COMMUNICATION AND COMPUTER NETWORKINTRODUCTION:Today computer is available in many offices and homes and therefore there is a need to share data and programs among various computers with the advancement of data communication facilities. The communication between computers has increased and it thus it has extended the power of computer beyond the computer room. Now a user sitting at one place can communicate computers of any remote sites through communication channel. The aim of this chapter is to introduce you the various aspects of computer network.DATA COMMUNICATION:We all are acquainted with some sorts of communication in our day to day life. For communication of information and messages we use telephone and postal communication systems. Similarly data and information from one computer system can be transmitted to other systems across geographical areas. Thus data transmission is the movement of information using some standard methods. These methods include electrical signals carried along a conductor, optical signals along an optical fibers and electromagnetic areas. Suppose a manager has to write several letters to various clients. First he has to use his PC and Word Processing package to prepare his letter. If the PC is connected to the entire client's PCs through networking, he can send the letters to all the clients within minutes. Thus irrespective of geographical areas, if PCs are connected through communication channel, the data and information, computer files and any other program can be transmitted to other computer systems within seconds. The modern form of communication like e-mail and Internet is possible only because of computer networking.BASIC ELEMENTS OF A COMMUNICATION SYSTEMThe following are the basic requirements for working of a communication system.1. A sender (source) which creates the message to be transmitted.2. A medium that carries the message.3. A receiver (sink) which receives the message.In data communication four basic terms are frequently used. They are:

1. Data: A collection of facts in raw forms that become information after processing.

2. Signals: Electric or electromagnetic encoding of data.3. Signaling: Propagation of signals across a communication medium.4. Transmission: Communication of data achieved by the processing of

signals.COMMUNICATION PROTOCOLS:You may be wondering how do the computers send and receive data across communication links. The answer is data communication software. It is this software that enables us to communicate with other systems. The data communication software instructs computer systems

and devices as to how exactly data is to be transferred from one place to another. The procedure of data transformation in the form of software is commonly called protocol. The data transmission software or protocols perform the following functions for the efficient and error free transmission of data.1.Data sequencing: A long message to be transmitted is broken into smaller packets of fixed size for error free data transmission.2.Data Routing: It is the process of finding the most efficient route between source and destination before sending the data.3. Flow control: All machines are not equally efficient in terms of speed. Hence the flow control regulates the process of sending data between fast sender and slow receiver.4. Error Control: Error detecting and recovering is the one of the main function of communication software. It ensures that data are transmitted without any error.DATA TRANSMISSION MODES:There are three ways for transmitting data from one point to another1.Simplex: In simplex mode the communication can take place in one direction. The receiver receives the signal from the transmitting device. In this mode the flow of information is Uni.-directional. Hence it is rarely used for data communication.2.Half-duplex: In half-duplex mode the communication channel is used in both directions, but only in one direction at a time. Thus a half-duplex line can alternately send and receive data.

3. Full-duplex: In full duplex the communication channel is used in both directions at the same time. Use of full-duplex line improves the efficiency as the line turn-around time required in half-duplex arrangement is eliminated. Example of this mode of transmission is the telephone line.COMPUTER NETWORK:A computer network is an interconnection of various computer systems located at different places. In computer network two or more computers are linked together with a medium and data communication devices for the purpose of communicating data and sharing resources. The computer that provides resources to other computers on a network is known as server. In the network the individual computers, which access shared network resources, are known as workstations or nodes.A network can consist of two computer connected together on a desk or it can consist of many Local Area Network (LANs) connected together to form a Wide Area Network (WAN) across a continent. In simple terms it is an interconnected set of some objects. For decades, we have been familiar with the Television, Railways, Banks and various other types of networks. In recent years, the computer network, a new form of network is becoming more and more visible in our day-to-day life. A computer network is an interconnected set of autonomous computers. Autonomous means each of them can function independent of others, i.e., each computer has individual processors. Simple we can say each computer (terminal, node) should not be a dumb terminal. The key is that two or more computers are connected together by a medium and are sharing resources. These resources can be files, printers, hard disk drives, CPU or data. By using a computer network, people can send and receive back information more quickly.NETWORK:A network is data communication systems that links terminals, microcomputers, minis or mainframes so that these can operate independently but also share data and other resources. In other words, a network is a way or means of transmitting or exchanging information from one or more source. Simply, network means connecting two or more computers together for sharing information and resources.TYPES OF NETWORKS:The entire computer network can be classified into following categories

Local Area Network. (LAN) Metropolitan Area Network. (MAN) Wide Area Network. (WAN)

LOCAL AREA NETWORKNetworks used to interconnect computers in a single room, rooms within a building or buildings on one site are called Local Area Network (LAN). LAN transmits data with a speed of several megabits per second (106 bits per second). The transmission medium is normally coaxial cables.

