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    • S4 CIT Computer Network Notes A. Introduction to Computer Network In the early stage of computer development, computers are standalone machines. What if you want to share a file with your classmate or you want to print a page but the printer did not connected to your computer? The answer is computer network. A computer network is a system in which two or more computers are connected, so that data communications and resources sharing (File sharing, hardware sharing e.g. printer) among the computers become possible. B. Types of Networks Networks can be distinguished by the distances that they cover. 1. LAN (Local Area Network) LAN is a network that links computer and other peripheral devices over short distances. Examples are network at home, in school campuses and offices. Very often, a LAN with an internal e-mail system, file server or message board is called Intranet. ____________________________________ P.1 S4/CIT/Computer Network/Notes
    • 2. WAN (Wide Area Network) A WAN covers a large geographic area using a communication channel such as dedicated telephone lines and radio waves. A WAN consists of two or more LANs. The Internet is the world’s largest WAN. C. Network Connection Protocol (協定 ) There are two types of Network Connection Protocol: Types of Diagram Description Network Connection Protocol Peer to - Every computer is connected peer directly to every other computer. - Each computer can use information from, and provide information to each other. - All computers are workstation (no server) - Security is weak Client / - Individual computers (clients or Server workstations) are connected to a central computer (server). - Server computer maintains the network user accounts - Clients can access programs or files stored on the server. - Clients request for services whereas servers provide services. - Since the server can give different levels of rights (權力) to different users, so security is high. ____________________________________ P.2 S4/CIT/Computer Network/Notes
    • D. Hardware Requirement for a Network 1. Network Interface Card (NIC / LAN Card) NIC is an expansion board that is added to the computer’s motherboard. It is the key hardware component for connecting the computer to a network (wired or wireless). It can be classified by: i. Types of connectors - RJ45 Connector - BNC Connector ii. Type of peripheral buses - PCI Bus - USB Bus - PCMCIA Bus iii. Size of bandwidth - 10Mbps, 100Mbps, 1Gbps (Intranet) - 11Mbps, 54Mbps (Wireless LAN) - 1.5Mbps, 3Mbps, 6Mbps (Connecting to the Internet) 2. Transmission Medium A network needs a transmission medium, such as: Transmiss Photo Connector Description ion Medium Twisted - Looks similar to Pair Wire telephone cable (UTP) - The most commonly used types of twisted pair wire is Cat5 cable RJ45 Connector Coaxial - Transmit at a higher Cable speed when compared with twisted-pair wires BNC Connector - Can transmit signal over long distances so they can be laid underground or underwater. ____________________________________ P.3 S4/CIT/Computer Network/Notes
    • Optical - Bundles of smooth, Fibre hair-thin strands of glass - Transmit data with high efficiency in terms of light beams - Can transmit signal over long distances - Largely replaced coaxial cables nowadays. Microwave - Electromagnetic waves which can wirelessly carry data through the atmosphere - It travels in straight line from one microwave relay station(轉發站) to another. 3. Connecting Devices Name Photo Description Hub - provides a central connection point for cables in a network - slower than switch Switch - advanced type of hubs - faster than hubs Router - connect multiple networks - connect LANs to form a WAN - connect a LAN to the Internet ____________________________________ P.4 S4/CIT/Computer Network/Notes
    • MC Exercise 1: 1. Which of the following statements about a network is correct? A. Files stored on the server are accessible to all users. B. The passwords of users on the network are unique. C. A printer is connected physically to all the computers on the network. D. User account is designed to prevent unauthorised access. 2. Computers on a local area network are joined physically together to one or more A. hub or switch B. printer C. scanner D. network interface card 3. Which of the following can be shared in a network? (1) files (2) Internet access (3) keyboard A. (1) only B. (2) only C. (1) and (2) only D. (1), (2) and (3) 4. A network is set up in a building with 15 computers and a powerful computer dedicated to managing disk drives and printers. What type of network is this? A. a peer-to-peer local area network B. a peer-to-peer wide area network C. a client-server local area network D. a client-server wide area network 5. Which of the following is the function of a router? A. connecting a computer to the Internet B. connecting a LAN to the Internet C. connecting a computer to a LAN D. connecting a LAN to a computer E. Wireless Network A Wireless Local Area Network (WLAN) uses microwave (Radio Frequency, RF) to transmit and receive data over air, providing all the features and benefits of wired LAN technologies without the limitations of a cable. A wired LAN connects with a wired LAN by means of an Access Point (AP). An Access Point is a device that transports data between a wireless LAN and a wired LAN. It also requires wireless NICs installed in each workstation. ____________________________________ P.5 S4/CIT/Computer Network/Notes
    • Wireless PCMCIA Wireless NIC Card Wireless Access Point USB Adapter Advantages of using wireless network over wired network: 1. Save the cost in laying network cables 2. No cluttering of cables 3. More flexible when adding workstations 4. Notebook computer can access network at anywhere under the coverage area F. Remote access to a LAN A MODEM (Modulator-demodulator) is needed for remote access to a LAN or the Internet via a telephone line. Data in computer are digital signal ( 數 碼 信 息 ) (i.e. 01001101). However, telephone line can only transmit analogue signal (模擬信息). So, a modem is needed to modulate a digital signal into analogue signal. Through the telephone line, the incoming signal is analogue, so a modem is needed to demodulate an analogue signal into digital signal. So, to connect computers through a telephone line, there should be at least two modems. The way to use modem to connect to the Internet is called dial-up access. The data transfer rate is low. G. Network Topology It is the logical layout or the way in which the computers and other devices are connected. 1. Ring Topology All devices are connected on a circular path along which data flows in one direction. When a computer sends data, the data travels to each computer on the ring until it reaches its destination. ____________________________________ P.6 S4/CIT/Computer Network/Notes
