Chapter 3: Introduction to Computer Networking PDF

Chapter 3 Introduction to COMPUTER NETWORKING 3.1.Introduction to Computer Networking 3.1.1. Benefit of Network Computing 3.1.2. Types of Network 3.2.Introduction to The Internet 3.2.1....

Chapter 3 Introduction to COMPUTER NETWORKING 3.1.Introduction to Computer Networking 3.1.1. Benefit of Network Computing 3.1.2. Types of Network 3.2.Introduction to The Internet 3.2.1. Technical Requirement of Internet 3.2.2. Importance of the Internet 3.2.2.1.Communication Service 3.2.2.2.Information Retrieval Service 3.2.2.3.The World Wide Web 3.1 Introduction to Computer Networking Stand-alone personal computers, first introduced in 1970s, gave users the ability to create documents, spreadsheets, and other types of data and save them for future use. For a small business user or home computer, this was great. For large companies and organizations, however, it was not enough. The larger the company, the greater the need to share information between offices. The stand-alone computer was not enough for the following reasons: Their small hard drive capacities were inefficient. To print, each computer required a printer attached locally. Sharing documents was cumbersome. People grew tired of having to save a diskette, then taking that diskette to the recipient. There was no e-mail. Instead, there was interoffice mail, which was not reliable and frequently was not delivered in a timely manner. To address the above problems, Computer networking was born. A computer network is a connection of two or more computers and other devices that communicate to share data, hardware, and software. Copying files onto removable medium and giving them to others to copy onto their computers was sometimes referred to as the "sneakernet” as illustrated in the figure below. This early form of computer networking is one that many of us have used and perhaps still use today. Note: A method of transmitting electronic information by carrying it physically from one location to another, usually on removable medium is called sneakernet. Page 1  3.1.1 Benefit of Network Computing Networks increase efficiency and reduce costs. Computer networks achieve these goals in the following primary ways: 1. Sharing Information (Or Data)-- like, Company Data Base 2. Sharing Hardware(Peripheral Devices) Printers, Faxes, FDD,HDD,CD- ROM-DRIVE etc 3. Sharing Internet Connection 4. Sharing Programs/Software Expensive Professional Software 5. Centralizing Administration And Support i. Centralized Software Update: like Antivirus ii. Centralized Storage and Backup iii. Centralized Processing and management of data, such as Airline reservation. The following figure illustrates the benefit of computer networking Sharing Programs/Software Application (software) sharing occurs when several users at different locations run application programs that are installed centrally in one location (application server). Software (applications) that are installed centrally can be managed and upgraded centrally. Such systems are mostly common in financial system where the main Page 2 application is stored centrally and users in different sections of the finance department are able to run the system from the network. It is effective for several reasons  Less disk storage space is required because the program is stored only once on the server, instead of being stored on the hard disk s of multiple stand alone computers  When a new version of the software is released, it is easier to update on copy of the program on the server than to update many copies stored on stand –alone computer.  Purchasing a software license for a network can be less expensive than purchasing a single- user license for every workstation on the network In short sharing programs on a network  Saves disk space  Reduces maintenance cost  Reduce licensing cost Sharing Hardware (Peripheral devices) Computer networks enable us to share expensive hardware resource among several computers. A typical example of shared resource is printer. For example, a company may prefer to acquire one expensive printer and connect it to the network to provide high quality printing to users. This avoids the need to have separate printer for each computer in the office. Central Disk storage – Network system provide the possibility of using a dedicated file server to store all the company data in one location. Users will be able to access their data over the network from their workstations. Using central disk storage system facilitates data backup operations from a central location guarantying complete data recovery in case of system failures. Sharing Information/data  Sharing data – users in a certain network environment have the liberty of sharing data and information across the network. Data sharing enables different users to work on a certain file concurrently. Few examples of data sharing are: o Database: - databases are often managed centrally and several users can have access to the database at same time. For example, in a networked banking system, different bank branches can have access to the central account database. This enables bank clients to carry their transactions on any branch bank office. o Email: email communication can be achieved over the network enabling networked users in the company to communicate messages across the network using email. Page 3 Centralizing Administration And Support The login (gain access to) Process: Even if your computer is physically connected to a network, you cannot typically use network resources until you log into the network. When you log in , you formally identify yourself to the network by providing your ID and password. Your user ID and password are the basis for your user account. A user account provides access to network resources and accumulates information about your network use by tracking when you log in and log out. A network administrator/supervisor is the person who is responsible for setting up user accounts and maintain a network. The network administrator provides each new user with a user ID and starter password. In this way centralized administration and support can be achieved. Note: Network administration tasks include:  Managing users and security.  