急求计算机网络方面的中英文对照

　　计算机网络－局域网（中英文对照）[1]
　　http://www.educity.cn 作者：IT英语 来源：学赛网 2008年3月13日 发表评论 进入社区
　　Local area data networks，normally referred to simply as local area networks or LANs，are used to interconnect distributed communities of computer-based DTEs located within a single building or localized group of buildings.For example，a LAN may be used to interconnect workstations distributed around offices within a single building or a group of buildings such as a university campus.Alternatively，it may be used to interconnect computer-based equipment distributed around a factory or hospital complex[1].Since all the equipment is located within a single establishment，however，LANs are normally installed and maintained by the organization.Hence they are also referred to as private data networks.

　　The main difference between a communication path established using 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 involved[2].In the context of the ISO Reference Model for OSI，however，this difference manifests itself only at the lower network dependent layers.In many instances the higher protocol layers in the reference model are the same for both types of network.

　　Before describing the structure and operation of the different types of LAN，it is perhaps helpful to first identify some of the selection issues that must be considered.A summary of some of the these issues is given in Fig. 14-5.It should be stressed that this is only a summary;there are also many possible links between the tips of the branches associated with the figure[3].

　　1.Topology

　　Most wide area networks，such as the PSTN，use a mesh(sometimes referred to as a network)topology.With LANs，however，the limited physical separation of the subscriber DTEs allows simpler topologies to be used.The four topologies in common use are star，bus，ring and hub.

　　The most widespread topology for LANs designed to function as data communication subnetworks for the interconnection of local computer-based equipment is the hub topology, which is a variation of the bus and ring[4].This is shown in Fig. 14-6，sometimes it is called hub/tree topology.

　　2.Transmission media

　　Twisted pair，coaxial cable and optical fiber are the three main types of transmission medium used for LANs.

　　3.Medium access control methods

　　Two techniques have been adopted for use of the medium access control in the LANs.They are carrier-sense-multiple-access with collision detection(CSMA/CD)，for bus network topologies，and control token，for use with either bus or ring networks[5].

　　CSMA / CD is used to control multiple-access networks.Each on the network“listens” before attempting to send a message，waiting for the“traffic”to clear[6].If two stations try to send their messages at exactly the same time，a“collision”is detected，an both stations are required to“step back”and try later.

　　Control token is another way of controlling access to a shared transmission medium that is by the use of a control(pertnission)token.This token is passed from one DTE to another according to a defined set of rules understood and adhered to by all DTEs connected to the medium.A DTE may only transmit a frame when it is in possession of the token and，after it has transmined the frame，it passes the token on to allow another DTE to access the tranamission medium.

　　NOTES

　　[1] computer-based是指由计算机控制的，或装有微处理器的。

　　[2] that引出表语从句。

　　[3] 是指局域网的问题之间有很多联系，如总线拓扑结构可以采用双绞线、同轴电缆和光纤这些传输媒体等。

　　[4] 很少文献资料提出“集线”拓扑，一般都归并为星形拓扑之列。

　　[5] CSMA / CD访问控制方式，由于集线器和双绞线的使用，它也用于星形拓扑结构;令牌控制方式用于总线拓扑结构时指令牌总线网(token bus network)。

　　[6] traffic：通信量，话务量，clear在通信中为“清0”，拆线，此处指网上通信量为0。
　　KEYWORDS

　　Local Area Network(LAN) 局域网

　　private data network 专用数据网

　　topology 拓扑

　　Public Switched Telephone Network(PSTN) 公用电话交换网

　　hub 集线器

　　transmission media 传输媒体

　　Shielded Twisted Pair(STP) 屏蔽双绞线

　　Unshielded Twisted pair(UTP) 非屏蔽双绞线

　　coaxial cable 同轴电缆

　　baseband 基带

　　broadband 宽带

　　optical fiber 光纤

　　Carrier-Sense-Multiple-Access/Collision Detection(CSMA/CD) 带有检测冲突的载波侦听多路存取

　　Token 令牌

　　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。

　　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在于：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。

　　在叙述几种不同类型的局域网的结构和工作之前先认识一下必须考虑的有关问题可能是很有帮助的。图14-5概括了其中的某些问题。应该强调的是，这仅仅是一个概括，因为图中各分支之间还可能有很多链路。

　　1.拓扑结构

　　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。

　　2.传输媒体

　　双绞线、同轴电缆和光纤是局域网采用的3种主要传输媒体。

　　3.媒体访问控制方法

　　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)和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，以便其他DTE访问该传输媒体。

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