Internet Technologies 简明教程
Internet Reference Models
Reference Model
参考模型提供了一种全球可接受的标准化方法。由于使用计算机网络的人员分布在广阔的物理范围内，并且他们的网络设备可能有异构的架构。为了在异构设备之间进行通信，我们需要一个标准化模型，即参考模型，该模型将为我们提供一种方法，无论其架构如何，这些设备都可以进行通信。
Reference Model offers a means of standardization which is acceptable worldwide. Since people using the computer network are located over a wide physical range and their network devices might have heterogeneous architecture. In order to provide communication among heterogeneous devices, we need a standardized model i.e. a reference model, which would provide us way how these devices can communicate regardless their architecture.
我们有两个参考模型，例如 OSI 模型和 TCP/IP 参考模型，然而，OSI 模型是一个假设模型，而 TCP/IP 是一个绝对实用的模型。
We have two reference models such as OSI model and TCP/IP reference model, however, the OSI model is a hypothetical one but the TCP/IP is absolutely practical model.
OSI Model
OSI 是 Open System Interface 的首字母缩写。此模型由 International organization of Standardization (ISO) 开发，因此也称为 ISO-OSI 模型。
OSI is acronym of Open System Interface. This model is developed by the International organization of Standardization (ISO) and therefore also referred as ISO-OSI Model.
OSI 模型由七层组成，如下面的图表所示。每层都有一个特定功能，但是每层都向上一层提供服务。
The OSI model consists of seven layers as shown in the following diagram. Each layer has a specific function, however each layer provide services to the layer above.
Physical Layer
物理层负责以下活动：
The Physical layer is responsible for the following activities:
-
Activating, maintaining and deactivating the physical connection.
-
Defining voltages and data rates needed for transmission.
-
Converting digital bits into electrical signal.
-
Deciding whether the connection is simplex, half duplex or full duplex.
Data Link Layer
数据链路层执行以下功能：
The data link layer performs the following functions:
-
Performs synchronization and error control for the information which is to be transmitted over the physical link.
-
Enables error detection, and adds error detection bits to the data which are to be transmitted.
Network Layer
以下是网络层的函数：
Following are the functions of Network Layer:
-
To route the signals through various channels to the other end.
-
To act as the network controller by deciding which route data should take.
-
To divide the outgoing messages into packets and to assemble incoming packets into messages for higher levels.
Transport Layer
传输层执行以下功能：
The Transport layer performs the following functions:
-
It decides if the data transmission should take place on parallel paths or single path.
-
It performs multiplexing, splitting on the data.
-
It breaks the data groups into smaller units so that they are handled more efficiently by the network layer.
Session Layer
会话层执行以下功能：
The Session layer performs the following functions:
-
Manages the messages and synchronizes conversations between two different applications.
-
It controls logging on and off, user identification, billing and session management.
Presentation Layer
表示层执行以下功能：
The Presentation layer performs the following functions:
-
This layer makes it sure that the information is delivered in such a form that the receiving system will understand and use it.
Application Layer
应用层执行以下功能：
The Application layer performs the following functions:
-
It provides different services such as manipulation of information in several ways, retransferring the files of information, distributing the results etc.
-
The functions such as LOGIN or password checking are also performed by the application layer.
TCP/IP Model
TCP/IP 模型是实用模型且用于互联网。TCP/IP 是传输控制协议和互联网协议的首字母缩写。
TCP/IP model is practical model and is used in the Internet. TCP/IP is acronym of Transmission Control Protocol and Internet Protocol.
TCP/IP 模型将两层（物理和数据链路层）合并为一层，即 Host-to-Network 层。下图显示了 TCP/IP 模型的各个层：
The TCP/IP model combines the two layers (Physical and Data link layer) into one layer i.e. Host-to-Network layer. The following diagram shows the various layers of TCP/IP model:
Application Layer
此层与 OSI 模型相同并执行以下功能：
This layer is same as that of the OSI model and performs the following functions:
-
It provides different services such as manipulation of information in several ways, retransferring the files of information, distributing the results etc.
-
The functions such as LOGIN or password checking are also performed by the application layer.
Transport Layer
它执行与 OSI 模型中传输层相同的功能。以下是要点关于传输层：
It does the same functions as that of transport layer in OSI model. Here are the key points regarding transport layer:
-
It uses TCP and UDP protocol for end to end transmission.
-
TCP is reliable and connection oriented protocol.
-
TCP also handles flow control.
-
The UDP is not reliable and a connection less protocol also does not perform flow control.
Internet Layer
该层的目的是允许主机将数据包插入网络，然后使其独立地传输到目的地。然而，接收数据包的顺序可能与发送它们的顺序不同。
The function of this layer is to allow the host to insert packets into network and then make them travel independently to the destination. However, the order of receiving the packet can be different from the sequence they were sent.