Knowledge points involved
What is OSI architecture, the concept of OSI architecture, three elements of network protocol, application layer, presentation layer, session layer, transport layer, presentation layer, data link layer, physical layer, functions of each layer of the Open Systems Interconnection Reference Model . In-depth understanding of the system interconnection reference model, the 7 layers of OSI, and the 7 models of OSI.
原创于：CSDN博主-《拄杖盲学轻声码》，更多内容可去其主页关注下哈，不胜感激
Article Directory
foreword
更多考试总结可关注CSDN博主-《拄杖盲学轻声码》
1. The concept of agreement
In December 1969, ARPANET, the packet-switching network of the US Defense Advanced Research Agency, was put into operation, and the development of computer networks has entered a new era since then. ARPANET had only four nodes at the time, in the US Department of Defense, the Atomic Energy Commission, the Massachusetts Institute of Technology and California. Obviously, it is not enough to carry out data communication between these 4 computers, only the channel for transmitting data, and some pre-agreed rules must also be followed, and these rules clarify the format of the exchanged data and related synchronization issues. People need to use the same language to talk to each other. If one person speaks Chinese and the other speaks English,
Then there must be a translator, otherwise the information between the two cannot be communicated. The communication process between computers
is very similar to the conversation process between people, except that the former is controlled by the computer, and the latter is controlled by the people participating in the conversation.
A computer network protocol is a set of agreements that must be followed by both communicating computers. For example, how to establish a connection, how to identify each other, etc. Only by following this convention can computers communicate with each other.
Usually the network protocol consists of 3 elements.
(1) Syntax, that is, the structure and format of control information or data;
(2) Semantics, that is, what kind of control information needs to be sent, what actions to complete, and what responses to make;
(3) Synchronization, that is, the sequence of events Detailed description.
2. Open System Interconnection Reference Model System Architecture
The practical experience of ARPANET shows that for very complex computer multi-networks, the structure is preferably hierarchical. According to this feature, the International Organization for Standardization ISO has introduced the Open System Interconnection Reference Model (ISO'OSIRM, OpenSystemInterconnectReferenceModel). This model defines the standard for the interconnection of different computers, and is the basic framework for designing and describing computer networks AISIN. The system structure of the open system mutual reference model is hierarchical, which is divided into 7 layers, as shown in Table 1-1. In this model, there is peer-to-peer communication between layers, and this kind of communication is only logical. The real communication is realized at the bottom layer—the physical layer. Each layer needs to complete the corresponding functions, and the next layer is The upper layer provides services, thus dividing the complex communication process into multiple independent sub-problems that are relatively easy to solve.
Table 1-1 OSIRMs
| layer number | English abbreviations | English name | Chinese name |
|---|---|---|---|
| 7 | A | ApplicationLayer | application layer |
| 6 | P | PresentationLayer | presentation layer |
| 5 | S | SessionLayer | session layer |
| 4 | T | TransportLayer | transport layer |
| 3 | N | NetworkLayer | Network layer |
| 2 | DL | DataLinkLayer | data link layer |
| 1 | PL | PhysicalLayer | physical layer |
Historically, two international organizations that have played a significant role in formulating computer network standards are: the International Organization for Standardization (ISO, International Standardization Organization) and the International Telegraph and Telephone Consultative Committee (CCITT, International Telephone and Telegraph Consultative Committee). The fields of work of ISO and CCITT are different. ISO is a global non-governmental organization and a very important organization in the field of international standardization. The mission of ISO is to promote standardization and related activities on a global scale, to facilitate the exchange of products and services internationally, and to develop international mutual cooperation in knowledge, science, technology and economic activities. CCITT is now renamed the International Telecommunication Union Telecommunication Standardization Sector (ITU-T).
Operational and tariff issues, publication of proposals. Although OSI was formulated by ISO at the beginning, many of the later standards were jointly formulated by ISO and CCITT. CCITT's proposal X.200 is to explain the reference model of open system interconnection.
3. Functions of each layer of the OSI reference model
1) Physical layer
The physical layer is the most important and basic layer in the OSI layered structure system. It is based on the transmission medium and realizes the physical interface between devices. The physical layer just receives and sends a stream of bits, regardless of the meaning and structure of the information. It includes the description of various mechanical, electrical and functional provisions of the equipment connected to the network, and also defines the
level of potential, the interval of change, the type of cable, the characteristics of the connector, etc. The unit of data at the physical layer is the bit.
The function of the physical layer is to realize the bit-by-bit transmission between entities, ensure the correctness of the bit-by-bit transmission, and provide a transparent bit stream transmission to the data link layer. Complete the establishment, maintenance and removal of data links between data terminal equipment, data communication and switching equipment and other equipment.
