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1、英文原文： Computer networkA computer network, often simply referred to as a network, is a collection of computers and devices interconnected by communications channels that facilitate communications among users and allows users to share resources. Networks may be classified according to a wide variety o
2、f characteristics. A computer network allows sharing of resources and information among interconnected devices. History ：Early networks of communicating computers included the military radar system Semi-Automatic Ground Environment (SAGE) and its relative the commercial airline reservation system Se
3、mi-Automatic Business Research Environment (SABRE),started in the late 1950s.12 When Russia launched His SPUTNIK Satellite in Space In 1957.The American Started Agency Names ADVANCE RESEARCH PROJECT AGENCY (ARPA) & launched THis 1st Satellite Within 18 Month After Establishment.Then Sharing Of TheIn
4、formation InAnother Computer They Use ARPANET.And This All Responsibility On Americas Dr.LIED LIEDER.Then in 1969,ARPANET Comes in INDIA And INDIAN Switched This Name To NETWORK. In the 1960s, the Advanced Research Projects Agency (ARPA) started funding the design of the Advanced Research Projects A
5、gency Network (ARPANET) for the United States Department of Defense. Development of the network began in 1969, based on designs developed during the 1960s.3 The ARPANET evolved into the modern Internet. Purpose ：Computer networks can be used for a variety of purposes: Facilitating communications. Us
6、ing a network, people can communicate efficiently and easily via email, instant messaging, chat rooms, telephone, video telephone calls, and video conferencing. Sharing hardware.：In a networked environment, each computer on a network may access and use hardware resources on the network, such as prin
7、ting a document on a shared network printer. Sharing files, data, and information. In a network environment, authorized user may access data and information stored on other computers on the network. The capability of providing access to data and information on shared storage devices is an important
8、feature of many networks. Sharing software.： Users connected to a network may run application programs on remote computers. information preservationSecurityNetwork classification The following list presents categories used for classifying networks. Connection method ：Computer networks can be classif
9、ied according to the hardware and software technology that is used to interconnect the individual devices in the network, such as optical fiber, Ethernet, wireless LAN, HomePNA, power line communication or G.hn. Ethernet as it is defined by IEEE 802 utilizes various standards and mediums that enable
10、 communication between devices. Frequently deployed devices include hubs, switches, bridges, or routers. Wireless LAN technology is designed to connect devices without wiring. These devices use radio waves or infrared signals as a transmission medium. ITU-T G.hn technology uses existing home wiring
11、(coaxial cable, phone lines and power lines) to create a high-speed (up to 1 Gigabit/s) local area network. Wired technologies ：Twisted pair wire is the most widely used medium for telecommunication. Twisted-pair cabling consist of copper wires that are twisted into pairs. Ordinary telephone wires c
12、onsist of two insulated copper wires twisted into pairs. Computer networking cabling consist 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 r
13、anges from 2 million bits per second to 100 million bits per second. Twisted pair cabling comes in two forms which are Unshielded Twisted Pair (UTP) and Shielded twisted-pair (STP) which are rated in categories which are manufactured in different increments for various scenarios. Coaxial cable is wi
14、dely used for cable television systems, office buildings, and other work-sites for local area networks. The cables consist of copper or aluminum wire wrapped with insulating layer typically of a flexible material with a high dielectric constant, all of which are surrounded by a conductive layer. The
15、 layers of insulation help minimize interference and distortion. Transmissionspeed range from 200 million to more than 500 million bits per second. Optical fiber cable consists of one or more filaments of glass fiber wrapped in protective layers. It transmits light which can travel over extended dis
16、tances. Fiber-optic cables are not affected by electromagnetic radiation. Transmission speed may reach trillions of bits per second. The transmission speed of fiber optics is hundreds of times faster than for coaxial cables and thousands of times faster than a twisted-pair wire.citation needed Wirel
17、ess technologies ：Terrestrial microwave Terrestrial microwaves use Earth-based transmitter and receiver. The equipment looks similar to satellite dishes. Terrestrial microwaves use low-gigahertz range, which limits all communications to line-of-sight. Path between relay stations spaced approx, 30 mi
18、les apart. Microwave antennas are usually placed on top of buildings, towers, hills, and mountain peaks. Communications satellites The satellites use microwave radio as their telecommunications medium which are not deflected by the Earths atmosphere. The satellites are stationed in space, typically
19、22,000 miles (for geosynchronous satellites) above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals. Cellular and PCS systems Use several radio communications technologies. The systems are divided to different geographic areas. Each area has
20、 a low-power transmitter or radio relay antenna device to relay calls from one area to the next area. Wireless LANs Wireless local area network use a high-frequency radio technology similar to digital cellular and a low-frequency radio technology. Wireless LANs use spread spectrum technology to enab
21、le communication between multiple devices in a limited area. An example of open-standards wireless radio-wave technology is IEEE. Infrared communication , which can transmit signals between devices within small distances not more than 10 meters peer to peer or ( face to face ) without any body in th
22、e line of transmitting. Scale： Networks are often classified as local area network (LAN), wide area network (WAN), metropolitan area network (MAN), personal area network (PAN), virtual private network (VPN), campus area network (CAN), storage area network (SAN), and others, depending on their scale,
23、 scope and purpose, e.g., controller area network (CAN) usage, trust level, and access right often differ between these types of networks. LANs tend to be designed for internal use by an organizations internal systems and employees in individual physical locations, such as a building, while WANs may
24、 connect physically separate parts of an organization and may include connections to third parties. Functional relationship (network architecture) ：Computer networks may be classified according to the functional relationships which exist amongthe elements of the network,e.g., active networking, clie
