmission Infrared Transmission: Infrared transmission uses low-frequency light wave, below thevisible spectrum, to transmit data through the air. Infrared transmitters are seldom usedto transmit data to and from portable or handheld computers. Microwave Transmission: Microwave is a high-frequency beam with short wavelength. Microwave can be transmitted over a direct line-of-sight path between any two

points. Satellite Transmission: In satellite transmission, singles are transmitted to a satellite500 to 22,000 miles is space.

One disadvantage of satellite transmission is the delaythat occurs in transmission, which is known as the propagation delay.

In a network data can be transmitted in two different ways, serial transmissionand in

parallel transmission .Serial Data Transmission: Bits are transmitted sequentially, one after theother.Parallel Data Transmission: Bits are transmitted through separate channelssimultaneously.

The standard unit of measure used to specify the speed of the data flow through anetwork is

bits per second (bps).

Network Interface Card (NIC)

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Types of NetworksThere are two primary types of networks.Local Area Networks (LAN)

Wide Area Networks (WAN): WAN is a communications network that has no

geographicallimit. A WAN may span hundreds or thousands of miles. In general, a WAN is made upof anumber of interconnected LANs. WANs can use either analog (telephone lines) or digitalsignals or a combination of both.

Local area networks are of two types: client server and peer-to-peer. A Clientserver LAN

consists of devices that provide services called servers, and devices that request servicesfromservers, called clients. Typically servers are the devices that manage shared devices such

as printers. In a peer-to-peer LAN there is no server, instead all devices communicatedirectlywith each other.

Peer-to-peer networks are also called workgroups . 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.

A backbone network is a network that connects many networks in a single site. Router: A router is a special device that enables the connection of two or morenetworks so that the computers in different networks can communicate with eachother. Bridge: A Bridge is an interface that enables similar networks to communicate. Gateway: A Gateway is an interface that enables dissimilar networks to communicate.

A network protocol is an agreement on how to converse. The four mostcommonly used

network protocols are TCP/IP, IPX/SPX, X.25 and SNA. Each protocol performsessentially

the same functions, but each protocol is incompatible with others unless there is a specialdevice to translate between them.

TCP/IP protocol uses 3 different types of addressing to move data betweencomputers:

physical, logical and port addresses. The physical addresses (MAC addresses) are utilizedto

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move data within a single LAN, logical addresses (IP addresses) are used to move datafromone LAN to another LAN across the network and the port address is used to transmit datatothe intended application.

The International network information centre (InterNIC) assigns to companies blocks of IP

addresses based on the size of their networks.

Computer VirusA computer virus is a program or piece of code designed to hide in the background and to

perform one or more of the following activities Replicate itself Attach to other programs or instructions Perform unintended actions

A program needs not to perform outright damage in order to be called a Virus.Currently there are more than14,000 known viruses.

The first common computer virus, called Brain virus, was written in 1986 bytwo brothers in

Pakistan. The majority of viruses are written by hackers. Some of the known viruses andtheir effects are given below. SCOLD.A : Send simple e-mails. CAP : One effect of CAP is that all documents are saved in the Word DOC format,regardless of the format you choose. For example, if a document is saved as an RTFfile, the extension of the document will become RTF but internally the file is still aDOC. Nimda : Nimda is a complex virus with a mass mailing worm component whichspreads itself in attachments named README.EXE. Blaster : Every 17th day of each month, the virus checks for existence of "c:\minny.log" file. If the file does not exist, the virus appends several commands tothe end of the "c:\autoexec.bat". These commands attempt to remove everything from"C:", "D:", "E:" and "F:" drives when the system is restarted. Jerusalem : This is one of the oldest and most common viruses around. It will infect

both .EXE and .COM files. Jerusalem activates on every Friday the 13th, deleting programs run on that day. 30 minutes after an infected program is run, the virus willalso cause a general slowdown of the computer and make a part of the screen scroll uptwo lines. Arab : The virus counts programs being executed on an infected system. When thecounter exceeds 256 the virus makes the hard disk unbootable Bin Laden : This virus does not have a destructive effect. After starting, it sometimesdisplays a message.

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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 enemys computer.

Virus SymptomsSome of the symptoms that a computer will show up when infected with a virus are listed

below. Programs take long time to load than normal. Computer's hard drive constantly runs out of free space. The floppy disk drive or hard drive runs when you are not using it. New files keep appearing on the system and you don not know where they came from. Strange sounds 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. Programs act erratically."

A virus comprises of four general components The replicator (or engine): replicates the virus. The safeguard : prevents the detection and the removal of the virus The trigger monitor : Monitors for a specific condition to deliver payload. Thesespecific conditions may be the arrival of a particular date, time, keyboard stroke, etc. The payload : the action it does apart from propagation. This action may be anamusing action or malicious action resulting in distraction from the current work or destruction of data.