LAN links computers, i.e., software and hardware, in the same area for the purpose of sharing information. Usually LAN links computers within a limited geographical area because they must be connected by a cable, which is quite expensive. People working in LAN get more capabilities in data processing, work processing and other information exchange compared to stand-alone computers. Because of this information exchange most of the business and government organizations are using LAN.MAJOR CHARACTERISTICS OF LAN

every computer has the potential to communicate with any other computers of the network

high degree of interconnection between computers easy physical connection of computers in a network inexpensive medium of data transmission high data transmission rate

ADVANTAGES The reliability of network is high because the failure of one computer

in the network does not affect the functioning for other computers. Addition of new computer to network is easy.

Application of ComputerCOMPUTER FUNDAMENTALS

COMMUNICATION AND COMPUTER NETWORK High rate of data transmission is possible. Peripheral devices like magnetic disk and printer can be shared by

other computers.DISADVANTAGES: If the communication line fails, the entire network system breaks down. USE OF LAN: Followings are the major areas where LAN is normally used

File transfers and Access Word and text processing Electronic message handling Remote database access Personal computing Digital voice transmission and storage

METROPOLITAN AREA NETWORK (MAN) -Interconnecting LAN’s in different cities, using various public or private facilities such as telephone system, Leased lines or satellite links forms a Metropolitan Area Network (MAN). The communication facilities are not owned by the organizations and are leased from the communication companies.WIDE AREA NETWORK -The term Wide Area Network (WAN) is used to describe a computer network spanning a regional, national or global area. For example, for a large company the headquarters might be at Delhi and regional branches at Bombay, Madras, Bangalore and Calcutta. Here regional centers are connected to headquarters through WAN. The distance between computers connected to WAN is larger. Therefore the transmission mediums used are normally telephone lines, microwaves and satellite links.CHARACTERISTICS OF WAN:Followings are the major characteristics of WAN.

1. Communication Facility:For a big company spanning over different parts of the country the employees can save long distance phone calls and it overcomes the time lag in overseas communications. Computer conferencing is another use of WAN where users communicate with each other through their computer system.

2. Remote Data Entry:Remote data entry is possible in WAN. It means sitting at any location you can enter data, update data and query other information of any computer attached to the WAN but located in other cities. For example, suppose you are sitting at Madras and

want to see some data of a computer located at Delhi, you can do it through WAN.

3. Centralized Information:In modern computerized environment you will find that big organizations go for centralized data storage. This means if the organization is spread over many cities, they keep their important business data in a single place. As the data are generated at different sites, WAN permits collection of this data from different sites and save at a single site.

Examples of WAN:1. Ethernet: Ethernet developed by Xerox Corporation is a famous

example of WAN. This network uses coaxial cables for data transmission. Special integrated circuit chips called controllers are used to connect equipment to the cable.

2. Arpanet: The Arpanet is another example of WAN. It was developed at Advanced Research Projects Agency of U. S. Department. This Network connects more than 40 universities and institutions throughout USA and Europe.

DIFFERENCE BETWEEN LAN AND WAN LAN is restricted to limited geographical area of few kilometers. But

WAN covers great distance and operate nationwide or even worldwide.

In LAN, the computer terminals and peripheral devices are connected with wires and coaxial cables. In WAN there is no physical connection. Communication is done through telephone lines and satellite links.3

Application of ComputerCOMPUTER FUNDAMENTALS

COMMUNICATION AND COMPUTER NETWORK Cost of data transmission in LAN is less because the transmission

medium is owned by a single organization. In case of WAN the cost of data transmission is very high because the transmission mediums used are hired either telephone lines or satellite links.

The speed of data transmission is much higher in LAN than in WAN. The transmission speed in LAN varies from 0.1 to 100 megabits per second. In case of WAN the speed ranges from 1800 to 9600 bits per second (bps).

Few data transmission errors occur in LAN compared to WAN. It is because in LAN the distance covered is negligible.

NETWORK TOPOLOGYThe term topology in the context of communication network refers to the way the computers or workstations in the network are linked together. According to the physical arrangements of workstations and nature of work, there are three major types of network topology. They are star topology, bus topology and ring topology.STAR TOPOLOGY:In star topology a number of workstations (or nodes) are directly linked to a central node (see, Fig. 4.3). Any communication between stations on a star LAN must pass through the central node. There is bi-directional communication between various nodes. The central node controls all the activities of the nodes. The advantages of the star topology are:

It offers flexibility of adding or deleting of workstations from the network.