25、ntserver, Wireless ad hoc network andpeer-to-peer (workgroup) architecture. Network topology ：Main article: Network topology Computer networks may be classified according to the network topology upon which the network is based, such as bus network, star network, ring network, mesh network. Network t
26、opology is the coordination by which devices in the network are arranged in their logical relations to one another, independent of physical arrangement. Even if networked computers are physically placed in a linear arrangement and are connected to a hub, the network has a star topology, rather than
27、a bus topology. In this regard the visual and operational characteristics of a network are distinct. Networks may be classified based on the method of data used to convey the data, these include digital and analog networks. Types of networks based on physical scopeCommon types of computer networks m
28、ay be identified by their scale. Local area network： A local area network (LAN) is a network that connects computers and devices in a limited geographical area such as home, school, computer laboratory, office building, or closely positioned group of buildings. Each computer or device on the network
29、 is a node. Current wired LANs are most likely to be based on Ethernet technology, although new standards like ITU-T G.hn also provide a way to create a wired LAN using existing home wires (coaxial cables, phone lines and power lines).4 Typical library network, in a branching tree topology and contr
30、olled access to resources All interconnected devices must understand the network layer (layer 3), because they are handling multiple subnets (the different colors). Those inside the library, which have only 10/100 Mbit/s Ethernet connections to the user device and a Gigabit Ethernet connection to th
31、e central router, could be called layer 3 switches because they only have Ethernet interfaces and must understand IP. It would be more correct to call them access routers, where the router at the top is a distribution router that connects to the Internet and academic networks customer access routers
32、. The defining characteristics of LANs, in contrast to WANs (Wide Area Networks), include their higher data transfer rates, smaller geographic range, and no need for leased telecommunication lines. Current Ethernet or other IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s. This is the d
33、ata transfer rate. IEEE has projects investigating the standardization of 40 and 100 Gbit/s.5 Personal area network :A personal area network (PAN) is a computer network used for communication among computer and different information technological devices close to one person. Some examples of devices
34、 that are used in a PAN are personal computers, printers, fax machines, telephones, PDAs, scanners, and even video game consoles. A PAN may include wired and wireless devices. The reach of a PAN typically extends to 10 meters.6 A wired PAN is usually constructed with USB and Firewire connections whi
35、le technologies such as Bluetooth and infrared communication typically form a wireless PAN. Home area network :A home area network (HAN) is a residential LAN which is used for communication between digital devices typically deployed in the home, usually a small number of personal computers and acces
36、sories, such as printers and mobile computing devices. An important function is the sharing of Internet access, often a broadband service through a CATV or Digital Subscriber Line (DSL) provider. It can also be referred to as an office area network (OAN). Wide area network :A wide area network (WAN)
37、 is a computer network that covers a large geographic area such as a city, country, or spans even intercontinental distances, using a communications channel that combines many types of media such as telephone lines, cables, and air waves. A WAN often uses transmission facilities provided by common c
38、arriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer. Campus network :A campus network is a computer network made up of an interconnection of local area networks
39、(LANs) within a limited geographical area. The networking equipments (switches, routers) and transmission media (optical fiber, copper plant, Cat5 cabling are almost entirely owned (by the campus tenant / owner: an enterprise, university, government etc.). In the case of a university campus-based ca
40、mpus network, the network is likely to link a variety of campus buildings including; academic departments, the university library and student residence halls. Metropolitan area network:A Metropolitan area network is a large computer network that usually spans a city or a large campus. Sample EPN mad
41、e of Frame relay WAN connections and dialup remote access. Enterprise private network :An enterprise private network is a network build by an enterprise to interconnect various company sites, e.g., production sites, head offices, remote offices, shops, in order to share computer resources. Virtual p
42、rivate network :A virtual private network (VPN) is a computer network in which some of the links between nodes are carried by open connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires. The data link layer protocols of the virtual network are said t
43、o be tunneled through thelarger network when this is the case. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of diff
44、erent user communities over an underlying network with strong security features. VPN may have best-effort performance, or may have a defined service level agreement (SLA) between the VPN customer and the VPN service provider. Generally, a VPN has a topology more complex than point-to-point. Internet
45、work :An internetwork is the connection of two or more private computer networks via a common routing technology (OSI Layer 3) using routers. The Internet is an aggregation of many internetworks, hence its name was shortened to Internet. Backbone network :A Backbone network (BBN) A backbone network
46、or network backbone is part of a computer network infrastructure that interconnects various pieces of network, providing a path for the exchange of information between different LANs or subnetworks.12 A backbone can tie together diverse networks in the same building, in different buildings in a camp
47、us environment, or over wide areas. Normally, the backbones capacity is greater than the networks connected to it. A large corporation that has many locations may have a backbone network that ties all of the locations together, for example, if a server cluster needs to be accessed by different depar
48、tments of a company that are located at different geographical locations. The pieces of the network connections (for example: ethernet, wireless) that bring these departments together is often mentioned as network backbone. Network congestion is often taken into consideration while designing backbon
49、es. Backbone networks should not be confused with the Internet backbone. Global area network:A global area network (GAN) is a network used for supporting mobile communications across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is handing off the user communications from one local coverage area to the next. In IEEE Project 802, thi