    • Advantage: - Easy and cheap to setup Disadvantage: - Data transmission in one direction. - Failure of a single device means all devices after cannot function. 2. Star Topology All devices on the network connect to a central computer (usually a server), thus forming a star. All data transferred from one computer to another passes through the server. Advantage: - One device failed will not affect the other devices Disadvantage: - If the server breaks down, the whole network fails. 3. Bus Topology All computers and other devices are connected to a single central cable. It is flexible such that computer can be attached to or detached from the network without disturbing the rest of the network. Advantage: - Failure of one device will not affect other devices. Disadvantage: - Failure of the cable means the whole network is broken down. Note: Practically, more than one topology is used in a network. For example, in a school network, a bus-star topology is used with a high bandwidth bus as the backbone (usually a optical fibre). The backbone connects a number of star networks such as a LAN inside the computer room. H. Network Operating System The most popular operating systems like Windows 98/ME, Windows 2000/XP include the software necessary to establish communication with a network server. They are called Network Operating System (NOS). It consists of two components: 1. Network Server Software - Controls file access from the server’s hard disk - Manages the print queue - Tracks user data such as user IDs and passwords Examples include Windows 2000 Server Edition, Windows 2003 Server Edition ____________________________________ P.7 S4/CIT/Computer Network/Notes
    • 2. Network Client Software - Gathers login information - Handles drive mapping - Directs printouts to the network printer Examples include Windows 2000/XP Home or Professional Edition Note: The server and the client software components do not necessarily have to match. Imagine a Windows NT computer in library and some Windows XP computer in classroom can login to the school’s Windows 2000 Server. I. Common Services on a Network Server Description Name File server - A high speed computer that stores the programs and data files shared by users. - Most file servers also store user accounts and the network information. - Data in a file server are important and must be back up regularly. Mail server - It provides e-mail services to users. - Each user has a mailbox which stores the incoming mails and copies of sent mails. Print - It manages one or more printers on a network. server - It allows a shared printer to be accessed by authorized users. Web server - A web server stores the contents of web pages. - It delivers web pages according to the requests from other users on the Internet. Proxy - It improves the efficiency of Internet access by temporarily storing copies of web server pages visited by previous users of the same LAN. - It also filters web pages to prevent users from accessing certain sites. J. Comparison of Speed in different network in terms of Data Transfer Rate The speed of a network is measured in the unit bps (bit per second). Note: The actual data transfer rate is much less than the bandwidth because of different factors. Can you find some reasons? _______________________________________________________________________ _______________________________________________________________________ Type of network Bandwidth(頻寬) LAN 10M bps Ethernet (乙太網) 100M bps Fast Ethernet 1G bps (~1000M bps) Gigabit Ethernet Wireless Bluetooth 1M bps / 10M bps LAN 802.11b 11M bps 802.11g 54M bps WAN T1 1.5M bps T2 44.7M bps ISDN 1.5 Mbps Modem Remote access through telephone line 56K bps ____________________________________ P.8 S4/CIT/Computer Network/Notes
    • MC Exercise 2: 1. A modem is needed by a computer to connect to a remote network through A. satellite B. power cable C. telephone line D. optical fibre 2. A network consists of a wireless access point. Which of the following may be used for a notebook computer to access the network? A. a wireless network interface card B. a wireless access point C. a TV antenna D. a radio tuner 3. A school is going to set up a wireless LAN. Which of the following is needed to maintain the user accounts? A. access point B. server C. switch D. hub 4. A modem converts signals from A. analog to digital only. B. binary to decimal only. C. analog to digital and from digital to analog. D. binary to decimal and from decimal to binary. 5. A wireless LAN can work without A. access point B. wireless NIC C. power cord D. UTP cable 6. Which of the following servers is needed by a web site? A. file server B. web server C. mail server D. proxy server 7. Which of the following servers can filter undesirable web sites? A. file server B. web server C. mail server D. proxy server ____________________________________ P.9 S4/CIT/Computer Network/Notes
    • 8. Which of the following statement(s) is / are correct? (1) LAN and WAN can be connected wither by cable or by wireless media. (2) The computer network in a school is LAN. (3) The network of LANs of schools in Hong Kong is WAN. A. (1) only B. (1) and (2) only C. (2) and (3) only D. (1), (2) and (3) 9. Which of the following statement(s) about file server is / are correct? (1) Examination papers should not be stored in file server since students can read the files in the server. (2) Teaching materials and class assignments can be stored in the file server and shared to all students. (3) Since file server is a centralized file storage, backup is very important. A. (3) only B. (1) and (2) only C. (2) and (3) only D. (1), (2) and (3) 10. Which of the following item is NOT the basic component of a network? A. Network Interface Card (NIC) B. Hub / Switch C. UTP Cable D. Secondary Storage ____________________________________ P.10 S4/CIT/Computer Network/Notes