2) Data link layer
The data link layer realizes the reliable transmission of data between entities. Links established through the physical layer can correctly transmit information with certain meaning and structure between entities, and at the same time provide effective services for the network layer above it. In the data link layer, the errors generated on the physical link are detected and corrected, and the error control technology is used to ensure the correctness of data communication; the data link layer also provides flow control services to ensure that the sender will not be caused by high speed The receiver did not have time to receive the data correctly. The data unit of the data link layer is a frame.
The function of the data link layer is to realize the correct transmission of binary information blocks between system entities. Provide reliable and error-free data information for the network layer. In the data link, the problems that need to be solved include: information mode, operation mode, error control, flow control, information exchange process control and communication control procedures.
3) Network layer
The network layer, also known as the communication subnet layer, is the interface layer between the high-level protocol and the low-level protocol. It is used to control the operation of the communication subnet and is the interface between the communication subnet and the resource subnet. The main task of the network layer is to provide routing and select an optimal path for the transmission of information packets. The network layer also has functions such as congestion control, information packet sequence control and network accounting. The unit of data exchanged at the network layer is the packet.
The function of the network layer is to provide services to the transport layer and accept services from the data link layer. Its main function is to realize the connection within the entire network system and provide the transport layer with a data transmission path between two end users within the entire network. aspects of the relationship. Therefore, the task of the network layer is to provide the means to establish, maintain and release communication connections, including switching methods, path selection, flow control, blockage and deadlock, etc.
4) Transport layer
The transport layer is established between the network layer and the conference layer, and is essentially an interface layer connecting the upper and lower layers in the network architecture. The transport layer is not only a separate structural layer, but also the core of the entire layered system protocol. Without the transport layer, the entire layered protocol is meaningless.
The transport layer obtains the services provided by the lower layer, including: sending and receiving a sequence of data block packets in the correct order, and using them to form
Transport layer data; obtain network layer addresses, including virtual channels and logical channels.
The services provided by the transport layer to the upper layer include: error-free and orderly message sending and receiving; providing transmission connections; and performing flow control.
The function of the transport layer is to receive data from the session layer, cut the data into smaller data slices as needed, and send the data to the network layer to ensure that the data slices arrive at the network layer correctly, so as to realize the transparent transmission of data between the two layers.
5) Session layer
The session layer is used to establish, manage and terminate sessions between two application systems. It is the interface through which users connect to the network. Its basic task is to be responsible for the transmission of original messages between two hosts.
The session layer provides services to the presentation layer and accepts services from the transport layer. In order to transfer data between presentation layer entities, session connections must be mapped to transport connections.
The functions of the session layer include: mapping from the session layer to the transport layer, flow control of the session connection, data transmission, recovery and release of the session connection, session connection management, and error control.
The services provided by the session layer to the presentation layer include: data exchange, isolation services, interaction management, session connection synchronization, and exception reporting.
The most important feature of the session layer is data exchange. Similar to a transport connection, a session is divided into link establishment, data exchange, and link release stages.
6) Presentation layer
The presentation layer serves the application layer upwards and accepts services from the session layer downwards. The presentation layer is to provide the service of the presentation method for the information transmitted between the application processes, and it only cares about the syntax and semantics of the information sent. The presentation layer needs to complete some specific functions, mainly including the conversion of different data encoding formats, providing data compression and decompression services, and encrypting and decrypting data.
The services provided by the presentation layer to the application layer include: syntax selection, syntax conversion, etc. Syntax selection is a means of providing an initial grammar and modifying this selection later. Syntax conversion involves code conversion and character set conversion, modification of data format, and adaptation of data structure operations.
7) Application layer
The network application layer is the window between communication users, providing users with services such as network management, file transfer, and transaction processing. It contains several independent and common user service protocol modules. The network application layer is the highest layer of OSI, providing dedicated programs for communication between network users. The content of the application layer mainly depends on the individual needs of users. The main issues involved in this layer are: distributed database, distributed computing technology, network operating system and distributed operating system, remote file transfer, email, terminal phone and remote job login and control etc. At present, there is almost no complete standard for the application layer in the world, and it is a wide range of research fields. Among the seven layers of OSI, the application layer is the most complex and contains the most application layer protocols, some of which are still being researched and developed.
Salary increase support area
I hope that everyone can learn more through this article, and the salary will increase year by year!
If you like bloggers, you can go to the list to find out. Bloggers specially set up a salary increase list for everyone to check. If you like it, you can click here to view it .
Summarize
更多资料大家可以关注csdn博主-《拄杖盲学轻声码》
The above is what I want to talk about today. This article mainly introduces what is the OSI architecture, a precise understanding of the OSI architecture, and I look forward to sharing more with you. Handsome guys and beauties give me a thumbs up, let’s work together in 2023! ! !