Encyclopedia of Computer Science and Technology

334 networkGoogle Groups, is the best-known and most complete suchsite.Further ReadingGoogle Groups. Available online. URL: http://groups.google.com.Accessed August 16, 2007.Hauben, Michael, and Ronda Hauben. Netizens: On the History andImpact of Usenet and the Internet. Los Alamitos, Calif.: IEEEComputer Society Press, 1997.Lueg, Christopher, and Danyel Fisher, eds. From Usenet to CoWebs:Interacting with Social Information Spaces. London: Springer,2003.Pfaffenberger, Bryan. The USENET Book: Finding, Using, and SurvivingNewsgroups on the Internet. Reading, Mass.: Addison-Wesley, 1995.Spencer, Henry, and David Lawrence. Managing Usenet. Sebastopol,Calif.: O’Reilly, 1998.networkIn the 1940s, the main objective in developing the first digitalcomputers was to speed up the process of calculation. Inthe 1950s, the machines began to be used for more generaldata-processing tasks by governments and business. By the1960s, computers were in use in most major academic, government,and business organizations. The desire for usersto share data and to communicate both within and outsidetheir organization led to efforts to link computers togetherinto networks.Computer manufacturers began to develop proprietarynetworking software to link their computers, but they werelimited to a particular kind of computer, such as a DEC PDPminicomputer, or an IBM mainframe. However, the U.S.Defense Department, seeing the need for a robust, decentralizednetwork that could maintain links between theircomputers under wartime conditions, funded the developmentof a protocol that, given appropriate hardware tobridge the gap, could link these disparate networks (seeInternet, local area network).Network ArchitectureToday’s networks are usually defined by open (that is, nonproprietary)specifications. According to the OSI (open systemsinterconnection) model, a network can be consideredto be a series of seven layers laid one atop another (see datacommunication).The physical layer is at the bottom. It specifies the physicalconnections between the computers, which can be anythingfrom ordinary phone lines to cable, fiber optic, orwireless. This layer specifies the required electrical characteristics(such as voltage changes and durations that constitutethe physical signal that is recognized as either a 1 or 0in the “bit stream.”The next layer, called the data link layer, specifies howdata will be grouped into chunks of bits (frames or packets)and how transmission errors will be dealt with (see errorcorrection).The network layer groups the data frames as parts of aproperly formed data packet and routes that packet fromthe sending node to the specified destination node. A varietyof routing algorithms can be used to determine the mostefficient route given current traffic or line conditions.The transport layer views the packets as part of a completetransmission of an object (such as a Web page) andensures that all the packets belonging to that object aresorted into their original sequence at the destination. Thisis necessary because packets belonging to the same messagemay be sent via different routes in keeping with trafficor line conditions.The session layer provides application programs communicatingover the network with the ability to initiate,terminate, or restart an interrupted data transfer.The presentation layer ensures that data formats areconsistent so that all applications know what to expect.This layer can also provide special services (see encryptionand data compression).Finally, the application layer gives applications highlevelcommands for performing tasks over the network,such as file transfer protocol (ftp).Most modern operating systems support this model.The Internet protocol (see TCP/ip) has become the linguafranca for most networking, so modern versions of MicrosoftWindows and the Macintosh Operating System as wellas all versions of UNIX provide the services that applicationsneed to make and manage TCP/IP connections.Networks that link computers remotely (such as overphone lines) are sometimes called wide area networks, orWANs. Networks that link computers within an office,home, or campus, usually using cables, are called localarea networks (LANs). See local area network for moredetails about LAN architecture and software.TrendsIt has become the norm for desktop and portable computersto have access to the Internet. A computer from whichone cannot send or receive e-mail or view Web pages almostgives the perception of being crippled, because so manyapplications now assume that they can access the network.For example, the latest antivirus programs regularly checktheir manufacturer’s Web site and download the latest virusdefinitions and software patches. Recent versions of Windows,too, include a built-in update facility that can obtainsecurity patches and newer versions of device drivers.The flip side of the power of networking to keep everyPC (and its user) up to date is the vulnerability to bothintrusion attempts and viruses (see computer crime andsecurity). Virtually all networks include a layer of softwarewhose job it is to attempt to block intrusions and protectsensitive information (see firewall).Besides attending to security, network administratorsand engineers must continually monitor the trafficon the network, looking for bottlenecks, such as an oftenrequesteddatabase being stored on a file server with a relativelyslow hard drive. Besides upgrading key hardware,another approach to relieve congestion is to adopt a distributeddatabase (see database management system)that stores “data objects” throughout the network and candynamically relocate them to improve access.