【正文】 el at Dartmouth College to his Complex Number Calculator in New York and received results back by the same means. In the late 1950s, early networks of municating puters included the military radar system SemiAutomatic Ground Environment (SAGE). In 1960, the mercial airline reservation system semiautomatic business research environment (SABRE) went online with two connected mainframes. In 1962, . Licklider developed a working group he called the Intergalactic Computer Network, a precursor to the ARPANET, at the Advanced Research Projects Agency (ARPA). In 1964, researchers at Dartmouth developed the Dartmouth Time Sharing System for distributed users of large puter systems. The same year, at Massachusetts Institute of Technology, a research group supported by General Electric and Bell Labs used a puter to route and manage telephone connections. Throughout the 1960s, Leonard Kleinrock, Paul Baran and Donald Davies independently conceptualized and developed network systems which used packets to transfer information between puters over a network. In 1965, Thomas Marill and Lawrence G. Roberts created the first wide area network (WAN). This was an immediate precursor to the ARPANET, of which Roberts became program manager. Also in 1965, the first widely used telephone switch that implemented true puter control was introduced by Western Electric. In 1969, the University of California at Los Angeles, the Stanford Research Institute, the University of California at Santa Barbara, and the University of Utah were connected as the beginning of the ARPANET network using 50 kbit/s circuits.[2] In 1972, mercial services using were deployed, and later used as an underlying infrastructure for expanding TCP/IP networks.Today, puter networks are the core of modern munication. All modern aspects of the public switched telephone network (PSTN) are putercontrolled. Telephony increasingly runs over the Internet Protocol, although not necessarily the public Internet. The scope of munication has increased significantly in the past decade. This boom in munications would not have been possible without the progressively advancing puter network. Computer networks, and the technologies that make munication between networked puters possible, continue to drive puter hardware, software, and peripherals industries. The expansion of related industries is mirrored by growth in the numbers and types of people using networks, from the researcher to the home user.An interconnected collection of autonomous puters (unique identity) is known as a puter network.PropertiesA puter network has the following properties:Facilitates interpersonal municationsPeople can municate efficiently and easily via , instant messaging, chat rooms, telephone, video telephone calls, and video conferencing.Allows sharing of files, data, and other types of informationAuthorized users may access information stored on other puters on the network. Providing access to information on shared storage devices is an important feature of many networks.Allows sharing of network and puting resourcesUsers may access and use resources provided by devices on the network, such as printing a document on a shared network printer. Distributed puting uses puting resources across a network to acplish tasks.May be insecureA puter network may be used by puter crackers to deploy puter viruses or puter worms on devices connected to the network, or to prevent these devices from accessing the network (denial of service).May interfere with other technologiesPower line munication strongly disturbs certain[3]forms of radio munication, ., amateur radio. It may also interfere with last mile access technologies such as ADSL and VDSL.May be difficult to set upA plex puter network may be difficult to set up. It may be costly to set up an effective puter network in a large organization.Communication mediaComputer networks can be classified according to the hardware and associated software technologies used to interconnect the individual network devices. These technologies include electrical cable (HomePNA, power line munication, ), optical fiber, and radio waves (wireless LAN). In the OSI model, these are located at levels 1 and 2.A widelyadopted family of munication media is collectively known as Ethernet. Defined by IEEE 802, it uses various standards and media to enable munication between devices. Wireless LAN technology is designed to connect devices without wiring. These devices use radio waves or infrared signals as a transmission medium.Wired technologiesThe order of the following wired technologies are, roughly, from slowest to fastest transmission speed.Twisted pair wire is the most widely used medium for telemunication. Twistedpair cabling consist of copper wires that are twisted into pairs. Ordinary telephone wires consist of two insulated copper wires twisted into pairs. Computer network cabling (wired Ethernet as defined by IEEE ) consists of 4 pairs of copper cabling that can be utilized for both voice and data transmission. The use of two wires twisted together helps to reduce crosstalk and electromagnetic induction. The transmission speed ranges from 2 million bits per second to 10 billion bits per second. Twisted pair cabling es in two forms: unshielded twisted pair (UTP) and shielded twistedpair (STP). Each form es in several category ratings, designed for use in various scenarios.Coaxial cable is widely used for cable television systems, office buildings, and other worksites for local area networks. The cables consist of copper or aluminum wire surrounded by an insulating layer (typically a flexible material with a high dielectric constant), which itself is surrounded by a conductive layer. The insulation helps minimize interference and distortion. Transmission speed ranges from 200 million bits per second to more than 500 million bits per second.ITUT technology uses existing home wiring (coaxi