Making resources available.  Maintaining applications and data.  Installing and upgrading application and operating system software. Terminologies  Stand-Alone Computer :A computer that is not connected to a network Peers are computers which use and provide network resources. Clients are computers that uses but do not provide network resources Serversare powerful computers that provide network resources Note: The server has:- Powerful processors Large size memory (RAM) Hard disk with High storage capacity (RAID) Media—the communication channel (bounded or unbounded) that make the physical connections. Network Resourcesany service or device, such as files, printers, or other items, made available for use by members of the network. Page 4  Example: Shared data,Shared printers and other peripherals, & shared applications Node: each device in the network 3.1.2 Types Of Network Network can be grouped in two major categories depending on the geographical scope they cover. Local Area Network (LAN) LAN refers to networks that have small geographical coverage usually within a building or a campus. LANs are found in most organizations, businesses, government offices, educational institutions, and in home computing environment. Wide Area Network (WAN) Wide area networks are networks that cover very large geographical area such as a country or the whole world. Typical WANs are the telephone networks operated by most long distance telephone companies. Large companies having offices scattered around the world use WAN to connect their computers. WAN usually use combination of several types of communication media such as cables, satellites, Microwaves, fiber- optics. Because a WAN has no geographical limitations, it can connect computers and other devices in separate cities or on opposite sides of the world. A multinational corporation with linked computers in different countries is using a WAN. Probably the ultimate WAN is the Internet. Examples of WAN:  Metropolitan Area Network( MAN): a network that transmits data and information over citywide distance and at a greater speed than LAN.  Internet It is a global collection of networks, both big and small. It is a giant network.  Intranet: Intranets are similar to World Wide Web (WWW) where centrally stored hypertext documents can be accessed using the web. Unlike the WWW, intranets are available only to user with in the company network system. Intranet is a very common service in large networked organizations.  Extranet: Although similar to intranet, extranet provides selected users from outside the organization to access data from the internal network. Extranets are commonly used by suppliers to provide data to company clients. Types of LAN Page 5  The three broad classification of LAN, based on how the computers in a network are configured and how they access information, or simply based on the roles of the computer found in the networkare:- 1. Peer-to-peer LAN/peer-based LAN/workgroups LAN 2. Client-server LAN/ server-based LAN 3. Hybrid LAN Note: Computers are categorized into three based on their role in LAN. These are client, server and peer. Peers are computers which use and provide network resources. Clients are computers that uses but do not provide network resources Servers are powerful computers that provide network resources I.PEER-TO-PEER LAN In such network configuration, all the connected systems (computers and printers) can communicate directly with each other without relying on other intermediary server. Implementation of Peer-to-Peer network is relatively simple, cheap and can connect up to 10 computers. Although appropriate for small networks, it lacks the ability to implement centralized management of users, files, and security features and tends to slow down performance as the number of connected computers increase. Although a peer-to-peer network might meet the needs of small organizations, it is not appropriate for all environments. In a typical peer-to-peer network, no network manager oversees administration for the entire network. Instead, individual users administer their own computers. All users can share any of their resources in any manner they choose. These resources include data in shared folder, printers, fax cards, and so on. On a computer network, security (making computers and data stored on them safe from harm or unauthorized access) consists of setting a password on a resource, such as a folder, that is shared on the network. All peer-to-peer network users set their own security, and shared resources can exist on any computer rather than on a centralized server only; consequently, centralized control is very difficult to maintain. This lack of control has a big impact on network security because some users may not implement any security measures at all. If security is an issue, a server-based network might be a better choice. Page 6  Advantages and disadvantages of Peer-to-peer /workgroupsLAN Advantage:  Users can control resource sharing  No reliance on other computers for their operation  Computers in the network are connected by a simple, easily visible cabling system.  Low initial cost-(no need of server and network operating system (NOS) software )  Easy to setup-(use client/desktop operating system like windows 95, 98,me….) Disadvantage  It lacks the ability centralized management of users & files  Lacks security features  Hard to locate network resources  Tends to slow down performance as the number of connected computers increase.  Additional load on computers because of resource sharing Where a Peer-to-Peer Network Is Appropriate? Peer-to-peer networks are good choices for environments where:  There are 10 users or fewer.  Users share resources, such as files and printers, but no specialized servers exist.  Security is not an issue.  