osi模型:七层模型介绍
Open Systems Interconnection Model or OSI Model is an architecture to specify computer networks functions in a clear and compatible way. OSI provides interoperability of diverse communication systems with standard communication and transmission architecture, layers, and protocols.
Open Systems Interconnection模型或OSI模型是用于以清晰且兼容的方式指定计算机网络功能的体系结构。 OSI提供具有标准通信和传输体系结构，层和协议的各种通信系统的互操作性。
OSI历史 (OSI History)
In the early- and mid- 1970s the computer networks largely owned and operated by the governments or related institutions. These computer networks were using proprietary standards, devices, and protocols related to IBM, DEC, etc. UK National Computing Center started working on “Why Distributed Computing” which resulted in ISO meeting about standardized networks. First meeting held in France in 1977 to create a standard that will lead to the OSI Model. After years of work and published drafts the OSI Reference Model is published in 1984 with the ISO 7498 standard.
在1970年代初和中期，计算机网络主要由*或相关机构拥有和运营。 这些计算机网络使用了与IBM，DEC等相关的专有标准，设备和协议。英国国家计算机中心开始致力于“为什么要使用分布式计算”，因此ISO召开了有关标准化网络的会议。 1977年在法国举行了第一次会议，以制定导致OSI模型的标准。 经过多年的工作和发布的草案，OSI参考模型于1984年以ISO 7498标准发布。
OSI层 (OSI Layers)
OSI Model simply refers to a model that proposes multiple logical layers that define functions, features, interoperability. OSI model consist of 7 layers each of them provide unique network service and function to create a complete model for network communications. The OSI Referance model works as top-down approach where a transmission starts from top to down.
OSI模型仅是指提出多个逻辑层的模型，这些逻辑层定义了功能，特征和互操作性。 OSI模型由7层组成，每个层提供独特的网络服务和功能，以创建用于网络通信的完整模型。 OSI引用模型是自上而下的方法，其中传输从上到下开始。
- Layer 7 – Application Layer 第7层–应用层
- Layer 6 – Presentation Layer第6层–表示层
- Layer 5 – Sessions Layer第5层-会话层
- Layer 4 – Transport Layer第4层–传输层
- Layer 3 – Network Layer第三层–网络层
- Layer 2 – Data Link Layer第2层–数据链路层
- Layer 1 – Physical Layer第1层-物理层
主机层(Host Layers)
There are some grouping about these layers where from Layer 7 to Layer 4 are called as Host Layers. Application Layer (Layer 7), Presentation Layer (Layer 6), Sessions Layer (Layer 5), Transport Layer (Layer 4) are called Host Layers because the host or operating system and network stack is responsible with these layers.
关于这些层有一些分组，其中从第7层到第4层称为Host Layers 。 应用程序层(第7层)，表示层(第6层)，会话层(第5层)，传输层(第4层)被称为主机层，因为主机或操作系统以及网络堆栈负责这些层。
媒体层(Media Layers)
Layers from Layer 3 to Layer 1 are called as Media Layers .Network Layer (Layer 3), Data Link Layer (Layer 2), Physical Layer (Layer 1) are called Media Layers because these layers are implemented on the Network Interface Card and Media and mainly run on inter-network devices like Switch, Router etc.
从第3层到第1层的层称为Media Layers 。网络层(第3层)，数据链路层(第2层)，物理层(第1层)被称为媒体层，因为这些层是在网络接口卡和媒体上实现的并主要在交换机，路由器等网络设备上运行。
第七层：应用层 (Layer 7: Application Layer)
Application Layer is the closest layer to the end-user. Applications layer mainly operated by applications the user is using. Browsers, Twitter Client, Zoom, Games, etc. run on the applications layer of the OSI Reference Model. Applications layer uses popular protocols like HTTP, HTTPS, NTP, SMB, VNC, RDP which is mainly run by the user side application.
Application Layer是最接近最终用户的层。 应用程序层主要由用户正在使用的应用程序操作。 浏览器，Twitter客户端，Zoom，游戏等在OSI参考模型的应用程序层上运行。 应用层使用流行的协议，例如HTTP，HTTPS，NTP，SMB，VNC，RDP，这些协议主要由用户端应用程序运行。
第六层：表示层 (Layer 6: Presentation Layer)
Presentation Layer is located as Layer 6. Presentation layer is mainly designed to provide different presentations of the data between different applications for the same applications layer protocols. This layer is mainly used for encoding like ASCII, EBCDIC, Serialization, Compression etc.
Presentation Layer位于第6层。表示层主要用于为同一应用程序层协议在不同应用程序之间提供数据的不同表示形式。 该层主要用于ASCII，EBCDIC，序列化，压缩等编码。
第5层：会话层 (Layer 5: Session Layer)
Session Layer controls the connections/sessions between hosts/computers. This layer manage the communication to be full-duplex, half-duplex, simplex and operations like session suspension, restart, termination, start etc for the Layer 4 protocol like TCP and UDP.