Breakdown of one station does not affect any other device on the network.

The major disadvantage of star topology is that failure of the central node disables communication throughout the whole network.

Bus Topology:In bus topology all workstations are connected to a single communication line called bus. In this type of network topology there is no central node as in star topology. Transmission from any station travels the length of the bus in both directions and can be received by all workstations. The advantage of the bus topology is that

It is quite easy to set up. If one station of the topology fails it does not affect the entire

system.The disadvantage of bus topology is that any break in the bus is difficult to identify.

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Application of ComputerCOMPUTER FUNDAMENTALS

COMMUNICATION AND COMPUTER NETWORK

Ring Topology:In ring topology each station is attached nearby stations on a point to point basis so that the entire system is in the form of a ring. In this topology data is transmitted in one direction only. Thus the data packets circulate along the ring in either clockwise or anti-clockwise direction. The advantage of this topology is that any signal transmitted on the network passes through all the LAN stations. The disadvantage of ring network is that the breakdown of any one station on the ring can disable the entire system.

Network Application -A network can convert a group of isolated users into a coordinated multi-user system. Anyone one who is having access to any network can easily and legally access or share any resources like data, peripherals or

software's with other user. Networks thus, can help any organization run more effectively, efficiently and economically.The applications of networks are:-

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Application of ComputerCOMPUTER FUNDAMENTALS

COMMUNICATION AND COMPUTER NETWORK1. Distributed Computer system –

The most important advantage of network is that all computers are not necessarily placed at one particular location but can be located at different sites.

2. Remote Data Entry –

Data entry can be done from any one of the computers connected to the network.

3. Remote Data Inquiry –

From any one of the computers, the date can be accessed and viewed.

4. Remote Job Entry –

The programs can be written or run from any computer connected to the network.

5. Database Sharing –

Many users can access simultaneously a file without corrupting the data.

6. Network Resources Sharing –

Network resources like printers, plotters, storage spaces etc. can be shared by the computers hooked to the network.

7. Electronic- Mail (E-Mail) –

E-Mail system basically used for sending message electronically to individuals or group of individuals in an inter and intra office environment. Other application based on it, include multimedia mail, database access, document sharing, fax routing etc.

8. Subscriber Services –

Many Subscribers can access a particular type of database provided by some agency on the payment of some fees.

9. Banking Services –

Different branches of any bank located at different places, connected to each other through network provide instant transaction from any one of these branches.

10. Telecommunicating –

In the latest trend of running business, workers at home are linked to their company’s computer and they are performing their jobs from their homes only.

11. Advertising –

Advertising of your products and services can be done through one of the popular network available.

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Application of ComputerCOMPUTER FUNDAMENTALS

COMMUNICATION AND COMPUTER NETWORKCOMPONENTS OF A NETWORK:A network is made up of hardware as well as software components specialized in nature to perform several functions of network.HARDWARE - The basic hardware components of network are-Server Workstations NICs Cabling Shared resources and peripheralsSERVER: -A server runs the network operating system and offers network services to users at their individual workstations. It offers services such as, file storage, security, resource management, user management etc. The most common type of server is the File Server. The file server does not need any special hardware. It is just a computer with one or more hard disks. Since the server's hard disk is shared, it is typically of a much larger capacity. Users access files stored on the file server as if they were using their own hard disks. This allows for sharing of programs as well as data. If sharing is not desired, private disk areas can be set up for individual users. File server perform another important function on the network. Printers connected to the server can be accessed directly by all the users of the network. When the program running on a client CPU generates a print command, the file to be printed is transparently diverted to the server and sent to the shared printer. If the printer is busy with some other printing job, each user's file is converted to a separate file and put in a print queue. File server can be non-dedicated and dedicated.

Non-Dedicated server: -A non-dedicated server is a server that plays a dual role, server as well as workstation. A user can sit on a non-dedicated server and work as if he/she is working on workstations. When workstation request for services a non-dedicated server services their requests, performing the duties of a server. A non-dedicated server when used in a client-server environment needs a very high capacity hard disk, and an extremely high performance CPU. This is essential because when a user (usually the administrator) works on the non-dedicated server he uses its resources such as CPU, hard disk etc. At the same time, the server has to tackle the client requests, which largely increases the load on the server. Microsoft's Windows for workgroup, Windows 95, Windows NT, and all forms of UNIX operating system feature non-dedicated server.

