OSI reference model and TCP/IP model
OSI reference model
The OSI reference model (Open System Interconnection/Reference Model) was proposed by the International Standards Organization (OSI).
The OSI reference model is divided into seven layers ( the lower layer serves the upper layer, and the upper layer uses the lower layer's services ):
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physical layer
Provide transparent bit stream transmission;
The data unit is bit (bit), that is, a binary bit "0" or "1", 1 Byte (byte B) = 8 bit (bit b) ;
The mechanical characteristics, electrical characteristics, functional characteristics, and regulatory characteristics of the network are defined through various protocols;
Important concept 1——Data terminal equipment DTE: It has certain data processing capabilities and data sending and receiving capabilities, and is used to provide or receive data. Common DTE devices include routers, PCs, terminals, etc.;
Important concept 2 - data communication equipment DCE: provide signal conversion and encoding functions between DTE and transmission lines, and are responsible for establishing, maintaining and releasing link connections. Common DCE equipment includes CSU/DSU, NT1, WAN switch, MODEM, etc.;
DCE provides clock, DTE does not provide clock .
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data link layer
The data unit is frame, with flow control function;
A data link is not equal to a link, a link is a physical line between two adjacent nodes, a data link is a data channel, and a logical circuit composed of a physical link + a necessary communication protocol;
The functions that the data link should have are: 1 link connection establishment, removal and separation, 2 frame delimitation and frame synchronization, 3 sequence control, 4 error detection and recovery, 5 link representation, flow/congestion control;
The data link layer in the LAN can be divided into two sublayers: logical link control LLC (only used when 802.3 format is used) + medium access control MAC (Ethernet frame format).
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Network layer
The main function of controlling the communication of the subnet is to provide routing, that is, to select the optimal path to the destination host and transmit data packets along this path;
Its functions include: road selection and relay, activation and termination of network connections, link multiplexing, error detection and recovery, flow/congestion control, etc.
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transport layer
By realizing reliable end-to-end data transmission, data segmentation, transmission and assembly can be realized, and error control and flow/congestion control are also provided.
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session layer
Allows sessions to be established between users on different machines;
Session refers to various services, including dialog control, token management, and synchronization functions.
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presentation layer
Provide a common data description format to facilitate information conversion and interoperability between machines in different systems;
The main functions are: data syntax conversion, syntax representation, data encryption and decryption, data compression and decompression.
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application layer
The application layer provides services to the application, and these services are grouped into service elements according to the characteristics they provide to the application;
Service elements are divided into: public application service element CASE + specific application service element SASE.
TCP/IP Reference Model
The correspondence between the TCP/IP reference model and the OSI reference model is as follows:
The hierarchical relationship diagram of the main protocol is as follows:
