Unit-1 Introduction to Computer Networks 2.pdf

Unit-1 Introduction to Computer Networks Lecture Outline  Introduction & Historical Background  Computer Networks  Categorization of Computer Networks  Computer Network vs. Distributed System  Uses of Computer Networks  Introduction & Historical Background ...

Unit-1 Introduction to Computer Networks Lecture Outline  Introduction & Historical Background  Computer Networks  Categorization of Computer Networks  Computer Network vs. Distributed System  Uses of Computer Networks  Introduction & Historical Background  Each of the past three centuries was dominated by a single new technology.  The 18th century was the era of the great mechanical systems accompanying the Industrial Éy  Revolution. The 19th century was the age of the steam engine. During the 20th century, the key technology was µ  information gathering, processing, and distribution.  Among other developments, we saw the installation of worldwide telephone networks, the invention of radio and television, the birth and unprecedented growth of the computer industry, the launching of communication satellites, and, of course, the Internet. Cont.…  As a result of rapid technological progress, these areas are rapidly converging in the 21st century and the differences between collecting, transporting, storing, and processing information are quickly disappearing.  Organizations with hundreds of offices spread over a wide geographical area routinely expect to be able to examine the current status of even their most remote outpost at the push of a button.  As our ability to gather, process, and distribute information grows, the demand for ever more sophisticated information processing grows even faster. Cont.…  Although the computer industry is still young compared to other industries (e.g., automobiles and air transportation), computers have made spectacular progress in a short time.  During the first two decades of their existence, computer systems were highly centralized, usually within a single e large room.  Not infrequently, this room had glass walls, through which visitors could gawk at the great electronic wonder inside. A medium-sized company or university might have had one or two computers, while very large institutions had at most a few dozen.  The idea that within forty years vastly more powerful computers smaller than postage stamps would be mass produced by the billions was pure science fiction. Computer Networks  A computer network consists of two or more computers or other intelligent devices linked by communication media (e.g., cable or wireless media) to achieve successful communication.  I Computer networking is used in many aspects of our lives, and its applications are proliferating.  For example, computer networks can be found in universities, secondary schools, and colleges, while in the corporate world, networks link geographically separated offices.  Local and state government offices use computer networks, as do military organizations, medical facilities, and the Internet.  Categorization of Computer Networks  Computer networks can be categorized as:  Local area networks (LANs),  Metropolitan area networks (MANs), and  Wide area networks (WANs).  The fundamental differences among what LANs, MANs, and WANs are distance 0 coverage, transmission speed, media, and error rate. Local Area Network (LAN)  A LAN is a class of computer network that covers a relatively small geographic area, for example, a room, a building or a campus.  A LAN is owned by a single organization and physically located within the organization’s premises. Metropolitan Area Network  A MAN is a backbone network that links multiple LANs in a large city or a metropolitan region covering up to few km. Wide Area Network  A WAN is a class of network that covers a large geographical area (e.g., a country or a continent).  Telephone networks and the Internet are examples of WANs.  ftp.wpiwdm gf.de ApHlpimidgi Computer Network vs. Distributed System  There is considerable confusion in the literature between a computer network and a distributed system. The key distinction is definition that in a distributed system, a collection of independent computers appears to its users as a single coherent system. adf.mniIteItsing  Usually, it has a single model or paradigm that it presents to the users. Often a layer of software on top of the operating system, called middleware, is responsible for implementing this model. A well-known example of a distributed system is the World Wide Web. It runs on top of the Internet and presents a model in which everything looks like a document (Web page). www.aiiiiiiiiii.Ei.ir  In a computer network, this coherence, model, and software are absent. Users are exposed to the actual machines, without any attempt by the system to make the machines look and act in a coherent way. If the machines have different hardware and different operating systems, that is fully visible to the users. If a user wants to run a program on a remote machine, he† has to log onto that machine and run it there. Cont.….  on top of a network. to In effect, a distributed system is a software system built The software gives it a high degree of cohesiveness and T  transparency. Thus, the distinction between a network and a distributed system lies with the software (especially the operating system), rather than with the hardware.  Nevertheless, there is considerable overlap between the two subjects. For example, both distributed systems and computer networks need to move files around. The difference lies in who invokes the movement, the system or the user. I 8  Uses of Computer Networks  Business Applications: Most companies have a p substantial number of computers. For example, a company may have a computer for each worker and use them to design products, write brochures, and do the payroll. Initially, some of these computers may have worked in isolation from the others, but at some point, management may have decided to connect them to be able to distribute information throughout the company.  Put in slightly more general form, the issue here is resource sharing. The goal is to make all programs, equipment, and especially data available to anyone on the network without regard to the physical location of the resource or the user. Cont.