Data Communication Computer Network 简明教程

Computer Network Models

网络工程是一项复杂的任务，涉及软件、固件、芯片级工程、硬件和电子脉冲。为了简化网络工程，整个网络概念被划分为多个层。每层涉及一些特定任务，且独立于所有其他层。但作为一个整体，几乎所有网络任务都依赖于这些层。层之间共享数据，它们彼此依赖才能接收输入并发送输出。

Networking engineering is a complicated task, which involves software, firmware, chip level engineering, hardware, and electric pulses. To ease network engineering, the whole networking concept is divided into multiple layers. Each layer is involved in some particular task and is independent of all other layers. But as a whole, almost all networking tasks depend on all of these layers. Layers share data between them and they depend on each other only to take input and send output.

Layered Tasks

在网络模型的分层架构中，整个网络进程被划分为小任务。然后，将每个小任务分配给一个特定层，该层只致力于处理任务。每层只做特定工作。

In layered architecture of Network Model, one whole network process is divided into small tasks. Each small task is then assigned to a particular layer which works dedicatedly to process the task only. Every layer does only specific work.

在分层通信系统中，一台主机的某个层处理由远程主机所做或准备由远程主机所做的任务，而且处理的任务位于远程主机同一级别的对等层。任务要么由最低层要么由最高层的层启动。如果任务由最高层启动，则向下传递给下一层以便进一步处理。下层做同样的事情，它处理任务并将其传递给下层。如果任务由最低层启动，则采取相反的路径。

In layered communication system, one layer of a host deals with the task done by or to be done by its peer layer at the same level on the remote host. The task is either initiated by layer at the lowest level or at the top most level. If the task is initiated by the-top most layer, it is passed on to the layer below it for further processing. The lower layer does the same thing, it processes the task and passes on to lower layer. If the task is initiated by lower most layer, then the reverse path is taken.

layered tasks

每一层都将执行其任务部分所需的所有过程、协议和方法集合在一起。所有层通过封装头和尾来识别其对应的层。

Every layer clubs together all procedures, protocols, and methods which it requires to execute its piece of task. All layers identify their counterparts by means of encapsulation header and tail.

OSI Model

开放系统互连是一种针对所有通信系统制定的开放式标准。OSI 模型由国际标准化组织 (ISO) 建立。此模型具有七层：

Open System Interconnect is an open standard for all communication systems. OSI model is established by International Standard Organization (ISO). This model has seven layers:

OSI model

  1. Application Layer: This layer is responsible for providing interface to the application user. This layer encompasses protocols which directly interact with the user.

  2. Presentation Layer: This layer defines how data in the native format of remote host should be presented in the native format of host.

  3. Session Layer: This layer maintains sessions between remote hosts. For example, once user/password authentication is done, the remote host maintains this session for a while and does not ask for authentication again in that time span.

  4. Transport Layer: This layer is responsible for end-to-end delivery between hosts.

  5. Network Layer: This layer is responsible for address assignment and uniquely addressing hosts in a network.

  6. Data Link Layer: This layer is responsible for reading and writing data from and onto the line. Link errors are detected at this layer.

  7. Physical Layer: This layer defines the hardware, cabling wiring, power output, pulse rate etc.

Internet Model

互联网使用 TCP/IP 协议套件，也称为 Internet 套件。这定义了包含 4 层架构的 Internet 模型。OSI 模型是通用通信模型，但 Internet 模型是互联网用于其所有通信的方式。互联网与其底层网络架构无关，其模型也是如此。此模型具有以下层：

Internet uses TCP/IP protocol suite, also known as Internet suite. This defines Internet Model which contains four layered architecture. OSI Model is general communication model but Internet Model is what the internet uses for all its communication.The internet is independent of its underlying network architecture so is its Model. This model has the following layers:

Internet model

  1. Application Layer: This layer defines the protocol which enables user to interact with the network.For example, FTP, HTTP etc.

  2. Transport Layer: This layer defines how data should flow between hosts. Major protocol at this layer is Transmission Control Protocol (TCP). This layer ensures data delivered between hosts is in-order and is responsible for end-to-end delivery.

  3. Internet Layer: Internet Protocol (IP) works on this layer. This layer facilitates host addressing and recognition. This layer defines routing.

  4. Link Layer: This layer provides mechanism of sending and receiving actual data.Unlike its OSI Model counterpart, this layer is independent of underlying network architecture and hardware.