Lesson 2 Network Topology

Network topology is a usually schematic description of the arrangement of a network, including its nodes and connecting lines． There are two ways of defining network geometry: the physical topology and the logical (or signal) topology．

Physical topology is the physical layout of the network, how the cables are arranged and how the computers are connected．

Logical topology refers to the nature of the paths the signals following from node to node.

In many instances, the logical topology is the same as the physical topology． But this is not always the case． For example, some networks are physically laid out in a star configuration, but they operate logically as bus networks．

Network topology is technically a part of graph theory． Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ in two networks and yet their topologies may be identical．

There are several common physical topologies, as described below and as shown in Figure 2-1．

Bus Topology-in which all of the nodes of the network are connected to a common transmission medium which has exactly two endpoints(this is the “bus”, which is also commonly referred to as the backbone, or trunk)． All data that is transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network virtually simultaneously． The two endpoints of the common transmission medium are normally terminated with a device called a terminator that exhibits the characteristic impedance of the transmission medium and which dissipates or absorbs the energy that remains in the signal to prevent the signal from being reflected or propagated back onto the transmission medium in the opposite direction, which would cause interference with and degradation of the signals on the transmission medium.

Star Topology-in which each of the nodes of the network is connected to a central node with a point-to-point link in a “hub” and “spoke” fashion, the central node being the “hub” and the nodes that are attached to the central node being the “spokes” (e．g．, a collection of point-to-point links from the peripheral nodes that converge at a central node) - all data that is transmitted between nodes in the network is transmitted to this central node, which is usually some type of device that then retransmits the data to some or all of the other nodes in the network, although the central node may also be a simple common connection point without any active device to repeat the signals．

Ring Topology—in which each of the nodes of the network is connected to two other nodes in the network and with the first and last nodes being connected to each other, forming a ring—all data that is transmitted between nodes in the network travels from one node to the next node in a circular manner and the data generally flows in a single direction only．

Mesh Topology—in which each of the nodes of the network is connected to each of the other nodes in the network with a point-to-point link — this makes it possible for data to be simultaneously transmitted from any single node to all of the other nodes． The physical fully connected mesh topology is generally too costly and complex for practical networks, although the topology is used when there are only a small number of nodes to be interconnected.

Tree (or Hierarchical) Topology—in which a central “root” node (the top level of the hierarchy) is connected to one or more other nodes that are one level lower in the hierarchy (i．e．, the second level) with a point-to-point link between each of the second level nodes and the top level central “root” node, while each of the second level nodes that are connected to the top level central “root” node will also have one or more other nodes that are one level lower in the hierarchy (i．e．, the third level) connected to it, also with a point-to-point link, the top level central “root” node being the only node that has no other node above it in the hierarchy — the hierarchy of the tree is symmetrical, each node in the network having a specific fixed number “f”, being referred to as the “branching factor” of the hierarchical tree.

Hybrid Topology is a type of network topology that is composed of one or more interconnections of two or more networks that are based upon different physical topologies mentioned above.