The organization and the network will experience only limited growth within the foreseeable future. II.CLIENT-SERVER LAN: Client/Server architecture consists of client computers (workstations) that require a network service and servers that provide the network service. Servers are usually very powerful computers both in processing power and in their storage capacity. Different dedicated servers can be configured depending on the need of the organizations, such as:  File servers – servers that provide centralized data storage which will be shared among the LAN users. Page 7   Print server – servers that control one or more printers and provide print service to LAN users.  Database server – servers that centrally store database and provide data to client application in the LAN.  Web server – servers that store web pages that will be viewed using browsers in the network.  Email server – servers that are used to manage and control email communications across the network. Note: A dedicated server is one that functions only as a server and is not used as a client or workstation. Servers are described as "dedicated" because they are not themselves clients, and because they are optimized to service requests from network clients quickly and to ensure the security of files and folder. Server- based networks have become the standard models for networking. Advantages and disadvantages of Client-Server Network Advantage  Strong central Security  Has central file storage, which allows all users to work from the same set of data and provides easy backup of critical data  Ability of servers to pool available hardware and software, lowering overall costs  Ability to share expensive equipment, such as laser printers and mass storage  Easy manageability of a large number of users (can support thousand of users  Freeing of users from the task of managing the sharing of resources  Optimized dedicated servers, which are faster than peers at sharing network resources Page 8  Disadvantage Expense – Requires high initialinvestment in dedicated server and expensive Network operating system software and client license Maintenance - Large networks will require a staff (a dedicated network administrator) to ensure efficient operation. Dependence - When server goes down, operations will cease across the network. Complex to install, configure and manage III. Hybrid LAN In hybrid LAN while the basic network will be peer-to-peer, we will also install server. Access to the file server requires an account and permissions, while access to other computers on the network is shared equally. Can you think about the advantage and disadvantage of this type of LAN? Note: The security advantage of peer-to-peer LAN is that each user controls access to their own resources. The security disadvantage of peer-to-peer LAN is that you can not differentiate among LAN users when you allow access to a resource. The security advantage of server-base LAN is that each user is allowed accessto only those resources that the user has the privilege to access. A disadvantage is that someone must centrally administer the security on your LAN. Note: The type of LAN you choose to implement will depend on factors such as the:  Size of the organization.  Level of security required.  Type of business.  Level of administrative support available.  Amount of network traffic.  Needs of the network users.  Network budget. 3.2 Introduction to Internet By the turn of the century, information, including access to the Internet, will be the basis for personal, economic, and political advancement. The popular name for the Internet is the information superhighway. Whether you want to find the latest financial news, browse through library catalogs, exchange information with colleagues, or join in a lively political debate, the Internet is the tool that will take you beyond telephones, faxes, and isolated computers to a growing networked information frontier. The Internet supplements the traditional tools you use to gather information, Data Graphics, News and correspond with other people. Used skillfully, the Internet Page 9 shrinks the world and brings information, expertise, and knowledge on nearly every subject imaginable straight to your computer. Internet is computer-based global information system. The Internet is composed of many interconnected computer networks. Each network may link tens, hundreds, or even thousands of computers, enabling them to share information with one another and to share computational resources such as powerful supercomputers and databases of information. The Internet has brought new opportunities to government, business, and education. Governments use the Internet for internal communication, distribution of information, and automated tax processing. In addition to offering goods and services online to customers, businesses use the Internet to interact with other businesses. Educational institutions use the Internet for research and to deliver courses and course material to students. It has made it possible for people all over the world to communicate with one another effectively and inexpensively. Internet service providers (ISPs) provide Internet access to customers, usually for a monthly fee. A customer who subscribes to an ISP’s service uses the ISP’s network to access the Internet. Because ISPs offer their services to the general public, the networks they operate are known as public access networks. In Ethiopia, there is only one ISP – The Ethio telecom that also owns most of the public networks. Page 10 3.2.1 Technical Requirement of the Internet The basic building blocks to the Internet are 1) Clients 2) Hosts/internet servers 3) IP Routers and 4) Connections/internet access Dialup connection Direct/leased line connection Note: internet is based on the concept of client/server computing The Clients The client refers to a computer or any device that connects to the Internet temporarily to get one of the services. Usually, a user runs a client application, such as a Web browser, that contacts one server at a time to obtain information. Because