通信英语The - Principle - of - PCM译文

typically aren’t built with the OSI protocols, although the OSI model is used as a guide. 不同点：

? TCP/IP将表示层和会话层的功能全部并入到它网的应用层中。 ? TCP/IP将OSI的数据链路层和物理层合成了一层。

? TCP/IP显得更为简单，因为其层数少，但是这只是一种错觉。OSI参考模型具有

多层但却不复杂，因而在开发和故障诊断方面更为简单。

? 由于TCP/IP协议是当时开发建设互联网所基于的标准，因而TCP/IP模型由于其

协议获得了广泛的信任。与之相反，尽管OSI模型被用来引导入门，但以OSI协议所建设的网络却不典型。

Many networking professionals have different opinions on which model to use. You should become familiar with both. You can use the OSI model as the microscope through which to analyze networks, but you also can use the TCP/IP protocols in common situation. Remember that there is a difference between a model (that is, layers interfaces, and protocol specifications) and an actual protocol that is used in networking.

许多互联网专家对采用那种模型存在意见分歧。你应对这两者都熟悉为好。可将OSI模型用作显微镜，通过它去分析网络，但在通常情况下，也能用TCP/IP协议来讨论问题。应记住，在模型（即分层、接口和协议规范）和在联网中采用的实际协议之间，是有区别的。

UNIT 5

Local-Area Networks (1) 局域网（1）

The focus of this chapter is local-area networks (LANs). 本章的重点是局域网。

As you know, LANs are high-speed, low-error data networks that cover a relatively small geographic area (up to a few thousand meters). LANs connect workstations, peripherals, and other devices in a single building or other geographically limited area.

我们知道，局域网是一个覆盖在地理上相对面积不大（不超过几千米）的高速的、低误码的数据网络。局域网连接工作站、外部设备和其他设施，这些设施分布在一座大楼，或其他有限面积的地段。

Topology defines the structure of the network. The topology definition contains two parts: the physical topology, which is the actual layout of the wire (media), and the logical topology, which defines how the media is accessed by the hosts. The physical topologies that are used commonly are bus, ring, star, extended star, hierarchical, and mesh (see figure 5-1).

拓扑定义网络的结构。拓扑的定义含有两个部分：物理拓扑，它是线路（介质）的实际安排；逻辑拓扑，它定义了媒体如何接入主机。通常使用的物理拓扑有总线型、环形、星型、扩展式星型、分层和网络结构（如图5-1示）

The commonly used physical topologies are:

? Bus topology—Uses a single backbone segment (length of cable) to which all the hosts

directly connect.

? Ring topology—Connects one host to the next and the last host to the first. This creates

a physical ring of cable.

? Star topology—Connects all cables to a central point of concentration. This point is

usually a hub or a switch.

? Extended star topology—Uses the star topology. It links individual stars together by

linking the hubs/switches. This extends the length and the size of the network.

? Hierarchical topology—Similar to an extended star topology, but instead of linking the

hubs/switches together, each secondary (subordinate) system is linked to a primary computer that controls the traffic on the topology.

? Mesh topology—Used when there can be absolutely no break in communication. As you

can see in figure5-1, with a full mesh topology, each host has its own connections to all other hosts. A partial mesh reflects the design of the Internet, which has multiple paths to any one location, although not a connection from every host to every other host. 常用的拓扑是：

? 总线拓扑－采用单一的主干线路段（即电缆长度），所有的主机都直接接到这段骨

干线上。

? 环型拓扑—将一个主机连接到下一个主机，而最后一个主机又连到第一个主机上。

这样就产生了一个物理的电缆环。

? 星型拓扑－将所有的电缆都连接到一个中心点上集中，这个点通常是一个集线器

或交换器。

? 扩展式星拓扑－使用星型拓扑。它采用连接集中器或交换器的办法将各个星型连

接到一起。这样就扩展了网络的长度和大小。

? 分层拓扑－类似于扩展式星型拓扑，但不是将集中器或交换器连接到一起。每个

第二级（下级）系统被连到一个主机上，由该主机控制拓扑中的流量。

? 网状拓扑――在要求绝对没有通信中断的条件下，使用这种拓扑。正如图4-1所示

的那样，在完全往事的拓扑中，每一个主机都有连线到任何其他主机。互联网（internet）的设计表现为部分网状拓扑。在互联网中，到任意一点具有多条路径，但不是每个主机到每个其他的主机都有连线。

