【正文】 SI 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 microwave– Terrestrial microwave munication uses Earthbased transmitters and receivers resembling satellite dishes. Terrestrial microwaves are in the lowgigahertz range, which limits all munications to lineofsight. Relay stations are spaced approximately 48km (30mi) apart.Communications satellites– Satellites municate via microwave radio waves, which are not deflected by the Earth39。s atmosphere. The satellites are stationed in space, typically in geosynchronous orbit 35,400km (22,000mi) above the equator. These Earthorbiting systems are capable of receiving and relaying voice, data, and TV signals.Cellular and PCS systems use several radio munications technologies. The systems divide the region covered into multiple geographic areas. Each area has a lowpower transmitter or radio relay antenna device to relay calls from one area to the next area.Radio and spread spectrum technologies– 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: IP over Avian Carriers was a humorous April fool39。s Request for Comments, issued as RFC 1149. It was implemented in real life in 2001.[5] Extending the Internet to interplanetary dimensions via radio waves.[6]Both cases have a large roundtrip delay time, which gives slow twoway munication, but doesn39。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。s personal puter when the user is surfing the web.Communication protocols have various characteristics. They may be connectionoriented or connectionless, they may use circuit mode or packet switching, and they may use hierarchical addressing or flat addressing.There are many munication protocols, a few of which are described below.EthernetEthernet is a family of protocols used in LANs, described by a set of standards together called IEEE 802 published by the Institute of Electrical and Electronics Engineers. It has a flat addressing scheme. It operates mostly at levels 1 and 2 of the OSI model. For home users today, the most wellknown member of this protocol family is IEEE , otherwise known as Wireless LAN (WLAN). The plete IEEE 802 protocol suite provides a diverse set of networking capabilities. For example, MAC bridging (IEEE ) deals with the routing of Ethernet packets using a Spanning Tree Protocol, IEEE describes V