Session Layer控制主机/计算机之间的连接/会话。 该层将通信管理为全双工，半双工，单工以及针对第4层协议(如TCP和UDP)的操作，如会话挂起，重启，终止，启动等。
第四层：传输层 (Layer 4: Transport Layer)
Transport Layer is responsible for transmitting data to the remote computer/host by using protocols TCP and UDP. Transport layer provides operations like flow control, segmentation, transmission connection management, re-transmission, transmission error detection, transmission error recover, timeout management, reliable transport etc. TCP and UDP are two popular transport layer protocols used today computer networks.
Transport Layer负责使用TCP和UDP协议将数据传输到远程计算机/主机。 传输层提供诸如流控制，分段，传输连接管理，重传，传输错误检测，传输错误恢复，超时管理，可靠传输等操作TCP和UDP是当今计算机网络中使用的两种流行的传输层协议。
第三层：网络层 (Layer 3: Network Layer)
Network Layer mainly responsible to transmit the data by the transport layer to the destinated remote computer/host. The network layer uses addresses to define different networks and hosts and routes between them. A network contains a single or multiple hosts or nodes which can communicate with other hosts/nodes located in different networks. The network layer provides:
Network Layer主要负责通过传输层将数据传输到目的地的远程计算机/主机。 网络层使用地址来定义不同的网络和主机以及它们之间的路由。 网络包含一个或多个主机或节点，可以与位于不同网络中的其他主机/节点进行通信。 网络层提供：
- Host Addressing 主机寻址
- Network Addressing网络编址
- Data Transfer Between Hosts/Nodes主机/节点之间的数据传输
- Routing路由
- Hop by Hop Transmission逐跳传输
- Network Segmentation网络细分
IP is the most popular network layer protocol used for 35 years. IP is used with the TCP and UDP protocols in a very integrated way and dominated the computer network in today’s world.
IP是35年来使用最广泛的网络层协议。 IP与TCP和UDP协议非常集成地使用，并且在当今的计算机网络中占主导地位。
第2层：数据链路层 (Layer 2: Data Link Layer)
Data Link Layer is used to transfer data from Node to Node in the same network which are connected direct link. Data Link Layer works in the same network. Data Link layer provides:
Data Link Layer用于在直接连接的同一网络中的节点之间传输数据。 数据链路层在同一网络中工作。 数据链路层提供：
- Node to Node datagram transmission 节点到节点数据报传输
- Data Flow Control数据流控制
Ethernet or Medium Access Control (MAC) protocols are the most known and used protocols in computer networks. Ethernet is defined with IEEE 802.3 standard. Wi-Fi is another popular protocol that is mostly used for laptops, tablets, smartphones, TVs to connect to LAN devices wireless. Wi-Fi is defined with IEEE 802.11 standard. Bluetooth and USB are another popular data link layer protocols used in computer networks.
Ethernet或Medium Access Control (MAC)协议是计算机网络中最知名和使用的协议。 以太网是使用IEEE 802.3标准定义的。 Wi-Fi是另一种流行的协议，主要用于笔记本电脑，平板电脑，智能手机，电视以无线方式连接到LAN设备。 Wi-Fi由IEEE 802.11标准定义。 Bluetooth和USB是计算机网络中使用的另一种流行的数据链路层协议。
第1层：物理层 (Layer 1: Physical Layer)
Physical Layer provides transmission medium for the data link layer. Physical layer transmit the datagrams over it with different type of signals. Physical Layer provides:
Physical Layer为数据链路层提供了传输介质。 物理层通过不同类型的信号在其上传输数据报。 物理层提供：
- Encoding 编码方式
- Signalling发信号
- Error Detection错误检测
The physical layer can operate over a copper wire, air, USB cable, fiber cable. The physical layer also uses simplex, half-duplex and full-duplex signal transmission modes.
物理层可以通过铜线，空气，USB电缆，光纤电缆运行。 物理层还使用单工，半双工和全双工信号传输模式。
OSI层和协议的示例数据包 (Example Packet For OSI Layers and Protocols)
Below you will find a network packet OSI layers. Frame is related with the physical layer andEthernet II is the data link layer. Internet Protocol Version 4 is the network layer and Transmission Control Protocol is transmission layer.
在下面，您将找到网络数据包OSI层。 Frame与物理层有关，而Ethernet II是数据链路层。 Internet Protocol Version 4是网络层， Transmission Control Protocol是传输层。
翻译自: https://www.poftut.com/what-is-osi-open-systems-interconnection-model/
osi模型:七层模型介绍