Figure 5-2 shows many topologies. It shows a LAN of moderate complexity that is typical of a school or a small business. It has many symbols, and it depicts many networking concepts that you will learn.

图5-2表示了多种拓扑。它画出了一个适度复杂的局域网，这种局域网典型的用于学校或小的机关。图中有多种符号，它表示了许多将要学到的联网知识。

Devices that connect directly to a network segment often are referred to as hosts. These hosts include computers (both clients and servers), printers scanners, and many other user devices. The host devices can exist without a network, but its capabilities are greatly limited.

直接连到一个网络段的部件常被称为主机。这些主机包括计算机（无论客户机或服务器）、打印机、扫描仪和许多其他用户设备。主机设备可以离开网络单独存在，但其能力将大打折扣。

Host devices are not part of any layer. They have a physical connection to the network media by having a network interface card (NIC) and the functions of the other OSI layers are performed in software inside the hose. This means that they operate at all seven layers of the OSI model. Host devices perform the entire process of the encapsulation and de-encapsulation to do their job of sending e-mails, printing reports, scanning pictures, or accessing databases. For those who are familiar with the inner workings of PCs, think of the PC itself as a tiny network that connects the bus and the expansion slots to the CPU, the RAM, and the ROM.

主机设备并不属于某一层。他们到网络介质的物理连接有网络接口卡（NIC）来完成，

而OSI其他层的功能由主机中的软件来完成。这就意味这他们运行于OSI模型的整个7层中。主机设备实现着打包和拆包的全过程，以完成诸如发送电子邮件、打印报告、扫描图片或接入数据库等工作。对熟悉个人计算机内部工作原理的人来说，可将PC机本身看成一个小型的网络，它将总线和扩展口连接到CPU，RAM和ROM.

There are no standardized symbols within the networking industry for hosts, but they usually are fairly easy to figure out. As shown in figure 5-2, they look like the real device so you are constantly reminded of that device.

在网络业内部，并没有标准化的符号表示主机，但它们通常极易识别。如图5-2所示，它们很像实际设备，因而记住它们是不成问题的。

Some important devices depicted in Figure 5-2 are explanted as following. 在图5-2中描绘了一些重要部件可说明如下。 NICs 网络接口卡

So far in this chapter, you deal with layer 1 devices and concepts. Starting with the network interface card (NIC), the discussion moves to layer two, the data link layer, of the OSI model. NICs are considered layer 2 devices because each individual NIC throughout the world carries a unique code, called a Media Access Control (MAC) address. This address is used to control data communication for the host on the network.

本章到现在为止，仅涉及到第一层的部件和概念。由网络接口卡开始，讨论进入第二层，即OSI模型的数据链路层。网络接口被认为是第二层的部件是由于世界上每一个网络接口卡都携带有一个独特的码，即媒体接入控制（MAC）地址。这个地址被用来控制网上主机的数据通信。

In terms of physical appearance, a NIC is a printed circuit board, that fits into the expansion slot of a bus on a computer’s motherboard or peripheral device. It is also called a network adapter. Its function is to adapt the host device to the network medium.

就物理外形来看，网络接口卡是一个印刷电路板，它可插入计算机主板或外设的总线扩展槽里。它亦被称为网络适配器，其功能是使主机适应网络介质。

The NIC is the basic hardware component of network communications. It translates the parallel signal produced by the computer into the serial format that is sent over the network cable. The 1s and 0s of binary communication are turned into electrical impulse, pulses of light , radio waves , or whatever signaling scheme he network media uses.