…. Business Applications  Server: In a technical sense, a server is an instance of a computer program that accepts and responds to requests made by another program, known as a client.  Less formally, any device that runs server software could be considered a server as well. Servers are used to manage network resources. if For example, a user -may setup a server to control access to a network, send/receive e-mail, manage print jobs, or host a website. Client Server Model  This whole arrangement is called the client-server model. It is widely used and forms the basis of much network usage.  The most popular realization is that of a Web application, in which the server generates Web pages based on its database in response to client requests that may update the database.  The client-server model is applicable when the client and server are both in the same building (and belong to the same company), but also when they are far apart. For example, when a person at home accesses a page on the World Wide Web, the same model is employed, with the remote Web server being the server and the user’s personal computer being the client. Under most conditions, one server can handle a large number (hundreds or thousands) of clients simultaneously. Cont.…  A computer network can provide a powerful communication medium among employees. Virtually every company that has two or more computers now has email (electronic mail), which employees generally use for a great deal of daily communication.  Telephone calls between employees may be carried by the computer network instead of by the phone company. This technology is called IP telephony or Voice over IP (VoIP) when Internet technology is used.  Desktop sharing: lets remote workers see and interact with a graphical computer screen. This makes it easy for two or more people who work far apart to read and write a shared blackboard or write a report together. When one worker makes a change to an online document, the others can see the change immediately, instead of waiting several days for a letter. Cont.…. Business Applications  Example: More ambitious forms of remote coordination such as telemedicine are only now starting to be used (e.g., remote patient monitoring) but may become much more important.  Third goal for many companies is doing business electronically, especially with customers and suppliers. This new model is called e-commerce (electronic commerce) and it has grown rapidly in recent years.  Airlines, bookstores, and other retailers have discovered that many customers like the convenience of shopping from home. Consequently, many companies provide catalogs of their goods and services online and take orders online.  Manufacturers of automobiles, aircraft, and computers, among others, buy subsystems from a variety of suppliers and then assemble the parts. Using computer networks, manufacturers can place orders electronically as needed. This reduces the need for large inventories and enhances efficiency.  Uses of Computer Networks : o Home Applications  Access to remote information comes in many forms. It can be surfing the World Wide Web for information or just for fun.  Information available includes the arts, business, cooking, government, health, history, hobbies, recreation, science, sports, travel, and many others. Fun comes in too many ways to mention, plus some ways that are better left unmentioned.  Many newspapers have gone online and can be personalized. For example, it is sometimes possible to tell a newspaper that you want specific news you want.  Information of hospitals may be available for those who wanted to take information from home through internet. Cont.…. Home Applications  Much of this information is accessed using the client- server model, but there is different, popular model for accessing information that goes by the name of peer-to- peer communication (Parameswaran et al., 2001). In this form, individuals who form a loose group can communicate with others in the group, as shown in Fig. Every person can, in principle, communicate with one or more other people; there is no fixed division into clients and servers. Cont.…. Home Applications  Between person-to-person communications and accessing information are social network applications. One of the most popular social networking sites is Facebook.  Even more loosely, groups of people can work together to create content. A wiki, for example, is a collaborative Web site that the members of a community edit. The most famous wiki is the Wikipedia, an encyclopedia anyone can edit, but there are thousands of other wikis.  The third category is electronic commerce in the broadest sense of the term. Home shopping is already popular and enables users to inspect the online catalogs of thousands of companies. Uses of Computer Networks : Mobile Users  Mobile Users: Mobile computers, such as laptop and handheld computers, are one of the fastest-growing segments of the computer industry.  Their sales have already overtaken those of desktop computers. Why would anyone want one? People on  the go often want to use their mobile devices to read and send email, tweet, watch movies, download music, play games, or simply to surf the Web for information.  They want to do all of the things they do at home and in the office. Naturally, they want to do them from anywhere on land, sea or in the air.  Cont.…. Mobile Users  Connectivity to the Internet enables many of these mobile uses. Since having a wired connection is impossible in cars, boats, and airplanes, there is a lot of interest in wireless networks.  Cellular networks operated by the telephone companies are one familiar kind of wireless network that blankets us with coverage for mobile phones. Wireless hotspots based on the 802.11 standard are another kind of wireless network for mobile computers.  Although wireless networking and mobile computing are often related, they are not identical, as Fig. shows. Here we see a distinction between fixed wireless and mobile wireless networks. Conversely, some wireless computers are not mobile.