New Words and Expressions

schematic adj． 概要的，图解的

arrangement n． 安排，排列，布置

node n． 节点

geometry n． 几何学，几何体，几何

define v． 定义，详细说明

physical adj． 外在的，物理的，有形的，实体的，自然的

logical adj． 逻辑的，合理的

signal n． 信号

configuration n． 配置

transmission n． 传输，传递

identical adj． 相同的，同样的

medium n． 媒介

endpoint n． 端点，终接点

backbone n． 主干网络，支柱，骨干

virtually adv． 几乎，事实上，实际上

simultaneously adv． 同时地

terminate v． 终接，结束

terminator n． 终接器

impedance n． 阻抗

dissipate v． 驱散

absorb v． 吸收

propagate v． 传播

interference n． 干涉，干扰

degradation n． 减弱，递减

peripheral adj． 外围的

converge v． 聚集，聚合

circular adj． 圆形的，循环的

practical adj． 实际的，实用的

complex adj． 复杂的

hierarchy n． 层次，层次结构

hierarchical adj． 分层的，等级的

symmetrical adj． 对称的

refer to A as B将A称为B

lay out排列，布置

refer to指的是，参考，涉及

be attached to附属于，连接

be composed of由……组成

peripheral device外围设备

Hybrid Topology混合拓扑结构

Bus Topology总线拓扑结构

Star Topology星状拓扑结构

Ring Topology环状拓扑结构

Mesh Topology网状拓扑结构

Exercises

I. Translate the following Chinese terms into English.

1． 节点

2． 拓扑结构

3． 星状拓扑

4． 媒介

5． 主干

6． 端点

7． 信号

8． 传输

9． 排列

10． 干扰

II. Translate the following English terms into Chinese.

1． Impedance

2． Hierarchy

3． Converge

4． Mesh Topology

5． Hybrid Topology

6． Symmetrical

7． Degradation

8． Terminator

9． Propagate

10． Peripheral

III. Choose the best answer from the four choices according to the passage.

1． How many ways of topologies are mentioned in this passage?

A． One

B． Two

C． Three

D． Four

2． Which of the following statements is true?

A． The logical topology is always the same as the physical topology．

B． The logical topology is always different from the physical topology．

C． The logical topology is identical to the physical topology in many cases．

D． The statements above are wrong．

3． How many physical topologies are mentioned in this passage?

A． Six

B． Five

C． Three

D． Four

4． Which of the following statements is true about Hybrid Topology?

A． Hybrid Topology refers to the nature of the paths the signals follow from node to node．

B． Hybrid topology is a type of network topology consisting of one or more interconnections of two or more networks．

C． Central “root” node is connected to one or more other nodes that are one level lower in the hierarchy in Hybrid Topology．

D． Each of the nodes of the network is connected to two other nodes in the network in Hybrid Topology．

5． What is mainly introduced in this passage?

A． Logical Topology

B． Star Topology

C． Bus Topology

D． Physical Topology

IV. Translate the following sentences into Chinese.

1． Physical topology is the physical layout of the network, how the cables are arranged and how the computers are connected．

2． Network topology is a usually schematic description of the arrangement of a network, including its nodes and connecting lines．

3． Some networks are physically laid out in a star configuration, but they operate logically as bus networks．

4． The two endpoints of the common transmission medium are normally terminated with a device called a terminator that exhibits the characteristic impedance of the transmission medium．

5． Mesh Topology-in which each of the nodes of the network is connected to each of the other nodes in the network with a point-to-point link．

6． The hierarchy of the tree is symmetrical, each node in the network having a specific fixed number “f”．

7． Hybrid Topology is a type of network topology that is composed of one or more interconnections of two or more networks that are based upon different physical topologies mentioned above．

8． Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ in two networks and yet their topologies may be identical．

参考译文

网络拓扑结构

网络拓扑通常是网络结构的图形描述，包括其节点和连接链路。有两种方式定义网络几何结构：物理拓扑结构和逻辑（或信号）拓扑结构。

物理拓扑是网络实际布局图，表示电缆是如何排列的以及计算机是如何相互连接的。

逻辑拓扑指的是信号从一个节点流向另一个节点的路径的性质。

在许多情况下，逻辑拓扑与物理拓扑相同，但是并非总是如此。例如，一些网络的物理排列是星状结构，但它们的逻辑操作却是总线结构。

网络拓扑在技术上是图形理论的一部分。在两个网络中，节点间的距离、物理互连方式、传输速率和（或）信号类型可以不同，但它的拓扑结构可以完全一致。

有几种常见的物理拓扑结构，如图2-1所示，描述如下。

在总线拓扑结构中，网络的所有节点被连接到一个公共传输介质，此介质只具有两个端点（这是“总线”，也通常简称为主干，或树干）——网络中的节点之间传输的数据都通过这个公用传输介质传输，并且几乎能同时被网络中所有节点收到。公共传输介质的两端都是以一个被称做终接器的设备结束的，它具有阻抗传输媒介的特征，并能消散或者吸收保留在信号中的能量，以防止信号被反射或以相反的方向重新传播回传输介质。如果没有终接器，传输媒介中的信号将会被干扰和减弱。

在星状拓扑结构中，每一网络节点都以一种点对点的连接方式连接到中央节点，中央节点是“集线器”，而与中央节点相连的各节点是“辐条”（比如，一系列从周边的节点汇聚到中央节点的点对点连接）——网络节点之间传输的数据都传输给这个中央节点，它通常是某种类型的设备，然后重新把数据传输给一些或全部的其他网络节点，但是中央节点也可能是一种简单的公共连接点，而没有任何活动设备来重复信号。

