Chapter Seven Network Topology [tə'p ɒ ləd ʒɪ ]

In networking, the term “topology” refers to the layout of connected devices on a network. This article introduces the standard topology of computer networking. One can think of a topology as a network’s virtual shape or structure. 在网络中， “ 拓扑结构 ” 指的是在网络上链接的设备的布局。 这篇文章介绍计算机网络中标准的拓扑结构。人们可以认为 拓扑结构就是网络一个虚拟的模型或者结构。

This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle in a family room, but it would be highly unlikely to find an actual ring topology there. 这种模型不一定要跟在网络上的实际物理设备的布局一 致。例如，电脑在家庭的局域网中可以以环形被安排在一 个房间里，但是这将和实际的环状拓扑有很大的不同。

Network topologies are categorized into the following basic types: bus, ring, star, tree, and mesh. More complex networks can be built as hybrids of two or more of the above basic topologies. 网络拓扑可以被分成一下几种基本的类型：总线，环状，星 形，树和网。更复杂的网络可以由以上基础网络中的两种或 者更多的杂交。

1.Bus Topology A bus topology connects computers along a single or more cable to connect linearly as Figure A network that uses a bus topology is referred to as a “bus network” which was the original form of Ethernet networks. Ethernet 10Base2 (also known as thinnet 长度不超过 200 米的细同轴 电缆规范 ) is used for bus topology. 1. 总线拓扑 一个总线拓扑通过一根或多根电缆线形的连接计算机，如 图 所示。使用了总线拓扑的网络指 “ 总线网络 ” ，它是以太 网的原始形式。总线拓扑使用 Ethernet 10Base2 ( 也被叫做细 电缆 ) 规范。

Bus topology is the cheapest way of connecting computers to form a workgroup or departmental LAN, but it has the disadvantage that a single loose connection or cable break can bring down the entire LAN. Termination is an important issue in bus networks. The electrical signal from a transmitting computer is free to travel the entire length of the cable. 对于总线网络来说，终断是一个重要的问题。从一个计算机发 射出的电子信号自由地在整个电缆上传输。

Without the termination, when the signal reaches the end of the wire, it bounces back （ [ba ʊ ns] 反弹） and travels back up the wire. When a signal echoes （回响） back and forth along an unterminated bus, it is called ringing （振铃）. The terminators absorb the electrical energy and stop the reflections. Advantages of bus topology:  Bus is easy to use and understand and inexpensive simple network.  It is easy to extend a network by adding cable with a repeater （中继器） that boosts （增加） the signal and allows it to travel a longer distance.

Disadvantages of bus topology: *A bus topology becomes slow by heavy network traffic with a lot of computers because networks do not coordinate with （协调） each other to reserve （预留） times to transmit. *It is difficult to troubleshoot （排解故障） a bus because a cable break or loose connector will cause reflections and bring down the whole network.

2 ． Star Topology A star topology links the computers by individual cables to a central unit, usually a hub as in Figure When a computer or other networking components transmits a signal to the network, the signal travels to the hub. Then, the hub forwards （ V. ） the signal simultaneously( 同时 ) to all other components connected to the hub. Ethernet 10BaseT （双绞线连接的 10M 以太网规范） is a network based on the star topology. Star topology is the most popular way to connect computers in a workgroup or departmental network.

Advantages of star topology: * The failure of a single computer or cable doesn’t bring down the entire network. * The centralized networking equipment can reduce costs in the long run by making network management much easier. 集中式网络设备可以更容易的通过网络管理减少在 长期运行中的成本。

*It allows several cable types in same network with a hub that can accommodate multiple cable type. Disadvantages of star topology: * Failure of the central hub causes the whole network failure. * It is slightly more expensive than using bus topology.

3. Ring Topology A ring topology connects the computers along a single path whose ends are joined to form a circle as Figure The circle might be logical only but the physical arrangement of the cabling might be similar to start topology, with a hub or concentrator at the center. 环形拓扑网，即将所有计算机沿着一条路径相互连接在一起， 形成的网络结构，如图 所示。环形可以是逻辑上的环形， 但是物理的布线要像拓扑形状，中间还要有一个集线器或集 中器。

 The ring topology is commonly used in token ring networks （令牌环网） that the ring of a token ring network is concentrated inside a device called a Multistation Access Unit (MAU) and Fiber Distributed Data Interface (FDDI) networks that the ring in this case is both a physical and logical ring and usually runs around a campus or collection of the buildings to form a high-speed backbone network. 环形拓扑网一般用在令牌环形网络中，而且该令牌环网的环 集中到称为 MAU 的设备和 FDDI 网络，这种情况下环形是物 理和逻辑上的环形，大多用在校园或楼群里，为了形成高速 的主干网。

Advantages of ring topology: ·One computer cannot monopolize （ [mə'n ɑ pə'la ɪ z] 垄断） the network. ·It continues to function after capacity is exceeded but the speed will be slow. Disadvantages of ring topology: ·Failure of one computer can affect the whole network. ·It is difficult to troubleshoot. ·Adding and removing computers disrupts the network.

4.Tree Topology A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable. Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.

Advantages of a tree topology: *Point-to-point wiring for individual segments （网段）. *Supported by several hardware and software venders. Disadventages of a tree topology: *Overall length of each segment is limited by the type of cabling used. *If the backbone line breaks, the entire segment goes down. *More difficult to configure and wire than other topologies.

5.Mesh Topology In a mesh topology, each computer on network has redundant data paths as showing in Figure The mesh topology provides fault tolerance. If a wire, hub, switch, or other component fails, data can travel along an alternate path. A diagram of a mesh networks looks like a fishing net. A mesh topology is most often used in large backbone networks in which failure of a single switch or router can result in a large portion of the network going down.

网状拓扑结构 在一个网络拓扑结构中，网络上的每一台电脑 都有冗长的数据通路，就像图 所显示的。网状 拓扑结构支持容错。如果一根电线，一个枢纽，一 个开关或者是其他的部件出现故障，数据可以沿着 另一条途径传送。一个网络拓扑结构图就像一张渔 网。网络拓扑结构经常被用在大的骨干网络中，其 中一个单独的开关或者路由的损坏都会导致大部分 网络的瘫痪。

Advantages of mesh topology: *Point-to-point wiring for individual segments. *Provides increased redundancy and reliability as well as ease of troubleshooting. Disadvantages of mesh topology: *The implementation of mesh topology is expensive than other types of topology.

6.Considerations When Choosing a Topology ◆ Money. A linear bus network may be the least expensive way to install a network; you do not have to purchase concentrators. ◆ Length of cable needed. The linear bus network uses shorter lengths of cable. ◆ Future growth. With a star topology, expanding a network is easily done by adding another concentrator. ◆ Cable type. The most common cable in schools is unshielded twisted pair （非屏蔽双绞 线）, which is most often used with star topologies.