it only needs to access one server at a time, client software can run on almost any computer, including smallhandheld devices such as personal organizers and cellular telephones (these devices are sometimes called Web appliances). Client get temporary IP addresses when connected. For instance when you try to establish a dial-up connection your computer – the clients is assigned a temporary IP address from a pool maintained by your ISP. When it is disconnected, IP address goes back into the pool and it is available for use by other dialup users. Page 11 Host/internet servers Host / Internet sever refers to computers with a permanent link to the Internet and always connected and on-line. To supply information to others, an internet server computer must run a server application. Although server software can run on any computer, most companies choose large, powerful computers to run server software because the company expects many clients to be in contact with its server at any given time. A faster computer enables the server program to return information with less delay. The internet hosts can be found with ISP, commercial business, Universities, Governmental agencies, International organizations, Military organizations, Professional and non-profit organizations more that avail their services on the Internet. IP Router IP Router is a special device, which basically sit at key points on the Internet and act like traffic police at an intersection of hundreds of streets. Data is sent from your computer in the form of a "packet". You can think of a packet to be similar to an envelope; it surrounds your data and contains both a return and destination address. Your computer handles the packets for you; it's all done in the background, without your knowledge. This technique is called packet switching. The Router basically reads the destination address on the packets being sent by your computer and then forwards the packet to the appropriate destination. The IP router finds the best path to your packets, based on current traffic and quality of transmission paths. In some cases your data will travel through several routers before reaching its ultimate destination as illustrated in the figure below. How IP routers are connected forming a complex network Software on the sending computer divides a large document into many packets for transmission; software on the receiving computer regroups incoming packets into the original document. A series of rules known as computer communication protocols specify how packet are assembled in the sending computers and re-assembled to Page 12 original messages in the receiving computers. The TCP/IP protocol handles such packet management on the Internet. In addition to handling packets the TCP/IP protocols enable the Internet to automatically detect and correct transmission problems. For example, if any network or device malfunctions, protocols detect the failure and automatically find an alternative path for packets to avoid the malfunction. Protocol software also ensures that data arrives complete and intact. If any packets are missing or damaged, protocol software on the receiving computer requests that the source resend them. Only when the data has arrived correctly does the protocol software make it available to the receiving application program, and therefore to the user. Connections/internet access The access to these networks is provided by Internet Service Providers (ISPs) that let you subscribe for either of the two modes of connections: dial-up and high- speed/dedicated connections The Dialup connection: sometimes referred to as on-demand connection is a temporary connection that is established using modem though a telephone line. To use a dialup connection you need a computer, modem, telephone line and a subscription /registrations to ISP. The subscription usually contains a username and password to be used by the user when dialling to the ISP. The username is used to uniquely identify you as a user and also be part of your e-mail address. The password is like a key – which should be kept secret so that no one gets connected using the account you have bought from the ISP. The ISP charges you per connection time just like telephone bills. The modem is a device that converts digital signal produced by the computer to analogue signal transmitted through telephone networks. This is because computers represent data in digital form and the telephone system transmits data using continuous, or analog, wave forms, their combination must include equipment that converts signals from one to another. The unit that does this is the modem, short for modulator-demodulator. Modulation converts digital signal into analog wave pattern. Demodulation reverses the process. For example In the figure below a sending modem MOdulates digital signals into analog signals, and a receiving modem DEModulatesanalog signals back into digital signals Page 13  The process of converting digital signal to analogue signal and v.v. Dialup connection is the cheaper and least convenient connection. It is also called a narrowband connection where the maximum speed that can be attained over a dialup connection is 56.6kbps (kilo bits per-second). As a result dialup connection cannot be used for advanced applications on the Internet that involve transmission of audio and video data. High-Speed / dedicated connection: also know by different names as leased line / direct/ broadband connection is a connection that does not require dialing to the ISP server but once setup, it is connected at all times. The connectivity is given through a high speed media such as Fiber optics, wireless (radio signals) or telephone lines with special device called ADSL modem. This connection is more quality connection that can provide a speed as a multiple of 64kbps. The ISP charges such a connection a fixed fee per month based on the speed you want to buy. Using this connection advanced applications that involve audio and video data can be used. 3.2.2Importance of internet IP