What Is Open Systems Interconnection?

Open System Interconnection (OSI) is initiated by ISO. Its task is to formulate international computer communication standards, especially to promote interconnection between incompatible systems. The OSI model divides computer communication protocols into seven layers: physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer.

Chinese name: Open systems interconnection
Foreign name: Open System Interconnection, OSI

Nature: Open System Interconnection Reference Model
Time: 1981

1. OSI Open System Interconnection 1. OSI Model

With the advancement of network technology and the continuous emergence of various network products, there is an urgent need to solve the problem of interconnection of different systems. In 1977, the International Organization for Standardization (ISO) set up a committee to propose a standard framework for heterogeneous system interconnection, namely the Open System Interconnection Reference Model (OSI / RM). This model divides the work of network communication into 7 layers, namely the physical layer, Data link layer, network layer, transport layer, session layer, presentation layer and application layer. Layers 1 to 4 are considered lower layers, and these layers are closely related to data movement. Levels 5 to 7 are high-level and contain application-level data. Each layer is responsible for a specific job and then transfers the data to the next layer. Each level must not separate their work content absolutely, and must work closely together.

There is a common corresponding method between the OSI model and the TCP / IP model. Generally, the IP layer is regarded as the OSI network layer protocol, and TCP is regarded as the OSI transport layer protocol. In this way, the OSI model is associated with commonly used communication protocols.

OSI is the abbreviation of Open System Interconnection, which means the open system interconnection reference model. Before the emergence of OSI, there were many architectures in computer networks. In order to solve the interconnection problem of networks of different architectures, the International Organization for Standardization ISO (1981) formulated the Open System Interconnection Reference Model (OSI / RM ). This model divides the work of network communication into 7 layers. From low to high, they are the physical layer, the data link layer, the network layer, and the transport layer. Session layer, the presentation layer and the application layer.

The first to third layers belong to the lower three layers of the OSI reference model and are responsible for creating links for network communication connections; the fifth to seventh layers are the upper three layers of the OSI reference model and are specifically responsible for end-to-end data communication; The layer is responsible for the connection of the upper and lower layers. Each layer completes certain functions, each layer directly provides services to its upper layers, and all layers support each other, and network communication can be performed in both directions from top to bottom (on the sending end) or from bottom to top (on the receiving end). . Of course, not every communication needs to go through all seven layers of OSI, and some even only need a certain layer corresponding to both parties. The transfer between physical interfaces and the connection between the repeater and the repeater need only be performed in the physical layer; and the connection between the router and the router only needs to pass through three layers below the network layer can. Generally speaking, communication between the two parties is carried out at the peer level, and communication cannot be carried out at the asymmetric level.

The method adopted in the OSI standard development process is to divide the entire large and complex problem into several small problems that are easy to handle. This is the layered architecture approach. In OSI, three levels of abstraction are used, namely architecture, service definition, and protocol specifications.

For the convenience of memory, you can think of the seven layers from high to low: All People Seem To Need Data Processing. Each capital letter corresponds to the first letter of the seven-layer name.

2. OSI Open Systems Interconnection 2. OSI layering principles

Every node in the network has the same level

Different nodes have the same function at the same level

Communication between adjacent layers of the same node through interfaces

Each layer can use the services provided by the lower layer and provide services to the upper layer

Communication between peer layers is achieved through protocols between the same layers of different nodes

3. OSIOSI/RM Open system interconnection 3. OSIOSI / RM layering

The essence of peer-to-peer communication:

Virtual communication between entities at the peer layer; the lower layer provides services to the upper layer; the actual communication is completed at the lowest layer; the sender data is gradually passed from the highest layer to the lower layer, and the receiver data is gradually passed from the lowest layer to the upper layer.

Protocol data unit PDU: In the OSI reference model, the information units exchanged between peer layer protocols are collectively referred to as protocol data units (PDUs).

The PDUs at the transport layer and the following layers have their own specific names:

Transport Layer-Segment

Network layer-packet (Packet)

Data Link Layer-Data Frame

Physical layer-Bit

4. OSI Open system interconnection 4. OSI seven-layer structure

Floor	Data Format	Function and connection	Typical equipment
Application layer	Data	An interface between web services and consumer applications	Terminal equipment (PC, mobile phone, tablet, etc.)
Presentation layer	Data	Data representation, data security, data compression	Terminal equipment (PC, mobile phone, tablet, etc.)
Session layer	Data	Mapping of session layer connection to transport layer; flow control of session connection; data transmission; session connection recovery and release; session connection management, error control	Terminal equipment (PC, mobile phone, tablet, etc.)
Transport layer	Data is organized into segments	Use an addressing mechanism to identify a specific application (port number)	Terminal equipment (PC, mobile phone, tablet, etc.)
Network layer	Split and reassemble packets	Path selection between different network systems based on network layer address (IP address)	router
Data Link Layer	Encapsulating bit information into a data frame	Functions such as establishing, revoking, identifying logical links and link multiplexing, and error checking on the physical layer. Addressing by using the hardware or physical address of the receiving system	Bridges, switches, repeaters
Physical layer	Bit stream	Establish, maintain, and cancel physical connections	Optical fiber, coaxial cable, Twisted pair, network card

4.1 Open System Interconnection 4.1 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. It plays the role of establishing, maintaining, and canceling physical connections to realize the physical interface between devices. The physical layer receives and sends a series of bit streams, regardless of the meaning and structure of the information.

The physical layer includes provisions that describe the various mechanical, electrical, and functional aspects of devices connected to the network. Specifically, the mechanical characteristics specify the specifications, pin number, and arrangement of the connectors required for network connection; the electrical characteristics specify the signal level on the line, impedance matching, Transmission rate, distance limitation, etc .; functional characteristics refer to the precise signal meaning assigned to each signal, that is, the functions of each line between DTE (data terminal equipment) and DCE (data communication equipment) are defined; process characteristics define the use of signal lines A set of operating procedures for bit stream transmission refers to the series of actions of both DTE and DCE on each circuit when the physical connection is established, maintained, and information is exchanged. The data unit of the physical layer is bits.

The main functions of the physical layer:

Provide a data transmission path for data-end equipment. The data path can be a physical medium or multiple physical media connected. A complete data transmission, including activating the physical connection, transmitting data, and terminating the physical connection. The so-called activation means that no matter how much physical media is involved, it must be connected between the two data terminal devices communicating to form a path.
data transmission

The physical layer should form an entity suitable for data transmission needs and serve data transmission:

1) To ensure the correctness of data transmission in bits;

2) Provide a transparent bit transmission to the data link layer;

3) Provide sufficient bandwidth (bandwidth refers to the number of bits (BIT) that can be passed per second) to reduce congestion on the channel. The data transmission method can meet the needs of point-to-point, point-to-multipoint, serial or parallel, half-duplex or full-duplex, synchronous or asynchronous transmission.

Complete some management work at the physical layer, such as completing the establishment, maintenance and removal of data links between data terminal equipment, data communication and switching equipment.

Typical equipment at the physical layer: optical fiber, coaxial cable, twisted pair, repeater, and hub.

4.2 Open System Interconnection 4.2 Data Link Layer

Based on the bitstream service provided by the physical layer, the bit information is encapsulated into a data frame Frame, which functions to establish, cancel, identify logical links and link multiplexing, and error check on the physical layer. Addressed by using the hardware or physical address of the receiving system. Establish a data link between adjacent nodes, provide error-free transmission of data frames (Frame) on the channel through error control, and provide effective services for the network layer above it.

The data link layer provides reliable transmission over unreliable physical media. The functions of this layer include: physical address addressing, data framing, flow control, data error detection, and retransmission.

At this layer, the unit of data is called a frame.

Representatives of data link layer protocols include: SDLC, HDLC, PPP, STP, Frame Relay, etc.

The main functions of the link layer:

The function of the link layer is to achieve the correct transmission of binary information blocks between system entities
Provide reliable and error-free data information for the network layer
Solve information mode, operation mode, error control, flow control, information exchange process and communication control procedures in the data link

The link layer provides data transmission services for the network layer. This service depends on the functions provided by this layer. The link layer should have the following functions:

The establishment, dismantling and separation of link connections.
Frame delimitation and frame synchronization. The data transmission unit of the link layer is a frame. Different protocols have different frame lengths and interfaces. However, frames must be delimited anyway.
Sequence control refers to the control of the sending and receiving sequence of frames.
Error detection and recovery. There are also link identification, flow control, and so on. Error detection uses square matrix code checking and cyclic code checking to detect data bit errors on the channel, while frame loss and so on use serial number detection. The recovery of various errors is often accomplished by feedback retransmission technology.

Typical equipment at the data link layer: Layer 2 switches, bridges, and network cards

4.3 Open System Interconnection 4.3 Network Layer

The network layer is also called the communication subnet layer. It is the interface layer between higher-level protocols. It is used to control the operation of the communication subnet. It is the interface between the communication subnet and the resource subnet. Two computers communicating in a computer network may go through many data links or may go through many communication subnets. The task of the network layer is to select the appropriate network routing and switching nodes to ensure timely transmission of data. The network layer will decapsulate the frames received by the data link layer, extract the data packet, and the network layer header is encapsulated in the packet, which contains the network address of the source address and destination address of the logical address information.

If you are talking about an IP address, then you are dealing with layer 3 issues, which are "packet" issues, not layer 2 "frames". IP is part of a Layer 3 problem, and there are routing protocols and Address Resolution Protocols (ARPs). Everything about routing is handled at layer 3. Address resolution and routing are important goals of Layer 3. The network layer can also implement functions such as congestion control, internetworking, packet sequence control, and network accounting.

The unit of data units exchanged at the network layer is the division and reassembly of packets.

Network layer protocols include: IP, IPX, OSPF, etc.

