Data Communication Computer Network 简明教程

Computer Network Topologies

网络拓扑是连接计算机系统或网络设备的排列方式。拓扑可能定义网络的物理和逻辑方面。在同一个网络中，逻辑拓扑和物理拓扑可能相同，也可能不同。

A Network Topology is the arrangement with which computer systems or network devices are connected to each other. Topologies may define both physical and logical aspect of the network. Both logical and physical topologies could be same or different in a same network.

Point-to-Point

点对点网络包含恰好两个主机，例如计算机、交换机或路由器，使用一根电缆背靠背连接的服务器。通常，一个主机的接收端连接到另一个主机的发送端，反之亦然。

Point-to-point networks contains exactly two hosts such as computer, switches or routers, servers connected back to back using a single piece of cable. Often, the receiving end of one host is connected to sending end of the other and vice-versa.

如果主机在逻辑上是点对点连接的，那么可能有多个中间设备。但是，最终主机不知道底层网络，并且会将对方视为直接连接。

If the hosts are connected point-to-point logically, then may have multiple intermediate devices. But the end hosts are unaware of underlying network and see each other as if they are connected directly.

Bus Topology

在总线拓扑的情况下，所有设备共享单条通信线路或电缆。当多个主机同时发送数据时，总线拓扑可能会出现问题。因此，总线拓扑使用 CSMA/CD 技术或识别一个主机为总线主设备来解决该问题。这是网络连接的一种简单形式，其中一台设备的故障不会影响其他设备。但是，共享通信线路的故障会导致所有其他设备停止运行。

In case of Bus topology, all devices share single communication line or cable.Bus topology may have problem while multiple hosts sending data at the same time. Therefore, Bus topology either uses CSMA/CD technology or recognizes one host as Bus Master to solve the issue. It is one of the simple forms of networking where a failure of a device does not affect the other devices. But failure of the shared communication line can make all other devices stop functioning.

bus topology

共享信道的两端都有线路终端器。数据仅在一个方向发送，并且一旦到达最远端，终端器就会从线路中删除数据。

Both ends of the shared channel have line terminator. The data is sent in only one direction and as soon as it reaches the extreme end, the terminator removes the data from the line.

Star Topology

星形拓扑中的所有主机都使用点对点连接连接到一个称为集线器设备的中央设备。也就是说，主机和集线器之间存在点对点连接。集线器设备可以是以下任一种设备：

All hosts in Star topology are connected to a central device, known as hub device, using a point-to-point connection. That is, there exists a point to point connection between hosts and hub. The hub device can be any of the following:

  1. Layer-1 device such as hub or repeater

  2. Layer-2 device such as switch or bridge

  3. Layer-3 device such as router or gateway

star topology

与总线拓扑一样，集线器充当单点故障。如果集线器发生故障，则所有主机与其他所有主机的连接都会失败。主机之间的每项通信都仅通过集线器进行。星型拓扑不需要高成本，因为连接一台主机只需要一根电缆，而且配置也很简单。

As in Bus topology, hub acts as single point of failure. If hub fails, connectivity of all hosts to all other hosts fails. Every communication between hosts, takes place through only the hub.Star topology is not expensive as to connect one more host, only one cable is required and configuration is simple.

Ring Topology

在环形拓扑中，每台主机连接到另外两台主机，创建了一个环形网络结构。当一台主机尝试与不与其相邻的主机通信或向其发送消息时，数据将通过所有中间主机进行传输。要在现有结构中连接一台主机，管理员可能只需要再使用一根电缆。

In ring topology, each host machine connects to exactly two other machines, creating a circular network structure. When one host tries to communicate or send message to a host which is not adjacent to it, the data travels through all intermediate hosts. To connect one more host in the existing structure, the administrator may need only one more extra cable.

ring topology

任何主机的故障都会导致整个环路的故障。因此，环路中的每个连接都是故障点。有些方法采用了另一个备份环路。

Failure of any host results in failure of the whole ring.Thus, every connection in the ring is a point of failure. There are methods which employ one more backup ring.

