局域网操作分析研究（文献翻译）

1、局域网操作分析研究网络有两类：局域网（lans）和广域网（wan）。在这里主要谈一下局域网。局域网就是在一个有限的地域范围内，典型的如在办公大楼或校园中，将计算机及相关外设（如打印机）用电缆进行物理连结而组成的一个系统。网络的拓扑结构可以是环形的、星形的或总线形的。在局域网中，可用光纤或同轴电缆连结多台计算机，且每台计算机的硬盘上必须装有专门的通信软件。允许具有不同平台的计算机共存于同一局域网中，且能进行数据交换的软件早已开发出来了。局域数据网一般简称为局域网，用于在一幢楼内或局限在楼群范围内把分散的基于计算机数据终端设备互连在一起。例如，一个局域网可以把一幢楼里或像大学校园内各个楼里的各个办

2、公室的工作站连在一起。另外，局域网也可以把分布在工厂或医院建筑群中的计算机设备连在一起。由于所有这些设备都是一个单位的，因此局域网一般也由这个单位负责安装和维护。于是这类局域网也叫做专用数据网。大多数广域网，如公共电话交换网（pstn）就使用网状（有时称为网络）拓扑结构。而局域网，由于用户数据终端设备相距很近，可采用简单的拓扑结构。常用的有星形、总线、环形和集线器等4种拓扑结构。应用最广的、用于本地计算机设备互连以进行数据通信的局域网拓扑结构是集线器拓扑结构。这种拓扑结构是总线和环形拓扑结构的变种。图14-6展示了为这种拓扑结构，有时也称为集线器树形拓扑结构。局域网的传输媒体。双绞线、同轴电缆

3、和光纤是局域网采用的3种主要传输媒体。局域网中采用了两种媒体访问控制技术。它们是用于总线网络拓扑结构的带冲突检测的载波侦听多路存取（csmacd）既可用于总线又可用于环形网络的令牌控制技术。csma/ cd用来控制多路存取网络上的每个站点在试图发送信息前先“侦听”，等待通路空闲。如果两个站点在同一时刻要发送信息，将会检测到冲突，这两个站点必须各自“后退”一步，以后再重试。令牌控制是另一种用令牌去控制访问共享传输媒体的方法。该令牌按所有连入媒体的dte都知道并遵守的一套确定的规则从一个dte传向另一个dte。当某一dte掌握这一令牌时，它才能传送一帧数据，而在传送完该帧之后，要将令牌释放，以便其

4、他dte访问该传输媒体。文件服务器用于局域网。许多网络称为文件服务器的计算机来管理。文件服务器具有大容量的硬盘和一些用来管理对网上文件访问的专门软件。它控制着网上用户如何共享数据和数据库，以及用户如何存取中央硬盘上的数据和应用软件的主拷备。确保两个用户不能偶然地在同一时刻对同一文件进行更新或争夺数据也是文件服务器的工作。文件服务器还可以管理对昂贵硬件（如激光打印机）的访问。当文件服务器用于局域网时，大型数据库就被存储在服务器上，而且用户也可以将其所有的工作文件存放在上面。这就像是一个人每天手工收集所有的数据，放到一个文件抽屉中，然后等需要时，再将这些数据发到工作人员手中。net ware局域网

5、操作系统。novell公司的net ware操作系统已经成为ibm公司特有的网络操作环境最强有力的竞争对手，它在对局域网络的支持方面也早于ibm公司。事实上，除了自己的一部分软件外，ibm公司也为所有的pc网和令牌环网购买了net ware操作系统。net ware操作系统的最主要部分是文件服务器软件，服务器上的目录被映射到网络驱动器，这样在用户看来，它运行起来就如同是虚拟磁盘。它的服务器软件是一个多任务的操作系统，文件结构也与dos 2.x和dos 3.x的文件结构一致。工作站上的 net ware外壳程序拦截了dos的21h中断，并重新走向为对服务器的调用。net ware操作系统的主要优

6、点是能够运行在多种网络硬件上。不论是选择ibm网络，corves公司的omninet网络，还是att公司的星形局域网络等，都可获得net ware的操作环境。用局域网方式建立的通信线路和通过公用数据网连接的线路两者的主要区别。因为局域网连接的设备之间相对来说距离较近，因而数据传输速率高得多。但是在iso的osi参考模型中，这种区别只在较低的与网络相关的几层比较明显。而很多实例表明，在该参考模型高层协议中，这两种网络是没有区别的。 附：英文原文：local area network csiec there are two categories of networks: local area ne