Main Phases of Virus ActivityThe main phases a virus generally follows are; Gains access to the system Lies dormant Propagates Virus is triggered by some event Virus Action

There are three main types of viruses

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Boot Virus File Virus (Parasitic Virus)

Macro Virus Macro Virus

A macro is a collection of application specific instructions coded to automate somemanual

process such as formatting a word document. Macro Viruses are either complete macrosor macro segments embedded as parts of legitimate macros. They may come straight fromthesoftware manufacturers original disks. Once a computer is infected with a macro virusthefollowing actions may result. Unusual messages Unknown macros listed in macros list Save data in unintended formats Lost data

How to Protect a Computer from Computer VirusesThere are many ways of preventing a computer from virus attack. Some of these

precautionsare listed below. Verbal Warning Use of anti-virus software

Avoiding the use of unchecked disks Avoiding the use of unauthorized or unchecked software Write protecting all standard software Use only CD-ROM versions of software whenever possible Using firewalls

Anti-virus software contains anti-virus scanners, which look for viruses. Theseanti-virus

scanners work by searching for fingerprints: short patterns, usually 16 bytes, recognizedtoexist in virus programs. There are two types of virus scanners; On-access scanners: automatically check the hard disk and diskette each time thesystem starts up. On-demand scanners: Executes only on users demand and can be scheduled to run atvarious times

Trojan HorseA program that performs unsolicited actions. However, they do not replicate and neither do

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they attach to other programs.

Internet The Internet grew out of a United States Department of Defense project, the

ARPANET (Advanced Research Project Agency Network) started in late 1960s. Its main

objective was to provide network links between universities, research organizations andremote computer centers.

anongovernmental internationalvolunteer organization called theInternet Society (ISOC) controlsthe standards and the future of theInternet.

On the Internet information is available mainly through three mediums: the World

Wide Web(WWW), Usenet News (News) and Electronic Mail (e-mail).

Types of resource available on the Internet .Different types of resources are available on the Internet. Some of the common types of information available on the Internet are: Different types of information such as text, sounds, pictures, and short movie clips. Information at all levels -- storybooks and games for children, university coursematerials, business information, fine art, and lots of things that are just plain fun. Actual business information -- including electronic shopping malls. Games that you can play alone or with other people all over the world. Programs to

help you find the information that you want.

What is an URL?To understand how a singe page is kept distinct from millions of other pages available ontheweb, you should learn to recognize its URL (short for Uniform Resource Locator ).Every

page on the Web can by uniquely identified by using its URL. An URL is simply acollectionof text segments that identifies an item in a computer network. You can retrieve a page

bytyping its URL in the Location field of the browser and pressing Enter (or by using OpenPage... in the File menu).Here is an example of a typical URL:Computer FundamentalsFaculty of Information Technology-University of Moratuwahttp://www.ucsc.cmb.ac.lk/centre/csc/index.htm

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http://www.ucsc.cmb.ac.lk/centre/csc/index.htmhttp://www.ucsc.cmb.ac.lk/centre/csc/index.htm


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Within the URL text are components that specify the protocol (the format that Netscapecanexpect the information to be in), the server (the computer that holds the item), and the

pathname of the item.

Component MeaningThe protocole.g. http:A colon follows the protocol. Many Internet pages use http(short for HyperText Transfer Protocol). You might also comeacross ftp (another way used to transfer files), news (the

protocol used by the Usenet news groups), and Gopher (a protocol that was briefly popular, but then supplanted by Netscape-type products).The server e.g.

//www.ucsc.cmb.ac.lk The server identifies the computer system that stores theinformation you see. Each server on the Internet has a uniqueaddress. You can often tell what country the server is in by itsaddress. The server name is preceded by two slashes.The path namee.g./centre/csc/index.htmThe path name identifies the location of an item on the server.Each segment of a path name is preceded by a single slash.This path name usually tells you the name of the filecontaining the page, as well as the names of the folders andsub-folders that store the file. For example, in this URL theindex.html page is stored in the csc folder that is stored inthe centre folder (Refer to Page number49)

If you try to go to a page that is not there, or you have typed in an incorrect URLfor a page,

then you will receive a message saying that the file was missing. This is called "Error404

File not found"

If you have typed the name of the server incorrect, or the server machine is not currentlyworking then you will see a different message, possibly like "Error 204 DNS NameLookupFailure."

There are two standard picture formats used in the Web, JPG and GIF. JPG, or jpeg, files are

normally used for pictures and GIF files are normally used for graphics and drawings.

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The Internet also contains brief videos that you can see, stored as QuickTime (Macintosh) or AVI (IBM compatible)formatmovies. They have to be brief, because each movie takes up large amount of storage

space. Browsers are often referred to as "client software." Client

Any computer linked to Internet and capable of receiving information from a server.

DomainThe top-level identifier for a particular Internet server (e.g., yahoo.com). Domains are of various types and they are categorized by one or more extensions. Users often can learnmuchabout an Internet site by studying its domain name.(Plese read page number 51 of GIT

book) FTP

The abbreviation for File Transfer Protocol, a method of transferring files from onecomputer to another via the Internet. Some Internet sites specialize in just this kind of transfer.Users can sign on to this type of server and copy files to their own computers. FTPserversusually require user Ids and passwords, or may allow you to log on via "anonymous ftp."

