What is the OSI network architecture
Source: Shangpin China |
Type: website encyclopedia |
Time: June 23, 2014
The first network architecture in the world was the SNA (System Network Architecture) proposed by IBM in 1974, that is, the system network architecture developed by IBM. Later, other companies have also proposed their own network architecture, such as DEC's DNA (Digital Network Architecture), the digital network architecture developed by American Digital Equipment Company, and the TCP/IP network architecture of the U.S. Department of Defense. For each company or department Beijing website construction The architecture reflects their own network standards and functions. ISO, the international standards organization, introduced the network architecture of OSI in 1983. Although there is no specific standard in OSI and no basis for practical application, it gives the problems to be solved when formulating network architecture standards and the methods or basic technical references to solve related problems. Therefore, OSI-RM network architecture is considered as a conceptual framework of computer network, which is a high generalization and complete description of computer network architecture. OSI, as an international standard network architecture, has great historical significance in promoting the development of networks. Although Internet networks are popular today, its network architecture based on TCP/IP protocol is only an industrial standard or de facto standard. 1. The layering idea of OSI First, analyze how OSI solves the problem of heterogeneous network interconnection. In many different network systems, the International Organization for Standardization (ISO) has adopted the idea of transforming complexity into simple structure and layering, that is, the complex network is divided into network layers according to functions, and the functions that the corresponding layers should have are analyzed one by one. The hierarchical idea is as follows. (1) Each layer should have clear functional boundaries and be strictly built on its lower layer. (2) In addition to realizing the functions of this layer, each layer also provides certain services for the upper layer (3) The details or implementation method of the service provided by the lower layer for the upper layer should be independent of the upper layer. (4) There should be mutually compatible communication rules or agreements between the same layers. According to the hierarchical idea of OSI, a complete computer network architecture should be composed of two parts, namely "network hierarchy model" and "network communication protocol" owned by each layer. Network hierarchy model: it clearly defines the functional boundaries of each layer, as well as the interfaces and service methods between adjacent layers. Network communication protocol: specifies the rules or agreements established during communication between the same levels. [Example] A real life example - the hierarchical application of "telegraph transmission process" is shown in Figure 3-1. The service system can be divided into four levels in the whole transmission process of the telegram. First, there are corresponding communication rules between the same layers. For example, the top layer "Message Cognition and Understanding Rules" stipulates which text and format the sender uses to write the message, otherwise the receiver cannot understand it; Secondly, there are service requirements for upper and lower connection between adjacent layers, such as the requirements of Layer 3 message coding layer on the number of words written by the upper layer sender. The communication parties do not need to face each other, and the sender only needs to transfer information from top to bottom according to the communication rules of each layer; The message can be transmitted by the receiver from bottom to top according to the communication rules at all levels. OSI, the idea of network layering, plays a very good guiding role in analyzing and solving problems encountered in daily work and how to turn complexity into simplicity. 2. Hierarchical model of OSI The OSI hierarchy model is divided into seven layers according to the network communication function, which are the physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer from low to high. As shown in Figure 3-2, it stipulates the basic framework for the interconnection, interworking and interoperability of open systems. Before analyzing the OSI seven layer network architecture, first explain several important concepts. Entity: In each layer, the software or hardware that implements the functions of this layer. Service access point: the communication interface that the lower entity provides services to the upper entity. It can also be said that the upper entity transfers data through the service access point of the lower entity and receives the services provided by the lower entity. Network communication protocol elements: Network communication protocols generally include the following three elements. (1) Syntax: specifies the structure and format of communication data and control information, such as the data format, coding and signal level used in communication. (2) Semantics: determine the communication control of both parties. For example, specify what control information to send, what control actions to complete, and what responses to make, and determine the control information used for coordination and error handling. (3) Timing: indicates the order of information interaction between two communication parties, such as event sequence and speed matching. Protocol Data Unit (PDU); It refers to the data unit transferred between peer levels. OSI is divided into seven layers. Each layer has its own PDU. That is, the PDU of the physical layer is a data bit The PDU is a frame, the network layer PDU is a packet, the transport layer PDU is a segment, and other higher level PDUs can be called messages, 1) First layer: Physical Layer The physical layer is directly connected to the communication media and is the physical interface layer for data transmission. This layer provides transparent bit stream transmission services for the upper layer (data link layer), and the basic unit of transmission data is bit. The function of the physical layer is to provide the data end device with a path to transmit data and realize the transmission of bit streams. The main contents of physical layer definition are as follows. (1) Mechanical characteristics: indicate the shape, size, lead number and arrangement of connectors used for interfaces (2) Electrical characteristics: indicate the range of voltage appearing on each line of interface cable (3) Functional characteristics: indicate the meaning of the voltage at a certain level on a line (4) Procedure characteristics: indicates the occurrence sequence of various possible events for different functions, specifically