Data Communication and Networking

Network Topologies

As we have seen earlier, topology is the geometric arrangement of the computers in a network. Common topologies include star, ring and bus.

Star Network

The star network as shown in Fig 5.6 is frequently used to connect one or more small computers or peripheral devices to a large host computer or CPU. Many organizations use the star network or a variation of it in a time-sharing system, in which several users are able to share a central processor.

In a time-sharing setup, each terminal receives a fixed amount of the CPU's time, called a time slice. If you are sitting at a terminal and cannot complete your task during the time slice, the computer will come back to you to allow you to do so. Actually, because the CPU operates so much faster than terminals, you will probably not even notice that the CPU is away. By establishing time-sharing, many people in a large organization can use a centralized computing facility. Time-sharing can also be purchased from an outside service, which is an economical way to operate for a small company that cannot afford its own large computer.

Star network is frequently used in a LAN to connect several microcomputers to a central unit that works as a communications controller. If the user of one microcomputer wants to send a document or message to a user at another computer, the message is routed through the central communications controller. Another common use of the star network is the feasibility of connecting several
microcomputers to a mainframe computer that allows access to an organization's
database. Access and control of star network typically is maintained by a polling system. Polling means that the central computer, or communications controller "polls" or asks each device in the network if it has a message to send and then allows each in turn to transmit data.

Ring Network

The ring network (see Fig. 5.7) is a Local Area Network (LAN) whose topology is a ring - can be as simple as a circle or point-to-point connections of computers at dispersed locations, with no central host computer or communications controller. That is, all of the nodes are connected in a closed loop. Messages travel around the ring, with each node reading those messages addressed to it.
One of the advantages of ring networks is that they can span larger distance than other types of networks, such as bus networks, because each node regenerates messages as they pass through it.

Access and control of ring networks are typically maintained by a "token-passing" system. IBM's Token-Ring network is thought by some observers to be a watershed event comparable to the development of the IBM PCV itself, because the Token-Ring network is designed to link all types of computers together, including not only personal computers but also possible mini computes and
mainframes.

A Token-Ring network as shown in Fig. 5.7 resembles a merry-go-round. To deliver a message, you would hand over your addressed note to a rider (the token) on the merry-go-round, who would drop it off at the appropriate place.

Bus Network

Bus networks (see Fig. 5.8) are similar to ring network that the ends are not connected. All communications are carried on a common cable or bus and are available to each device on the network. Access and control of bus networks are typically maintained by a method called contention, whereby if a line is unused, a terminal or device can transmit its message at will, but if two or more terminals initiate messages simultaneously, they must stop and transmit again at different intervals.

Network Protocols

A protocol is an agreed-upon format for transmitting data between two devices.
The protocol determines the following :

The type of error checking to be used.
 Data compression method, if any
 How the sending device will indicate that it has finished sending a

message 

 How the receiving device will indicate that it has received a message There are a variety of standard protocols from which programmers can choose ach has it own particular advantages and disadvantages; for example, some are simpler than the others, some are more reliable, and some are faster. From a user' s point of view, the only interesting aspect about protocols is that
your computer or device must support the right ones if you want to communicate with other computers. The protocol can be implemented either in hardware or in software. Some of the popular protocols are TCP/IP, HTTP, FTP, SMTP, POP, Token-Ring, Ethernet, Xmodem, Kermit, MNP, etc.

Network Architecture

The term architecture can refer to either hardware or software, or a combination of hardware and software. The architecture of a system always defines its broad outlines, and may define precise mechanisms as well. An open architecture allows the system to be connected easily to devices and
programs made by other manufacturers. Open architectures use off-the-shelf components and conform to approved standards. A system with a closed architecture, on the other hand, is one whose design is proprietary, making it difficult to connect the system to other systems. As we have seen before, network architectures can be broadly classified as using either peer-to-peer or
client/server architecture.

Peer-to-peer Architecture

This is a type of network in which each workstation has equivalent capabilities and responsibilities. This differs from client/server architecture, in which some workstations are dedicated to serving the others. Peer-to-peer networks are generally simpler and less expensive, but they usually do not offer the same performance under heavy loads.

Client/Server Architecture

This is a network architecture in which each computer or process on the network is either a client or a server. Servers are powerful computers or processors dedicated to managing disk drives (file servers), printers (print servers), or network traffic (network servers). Clients are less powerful PCs workstations on which users run applications. Clients rely on servers for resources, such as
files, devices, and even processing power.

Important terms used in Networking

(a) Internet

The newest type of network to be used within an organisation is an internet or internet web. Such networks enable computers (or network) of any type to communicate easily. The hardware and software needs are the same as for the internet, specifically TCP/IP, server and browser software used for the World Wide Web. Because most organisations have a need for more dynamic ways to
link people and information, the internet market is expanding day by day. Moreover, there is no need to adjust the network when a new user joins in. With the help of Internet, all computers of an organisation can work as stand-alone systems, connected to a mainframe, or part of a LAN or WAN.

(b) 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 without the help of postage stamp we can transfer mails anywhere in the world. With electronic mail the service is similar. But here data is transmitted through Internet and therefore within minutes the message reaches the destination may it be anywhere in the world. Therefore the mailing system through e-mail is excessively fast and is being used widely for mail transfer.

(c) Voice Messaging

It is a new communication approach which is similar to electronic mail except that it is audio message rather than text messages that are processed. A sender speaks into a telephone rather than typing, giving the name of the recipient and the message. That sender's voice signal is then digitised and stored. The system can then either deliver the message at a specified time in future or it can be
retrieved from a database by the recipient. The message is reconverted back into its analog format when it is delivered or retrieved so that the recipient hears it as the original sender's voice on a telephone. Voice messaging requires a computer with an ability to store the audio messages in digital form and then convert them back in an audio form upon verification. Each user has a voice mailbox in secondary storage and special equipment converts the audio message to and from the digital form. The main advantage of voice mail over electronic mail is that the sender does not have to type. Voice mail also makes it easy to include people in the firm's environment in an communication network.

(d) E-Commerce

Electronic commerce or e-commerce as it is popularly known refers to the paperless exchange of business information using Electronic Data Interchange, Electronic mail, Electronic Bulletin Boards, Electronic Fund Transfer and other network based technologies. Electronic Commerce (EC) not only automates manual process and paper transactions, but it also helps organisations to move into a fully electronic environment and change the way they usually operate. Few organisations have recently started conducting EC over Internet, the network of networks. Internet has also helped EC to boost up because it is a low cost alternative to the proprietary networks. EC standards are however under
development. Electronic Data Interchange (EDI) is still the dominant part of
EC.

Information Technology has transformed the way people work. Electronic Commerce (EC) has unearthed yet another revolution which is changing the way business houses buy and sell products and services. EC is associated with buying and selling of products and services over computer communication networks. EC transfers information electronically from computer to computer in
autonomous way. EC has, in fact, transformed the way organisations operate.

(e) Electronic Data Interchange (EDI)

EDI is the computer-to-computer exchange of business documents in a standard format. These formats look much like standard forms and are highly structured.

(f) Teleconferencing

It refers to electronic meetings that involve people who are at physically different sites. Telecommunication technology allows participants to interact with one another without travelling to the same location.