Address Vs Domain Name To be connected to the Internet, every computer must be assigned a unique number, known as its IP (Internet Protocol) address. As discussed above ach packet sent over the Internet contains the IP address of the computer to which it is being sent. Intermediate IP routers use the address to determine how to forward the packet. That means to request a service from an Internet host, you should know the IP address of the host where the service you want is provided. IP address is difficult to remember for people. It is made up of four 8-bit numbers separated by periods, something like 199.95.72.10. Because of this many host computers also have an easy to remember name like coca- cola.com, microsft.com, etc. Such a name is called Domain Name. Domain name is a natural language translation of IP address. It is maintained by Domain Name Servers (DNS) that translates the domain name to IP address. Hence DNS is one important service for the proper functioning of the Internet. By convention you should type Page 14 domain names using all lower case letters. A domain name ends with a three letter extension that indicates its top level domain. For example, in the domain name cocacola.com, the.com indicates that the computer is maintained by a commercial business. Table that shows common First-Level Domain Name extenstions and their description First-level domain Organization Type/description.com Commercial Business.edu Educational institutes.org Organization/Association.net Internet administrative organizations.gov Government.mil Military organizations.int Organizations established by international After the three letter extension, some treaties domain names (residing outside of the United States) also end with two letter country code such as uk, fr, et, za, etc. This country codes also serve as a top level Example: aau.edu.et, bbc.com.uk, yahoo.com.fr The domain name is widely used in Internet addresses that indicate Internet sites round the world that provide different Internet services. A computer with domain name is referred to as a Site. Activity If you are using a computer that has access to the Internet, you can find out the IP addresses of any host using a simple step 1. Click on Start button and select Run 2. Type in cmd on the Run box 3. Click Ok button on the Run box 4. Type ping domainName, Then the first thing you will see is a line of text that starts with "Pinging domainName [IP address]...." Find out the IP addresses of commonly used hosts such as yahoo.com, aau.edu.et, and bbc.com Internet services/applications can be classified in to three categories: 1. World Wide Web 2. Communication and 3. Information Retrieval services. 1. The World Wide Web (WWW) The World Wide Web, or just the Web is an internet service that links documents and information from the host computers distributed all over the world. WWW enables peoples around the world to share and exchange information. It represents billions of electronic documents called web pages from a verity of servers maintained by government Page 15 organizations, educational institutions, science research centers, private businesses etc. Currently the web pages present information in a graphical user interface that includes text, graphics, animations, and most importantly hyperlinks. Hyperlink is a special type of formatting used in web pages to link one page to the other. Once you start to view a web page from the web, the hyperlinks will take you to other pages / documents as you select (press button on) them using your mouse. It is the hyperlink that makes up the web – a complex link among the different nodes, in this case the web pages. WWW began in the late 1980's when physicist Dr. Timothy Berners-Lee wrote a small computer program for his own personal use. This program allowed pages, within his computer, to be linked together using keywords. It soon became possible to link documents in different computers, as long as they were connected to the Internet. The document formatting language used to link documents is called Hypertext Markup Language (HTML). The www, just like most of other internet services is based on the client server model. The web pages are stored in host computers called web servers round the world. These hosts are referred by a web site address that serves to locate the host computer. Web site address has a format that specifies the service and the host computer providing the service. The generic format for a web site address is: www.domain_name.ext.cu Where the first three letters (www) indicate the service followed after a period is domain name (with extension and country code) to identify the host computer. A web site is a collection of related web pages. Examples: www. yahoo.com, www.bbc.co.uk, www.aau.edu.et, On the other hand, there has to be client computers that run one of the web browser software such as Netscape Navigator,Internet Explorer, Fire Fox, Opera etc. A browser is a computer program that resides on your computer enabling you to use the computer to view WWW documents and access the Internet taking advantage of text formatting, hypertext links, images, sounds, motion, and other features. The browser requests web pages form the web servers. The web server responds by transmitting the web page to the requesting client computer where the web browser views the content of the web page. This exchange of hypertext documents is done by the use of a standard protocol called HyperText Transfer Protocol (HTTP). Today’s browsers made it easier to access the different web sites that had started to appear, containing more than just text. They also had sound and video files. They are also versatile in that the browsers also give you additional services such as e-mail, ftp etc. As it was discussed above a web site is composed of a series of web pages that contain information. Each page is stored as a separate file and referred to by a unique Uniform Resource Locator (URL). A URL is the address of a document on a computer. The components of URL include the web protocol, the web server name, the folder in which the page is stored, and the filename of the page as illustrated in the figure below. Page 16  An example of URL of a document on the world wide web Web page URLs begin with http:// the protocol, communication system that allows web clients to communicate with web servers. Many of today’s web browsers assume that all web address begin with http://. If you are using such a browser, you can omit http:// when you type a URL. The next part of URL is the web server name. A web server is a computer that uses special software to transmit web pages over the internet. Many web server names are domain name prefixed with www as in the case of web site address. The web server names for your favorite university might be something like www.aau.edu.et. Suppose you indicate to your web browser that you want to access www.aau.edu.et by entering the web server name, you access the site’s home page. A home page is similar to the title page in a book. It is the introductory page of a website that normally has a master menu of documents on that site. It identifies the site and contains links to other pages at the site. The URL of a web page reflects the name of any folder in which it is stored. For example, suppose that the university has a page listing degree programs. The listing is stored in a file called degree.html in a folder called information. The URL for this page would be: www.aau.edu.et/information/degree.html. File name extension for web pages are typically htm or html. This is because as mentioned earlier, web pages are written using a document formatting language called HTML. Surfing / web browsing Once you've made your first connection to your Internet Service provider, you are ready to begin surfing. Surfing is the process of reading web pages and moving from one web site to another. When you are surfing the web, you can choose from millions of websites to visit. Your browser has several ways of allowing you to do this. Web users are sometimes called web surfers. You may need to surf the web looking for information on a particular topic, or you may want to compile some data from the www. The problem with surfing is that, you have to know URL of every a web page containing information you need in order to get access to it. However, locating such a page among the billions of pages is not imaginable without a search engine or search directories. Page 17  Home page of The AAU's Official Web site Activity Go to any Internet cafe or where you can get an Internet access. Open the browser and type in the url "http://www.aau.edu.et. You must get the page shown in the figure above. Surf through it selecting the links listed at the left corner of the page and top of the page. Learn as much as you can about the University just be visiting the website. Search engine / Web Crawler/ A search engine is a database system designed to index Internet addresses (urls, usenet, ftp, image locations etc). The typical search engine contains a special program often called a spider (also sometimes called a "bot" or "crawler" or robots). The spider goes though the Internet, crawling page to page collecting keywords of web pages. The search engine indexes the pages with their keywords and url in a big database. When you want to look for a web page on a particular topic, all you need to do is give your keyword to the search engine. The search engine database will provide you with urls of pages indexed and contain the keyword you have requested. Examples of search engines / crawlers are:  Infoseek (http://ultra.infoseek.com) Page 18   AltaVista (http://www.altavista.digital.com)  Google (http://www.google.com)  Lycos(http://www.lycos.comd)  Yahoo[http://www.yahoo.com)  Excite[http://www.excite.com] Search directories Creates directories of web sites and their address. Directories are collection of resources. People or organization submit sites of interest to search service provider like:  Yahoo [http://www.yahoo.com],  Linkstar [http://www.linkstar.com],  LookSmart [http://www.looksmart.com]  Librarian's Index to the Internet [http://www.lii.org] During submission process, the people or organizations should supply the following information about their web sites such as URL, Title, Short summery. The search service provider will classify these directories according to the rules of a particular search service. A web directory lists resources, organized by subject area. To Search the directory, you just click / select the subject you want under the categorized list of topics rather than entering keyword. Or you can also enter one or more keywords and see a list of categories and sites displayed with those keywords in the title. However, the distinction between search engines and directories is not as clear cut as it used to be. All the major search directories will feed you results from a search engine if they cannot find what you are looking for in their own directory. On the other hand, some of the search engines will serve information from search directories before giving you data from the search engine's database. Search engines should be your first choice when you know exactly what you are looking for. They also cover a much larger part of the Web than the search directories. Comparison of Search engines and Web directories Activity Yahoo.com is both a search engine and a web directory. Visit yahoo.com and see information about Ethiopian tourist attractions from search engines and web directories listed above. Search for "Ethiopian tourism" in google.com, then Page 19  search for Ethiopia - tourism" in google.com. Compare the resulted list sites for the two search and write what it means by adding minus sign in keywords. 