ࡱ> /1.7 bjbjUU ""7|7|lJJJJ V2~~~~~~~~FHHHHHH$ ^l~~~~~l~~~~~~~~~F~~F~Z~rFTF~r AJJ~F0w~wF~Network Environments LAN - Local Area Network WAN - Wide Area Network Standalone - computer not on a network Advantages of LAN Networks Resources (e.g. printers, scanners, modems) can be shared Information can be shared (e.g. transfer of files, an Intranet) Applications can be installed on one computer and be made available across a network E-mail can be sent between users Easy to set up new users and equipment Different types of computer can communicate with each other Disadvantages of LAN Networks Users are dependant on the server. If this fails, certain applications or hardware devices may not be available A badly managed network is worse than a standalone machine. Much depends on the skill of the network manager. Networks have more security issues than standalones (e.g. hackers) Network traffic can degrade performance Server Based v Peer-to-Peer Networks Server Based Network - Different devices on the network are treated as clients or servers. The client devices send requests for service e.g. printing or retrieval of data. The server devices can be peripherals such as a printer, or it can be the file server itself. On a server-based network (e.g. Novell Netware) software and the backing store are held separately. If the server goes down, everyone is affected. User IDs and passwords are held centrally. Some processing tasks are performed by the server and some by the desktop computer. Peer-to-Peer Network - Each workstation communicates directly with every other workstation, without going through a server. On a peer-to-peer network (e.g. Windows for Workgroups) storage facilities are distributed throughout the network. Software is held on individual machines. Security is not centrally controlled. Backup is the responsibility of individual users. This type of network is less expensive. A peer-to-peer network is suitable for small offices where there are only 3-4 machines on the network. One disadvantage is that, if one workstation is switched off, the files on that station cannot be accessed from the rest of the network. The Network Manager Any network has to be managed. The network manager should ensure that: Response time is adequate Hardware is upgraded when necessary The required software is loaded Software is upgraded when necessary and users are informed of changes A system of regular backups is adhered to Network security is maintained WANs - Wide Area Networks Communication can be via microwave, satellite link or telephone line. The use of "global networks" e.g. the Internet has grown because of the changeover from analogue to digital lines, improved file compression techniques (both of which make downloading files faster) and a reduction in the cost of computers. Communication Links Telephone lines may be public (a fee is paid to the telephone company for time spent using the service) or private/leased lines (where there can be unlimited usage in return for an annual fee). Communication Media Twisted Pair (copper cable) - e.g. telephone lines Coaxial Cable - High quality insulated cable that transmits data at high speeds Fibre Optic Cable - Pulses of light (instead of electricity) are sent in digital form Microwave (e.g. mobile phone networks) - send signals in straight lines. Therefore, stations must be close together because of the earth's curvature. Communications Satellite - Hundreds of satellites are in geosynchronous orbit about 22,000 miles above earth. Bandwidth Bandwidth is the range of frequencies that the line can carry. ISDN and ASDL lines provide greater bandwidth than a standard telephone line. Bridges and Gateways Bridges - connect two LANs Gateways - computers that connect parts of a WAN Modems These convert from analogue signals to digital format. Modem = MOdulator/DEModulator Synchronous and Asynchronous Transmission Asynchronous transmission is when one character at a time is sent, with each character being preceded by a start bit and followed by a stop bit. A parity bit is usually also included (this checks for errors). Synchronous transmission - there is no need for the start and stop bits. Timing signals from the computer's internal clock control the rate of transmission. This is usually less error-prone. Protocols These are rules that allow different types of computers to communicate with each other on a network. Any equipment that uses the same protocol can be linked together. 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