A computer network, or simply a network, is a collection of computers and other

hardware by communication channels that allow sharing of resources and

information.Where at least one process in one device is able to send/receive data to/from

at least one process residing in a remote device, then the two devices are said to be in a

network. A network is a group of devices connected to each other. Networks may be

classified into a wide variety of characteristics, such as the medium used to transport the

data, communications protocol used, scale, topology, benefit, and organizational scope.

Communications protocols define the rules and data formats for exchanging information

in a computer network, and provide the basis for network programming. Well-known

communications protocols include two Ethernet, a hardware and link layer standard that

is ubiquitous in local area networks, and the Internet protocol suite, which defines a set of

protocols for internetworking, i.e. for data communication between multiple networks, as

well as host-to-host data transfer, and application-specific data transmission formats.

Computer networking is sometimes considered a sub-discipline of electrical engineering,

telecommunications, computer science, information technology or computer engineering,

since it relies upon the theoretical and practical application of these disciplines.

History

Before the advent of computer networks that were based upon some type of

telecommunications system, communication between calculation machines and early

computers was performed by human users by carrying instructions between them. Many

of the social behaviors seen in today's Internet were demonstrably present in the 19th

century and arguably in even earlier networks using visual signals.

In September 1940, George Stibitz used a Teletype machine to send instructions

for a problem set from his Model at Dartmouth College to his Complex Calculator

in New York and received results back by the same means. Linking output

systems like teletypewriters to computers was an interest at the Advanced

Research Projects Agency (ARPA) when, in 1962, J.C.R. Licklider was hired and

developed a working group he called the "Intergalactic Computer Network", a

precursor to the ARPANET.

Early networks of communicating computers included the military radar system

Semi-Automatic Ground Environment (SAGE), started in the late 1950s.

The commercial airline reservation system semi-automatic business research

environment (SABRE) went online with two connected mainframes in 1960.

In 1964, researchers at Dartmouth developed the Dartmouth Time Sharing System

for distributed users of large computer systems. The same year, at Massachusetts

Institute of Technology, a research group supported by General Electric and Bell

Labs used a computer to route and manage telephone connections.

Throughout the 1960s Leonard Kleinrock, Paul Baran and Donald Davies

independently conceptualized and developed network systems which used packets

that could be used in a network between computer systems.

1965 Thomas Marill and Lawrence G. Roberts created the first wide area network

(WAN). This was an immediate precursor to the ARPANET, of which Roberts

became program manager.

The first widely used telephone switch that used true computer control was

introduced by Western Electric in 1965.

In 1969 the University of California at Los Angeles, the Stanford Research

Institute, University of California at Santa Barbara, and the University of Utah

were connected as the beginning of the ARPANET network using 50 kbit/s

circuits.[2]

Commercial services using X.25 were deployed in 1972, and later used as an

underlying infrastructure for expanding TCP/IP networks.

Today, computer networks are the core of modern communication. All modern aspects of

the public switched telephone network (PSTN) are computer-controlled, and telephony

increasingly runs over the Internet Protocol, although not necessarily the public Internet.

The scope of communication has increased significantly in the past decade, and this

boom in communications would not have been possible without the progressively

advancing computer network. Computer networks, and the technologies needed to

connect and communicate through and between them, continue to drive computer

hardware, software, and peripherals industries. This expansion is mirrored by growth in

the numbers and types of users of networks, from the researcher to the home user.

Interconnected collection of autonomous computers(unique identity) is known as

computer network.

Types of networking

Local area network

A local area network (LAN) is a network that connects computers and devices in a

limited geographical area such as home, school, computer laboratory, office building, or

closely positioned group of buildings. Each computer or device on the network is a node.

Current wired LANs are most likely to be based on Ethernet technology, although new

standards like ITU-T G.hn also provide a way to create a wired LAN using existing home

wires (coaxial cables, phone lines and power lines).[9]

LAN Advantages:

• Workstations can share peripheral devices like printers. This is cheaper than

buying a printer for every workstations.

• Workstations do not necessarily need their own hard disk or CD-ROM drives

which make them cheaper to buy than stand-alone PCs.

• User can save their work centrally on the network’s file server. This means that

they can retrieve their work from any workstation on the network.

• They don’t need to go back to the same workstation all the time.

• Users can communicate with each other and transfer data between

workstations very easily.

• One copy of each application package such as a word processor, spreadsheet

etc. can be loaded onto the file and shared by all users.

