First aid! Understanding the seven layer structure of network OSI!

The Open Systems Interconnection Model (OSI) is a reference model proposed by the International Organization for Standardization (ISO) in 1984. As a conceptual framework, it is a standard for devices and application software from different manufacturers to communicate in a network. Now this model has become the main structural model of Mt. Yama for communication between computers and networks.

The structure of most of the network communication protocols currently in use is based on the OSI model. OSI defines the communication process as seven layers, that is, the task of transferring information between networked computers into seven smaller, more manageable task groups. Each task or task group is assigned to each OSI layer.

Each layer exists independently, so the tasks assigned to each layer can be executed independently. This makes it possible to change the scheme provided by one of the layers without affecting the others.

Each layer of the OSI seven-layer model has clear features. Basically, Layers 7 to 4 handle end-to-end communication between the data source and the data destination, while Layers 3 to 1 handle communication between network devices. In addition, the seven layers of the OSI model can also be divided into two groups:

Upper (Layer 7, Layer 6, and Layer 5) and Lower Layers (Layer 4, Layer 3, Layer 2, and Layer 1). The upper layer of the OSI model deals with application problems and is usually applied only to software. The layer, i.e., the application layer, is the closest to the end user.

The lower layer of the OSI model is to handle data transfer. The physical layer and the data link layer are applied on both hardware and software. The lowest layer, the physical layer, is the closest to the physical network medium (e.g., wires) and is responsible for sending data on the medium.

The specific descriptions of each layer are as follows:

Layer 7: Application Layer defines the interface for communication and data transmission in the network - user programs;

Provision of standard services, such as the transfer and processing of virtual terminals, files and tasks;

Layer 6: Presentation Layer masks the differences in data formats between different systems;

Specify the data transmission format of the independent structure;

encoding and decoding of data; encryption and decryption; Compression and decompression Layer 5: Session Layer manages user sessions and conversations;

control the establishment and hanging up of logical connections between users;

Report an error that occurred in the previous layer.

Layer 4: Transport Layer manages end-to-end information transmission in the middle of the network;

Provide reliable and orderly packet delivery through error correction and flow control mechanisms;

Provide connection-free packet delivery;

Layer 3: Network Layer defines how data is transferred between network devices.

route packets based on the address of the network device;

Provides flow and congestion control to prevent loss of network resources.

Layer 2: Data Link Layer defines the program that operates the communication connection;

Encapsulating packets as data frames;

Monitor and correct packet transmission errors.

Layer 1: Physical Layer defines the physical way in which data is sent through network devices;

as an interface between network media and devices;

Define optical, electrical, and mechanical properties.

Answer] :(1) Physical layer.

The unit of data processed by the physical layer is bits, the physical layer provides a physical link for the data link layer upwards to achieve transparent bitstream transmission services, and the physical layer is connected to the physical layer downwards to determine the mechanical, electrical, functional and process characteristics of the network interface connected to the physical ***.

2) Data link layer.

The data link layer is responsible for transmitting data with frames as PDUs between nodes on a single link, enabling reliable data transmission on less reliable physical links. The main functions of the data link layer include: establishing, maintaining, and releasing data link connections, link access control, flow control, and error control.

3) Network layer.

The PDUs transmitted at the network layer are called packets or packets, which transmit packets between physical networks and are responsible for transmitting packets from the source host to the destination through intermediate nodes. The network layer is the highest layer of the communication subnet, providing both virtual circuit and datagram services to the host. The network layer is mainly responsible for packet ** and route selection, according to the routing table, the packet is transmitted from the origin server to the destination station hop by hop, and can adapt to the change of network load and topology, and dynamically update the routing table.

4) Transport layer.

The transport layer provides end-to-end reliable transmission services between user processes at the source and destination nodes. The end-to-end transmission refers to the transmission between the two transport layer entities of the source node and the destination node, and does not involve intermediate nodes such as routers. In order to ensure reliable transport services, the transport layer has the following functions:

Network quality of service selection for connection, flow control and congestion control, error control, etc.

5) Session layer.

The session layer adds a mechanism for controlling the session on the basis of the transport layer service, and establishes, organizes and coordinates the interaction process between application processes. The types of services provided by the session layer include duplex, half-duplex and simplex modes. One way of session management is token management, where only the token holder can perform an action.

The session layer provides synchronization control of the session, and when a failure occurs, the session activity is repeated at the synchronization point before the failure point, rather than having to start from scratch.

6) Presentation layer.

The presentation layer defines the format of data exchange between users or applications, and provides conversion services between data representations to ensure that the meaning of the transmitted information remains unchanged after it reaches the destination.

7) Application layer.

The application layer directly faces user applications and provides users with convenient access to various network resources.