Mesh Topology

在这种类型的拓扑中，一台主机连接到一个或多个主机。该拓扑具有点对点连接的所有其他主机的点对点连接中的主机，或者也可能具有仅点对点连接到少数主机的点对点连接中的主机。

In this type of topology, a host is connected to one or multiple hosts.This topology has hosts in point-to-point connection with every other host or may also have hosts which are in point-to-point connection to few hosts only.

mesh topology

网状拓扑中的主机还充当其他没有直接点对点链路的主机的中继。网状技术分为两种类型：

Hosts in Mesh topology also work as relay for other hosts which do not have direct point-to-point links. Mesh technology comes into two types:

  1. Full Mesh: All hosts have a point-to-point connection to every other host in the network. Thus for every new host n(n-1)/2 connections are required. It provides the most reliable network structure among all network topologies.

  2. Partially Mesh: Not all hosts have point-to-point connection to every other host. Hosts connect to each other in some arbitrarily fashion. This topology exists where we need to provide reliability to some hosts out of all.

Tree Topology

它也被称为层次拓扑，这是目前使用最广泛的网络拓扑形式。该拓扑模拟为扩展星形拓扑并继承总线拓扑的特性。

Also known as Hierarchical Topology, this is the most common form of network topology in use presently.This topology imitates as extended Star topology and inherits properties of bus topology.

该拓扑将网络划分为多个网络层面/层级。主要在局域网中，网络分为三类网络设备。最下层是接入层，计算机附加在此处。中间层被称为分配层，它充当上层和下层之间的调解器。最高层被称为核心层，是网络的中心点，也是树的根，所有节点从中分叉。

This topology divides the network in to multiple levels/layers of network. Mainly in LANs, a network is bifurcated into three types of network devices. The lowermost is access-layer where computers are attached. The middle layer is known as distribution layer, which works as mediator between upper layer and lower layer. The highest layer is known as core layer, and is central point of the network, i.e. root of the tree from which all nodes fork.

tree topology

所有邻近主机在其之间进行点对点连接。类似于总线拓扑，如果根节点关闭，那么整个网络都会受到影响，即使它不是单点故障。每个连接都充当故障点，故障会导致网络划分为无法访问的片段。

All neighboring hosts have point-to-point connection between them.Similar to the Bus topology, if the root goes down, then the entire network suffers even.though it is not the single point of failure. Every connection serves as point of failure, failing of which divides the network into unreachable segment.

Daisy Chain

这种拓扑以线性方式连接所有主机。类似于环形拓扑，除端点主机外，所有主机仅连接到两台主机。这意味着，如果菊花链中的端点主机连接，则表示环形拓扑。

This topology connects all the hosts in a linear fashion. Similar to Ring topology, all hosts are connected to two hosts only, except the end hosts.Means, if the end hosts in daisy chain are connected then it represents Ring topology.

daisy chain topology

菊花链拓扑中的每个链路都表示单点故障。每个链路故障都会将网络分成两个部分。每个中间主机充当其直接主机的中继。

Each link in daisy chain topology represents single point of failure. Every link failure splits the network into two segments.Every intermediate host works as relay for its immediate hosts.

Hybrid Topology

包含多种拓扑设计的网络结构称为混合拓扑。混合拓扑继承了所有包含拓扑的优点和缺点。

A network structure whose design contains more than one topology is said to be hybrid topology. Hybrid topology inherits merits and demerits of all the incorporating topologies.

hybrid topology

上面的图片表示任意混合拓扑。组合拓扑可能包含星形、环形、总线和菊花链拓扑的属性。大多数广域网都通过双环拓扑连接，而连接到它们的网络大多是星形拓扑网络。互联网是最大的混合拓扑的最佳示例

The above picture represents an arbitrarily hybrid topology. The combining topologies may contain attributes of Star, Ring, Bus, and Daisy-chain topologies. Most WANs are connected by means of Dual-Ring topology and networks connected to them are mostly Star topology networks. Internet is the best example of largest Hybrid topology