HyperText Markup Language. The computer "language" of the World Wide

Web,comprised of the coding, or tags, that indicate how text and images are to be displayed, aswellas how documents and files are to be linked.HTML was developed at the European Particle Physics Laboratory in Switzerland,known byits French acronym CERN.

HTTP HyperText Transfer Protocol. The basic computer rules and language that allow for thelinking and transferring of Web pages and graphics.

ModemA modem converts digital signals from your computer into audible tones that can betransmitted over regular analog phone lines. This conversion process is calledMo dulation.The modem on the other end then transfers those same audible tones back (or Dem odulates)into its original digital signals.

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A proxy server is a computer that provides access to the Internet. proxy server may alsohave a firewall

An automated Web Site that uses advanced searching software to locate other

Web Sites andInternet files based on search criteria entered by the user is called a Search Engine.Engine. Examplesof such sites include Alta Vista, HotBot, Infoseek, and Lycos,

Server A computer directly linked tothe Internet that holds filesready for downloading by a"client" machine, such as ahome PC is called a Server.

Downloaded files are either viewed or copied by the clientusing "client software," such asa Web browser.

URLUniform Resource Locator. The standard "address" format used by a server and thespecificfiles that make up the document or Web Site. Web Site URLs always start "http://".

World Wide Web

That part of the Internet operating under the HTTP protocol and using the HTMLlanguage. Itis usually referred to simply as "the Web." The Web is differentiated from Gopher andother text-only protocols by its "Graphic User Interface" (GUI) that allows the joining of text andimages on individual Web pages. However, like the earlier Gopher Protocol, it is amenudrivensystem for retrieving and accessing information.

E-Mail The standard protocol used for sending Internet e-mail is called SMTP, short for

Simple Mail Transfer Protocol. It works in conjunction with POP servers. POP stands for PostOffice Protocol. When you send an e-mail message, your computer routes it to an SMTPserver. The server looks at the e-mail address (similar to the address on an envelope),thenforwards it to the recipient's mail server, where it is stored until the addressee retrieves it.You

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can send e-mail anywhere in the world to anyone who has an e-mail address. Remember,almost all Internet service providers and all major online services offer at least one e-mailaddress with every account.

The top-level domain=TLD (Refer to E-Mail)

Data StructuresBits, Bytes, Nibbles and Word Length

NibbleA nibble is a group of FOUR bits. This gives a maximum number of 16 possibledifferent values . 2 4 = 16 (2 to the power of the number of bits)

EBCDIC (Extended Binary Coded Decimal Interchange Code)

Please Read GIT BOOK Page number 73 for Boolean Algebra and LogicGates.

Automatic Teller Machine(ATM)

Magnetic Resonance Imaging (MRI)

A computer will not be useful without making a computer system. Acomputer system is a combination of six elements:1. Hardware basic electronic and electro -mechanical parts in a computer2. Software instruction to hardware3. Data and Info rmation data as input and information as processed data

4. Procedures or instructions guidelines to use software and hardware5. People Those Operating the computer system6. Communication connection between computers to share data and information

Generally computerhardware can be divided into five categories: (1) input devices, (2) processors, (3)storage devices, (4) output devices, and (5) communications.

Please Read Chapter 1 Page number 25

The CPU consistsof three main parts:1. Control Unit2. Arithmetic/logic unit (ALU).3. Registers

Control Unit and ALU are connected by a kind of electronic roadway called abus

Registers:Registers store very small amounts of data and instructions for short periods of time.

They hold various types of information such as data, instructions, memory addressesand the intermediate results of calcula tions. Essentially, they hold the data and

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very brief periods of time: Data to be processed by the CPU Instructions for the CPU as to how to process the data Operating System programs that include instructions for computer howto function (details can be found in Section 1.3)

There are four main types of primary storage:RegistersRandom Access Memory (RAM)Read-Only Memory (ROM).Cache Memory

A computer-based information system involves six interdependent components:hardware (machines), software (instructions or programs), people (programmers,managers, or users), procedures (rules), data and information.

There are number of types of information systems corresponding to variouslevels of management. The three traditional levels of management are1) Lower or Operational Management2) Middle or Tactical Management3) Upper or Top or Strategic Management

Management uses several basic types of information systems:

1) Transaction Processing Systems (TPS) = Data Processing Systems (DPS).2) Management Information Systems (MIS)3) Decision Support Systems (DSS)

4) Executive Information Systems =(EIS) Executive Support System (ESS).5) Expert Systems (ES)

Systems Development Life Cycle (SDLC) SDLC is also known as Classic LifeCycle Model , Linear Sequential Model or Water Fall Model .

Following are the different phases of the SDLC:1. Problem Definition2. Feasibility Study

3. System Analysis4. System Design5. Software Development6. Testing7. Implementation8. Maintenance

During design, the analyst:

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Short Notes of GIT

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