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Transcript
Illawarra Grammar School • Computing networks at first consisted of a mainframe computer connected to dumb terminals which had no processing power themselves. Users shared the mainframe’s CPU, disk storage and other resources. The mainframe did all the work and this is referred to as a Shared Processor System and Centralised Processing. • Due to advancements of the personal PC and falling costs, companies have replaced their mainframes & minicomputers with networks of PCs. Each PC operates separately of the others, yet any PC can share files, printers and other resources with any other PC on the network. Referred to as Distributed Processing • An emerging technology is the attempt to combine the best of distributed computing and centralised computing (a hybrid of both). By storing all the programs and data on a central server, network administrators regain control over the organisations’ computing resources. Because users can’t install programs on their own machines, they can run only authorised software - this great reduces the risk of computer viruses and copyright infringements • PCs have their own processor, graphical display and disk storage. Special PCs (file servers) allow individual client PCs to use the shared disk, printers etc. This provides excellent performance and flexibility but at a high cost in equipment, software and ongoing maintenance. • Thin clients are PCs with no disk storage. This clients must load all their software from a central server and must save their work on the server. Users can’t install their own programs or make configuration changes. It combines the nature of mainframe computing with the power and graphical interface of personal computers. • A Network Model consists of: • Network Operating System (NOS). Consists of a suite of programs that run in networked computers. Some programs allow the sharing of files and printers. Computers that share resources are called servers. Both client and server software run at the same time. • Networked peripherals: Include printers & modems. Networked peripherals have internal specialised processors to run networking server software so they don’t have to be directly attached to a computer. • A Network Model consists of: • Network Interface Card (NIC or LAN adaptor): A network interface card changes the computer signals into more powerful signals that can cross a network cable. The NIC takes the data from the computer, packages the data for transmission and acts as a gatekeeper to control access to the shared network cable. • Network Cabling: electrical pulses can be sent over copper cable, pulses of light over fibre-optic or through the air using radio or light waves. Cabling installations use a wiring hub to isolate cable problems and improve reliability. Modem- Networked Device Networking Software Wiring Hub Printer- Networked Device Network Interface Card Networking Software • Networking software can be a separate product one adds to a system such as Novell’s Netware. Modern operating systems include inbuilt networking functions [Windows 95, 98, 2000, XP]. Apple Macintosh computers come with networking software that allows them to operate on Netware or Windows based networks. • Network Interface Cards links a computer to the cable system. It controls the flow of data between the computer’s internal data bus and the serial stream of data on the network cable. Some computers now come with their NIC built into the main circuit board. Printer- Networked Device Modem- Networked Device • Devices such as printers and modems can have their own network attachments. The networking software makes it possible to use these shared devices as if they were locally attached. Wiring Hub • One can choose from a variety of network cable types and arrange the cable in several ways. Cabling is a hidden cost that many people fail to consider when planning a network installation. One’s network is only as good as the cabling. • Client Computer: is not a server; its networking client software allows application programs to use disk drives, printers, etc. as if they were directly attached to the computer. In this case an application program (spreadsheet) has a print job. The print job goes from the application to Windows with instructions to print on a specific port such as LPT2. Windows send any print jobs addressed to LPT2 to a specific networked printer. • Redirector: the principle element of the client software. The redirector captures service request it has been programmed to recognise and routes them out of the PC and across the network for service. Typically, redirector software is a built-in part of the client’s operating system • Network communications protocols package the requests from the client computers and send the requests across the network. Protocols handle the nitty-gritty details of addressing, routing, ensuring delivery and ensuring accuracy. Typical protocols include Apple File Protocol (Apple), Microsoft’s NetBIOS Extended User Interface (NetBEUI), & Novell’s Sequental Packet Exchange and Internetwork Packet Exchange (SPX and IPX. Because the Internet is based on the TCP/IP protocol, many companies have adopted TCP/IP as their standard corporate