• When a new version comes out, it only has to be loaded onto the server instead

of onto every workstation.

LAN Disadvantages:

• Special security measures are needed to stop users from using programs and

data that they should not have access to;

• Networks are difficult to set up and need to be maintained by skilled

technicians.

• If the file server develops a serious fault, all the users are affected, rather than

just one user in the case of a stand-alone machine.

Typical library network, in a branching tree topology and controlled access to

resources

A sample LAN is depicted in the accompanying diagram. All interconnected devices

must understand the network layer (layer 3), because they are handling multiple subnets

(the different colors). Those inside the library, which have only 10/100 Mbit/s Ethernet

connections to the user device and a Gigabit Ethernet connection to the central router,

could be called "layer 3 switches" because they only have Ethernet interfaces and must

understand IP. It would be more correct to call them access routers, where the router at

the top is a distribution router that connects to the Internet and academic networks'

customer access routers.

The defining characteristics of LANs, in contrast to WANs (Wide Area Networks),

include their higher data transfer rates, smaller geographic range, and no need for leased

telecommunication lines. Current Ethernet or other IEEE 802.3 LAN technologies

operate at data transfer rates up to 10 Gbit/s. IEEE has projects investigating the

standardization of 40 and 100 Gbit/s.[10] LANs can be connected to Wide area network by

using routers.

Networks are often classified by their physical or organizational extent or their purpose.

Usage, trust level, and access rights differ between these types of networks.

Metropolitan area network

A metropolitan area network (MAN) is a computer network that usually spans a city or

a large campus. A MAN usually interconnects a number of local area networks (LANs)

using a high-capacity backbone technology, such as fiber-optical links, and provides up-

link services to wide area networks (or WAN) and the Internet.

The IEEE 802-2002 standard describes a MAN as being:[1]

“ A MAN is optimized for a larger geographical area than a LAN, ranging

from several blocks of buildings to entire cities. MANs can also depend on

communications channels of moderate-to-high data rates. A MAN might be

owned and operated by a single organization, but it usually will be used by

many individuals and organizations. MANs might also be owned and

operated as public utilities. They will often provide means for

internetworking of local networks.

”

Authors Kenneth C. Laudan and Jane P. Laudon (2001) of Management Information

Systems: Managing the Digital Firm 10th ed. define a metropolitan area network as:

{{cquote| A Metropolitan Area Network (MAN) is a large computer network that spans a

metropolitan area or campus. Its geographic scope falls between a WAN and LAN.

MANs provide Internet connectivity for LANs in a metropolitan region, and connect

them to wider area networks like the Internet.

ADVANTAGES AND DISADVANTAGES OF

METROPOLITAN AREA NETWORK

MAN (Metropolitan Area Network) falls in between the LAN and WAN. It therefore increases

the efficiency of handling data while at the same time saves the cost attached to establish a

wide area network. MAN (Metropolitan Area Network) offers centralized management of data.

It enables you to connect many fast LANs together. Telephone companies worldwide have

facilitated the transfer of data with the help of an underground optical fiber network. These

optical fibers increase the efficiency and speed of data transfer. The optical fibers enable you to

access a speed of almost 1000mbps. If you develop a WAN of 1.45 mbps its cost is more than

what it gives you. Whereas when you establish metropolitan area network it offers you the

speed of 1000mbps as a whole with the lowest cost involved.

Wide area network

A Wide Area Network (WAN) is a network that covers a broad area (i.e., any

telecommunications network that links across metropolitan, regional, or national

boundaries) using private or public network transports. Business and government entities

utilize WANs to relay data among employees, clients, buyers, and suppliers from various

geographical locations. In essence, this mode of telecommunication allows a business to

effectively carry out its daily function regardless of location. The Internet can be

considered a WAN as well, and is used by businesses, governments, organizations, and

individuals for almost any purpose imaginable.[1]

Related terms for other types of networks are personal area networks (PANs), local area

networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs)

which are usually limited to a room, building, campus or specific metropolitan area (e.g.,

a city) respectively.

Advantages of a WAN:

Covers a large geographical area so long distance businesses can connect on the one

network

Shares software and resources with connecting workstations

Disadvantages of a WAN:

Are expensive and generally slow

Need a good firewall to restrict outsiders from entering and disrupting the network

Microsoft Excel And describe its function.