网络接口卡是网络通信的基本硬件。它将由计算机产生的并行信号转换成串行格式，此串行信号才通过网络电缆发送。二进制通信的1和0被转换为电脉冲、光脉冲、无线电波或网络介质使用的不管何种信号体系。

NICs have no standardized symbol. It is implied that, whenever you see networking devices attached to network media, there is some sort of NIC or NIC-like device present (although it is generally not shown). Wherever you see a dot on a topology, there is either a NIC or an interface (port) that acts like at least part of a NIC.

网络接口卡没有标准化的符号。这就意味着，无论何时看到网络器件被连到网络介质上，就有某种网络接口卡或类似网络接口卡的部件存在（尽管通常不显示出来）。无论在哪个拓扑图中看到一个圆点，则要么有网络接口卡，要么就有一个接口，它至少部分起着网络接口卡的作用。

Media 介质

The symbols for media vary, as shown in figure 5-3. For example, the Ethernet symbol is typically a straight line with perpendicular lines projecting from it ; the Token Ring network

symbol is a circle with hosts attached to it ; and for FDDI, the symbol is two concentric circles with attached devices.

介质的符号有多种，如图5-3示。例如，以太网的典型符号是一条直线，并由它射出许多垂直线。令牌环网的符号是一个圆圈，许多主机与之相连。对FDDI，符号是两个同心圆并连有许多部件。

The basic functions of media are to carry a flow of information, in the form of bits, through a LAN. Other than wireless LANs (that use the atmosphere, or space, as the medium) and the new PANs (personal-area networks, which used the human body as a networking medium!), networking media confine network signals to wire, cable, or fiber. Networking media are considered Layer 1 components of LANs.

介质的基本功能以比特的形式通过局域网传送信息流。与无线局域网（它采用大气或空间作为媒体）和新的人体网（个人网络――它利用人体作为联网介质）不同，网络介质将网络信号局限于导线、电缆或光纤上。联网介质被认为是局域网的第一层部件。

You can build computer networks with many different media types. Each media has advantages and disadvantages. What is an advantage for one media (Category 5 cost) might be a disadvantage for another (fiber optic cost). Some of the possible advantages or disadvantages include the following:

你可用许多不同的介质类型构建计算机网络。每种介质都有有缺点。一种媒体的优点（如五类线的价格）可能是一种媒体的缺点（如光纤的价格）。一些可能的优缺点常包括：

? Cost

? Ease of installation

? Maximum cable length ? 价格

? 安装的难易 ? 最大电缆长度

Coaxial cable, optical fiber, and even free space can carry network signals. However, the principal medium you will study is called Category 5 unshielded twisted-pair (CAT 5 UTP) cable because it is the most-used medium in network installations.

同轴电缆、光纤，甚至自由空间都能传送网络信号。但是，你将学习的主要介质被叫做五类非屏蔽双绞线（CAT 5 UTP）, 因为它是网络安装中最常使用的介质。

Repeaters

As mentioned in the ―media‖ section, many types of media exist and each type has advantages and disadvantages. One of the disadvantages of the type of cable that we primarily use, CAT 5 UTP, is cable length. The maximum length for UTP cable in a network is 100 meters (approximately 333 feet). If you need to extend beyond the network limit, as shown in figure 5-4, you must add a device to your network. This device is called a repeater.

中继器

正如在介质一节所提及，存在着许多类型的介质，且每种都有其优缺点。我们主要使用的电缆，即CAT 5 UTP, 其缺点是电缆长度。在一个网络中UTP电缆长度的最大长度是100米（大约333英尺）。如果想要将网络延伸超过这个界限，如图5-4所示，就必须向网络添加设备，该设备被称为中继器。

The term repeater comes from the early days of visual communication, when a man situated on a hill received a signal from a person on the hill to his left, and then repeated the signal to the person on the hill to his right. It also comes from telegraph, telephone, microwave, and optical