refers to a group of operating procedures for bit stream transmission using signal lines, such as the establishment of physical connections, synchronization control, etc. 2) Layer 2: Data Link Layer The data link layer is for communication between adjacent nodes. The physical layer has provided necessary transmission channels and physical connections for data communication between terminal devices or adjacent nodes, but this physical channel is not reliable. Because the data transmitted on the physical layer is inevitably affected by various factors, resulting in errors or data loss. In order to ensure reliable data transmission, corresponding communication control must be carried out on the upper layer. That is to say, each communication of the physical layer can transmit the bit stream only after the communication link is established in the upper layer. After the data transmission is completed, the upper layer also needs to remove the communication link. This data receiving and sending relationship established by the upper layer is called the data link. The function of the link layer: it is mainly responsible for the establishment, maintenance and removal of the data link, and on the data link from adjacent nodes to nodes, Error free transmission of data frames on the channel is provided through error control. The main contents of the data link layer definition are as follows. (1) Framing: the encapsulation of data frames. The protocol header control information of this layer is added to the front end of the upper protocol data unit, and the end control information is added to the end. (2) Physical addressing: gives the addressing method for data transmission from one node to the next node. (3) Definition of flow control or error control. 3) Layer 3: Network Layer The network layer is the highest layer of the communication subnet in the computer network. This layer is more concerned with the communication control of the communication subnet, so it is also called the communication subnet layer. This layer packages the data units transmitted from the local host transmission layer and transmits them through the communication subnet, The network layer finally delivered to the destination host. The function of the network layer: to achieve host to host data transmission between two terminal systems. The main contents of network layer definition are as follows (1) Service: provide service modes such as connection oriented or connectionless oriented. (2) Logical addressing: refers to the addressing method for data transmission from the host at one end of the network to the host at the other end of the network. (3) Routing: select the best path to the destination host according to certain principles and algorithms. 4) Layer 4: Transport Layer The transmission layer shields the data communication details of the lower layer from the upper layer, which is responsible for the overall data transmission and data control. The function of the transport layer: to achieve end-to-end data transmission between two terminal systems. The main contents of the transport layer definition are as follows. (1) Process addressing: defines the addressing method between different application processes. (2) Grouping and reorganization of data. (3) Connection management: connected transmission or connectionless transmission. (4) Error control and flow control, etc. 5) Layer 5: Session Layer On the basis of end-to-end services provided by the transport layer, the session layer establishes and maintains a session between the two end session entities, and synchronizes the session. Functions of the session layer: provide a process oriented session service, and provide effective control and management for session activities. 6) The sixth layer: Presentation Layer The presentation layer deals with the information representation between the host systems at both ends of OSI. It defines a data type or data structure through an abstract method, and uses this abstract data structure to realize the conversion of data types and codes between the host systems at both ends. Functions of the presentation layer: data coding, data compression, data encryption, etc. 7) Layer 7: Application Layer The application layer is the interface between the computer network and the end user. It is also the highest layer, which is composed of various application programs. Different applications can meet different needs of users. The data message transmitted by the network is directly generated by various applications. Functions of the application layer: provide data exchange required between application processes, and realize basic functions such as remote operation or process proxy. 3. OSI-RM Overview (1) The highest layer of ISO-RM is the application layer, which provides users with application level services; The lowest layer is the physical layer, which is responsible for connecting the communication media to realize data transmission. The lower three layers can be seen as data transmission oriented control layer (belonging to communication subnet), which is mainly responsible for solving problems related to communication control and communication addressing, while the higher three layers can be seen as data processing oriented application layer (belonging to resource subnet), which is mainly responsible for solving problems such as data communication and data processing between application processes; The transport layer plays a connecting role in OSI/RM. As the interface between the communication subnet and the resource subnet, it is responsible for the overall data transmission and data control to ensure that the communication systems at both ends achieve end-to-end data transmission. It can be seen that the transport layer is the key layer of the entire network architecture. (2) The connection between the upper layer and the lower layer is realized through the service access points between the layers; The upper layer sends service requests to the lower layer through the service access point of the lower layer, and the lower layer provides services to the upper layer through the service access point of this layer. (3) Only the communication systems (hosts) at both ends have all seven layers of network structure. When the systems at both ends communicate through the network, there is no direct communication connection between the peer layers except the physical layer. The data transmission unit between the peer layers is controlled by the communication protocol of this layer, so it is called virtual communication. Only the data transmission of the physical layer is the real physical communication. (4) When data transmission needs to be transmitted through the communication subnet, the intermediate node equipment responsible for forwarding data is at most involved in applications at the lower three or two layers.
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