在环状拓扑结构中，每一网络节点连接到另两个网络节点上，并且第一个和最后一个节点相互连接形成一个环——所有在网络节点中传输的数据都按循环方式从一个节点到下一个节点传输，并且数据通常只在单一方向上流过。

在网状拓扑结构中，每个网络节点以点对点连接方式与其他节点相连，这使它可能从某单一节点同时向所有的其他节点传送数据。对于实际网络来说，虽然网络拓扑只在有少量节点连接时使用，但物理结构完整连接的网状拓扑通常昂贵而复杂。

在树状（或分层）拓扑结构中，一个中心“根”节点（分层中的最高层）以点对点连接方式连接到一个或多个（例如，第二层）的节点上，每一个和最高层中央“根”节点相连的二层节点也会以点对点方式和一个或多个低一层（例如，第三层）节点相连，最高层次的中央“根”节点是唯一的，并且没有高于它层次的其他节点。树状层次是对称的，各网络节点有特定的固定值“f”，这个数值“f”，被称为等级树的“分支因子”。

混合拓扑结构是一种由一个或多个基于上面所提的不同的物理拓扑结构的两个或更多的网络连接而成的一种网络拓扑结构。

Crazy Combat（疯狂实战）

1． A network admin wants to know every hop the packets take when he accesses cisco．com． Which command is the most appropriate to use?

A． path cisco．com

B． debug cisco．com

C． trace cisco．com

D． traceroute cisco．com

2． Which two addresses can be assigned to a host with a subnet mask of 255．255．254．0? (Choose two．)

A． 113．10．4．0

B． 186．54．3．0

C． 175．33．3．255

D． 26．35．2．255

E． 17．35．36．0

3． Which of the following statements describe the network shown in the graphic? (Choose two．)

A． There are two broadcast domains in the network．

B． There are four broadcast domains in the network．

C． There are six broadcast domains in the network．

D． There are four collision domains in the network．

E． There are five collision domains in the network．

F． There are seven collision domains in the network．

4． Which destination addresses will be used by Host A to send data to Host C? (Choose two．)

A． the IP address of Switch 1

B． the MAC address of Switch 1

C． the IP address of Host C

D． the MAC address of Host C

E． the IP address of the router's E0 interface

F． the MAC address of the router's E0 interface

5． Host 1 is trying to communicate with Host 2． The e0 interface on Router C is down． Which of the following are true? (Choose two．)

A． Router C will use ICMP to inform Host 1 that Host 2 cannot be reached．

B． Router C will use ICMP to inform Router B that Host 2 cannot be reached．

C． Router C will use ICMP to inform Host 1, Router A, and Router B that Host 2 cannot be reached．

D． Router C will send a Destination Unreachable message type．

E． Router C will send a Router Selection message type．

F． Router C will send a Source Quench message type．

6． Refer to the exhibit． Which destination addresses will Host A use to send data to Host B? (Choose two．)

A． the IP address of Switch1

B． the IP address of Router1 Fa0/0

C． the IP address of Host B

D． the MAC address of Switch1

E． the MAC address of Router1 Fa0/0

F． the MAC address of Host B

7． Refer to the exhibit． SW1 and SW2 have default configurations． What will happen if Host 1 sends a broadcast?

A． Hosts 2, 3, and 4 will receive the broadcast．

B． Hosts 1, 2, 3, and 4 will receive the broadcast．

C． Host 2 and the Fa0/0 interface of R1 will receive the broadcast．

D． Hosts 1, 2 and the Fa0/0 interface of R1 will receive the broadcast．

E． Hosts 1, 2, 3, 4 and interface Fa0/0 of R1 will receive the broadcast．

F． Hosts 2, 3, 4, and interfaces Fa0/0 and Fa0/1 of R1 will receive the broadcast．

8． Refer to the exhibit． In the communication between Host 1 and Host 2 over the point-to-point WAN, which protocol or technology is represented by dashed line A?

A． IP

B． T1

C． PPP

D． IEEE 802．3

参考答案

1.D 2.BD 3.AF 4.CF 5.AD 6.CE 7.C 8.A