2. Communication services Communication refers to the process of transferring messages from one network to another over the internet. Some of communication service on the internet includes: Electronic mail,Usenet (newsgroup),Keyboard chat (IRC), Video conferencing and Internet telephone. Using these services you will be able to keep in touch and send and receive messages to colleagues and friends. a. Electronic mail Electronic mail is a method for sending and receiving textual messages from one user to another through the internet. It is the electronic equivalent of sending a letter through the mail, but a more efficient means of communication than ground or air mail. Although email is mostly used to transfer text based messages, users can attach other files (such as graphics, audio, video…) with their emails. Email is also provided through the client server model. Email users have electronic mailboxes on e-mail servers that provide the email service. Each mailbox is identified by an email account composed of a username and password. An email address has two major components, the username and the domain name separated by the special character @ (username@domain-name). The username is the name assigned to the person or organization using the email, and the domain name is a unique name that identifies the name of the network system connected on the internet, usually called email server. For example, the domains name for an email address provided by Ethiopian Internet Center is ethionet.et. If you subscribe for an email using your name (such as Abebe) your email address will be [email protected] On client side email users use email client application programs to connect to the email server to enable them to compose, send and receive electronic messages. There are several client applications supplied by different software developers, such as Eudora, Pegasus, Ms-Outlook, Lotus CC:Mail etc. Most of these programs also have different features more than sending and receiving mails such as saving the address for future reference in an address book, reply to the original message, forward the message to a third party, check spelling, and few more features of word processors. E-mail server software uses a store-and-forward technology. Your workstation does not need to be turned on when someone sends e-mail to you. The e-mail server found in ISP provides e-mail services to the general public with monthly fee. Free e-mail accounts are also provided by advertiser-supported internet sites such as Yahoo, hotmail, gmail, and many more. Email Attachments An attachment is a computer file or files place inside an email message. Email was originally designed to handle only plain text (no formatting, ie. bold, centering, etc.) which was transferred from one computer to another in a format called ASCII. ASCII is a standard across all computer types which makes e-mail universal. Today many modern email programs allow HTML (hypertext markup language) to be included in email Page 20 messages which allow you to format your email messages with font sizes, bold, centre and such. The recipient must have an HTML capable email reader to see the formatting otherwise they will get a document marked up with HTML tags that looks garbage data unreadable to users as seen below in the figure. To attach a document to an email message drag the file attachment to the body of the message and drop it. An attachment can also be added by choosing File Attachment from the menu. Most modern email programs place a button on the Toolbar usually in the shape of a paper clip that you can click. A dialog box will open that allows you to browse and select the file that you want to attach. You can attach more than one file to a single email message. Activity Register for a free e-mail address from two sites. You can try from mail.yahoo.com, hotmail.com, and gmail.com.Send data for your self from one mail to another. Attach files, to your message by dragging and dropping it to the message and using the attach buttons. b. Usenet (newsgroup) Newsgroups are discussion groups that are going on all the time. You can write a message and post it to a newsgroup in the morning and come back to read a reply to the message in the afternoon. You can also reply to other's messages. The popular sites such as yahoo and google enable you to start a group and have a community of your interest Page 21  A typical interface of a Usenet where you can start your own discussion group There are thousands of news groups covering as many topics. Some are moderated, meaning that a person who has knowledge or an interest in a certain topic will receive the message and read it before posting it. This keeps the discussion on topic. Most are not moderated and the discussion can go anywhere but it can be a great way to get a quick reply to your questions. As well as the public newsgroups many organizations and companies will provide discussion groups of bulletin boards that focus on areas related to their own topics. This can be a great spot to find out more about an organization, find a solution to a problem you are having and participate in the discussions related to that company or organization. One problem with posting a message to a discussion or newsgroup is that unscrupulous companies mine the newsgroups for email addresses and then send unsolicited email (spam) to the poster. It is recommended that you disguise your email address using your email software preferences or the bulletin board settings. Many newsgroups have a publicly accessible faq. A faq is a collection of frequently asked questions that have been posted many times to a newsgroup. You may get a gentle reminder or a completely rude and insulting comment if you post a question that is on the faq. It is acceptable to post a question as where the faq is located and you may find that frequent posters will mention the faq in their message. Example of Usenet site: www.dejanews.com Page 22  Activity Search for a USNET / discussion group site and read peoples, comments on technology issues. To start with this, you can just type in a problem such as virus problems, or technology problems you have faced on the search engines. The result usually takes you to discussions group pages where similar questions are treated.Post your own questions and collect answers for your questions. This