Microsoft Excel is a spreadsheet application developed by Microsoft for Microsoft

Windows and Mac OS X. It features calculation, graphing tools, pivot tables, and a macro

programming language called Visual Basic for Applications. It has been a very widely

applied spreadsheet for these platforms, especially since version 5 in 1993, and it has

replaced Lotus 1-2-3 as the industry standard for spreadsheets. Excel forms part of

Microsoft Office. The current versions are 2010 for Microsoft Windows[1] and 2011 for

Mac OS X.[2]

Basic operation

Main article: Spreadsheet

Microsoft Excel has the basic features of all spreadsheets,[3] using a grid of cells arranged

in numbered rows and letter-named columns to organize data manipulations like

arithmetic operations. It has a battery of supplied functions to answer statistical,

engineering and financial needs. In addition, it can display data as line graphs, histograms

and charts, and with a very limited three-dimensional graphical display. It allows

sectioning of data to view its dependencies on various factors for different perspectives

(using pivot tables and the scenario manager).[4] It has a programming aspect, Visual

Basic for Applications, allowing the user to employ a wide variety of numerical methods,

for example, for solving differential equations of mathematical physics,[5][6] and then

reporting the results back to the spreadsheet. It also has a variety of interactive features

allowing user interfaces that can completely hide the spreadsheet from the user, so the

spreadsheet presents itself as a so-called application, or decision support system (DSS),

via a custom-designed user interface, for example, a stock analyzer,[7] or in general, as a

design tool that asks the user questions and provides answers and reports.[8][9][10] In a more

elaborate realization, an Excel application can automatically poll external databases and

measuring instruments using an update schedule,[11] analyze the results, make a Word

report or Power Point slide show, and e-mail these presentations on a regular basis to a

list of participants.

Use of a user-defined function sq(x) in Microsoft Excel. The named variables x & y are identified

in the Name Manager. The function sq is introduced using the Visual Basic editor supplied with

Excel.

Subroutine in Excel calculates the square of named column variable x read from the

spreadsheet, and writes it into the named column variable y.

Graph made using Microsoft Excel

Microsoft allows for a number of optional command-line switches to control the manner

in which Excel starts.[12]

Charts

Excel supports charts, graphs, or histograms generated from specified groups of cells.

The generated graphic component can either be embedded within the current sheet, or

added as a separate object.

These displays are dynamically updated if the content of cells change. For example,

suppose that the important design requirements are displayed visually; then, in response

to a user's change in trial values for parameters, the curves describing the design change

shape, and their points of intersection shift, assisting the selection of the best design.

Microsoft Excel functions

Listing of the most useful Microsoft Excel functions. Explanations on how to write each

function in Excel and examples of its use are provided.

1- LEFT ()- it returns the specified no. of characters from the left side of a text string, if the number is not specified ,it returns the first characters syntax=left(text, number of character)eg. left (chandani,7) will return chandani.

2- RIGHT()-it return the specified numbr of characters from the right side of a text string. If the number is not specified . if returns the last character. Syntax-right(string,number of character)eg.right(chandrawati,4) will return wati.

3- MID()-returns specified number of character sorting from a specified position in a text string. Syntax-MID(string,start number,number of character.)eg.MID(chandrawati 5,3) will return dra.

4- LEN()-it returns the length of a text string , spaces are counted as character. Syntax=Len (string) eg.LEN(chandrawati college) will return.

5- Proper()-it converted th first letter of each word in a text string to uppercase and remaining letters to lowercase. Syntax=proper(string).eg=proper(chandrwati college) will return chandrwati college.

6- UPPER()-it converts all lowercase letters in a text string to uppercase. Syntax=UPPER(chandrawati)will return eg. Chandrawati

7- LOWER()-it converts all uppercase letter in a text string to lowercase. Syntax=lower(string) eg. LOWER(chandrawati) will return chandrawati.

8- TRIM()-it removes all leading or trailing blanks from a string. Syntax=TRI(string) eg.TRIM(chandrawati college) will return chandrawati college.

9- EXACT(text1,text2)-it will return or false depending on whether text1 is exactly same as text2 or not syntax=EXACT(text1,text2)=exact (Chandra,chandu) will return false.

10- CONCATENATE()-this function joins several text string into on text string syntax=CONCATENATE text1,text2) we can give 2 to 30 items to be joined into a single text string.eg.=CONCATENATE(RAM,SHYAM) will return Ramshyam.