【正文】 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 (coaxial cable, phone lines and power lines) to create a highspeed (up to 1 Gigabit/s) local area network.An optical fiber is a glass fiber. It uses pulses of light to transmit data. Some advantages of optical fibers over metal wires are less transmission loss, immunity from electromagnetic radiation, and very fast transmission speeds of up to trillions of bits per second. One can use different colors of lights to increase the number of messages being sent over a fiber optic cable.Wireless technologiesMain article: Wireless networkTerrestrial microwavekm (30– Satellites municate via microwave radio waves, which are not deflected by the Earth39。km (22,000– Wireless local area networks use a highfrequency radio technology similar to digital cellular and a lowfrequency radio technology. Wireless LANs use spread spectrum technology to enable munication between multiple devices in a limited area. IEEE defines a mon flavor of openstandards wireless radiowave technology.Infrared munication can transmit signals for small distances, typically no more than 10 meters. In most cases, lineofsight propagation is used, which limits the physical positioning of municating devices.A global area network (GAN) is a network used for supporting mobile across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile munications is handing off user munications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial wireless LANs.[4]Exotic technologiesThere have been various attempts at transporting data over exotic media:s Request for Comments, issued as RFC 1149. It was implemented in real life in 2001.[5]t prevent sending large amounts of information.Communications protocols and network programmingA munications protocol is a set of rules for exchanging information over a network. In a protocol stack (also see the OSI model), each protocol leverages the services of the protocol below it. An important example of a protocol stack is HTTP running over TCP over IP over IEEE . (TCP and IP are members of the Internet Protocol Suite. IEEE is a member of the Ethernet protocol suite.) This stack is used between the wireless router and the home user39