7、tworks (lans) and wide area networks (wans). here id like to mainly talk about lan.a lan is a system of computers and associated peripherals such as printers that are physically connected by cable within a limited geographical area-typically in an office building or on a college campus. the topology

8、 of the network may be ring, star or bus. lans use fiber optics or coaxial cable to connect computers, and each computer must have special communications software installed oil its hard disk. software has been developed that allows computers with different platforms to coexist and exchange data on t

9、he same lan.local area data networks，normally referred to simply as local area networks or lans，are used to interconnect distributed communities of computer-based dte located within a single building or localized group of buildingsfor example，a lan may be used to interconnect workstations distribute

10、d around offices within a single building or a group of buildings such as a university campusalternatively，it may be used to interconnect computer-based equipment distributed around a factory or hospital complex1since all the equipment is located within a single establishment，however，lans are normal

11、ly installed and maintained by the organizationhence they are also referred to as private data networksmost wide area networks，such as the pstn，use a mesh（sometimes referred to as a network）topologywith lans，however，the limited physical separation of the subscriber dte allows simpler topologies to b

12、e usedthe four topologies in common use are star，bus，ring and hubthe most widespread topology for lans designed to function as data communication subnet works for the interconnection of local computer-based equipment is the hub topology, which is a variation of the bus and ring4this is shown in fig.

13、 14-6，sometimes it is called hub/tree topologyits transmission media. twisted pair，coaxial cable and optical fiber are the three main types of transmission medium used for lanstwo techniques have been adopted for use of the medium access control in the lansthey are carrier-sense-multiple-access with

14、 collision detection（csmacd），for bus network topologies，and control token，for use with either bus or ring networks5csma / cd is used to control multiple-access networkseach on the network “listens” before attempting to send a message，waiting for the “traffic” to clear6if two stations try to send the

15、ir messages at exactly the same time，a “collision” is detected，an both stations are required to “step back” and try latercontrol token is another way of controlling access to a shared transmission medium that is by the use of a control（pretension）tokenthis token is passed from one dte to another acc

16、ording to a defined set of rules understood and adhered to by all dte connected to the mediuma dte may only transmit a frame when it is in possession of the token and，after it has transmitted the frame，it passes the token on to allow another dte to access the transmission mediuma file server is used

17、 on a lan. many networks are managed by a computer called a file server. a file server has a largecapacity hard disk and special software that manages access to flies on the network. it controls how data and database are shared among users on the network and how users access master copies of data an

18、d application software on the centralized hard disk. it is the file servers job to make sure that users dont accidentally try to update a file at the same time and scramble the data. the file server may also manage the access to an expensive piece of hardware such as a laser printer. when a file ser

19、ver is used on a lan, large databases are stored on the server and users may store all of their work flies there as well. this operation is analogous to someones manually collecting all the data each day placing them in a file drawer, and then redistributing these data to the workers as needed.net w

20、are its the lan operating system. novells net ware has become the strongest competitor to ibms own network operating environment and predates ibm in its support for lans. in fact, ibm now markets net ware for both the pc network and for token ring in addition to its own software. the key to net ware

21、 is the file server software and directories on the server are mapped to network drives, thus operating like virtual diskettes from a users perspective. the server software is a multitasking operating system with a file structure consistent with dos 2.x and dos 3.x. the net ware shell in each workst

22、ation intercepts 21h dos calls and redirects appropriate calls to the server. perhaps the primary advantage of net ware is that it runs on a wide variety of networking hardware. regardless of whether you choose an ibm network, corves omninet, att star lan, etc., you can obtain net ware as the operat

23、ing environment. in fact, if you have one of each the user will not be able to tell the difference (unless there are some distinct performance differentials), and they can be interconnected through novells system of bridges and gateways. the main difference between a communication path established u

24、sing a lan and a connection made through a public data network is that a lan normally offers much higher data transmission rates because of the relatively short physical separations involved2in the context of the iso reference model for osi，however，this difference manifests itself only at the lower

25、network dependent layersin many instances the higher protocol layers in the reference model are the same for both types of networkfrom：恋爱运up一绪lets月亭方正！気持今flying get！dou都斗豆逗陡抖痘兜读蚪窦篼蔸乧侸兠凟剅吺唗投斣枓梪橷毭氀浢渎渎瞗窬窦脰艔豆读逾郖酘酡钭鋀钭閗闘阧餖饾斗鬦鬪鬬鬭du读度毒渡堵独肚镀赌睹杜督都犊妒顿蠹笃嘟渎椟牍黩髑芏儥凟剢剫匵厾噣土涂妬嬻剬塅媏彖断毈瑖碫篅簖缎专腶葮褍踹躖锻鍴mr najib met search c