Dedicated Server: -

A dedicated server is the one, all the power of which is dedicated for servicing workstations requests. Users (including administrator) can not work on a dedicated server as they do in case of non-dedicated server. Only network administrator uses the console of a dedicated server to carry out network administration. Novell's NetWare operating system features dedicated server.

TYPES OF SERVERS:-In large networks, there can be more than one servers configured to provide specific servers. Servers are called by their functions, E.g. E-mail server, database server, fax server etc.

File server: -

This server provides file storage and retrieval services including security features that control file access rights. In small networks the file server is configured to acts as a database server, backup & archive server, communication server etc.

E-mail server or E-mail gateway: -

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Application of ComputerCOMPUTER FUNDAMENTALS

COMMUNICATION AND COMPUTER NETWORKE-mail server provides local or enterprise-wide electronic mail services. The E-mail server is also configured to translate between different E-mail systems.

Database server: -

A database server is the server configured to handle user database requests and responses.

Fax server: -

This server manages incoming and outgoing faxes for network users. The users forward their faxes to the fax server, which then forwards them to other network users on the globe over communication links.

Print server: -

This server provides user access to printer attached to the network and manages print jobs through a print queue system.

Backup and archive server: -

This server is dedicated to backing up and archiving files on the network.

WORKSTATIONS:When a computer is connected to a network, it becomes a node on the network and is called a workstation or client. A workstation has its own local operating system depending on the machine type. Workstations can be DOS-based PCs, apple MACs running System, UNIX-based PCs, System running OS/2, or diskless workstations. Thus, a network connected computer behaves in its normal mode except that it can retrieve files from more places. Traditionally, a workstation's job has been to execute program files retrieved from the network, and the server's job to deliver those files to the workstation. Thus, most of the processing is performed at the client workstation and the server delivers the files to be processed. However, with the advent of network based client-server computing, the role of the server has changed drastically. In this distributed processing environment, the processing burden is shared by both components of the network. For example, in client server databases, the workstation provides the user interface while the database server or database engine performs the task of searching, indexing and modifying the database. When a computer is connected to a network, it becomes a node on the network and is called a workstation or client. These nodes may be of 3 types-

Dumb- These nodes can only receive and send data.Smart - These nodes can receive, send data and edit data as well.Intelligent - These nodes can receive, send data and edit data and can run programs independently.NETWORK INTERFACE CARDS (NIC):- Each computer attached to a network requires a network interface card that supports a specific networking scheme such as Ethernet, Arc Net, or Token Ring. The network cable is attached to the network interface card. It contains the rules and logic to access the LAN. Supporting a data transfer rate of 35 – 45 Mbps are also available, but that is very expensive.CABLING:TRANSMISSION CHANNEL: -Generally following four types of channels are used for data communication in a LAN –

1. Wire Pairs2. Coaxial Cables3. Microwave4. Twisted Pair Cable5. Fiber-Optic Cable6. Communication Satellite

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Application of ComputerCOMPUTER FUNDAMENTALS

COMMUNICATION AND COMPUTER NETWORKWire Pairs:Wire pairs are commonly used in local telephone communication and for short distance digital data communication. They are usually made up of copper and the pair of wires is twisted together. Data transmission speed is normally 9600 bits per second in a distance of 100 meter.Coaxial Cables:Coaxial cable is groups of specially wrapped and insulted wires that are able to transfer data at higher rate. They consist of a central copper wire surrounded by an insulation over which copper mesh is placed. They are used for long distance telephone lines and local area network for their noise immunity and faster data transfer.Microwave:Microwave system uses very high frequency radio signals to transmit data through space. The transmitter and receiver of a microwave system should be in line-of-sight because the radio signal cannot bend. With microwave very long distance transmission is not possible. In order to overcome the problem of line of sight and power amplification of weak signal, repeaters are used at intervals of 25 to 30 kilometers between the transmitting and receiving end.Twisted Pair Cable:As the name suggests, it is a coiled wire consisting of two insulate copper wires, twisted on each other. This type of wire is easiest one to lay and used for short and medium range network. Advantage of this cable is that of its noise interface and low transmission rate.Fiber-Optic Cable:This cable uses light impulses that travel through clear, flexible tubing half the size of human hair. These cables are reliable for high data transfer rate over a longer distance.Communication Satellite:The problem of line-sight and repeaters are overcome by using satellites which are the most widely used data transmission media in modern days. A communication satellite is a microwave relay station placed in outer space. INSAT-1B is such a satellite that can be accessible from anywhere in India. In satellite communication, microwave signal is transmitted from a transmitter on earth to the satellite at space. The satellite amplifies the weak signal and transmits it back to the receiver. The main advantage of satellite communication is that it is a single microwave relay station visible from any point of a very large area. In microwave the data transmission rate is 16 gigabits per second. They are mostly used to link big metropolitan cities.INTERNET

