Data Communication Computer Network 简明教程

Network Switching

交换是将来自一个端口的数据包转发到通向目标的端口的过程。当数据进入一个端口时，称为入口；当数据离开一个端口或发出时，称为出口。通信系统可能包括许多交换机和节点。总体来说，交换可以分为两类：

Switching is process to forward packets coming in from one port to a port leading towards the destination. When data comes on a port it is called ingress, and when data leaves a port or goes out it is called egress. A communication system may include number of switches and nodes. At broad level, switching can be divided into two major categories:

  1. Connectionless: The data is forwarded on behalf of forwarding tables. No previous handshaking is required and acknowledgements are optional.

  2. Connection Oriented:  Before switching data to be forwarded to destination, there is a need to pre-establish circuit along the path between both endpoints. Data is then forwarded on that circuit. After the transfer is completed, circuits can be kept for future use or can be turned down immediately.

Circuit Switching

当两个节点通过专用通信路径彼此通信时，这称为电路交换，需要一条预先指定的数据传输路线，不允许其他数据传输。在电路交换中，为了传输数据，必须建立电路，以便进行数据传输。

When two nodes communicate with each other over a dedicated communication path, it is called circuit switching.There 'is a need of pre-specified route from which data will travels and no other data is permitted.In circuit switching, to transfer the data, circuit must be established so that the data transfer can take place.

电路可以是永久的或临时的。使用电路交换的应用程序可能必须经历三个阶段：

Circuits can be permanent or temporary. Applications which use circuit switching may have to go through three phases:

  1. Establish a circuit

  2. Transfer the data

  3. Disconnect the circuit

circuit switching

电路交换是为语音应用程序设计的。电话是最适合的电路交换示例。在用户拨打电话之前，必须在网络上建立呼叫者和被叫者之间的虚拟路径。

Circuit switching was designed for voice applications. Telephone is the best suitable example of circuit switching. Before a user can make a call, a virtual path between caller and callee is established over the network.

Message Switching

这种技术介于电路交换和分组交换之间。在报文交换中，整个报文被视为一个数据单元，并完整交换/传输。

This technique was somewhere in middle of circuit switching and packet switching. In message switching, the whole message is treated as a data unit and is switching / transferred in its entirety.

使用报文交换工作的交换机首先接收整个报文，并将其缓冲，直到有可用于将其传输到下一个跳跃的资源。如果下一个跳跃没有足够的资源来容纳大尺寸报文，则存储报文，然后交换机等待。

A switch working on message switching, first receives the whole message and buffers it until there are resources available to transfer it to the next hop. If the next hop is not having enough resource to accommodate large size message, the message is stored and switch waits.

message switching

这种技术被认为是对电路交换的替代。与电路交换中的为两个实体完全阻塞路径不同，报文交换被分组交换所取代。报文交换具有以下缺点：

This technique was considered substitute to circuit switching. As in circuit switching the whole path is blocked for two entities only. Message switching is replaced by packet switching. Message switching has the following drawbacks:

  1. Every switch in transit path needs enough storage to accommodate entire message.

  2. Because of store-and-forward technique and waits included until resources are available, message switching is very slow.

  3. Message switching was not a solution for streaming media and real-time applications.

Packet Switching

消息交换的不足之处催生了一种分组交换的思想。整个消息被分解成更小的块，称为分组。交换信息被添加到每个分组的头部并独立传输。

Shortcomings of message switching gave birth to an idea of packet switching. The entire message is broken down into smaller chunks called packets. The switching information is added in the header of each packet and transmitted independently.

中间网络设备更容易存储小尺寸分组，而且它们在载波路径或交换机的内部存储器中也不会占用太多的资源。

It is easier for intermediate networking devices to store small size packets and they do not take much resources either on carrier path or in the internal memory of switches.

packet switching

分组交换提高了线路效率，因为可以将来自多个应用程序的分组多路复用到载波上。互联网使用分组交换技术。分组交换使用户能够根据优先级区分数据流。分组根据其优先级进行存储和转发，以提供服务质量。

Packet switching enhances line efficiency as packets from multiple applications can be multiplexed over the carrier. The internet uses packet switching technique. Packet switching enables the user to differentiate data streams based on priorities. Packets are stored and forwarded according to their priority to provide quality of service.