LOCAL AREA NETWORK 局域网
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．
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。
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．
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。
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]．
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。