networking protocol. TCP/IP is very flexible, and it can transport data on behalf of other protocols such as NETBIOS. • The driver software works between the network interface card and the network’s communications software. If an adaptor manufacturer supplies drivers conforming to Microsoft Network Driver Interface Standard (NDIS) or Novell’s Open Data-Link Interface (ODI), one can use the adaptor with a variety of network operating systems. • Netware Dedicated File Server: It can’t run applications. When a request for file access is made, the server software checks to make sure the person placing the request has been granted access to the file. Once validated, the request goes to the file server software. This software coordinates simultaneous requests for the same data, finds the data, and sends it back to the requesting client computer. • Client Computer & Print Server: The networking software in this computer gives it the ability to act as a client, so it can request files & send print jobs to other server computers, but it can also act as a print server. It can accept print jobs from other client computers and print them on its locally attached printer. • Client Computer: is running an application program, a database, that needs access to a file. The application sends to Windows the filename, along with the specified disk drive name, eg. F:. Windows reroutes any request for data on the F: drive to the file server. • Network operating systems package requests from the keyboard & from applications in a succession of data envelopes for transmission across the network. In this example, Novell’s NetWare packages a file request in an IPX packet, and the LAN adaptor packages the IPX request into an Ethernet frame. Each data envelope contains its own addressing & error control information RJ-45 Connector twisted pair wire Connector for Transceiver BNC coaxial connector Expansion bus connectors • RJ-45 Connector Unshielded Twisted Pair • BNC connector Coaxial Cable • IBM data connector Shielded Twisted Pair • ST fibre-optic connector • Network cabling should be at least Category 5 • LAN cables come in many physical configurations. One needs to consider resistance to crosstalk - electric currents between pairs of wires in the same cable - resistance to outside electrical fields caused by power lines, motors, relays, radio transmitters, other devices; and ease of installation. • Fibre-optic cables uses pulses of light and have total immunity from electrical noise. Fibre-optic cables carry signals faster and farther than any other type of cable. Cables with outside shields of copper braid or foil, such as coaxial and shielded twisted pair, offer good resistance to electrical noise, but because they are thicker, they are difficult to pull through the wiring conduits & walls. The thin, unshielded twisted-pair is easier to install but offers less resistance to electrical noise. This fibre-optic cable doesn’t fill conduits, but installers need special training and equipment. Internet Access point A Access point B Router Laptop with wireless card • Wireless LANs are used to provide LAN connections for portable PCs. Each laptop has a wireless access card. Server Printer • Bluetooth: named for a Viking king, Bluetooth is a new wireless protocol that promises to link virtually any type of communications equipment into a portable, small network. • Bluetooth uses radio technology similar to the 802.11 wireless LAN standard, but at a very much lower power level. The low power level allows Bluetooth devices to be very small, and it also reduces interference from other close Bluetooth devices. • Bluetooth devices have a range of about 10 metres and can operate at speeds up to about 700bps. • Think of Bluetooth as a wireless replacement for all the cables in your office. All Bluetooth devices can talk to each other, allowing them to exchange information wirelessly. Typical applications include wireless links among cell phones, laptop / handheld computers, scanners, keyboards, mice, and digital cameras. • Each client computer runs its own application programs, & networking software in each machine redirects requests for file & print services to the file server. Files created by the applications and perhaps the applications themselves, cross the network from the file server. • This is designed to act as a central server, is equipped with multiple hard drives, a tape drive (backup), CD-ROM drive. It uses specialised multitasking operating system and runs many management / monitoring programs • A non-windows Network computer (NC) terminal for mainframes or a NC to carry out web services or Java or other specialist services written in a specific langauge. The programs are executed in the clients but the data / files are sent by the file server. • Windows machines where they are used for input & output. The programs are actually executed at the shared CPU Server. • A thin network client solution reduces the number of user induced problems. • Any computer on the network can share its drives & printers while running application programs. Here different computers share their hard drives, tape drive & CD-ROM drive. Easy to install and economical. How Networks Work Derfler, Freed QUE publishers, ISBN: 0 7897 2445 6