The Internet is a network of networks. Millions of computers all over the world are connected through the Internet. Computer users on the Internet can contact one another anywhere in the world. If your computer is connected to the Internet, you can connect to millions of computers. You can gather information and distribute your data. It is very much similar to the telephone connection where you can talk with any person anywhere in the world. In Internet a huge resource of information is accessible to people across the world. Information in every field starting from education, science, health, medicine, history, and geography to business, news, etc. can be retrieved through Internet. You can also download programs and software packages from anywhere in the world. Due to the tremendous information resources the Internet can provide, it is now indispensable to every organization.Origin of Internet:In 1969 Department of Defense (DOD) of USA started a network called ARPANET (Advanced Research Projects Administration Network) with one computer at California and three at Utah. Later on other universities and R & D institutions were allowed to connect to the Network. APARNET quickly grew to encompass the entire American continent and became a huge success. Every university in the country wanted to become a part of ARPANET. So the network was broken into two smaller parts MILNET for managing military sites and ARPANET (smaller) for managing non-military sites. Around 1980, NSFNET (National Science Foundation Network) was created. With the advancement of modern communication facilities, other computers were also allowed to be linked up with any computer of NSFNET. By 1990 many computers were looking up to NSFNET giving birth to Internet.Internet functions:

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Application of ComputerCOMPUTER FUNDAMENTALS

COMMUNICATION AND COMPUTER NETWORKInternet is not a governmental organization. The ultimate authority of the Internet is the Internet Society. This is a voluntary membership organization whose purpose is to promote global information exchange. Internet has more than one million computers attached to it.E-mail:E-mail stands for electronic mail. This is one of the most widely used features of Internet. Mails are regularly used today where with the help of postage stamp we can transfer mails anywhere in the world. With electronic mail the service is similar. But here data are transmitted through Internet and therefore within minutes the message reaches the destination may it be anywhere in the world. Therefore the mailing system is excessively fast and is being used widely for mail transfer.Internet tools:

• Protocol -• A set of rules and conventions for sending information over a

network. These rules govern the content, format, timing, sequencing, and error control of messages exchanged among networkdevices.

• Transmission Control Protocol/Internet Protocol (TCP/IP) - A set of networking protocols widely used on the Internet that provides communications across interconnected networks of computers with diverse hardware architectures and various operating systems. TCP/IP includes standards for how computers communicate and conventions for connecting networks and routing traffic.

• Internet Protocol (IP) - A routable protocol in the TCP/IP protocol suite that is responsible for IP addressing, routing, and the fragmentation and reassembly of IP packets.

• Internet service provider (ISP) - A company that provides individuals or companies access to the Internet and the World Wide Web. An ISP provides a telephone number, a user name, a password, and other connection information so users can connect their computers to the ISP's computers. An ISP typically charges a monthly or hourly connection fee.

• Web server - A computer that is maintained by a system administrator or Internet service provider (ISP) and that responds to requests from a user's browser.

• Extensible Markup Language (XML) - A meta-markup language that provides a format for describing structured data. This facilitates more precise declarations of content and more meaningful search

results across multiple platforms. In addition, XML will enable a new generation of Web-based data viewing and manipulation applications.

• File Transfer Protocol (FTP) - A member of the TCP/IP suite of protocols, used to copy files between two computers on the Internet. Both computers must support their respective FTP roles: one must be an FTP client and the other an FTP server.

• Hypertext Transfer Protocol (HTTP) - The protocol used to transfer information on the World Wide Web. An HTTP address (one kind of Uniform Resource Locator [URL]) takes the form: http://www.microsoft.com.

• Telnet - A terminal-emulation protocol that is widely used on the Internet to log on to network computers. Telnet also refers to the application that uses the Telnet protocol for users who log on from remote locations.

• Workgroup - A simple grouping of computers intended only to help users find such things as printers and shared folders within that group. Workgroups in Windows do not offer the centralized user accounts and authentication offered by domains.

• Authentication - The process for verifying that an entity or object is who or what it claims to be. Examples include confirming the source and integrity of information, such as verifying a digital signature or verifying the identity of a user or computer.

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