osi七层网络层

介绍 (Introduction)

Have you ever wondered how data is sent through the network from one machine to another? If yes, then the Open System Interconnected model is what you are looking for.

您是否曾经想过如何通过网络将数据从一台机器发送到另一台机器？ 如果是，那么您正在寻找开放系统互连模型。

The OSI model is used to help standardize and characterize how data should flow from sender to receiver without taking into consideration the underlying internal structure of the endpoint (sender, receiver).

OSI模型用于帮助标准化和表征数据应如何从发送方流向接收方，而无需考虑端点(发送方，接收方)的内部结构。

The organisation that came up with this model is the International Standardisation Organisation and hence this model is formally referred to as ISO - OSI.

提出此模型的组织是国际标准化组织 ，因此该模型正式称为ISO-OSI 。

建筑 (Architecture)

As in the figure below the model divides the network into 7 layers. Data communication in the OSI model starts with the top layer ( Application Layer ) of the stack at the sending side, travels down the stack to the sender’s lowest layer ( Physical Layer ), then traverses the physical network connection to the bottom layer on the receiving side, and up its OSI model stack.

如下图所示，模型将网络分为7层 。 OSI模型中的数据通信从发送方的堆栈的顶层(应用程序层)开始，向下沿堆栈行进到发送方的最低层(物理层)，然后遍历物理网络连接到接收方的底层方面，以及其OSI模型堆栈。

We go for a layered approach because it is easy to design independent layers with dedicated functions which interact with each other as compared to a single complex model.

我们之所以采用分层方法，是因为与单个复杂模型相比，设计具有相互交互的专用功能的独立层很容易。

重要观察 (Important Observations)

• End-to-end layers: In the diagram above, you would notice that the upper layers of the protocol (Application - Transport), the sender’s and receiver’s layers are directly connected via arrows. This is because these layers are not aware of intermediate devices that are used to transport data (such as switches and routers). These layers appear to communicate directly with each other.

  端到端层：在上图中，您会注意到协议的上层(应用程序-传输)，发送者和接收者的层通过箭头直接连接。 这是因为这些层不知道用于传输数据的中间设备(例如交换机和路由器)。 这些层似乎彼此直接通信。

• Unit of Data: In the diagram above, to the extreme left is the unit of data that is used in each of the layer. The transport layer (and the layers below it) have a unique name for the unit of data being transferred from sender to receiver.

  数据单位：在上图中，最左端是每个图层中使用的数据单位。 传输层(及其下面的层)对于从发送方到接收方的数据单位具有唯一的名称。

图层功能 (Functions of Layers)

• Layer 1 - Physical Layer: The physical layer is the lowest of the OSI Layers and the most complex. This is because of the underlying hardware technologies used. The function of this layer is to define how the bit stream will be transmitted rather than the logical data packet. It deals with defining which frequency will the bit be transmitted on, what kind of modulation will be used, how the bits will be grouped and other low lying physical parameters needed for transmission of bits.

  第1层-物理层：物理层是OSI层中最低的，也是最复杂的。 这是因为所使用的基础硬件技术。 该层的功能是定义如何传输位流，而不是逻辑数据包。 它涉及定义将在哪个频率上传输比特，将使用哪种调制方式，如何对比特进行分组以及传输比特所需的其他低层物理参数。

• Layer 2 - Data Link Layer: The data link layer is responsible for transferring data to adjacent devices on the same Local Area Network (LAN). This layer also has provisions to make sure that error-free data is being passed on to the higher layers from the physical layer. Hence, it contains error detection and correction mechanisms to ensure the integrity of data is maintained.

  第2层-数据链路层：数据链路层负责将数据传输到同一局域网(LAN)上的相邻设备。 该层还提供了确保无错误数据从物理层传递到更高层的规定。 因此，它包含错误检测和纠正机制，以确保维持数据的完整性。

• Layer 3 - Network Layer: The network layer is responsible for forwarding packets to other networks. Usually a network is divided into multiple subnets and the network layer with the help of routers forwards packets between such networks to establish a Wide Area Network (WAN).

  第3层-网络层：网络层负责将数据包转发到其他网络。 通常，一个网络被分为多个子网，网络层借助路由器在这些网络之间转发数据包以建立广域网(WAN)。

• Layer 4 - Transport Layer: The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplication. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.

  第4层-传输层：传输层可确保按顺序无错误地传递消息，并且不会丢失或重复。 它使高层协议不必担心它们与对等方之间的数据传输。

• Layer 5 - Session Layer: The session layer allows session establishment between processes running on different stations.

  第5层-会话层：会话层允许在不同工作站上运行的进程之间建立会话。

• Layer 6 - Presentation Layer: The presentation layer formats the data to be presented to the application layer.

  第6层-表示层：表示层格式化要呈现给应用程序层的数据。

• Layer 7 - Application Layer: The application layer serves as the window for users and application processes to access network services.

  第7层-应用程序层：应用程序层充当用户和应用程序进程访问网络服务的窗口。

翻译自: https://www.freecodecamp.org/news/a-crash-course-in-osi-layers/

osi七层网络层