may require you to get registered first. c. Internet Relay Chat (IRC) IRC allows users to participate in real-time discussions with other users through the Internet. A user logs on to an IRC site and then types messages and replies to others messages as they are written. Often an IRC discussion has a topic arranged in rooms or virtual places and users will join to room to discuss the topic. Other IRC discussions are free flowing and regardless of how centred the discussion is it can take some time to sort out the different threads of discussion in a busy IRC. There is no restriction to the number of people that can participate in a given discussion room, or the number of rooms that can be formed on IRC. They are also used for online conferences with a famous or knowledgeable person contributing their time to answer questions and present information. Along the same theme are online game sites where Internet users can compete against other users on the Net. You can find free services of IRC on the following site  http://www.funcity.com/chat  http://www.theglobe.com  http://www.talkcity.com  http://www.wbs.net (web broadcasting services) d. Instant Messaging (IM) IM is a software, such as YahooMessanger and MSN allows users to find friends and contacts through the Internet and communicate with them in real time. Users maintain a list of contacts which the software will notify when they are online and contact is available. To use an instant messaging service it is necessary to have and register software available on the Internet. Instant messaging allows you to chat, send messages and files and various other features but only with people you choose. Often the software and registration is free. Some services also provide the ability to connect a microphone to your computer and talk to your contact. Your contact can talk back using their account and software which you can hear using the computer's speakers or an ear attachment. These days people are using the same IM software to have a live conversation using voice chat and video chat. Page 23  Activity Use the MSN messenger program to chat with your colleague / classmate. You must have a hotmail e-mail account to do that.Yahoostarted a web based messenger service for free at http://webmessenger.yahoo.com/. If you have yahoo email address, you can sign in and chat with it. Try it.Try to add your yahoo email address to your contacts in MSN messenger and v.v e. Video Conferencing Video conferencing is a communications technology that integrates video and voice to connect remote users with each other as if they were in the same room. Users see and hear each other in real-time. Video conferencing means making a conference on the Internet by individuals who live in different location. This takes place at the same time in different location. It is similar to conference in a hall except they are at distant. For participation in video conferencing (receive and send), each user needs  computer( with Sound video card )  Webcam  Headphone set with microphone  broadband (high speed multi-channel) internet connection  Client software program Video conferencing involves the visual communication of parties around the world. You can use Video conferencing to link up with specific parties or you can join Video conferencing communities that operate in the same way as Internet Chat does. f. Internet telephone The Internet can also be used to transfer telephone calls using an application known as IP-telephony. This application requires a special phone that digitizes voice and sends it over the Internet to a second IP telephone. You need special client software called audio conferencing programs. Newer software operates in full duplex conversation (i.e you can speak and hear the other person at the same time). In half duplex conversation only one person can speak at a time. 3. Information Retrieval Services Information retrieval service refers to the process of transferring large volume of data across the internet. Examples of information retrieval application on the internet include FTP and Telnet. Page 24  a. File Transfer Protocol FTP is method for transferring files from one internet site to another internet sites. FTP enables user to download or retrieve files from ftp sites and to upload files to ftp server. Like many internet applications, FTP is a client server system where there is FTP server that provides the service and users use FTP client programs to access resources from the FTP server. FTP service is a popular internet service which is widely used to download exciting games, large documents, program upgrades and other resources from the internet. Although some FTP sites have restricted access where only authenticated users are given access to the FTP server, there are still several anonymous FTP sites that allow anyone to transfer files without prior permission. The FTP application is often automatically invoked when a user downloads an updated version of a piece of software. Applications such as FTP have been integrated with the World Wide Web, making them transparent so that they run automatically without requiring users to open them. When a Web browser encounters a URL that begins with ftp:// it automatically uses FTP to access the item. Activity Find FTP sites on the Internet. You can use the search engines. Try to download files form the ftp service if you get a free one. b. Telnet (Terminal Networking) Telnet is an internet application and protocol that allows users to log onto a remote computer. Once connected to a remote computer using telnet application, users can access their files and document, work on their files and save their work on the remote computer as if they are directly connected to the server on the local area network. Telnet gives the appearance that the user’s keyboard and monitor are connected directly to the remote computer. For example, a businessperson who is visiting a location/ working at home that has Internet access can use Telnet to contact their office computer. Page 25