S4 CIT Computer Network In the past… Computers are standalone machine. What can we do if we want to… share a file with someone? print a file but the printer is connected to another computer? Computer Network It is a system in which two or more computers are connected so that data communications resources sharing (File sharing, hardware sharing e.g. printer) among the computers become possible. Types of Network LAN (Local Area Network) links computer and other peripheral devices over short distances. For examples: • network at home • network in school campuses and offices a LAN with an internal e-mail system, file server or message board is called Intranet. Types of Network WAN (Wide Area Network) 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. Network Connection Protocol Peer to Peer Every computer is connected directly to every other computer. Each computer can use information from, and provide information to each other. There is no server. Security is weak. Example is WinMX. Network Connection Protocol Client / Server Individual computers (clients or 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. Examples: School network, Playing on-line games Hardware Requirement for a Network - NIC NIC (Network Interface Card) also called LAN Card an expansion board added to the computer’s motherboard key hardware component for connecting the computer to a network (wired or wireless) Hardware Requirement for a Network - NIC NIC can be connected to the computer by means of: PCI Bus USB Bus PCMCIA Bus (In Notebook Computer) Hardware Requirement for a Network – Transmission Medium Data and signal need to be transmitted over a network through a medium (媒體) It can be a cable (wired) or through microwave (wireless) Transmission Medium – Twisted-pair Cable Sometimes called the UTP Cable (Unshielded Twisted Pair). Looks similar to telephone cable The most commonly used types of twisted pair wire is Cat5 cable. The connector of this kind of cable is called RJ45. Transmission Medium – Coaxial Cable Transmit at a higher speed when compared with twisted-pair cables Can transmit signal over long distances so they can be laid underground or underwater. Transmission Medium – Optical Fibre Bundles of smooth, 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. Transmission Medium – 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. Hardware Requirement for a Network – Connecting Devices Hub provides a central connection point for cables in a network slower than switch Hardware Requirement for a Network – Connecting Devices Switch advanced type of hubs faster than hubs Hardware Requirement for a Network – Connecting Devices Router connect multiple networks connect LANs to form a WAN connect a LAN to the Internet Wireless Network Usually a Wireless LAN (WLAN) Use microwave (Radio Frequency, RF) to transmit and receive data over air Have same features as a LAN, but without the limitations of a cable. Access Point (AP)is required in a WLAN. It is a device that transports data between a wireless LAN and a wired LAN. wireless NICs are also required in each workstation. Wireless Network Wireless PCMCIA Card Wireless NIC Access Point Wireless USB Adapter Advantages of using Wireless network Save the cost in laying network cables No cluttering of cables More flexible when adding workstations Notebook computer can access network at anywhere under the coverage area. MODEM MODEM – Modulator and Demodulator Modulate Convert a digital signal into analogue signal. Demodulate Convert an analogue signal into digital signal. MODEM Analogue Signal (模擬信息) Can be transmitted along telephone lines and UTP Digital Signal (數碼信息) ie: 01011010 Computer can only process digital signal Using MODEM to connect to the Internet Traditionally, Use telephone lines Speed 56kbps Nowadays Use telephone lines & UTP Cables Called ADSL MODEM Speed 1.5Mbps, 3Mbps, 6Mbps Network Topology It is the logical layout or the way in which the computers and other devices are connected. Types: Ring Star Bus Ring Topology All devices are connected on a circular path data flows in one direction. When a computer sends data, the data travels to each computer on the ring until it reaches its destination. Star Topology All devices on the network connect to a central computer (usually a server) All data transferred from one computer to another passes through the server. Bus Topology All computers and other devices are connected to a single central cable. It is flexible because computer can be attached to or detached from the network without disturbing the rest of the network. Comparison of different Topologies - Ring Ring Topology Advantage: • Easy and cheap to setup Disadvantage: • Data transmission in one direction. • Failure of a single device means all devices after cannot function. Comparison of different Topologies - Star Star Topology Advantage: • One device failed will not affect the other devices Disadvantage: • If the server breaks down, the whole network fails. Comparison of different Topologies - Bus Bus Topology Advantage: • Failure of one device will not affect other devices. Disadvantage: • Failure of the cable means the whole network is broken down. Practical Network Topology 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. Network Operating System Windows 98/ME, Windows 2000/XP, Linux, etc include the software necessary to establish communication with a network server. Old operating system like DOS do not have these network features. Brief Introduction to OS The most important program that runs on a computer. Every general-purpose computer must have an OS to run other programs. OS perform basic tasks recognizing input from the keyboard sending output to the display screen keeping track of files and directories on the disk controlling peripheral devices such as disk drives and printers. Network Server Software multi-user like a traffic cop different programs and users running at the same time do not interfere with each other. controls file access from the server’s hard disk manages the print queue responsible for security ensuring that unauthorized users do not access the system. tracks user data such as user IDs and passwords Examples include Windows 2000 Server Network Client Software Gathers login information Handles drive mapping Directs printouts to the network printer Examples include Windows 98/ME, Windows 2000/XP Home or Professional Edition Practical Consideration The server and the client software components do not necessarily have to match. A Windows NT computer in library and some Windows XP computer in classroom can login to the school’s Windows 2000 Server. Common Services on Network - 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. Common Services on Network - Mail server It provides e-mail services to users. Each user has a mailbox which stores the incoming mails and copies of sent mails. Note the differences of Yahoo! Mail and Outlook Express Common Services on Network - Print server It manages one or more printers on a network. It allows a shared printer to be accessed by authorized users. Sometimes, a Print Server may not be a computer. In most case, it is connected to the main server in the LAN. But a workstation can also act as a Print Server. Common Services on Network - 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. Web pages are delivered through a program called web server program. Eg: Microsoft IIS, Apache Common Services on Network - Proxy server It improves the efficiency of Internet access by temporarily storing copies of web pages visited by previous users of the same LAN. It also filters web pages to prevent users from accessing certain sites.