4.2LOCAL AREA NETWORKS

keywords:
lan 局域网
client server 客户服务器
peer to peer 对等方式
hub 集线器
switch 交换机
topology 拓扑
start topology 星星拓扑
ethernet 以太网
randomly 随机的
CSMA/CD 冲突检测
coaxial cable 同轴电缆
router 路由器
ASIC 专用集成电路
Gigabit ethernet 千兆单位以太网
fiber optic 光纤
FDDI 光纤分布数据接口
ATM 异步转移模式
ISM 工业科学 和 医药 频段
FHSS 调频 扩频率
DSSS直序扩频
MAC 媒体访问控制
WAP 无线应用协议
cell phone 蜂窝电话
pager 寻呼机
wireless bridge 无线网桥
leased line 租用线

Orignizations often need networks to connect gengrapgically close resources, sunch as PCs located in the same collage classroom or business office, sunch  networks are known as local arae networks or lans, most lans are of the client-server or peer- to - peer varitey.The most widespeared topology for LANs designed to function as data communicartion for interconnection of local computer-based equipment is the star topology, in which the major equipments used to connct a variety of computer-based devices are hubs or switches.typical lan product of using the star topology is the most famous ethermet, which dominates the lan world today.1.EthernetIEEE 802.3 supports a lan standard orginally developed by Xerox and later extended by a joint venture digital equipment corporatin, Intel corporation, and Xerox, This was called ethernet.(1)Access MethodHow can only one computer at a time be allowed to transmit on the network?　Access to the network - the right to transmit-can be allowed in two ways: randomly or in a deterministic order,carrier sense multiple Access is the random-access method used with Ethernet.This method requires a workstation to listen to the network media before attempting to initate a transmission, if the workstation "senses" on  the line, in defers its transmission for  a short period trying to access the network again, when it sences a clear line,it begins ,when two workstations can sense an unused line and begin transmitting at  approximately the same time,The result is collision, ethernet implements carrier sence multiple access with collision detection.2) 10 base-T and 100 Base - T: twisted-pair EnthernetThe most popular standard defined in the IEEE 803.3 series is 10Base -T,a star-topology lan using unshielded twisted pair(UTp) cable instead of coaxial cable,it supports a data rate of 10Mb/s and has a maximum length (hub to station) of 100 metersThe 100Base-T Ethernet which is known as fast Ethernet has the same topology as 10Base_t showing on fig 4-3 ,using this topology, we can build a fast Ethernet and need Nics of 100Mb.s and hub of 100mb/s and do not need cabling any more3)Gigabit EthernetWith the sucesses of Ethernet and fiber-optic communications, the logical evolution in Ethernet technology led to the development of the Gogabit Ethernet, which is an Ethernet  that operates at 1Gb/s.the cost performance of Gigabit Ethernet outweigts FDDI and ATM, Gigabit Ethernet one hundred time the speed of the regular Ethernet, ten times that of times that of fast Ethernet,and faster than FDDI operating at 150Mb/s2.switchswicth are basically multi-port bridgem but some charateristics with routers, Like touters, switches work by dividing up the network into a number og segments each of which can operate without interference from traffic local to any of the other segments,switching performed at layer2, the Mac address is used, which is dependent of protocol address, like bridge, a switch learns which address reside on each of its ports and then swithces data appropriately, A switch can be designed using conventional microprocessors or delicated ASIC technology.3,wireless local area networks(wlans)1)The 802.11 standard1n 1990, the IEEE 802 executive committee established the 802.11 working group to create a WLan standard, the standard specificed an operaring frequency in the 2.4GHz industrial, Scientific, and medical band and began laying the groundwork as code division- edge technology.Seven years later, the group approved IEEE 802.11 as the world's first WLAN standard with data rates of 1 and 2Mb/s,The 802,11 standard provided 1 or 2 MB/s transmission in the 2.4Ghz band using either a FHSS technique or DSSS also known as code division multiple AccessThe Exection committee anticipated the need for a more robust and faster technology to provided commectivilty,ALmost immediatly, the committee began work on another 802.11 extension that would satisfy the future needs of greater bandwidth and board acceptance,within 24 months, the working group approved two project Authorization requests for high rate , phsical-layer  extensions to 803,11 the two extensions were designed to work withe existing 802,11 medium access control layer, with one being the IEEE 802.11 a for 5Ghz and the other IEEE 802,11b for 2.4Ghz.2) wireless application protocolWap is a standard for providing cell phones, pagers and other handheld devices withe secure access to e-mail and text-board web pages.3)building to building wireless connectivitywithout a wired alternatives, organizations frequently resort to wireless access technologies to link together separate LANs, contracting with a local telephone provider for a leased line presents a variety of drawbacks, installation is typically expensive and rarely immdiate,Monthly fees are often quite high for bandwidth that by lan standards is very low, A wireless bridge can be inverstment is made, there are no recurring charges ,and today's wireless bridge provide the bandwidth one would except from a technology rooted in data, rather than the voice, communications.