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Preface: ISO/OSI reference model

Early 1980's-Mid 1980's, the rapid growth of the scale and number of computer networks. Many different specifications and implemented network products are difficult to interoperate, and the special system seriously hinders the development of computer network. Through the study of the existing computer network architecture at that time and drawing lessons from its essence, a standard was published in 1984 (file ISO 7048).

The network structure is divided into seven layers: physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer. Each layer has its own set of functions and interacts with the adjacent upper and lower layers.

Each layer of the OSI reference model has its own set of functions that must be implemented to ensure that datagrams can be transmitted from source to destination. The functions of the layers of the OSI reference model are shown in the table:

Physical layer of 1.OSI

Physical layer protocols have four characteristics:

Mechanical properties: specification, size, number of pins and arrangement of connectors required for physical connection

Electrical characteristics: signal level, data coding, impedance, transmission rate and distance limitation when transmitting bit stream on physical channel

Functional features: the exact meaning of each signal line, that is, the function of each signal line

Protocol characteristics: a set of operating procedures for bit stream transmission using signal lines, which refers to the exchange of data order between physical connections during the establishment, maintenance, and exchange of information.

two。 Data link layer

To achieve error-free transmission between two adjacent machines. Provide services to the network layer by strengthening the original bitstream transmission services provided by the physical layer

Framing: specifies the smallest logical unit of data transmission at the data link layer-the type and format of the frame. The information packets received from the network layer are formed into frames and transmitted to the physical layer, and then transmitted by the physical layer to the other party's data link layer.

Physical addressing and access to network topologies

Error control: there is a check field in the information frame, when the receiver receives the frame, it is checked according to the selected error control method, and the error is handled when the error is found.

Flow control: coordinate the data flow between the sender and the receiver so that the transmission rate does not exceed the receiver's rate.

Media access Control in shared Media Environment

Link management: establish, maintain, and release data links.

3. Network layer

It involves sending the data (packets) sent from the source to the destination through various paths. from the source to the destination, it may have to go through many intermediate nodes-interconnection and path selection.

(source network-destination network)

The highest layer of the communication subnet, but the lowest layer that handles end-to-end (host-host) data transfer.

Type and format of information grouping

Logical addressing

Routing and forwarding

Congestion control

4. Transport layer

Establish, maintain and dismantle transmission connections for high-level data transmission to achieve transparent end-to-end transmission (host-host)

In the true sense of the "end-to-end" layer from source to destination, a program on the source side communicates peer-to-peer with a "similar" program on the source side.

It shields the boundary between the upper three layers (application-oriented) and the lower three layers (data transmission), makes up for the lack of quality of service provided by the network, and provides reliable network services.

Segmentation (source side) and merging (destination side) of information

Flow control and error recovery

Multiplexing

5. Session layer

Establish, manage, and terminate sessions between applications or processes on different machines

Provide service session (Dialogue) management for the presentation layer:

Token (Token)

Synchronization of session

Checkpoint (Checkpoint

6. Presentation layer

The layers below the presentation layer are only concerned with reliable data transmission, while the presentation layer is concerned with the syntax and semantics of the transmitted data.

Complete the conversion of syntax format

Different computers may have different internal data representations (abstract syntax). After receiving some form of grammatical data from the application layer, the presentation layer converts it into data represented by a common syntax (transfer syntax) suitable for transmission between network entities. Including data representation format and conversion, data compression, encryption and decryption, protocol conversion, etc.

The type of common syntax used in negotiation with the recipient

Connection establishment, data transfer and connection release between peer-to-peer entities in the presentation layer

7. Application layer

The highest layer of OSI, which is also the nearest layer to the user, is the interface between the computer network and the user.

It consists of several application processes or programs. The network provides a variety of network services for users through the application layer.

Common application layer services:

Directory service

E-mail

File transfer

Virtual terminal

Layer-to-layer Communication of 8.OSI

The process of interaction between layers of the same computer and the process of communication between different computers on the same layer are interrelated.

Each layer provides services to the upper layers in its protocol specification.

Each layer exchanges some information with the software and hardware of the same layer in other computers.

An example of data encapsulation

The key to the whole process: although the actual transmission direction of the data is vertical, each layer is programmed as if the data were transmitted horizontally.

For example, when the sender's transport layer receives a message from the session layer, it adds a transport layer header and sends the message to the receiver's transport layer. From the point of view of the sender's transport layer, the message is actually sent to the network layer within the machine, but this fact is only an unimportant technical detail. For example, when a non-lingua franca diplomat spoke at the United Nations, he thought he was addressing other diplomats present. In fact, he was only speaking to his own translator, and perhaps only the translator could understand what he was talking about; however, this did not prevent him from communicating with other diplomats, as it was only a technical detail.

The name of each layer of data in the 9.OSI reference model

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