26、rews at pearce raaf base near perth on thursday morning, before their planes left for the day, and then later held talks with mr abbott.the disappearance of mh370 has tested our collective resolve, he told a news conference.faced with so little evidence, and such a herculean task, investigators from

27、 malaysia, the us, the uk, china, australia and france have worked without pause to reveal the aircrafts movements.he thanked both search teams and the australian government for their efforts in recent weeks, and said the search would go on.continue reading the main storymh370 - facts at a glance 8

28、march:malaysia airlines kuala lumpur-beijing flight carrying 239 people disappears planes transponder, which communicates with ground radar, was switched off as it left malaysian airspace satellite pings indicate plane was still flying seven hours after satellite contact was lost 24 march:based on n

29、ew calculations, malaysian pm says beyond reasonable doubt that plane crashed in southern indian ocean with no survivors what we know the search for flight mh370i know that until we find the plane, many families cannot start to grieve. i cannot imagine what they must be going through. but i can prom

30、ise them that we will not give up, he said.malaysian authorities have come in for heavy criticism over their management of the search, especially from relatives of the planes 153 chinese passengers.on thursday, eight military planes and nine ships were due to take part in the search.weather conditio

31、ns were fair, with visibility of approximately 10km (6 miles), the joint agency coordination centre (jacc) - which is overseeing the search - said.the british submarine hms tireless is also in the southern indian ocean and is due to be joined by royal navy ship hms echo.the australian navy ship ocea

32、n shield is heading to the region and has equipment for detecting the planes black-box flight recorder.experts say timing is critical as the flight recorder may only have enough battery power to send out a signal until 7 april.air chief marshall angus houston, head of the jacc, warned that the searc

33、h operation faced multiple difficulties.this is one of the most demanding and challenging search and rescue operations, or search and recovery operations, that i have ever seen and i think probably one of the most complex operations of this nature that the world has ever seen, he told mr najib and m

34、r abbott.on wednesday malaysian police chief khalid abu bakar said investigators had cleared all passengers of possible involvement in hijacking, sabotage or having personal or psychological problems that could have been connected to the disappearance.but he said that the criminal investigation coul

35、d go on and on and on. we have to clear every little thing.at the end of the investigations, we may not even know the real cause. we may not even know the reason for this incident, he added.the police chief said that more than 170 interviews had been conducted with family members of the pilots and c

36、rew members, and that even cargo and food served on the plane were being investigated in case ofdui对队堆兑敦镦碓怼憝兊兑垖埻塠夺夺対对嵟憞怼捶杸濧濻瀢瀩痽磓祋綐膭薱謉譈譵追鈗锐锐錞鎚镦鐜锐陮队頧鴭dul乧dun吨顿蹲墩敦钝盾囤遁不趸沌盹镦礅炖砘伅俊吨墪壿庉忳敦憞撉撴楯橔潡炖犜獤碷腞腯蜳豚踲蹾趸逇遯钝镦鐜顿驐duo多朵夺舵剁垛跺惰堕掇哆驮度躲踱沲咄铎裰哚缍亸仛兊兑兑凙刴剟剫吋喥嚉嚲垜埵堕墯夛夺奲媠嫷尮崜嶞憜挅挆捶揣敓敚敠敪朶杂杕枤柁柂柮桗棰椯椭毲沰沱泽痥硾缍茤袳詑誃貀趓跢跥跿躱軃郸郸鈬錞锗铎锗陀陊隋隋隓飿饳驮駄鬌鮵鵽点e饿哦额鹅蛾扼俄讹阿遏峨娥恶厄鄂锇谔垩锷阏萼苊轭婀莪鳄颚腭愕呃噩鹗屙亚亜亚伪佮侉偔伪伪僫匎匼卾吪呝咢咹哑唖啈啊啐哑恶囐囮垭垭垩堨堮妸妿姶娾娿媕屵岋峉峩崿庵廅悪恶戹搕搤搹擜曷枙桠櫮唉歞歹歺洝涐湂猡珴琧痷皒睋砈砐砨砵硆硪磀礘胺蒍蕚蘁蚅蝁覨讹咯誐谔譌讍豟轭軶輵迗遌遻邑鈋锇锷鑩閜阏阨阸隘頞頟额颚饿餩騀鬲魤魥鳄鰪鳄鴳鵈鹅