The main function of the network layer is to select paths between different network systems based on the network layer address (IP address).

In order to establish network connections and provide services to upper layers, the network layer should have the following main functions:

Routing and relaying;
Activate and terminate the network connection;
Multiple network connections are multiplexed on one data link, and time-division multiplexing technology is often adopted;
Error detection and recovery;
Sequencing, flow control;
Service selection
Network management
Introduction to network layer standards.

Typical device at the network layer: router

4.4 Open System Interconnection 4.4 Transport Layer

The transport layer is established between the network layer and the session layer. In essence, it is an interface layer that connects the high and low layers in the network architecture. An addressing mechanism is used to identify a specific application (port number). The transport layer is not only a separate structural layer, it is also the core of the entire layered system protocol. Without the transport layer, the entire layered protocol is meaningless.

The data unit of the transport layer is a segment of data organized by data. This layer is responsible for obtaining all the information. Therefore, it must track the fragmentation of data units, out-of-order packets and other dangers that may occur during transmission.

The services provided by the transport layer to the lower layers include:

Send and receive the correct data block packet sequence and use it to form the transport layer data;
Obtain network layer addresses, including virtual channels and logical channels.
Services provided by the transport layer to the upper layer include:
Orderless message sending and receiving without errors;
Provide transmission connections;
Perform flow control.

The transport layer provides end-to-end (end-user to end-user) transparent and reliable data transmission services for the upper layer. The so-called transparent transmission means that the transport layer shields the upper-layer specific details of the communication transmission system during the communication process.

Representatives of transport layer protocols include: TCP, UDP, SPX, etc.

The main function of the transport layer is to receive data from the session layer, cut the data into smaller pieces of data as needed, and transmit the data to the network layer to ensure that the data pieces reach the network layer correctly, so as to achieve the transparent transmission of the two layers of data.

The transport layer is the first end-to-end layer when two computers are communicating over the network for data communication and has a buffering effect. When the quality of service at the network layer cannot meet the requirements, it will improve the service to meet the requirements of the high level; when the quality of service at the network layer is good, it only takes very little work. The transport layer can also be multiplexed, that is, multiple logical connections are created on one network connection.

The transport layer is also called the transport layer. The transport layer only exists in the open-end system. It is a layer between the lower three layers of the communication subnet system and the upper three layers, but it is a very important layer. Because it is the last layer from source to destination to control data transmission from low to high.

There is an existing fact that there are large differences in performance among various communication subnets in the world. For example, telephone switching networks, packet switching networks, public data switching networks, LANs and other communication subnets can be interconnected, but they provide different throughput, transmission rates, and data delay communication costs. For the session layer , a constant-performance interface is required. The transport layer assumes this function. It uses shunting / merging and multiplexing / medial multiplexing technologies to adjust the differences in the above communication subnets, making the session layer invisible.

In addition, the transport layer must have functions such as error recovery and flow control to shield the session layer from details and differences in these aspects of the communication subnet. The data object of the transport layer pair is no longer the network address and host address, but the interface port with the session layer. The ultimate purpose of the above functions is to provide reliable and error-free data transmission for the session. Services at the transport layer generally have to go through three phases: the transmission connection establishment phase, the data transmission phase, and the transmission connection release phase to complete a complete service process. In the data transmission phase, it is divided into two types: general data transmission and accelerated data transmission. There are five types of transport layer services. It can basically meet various needs for transmission quality, transmission speed, and transmission cost.

4.5 Open Systems Interconnection 4.5 Session Layer

This layer can also be called the meeting layer or the dialogue layer. In the high level of the conversation layer and above, the unit of data transmission is no longer named separately, collectively called the message. The session layer does not participate in specific transmissions. It provides mechanisms for establishing and maintaining communication between applications, including access verification and session management. For example, the server authenticates the user login by the session layer.

The services provided by the session layer enable applications to establish and maintain sessions and synchronize sessions. The use of checkpoints at the session layer enables the communication session to resume communication from the checkpoints when the communication fails. This ability is extremely important for transferring large files. The session layer, presentation layer, and application layer form the upper three layers of the open system, and provide distributed processing, dialog management, information display, and recovery of the last error in the face of application processes. The session layer is also responsible for the application process service requirements, and the part of the work that the transport layer cannot complete gives the transport layer a functional gap to make up for. The main functions are dialog management, data stream synchronization and resynchronization. To complete these functions, a large number of service unit function combinations are required, and dozens of functional units have been formulated.

The main functions of the session layer:

Mapping of session layer to transport layer;
Flow control for session connections;
data transmission;
Session connection restoration and release;
Session connection management, error control.
To establish a connection between session entities and to establish a session connection for two peer session service users, the following tasks should be done:
Map the session address to a shipping address;
Select the required transport service quality parameters (QOS);
Negotiate session parameters;
Identify each session connection;
Transfer limited transparent user data;
Data transmission phase.

This phase is an organized, synchronized data transfer between the two session users. The user data unit is SSDU and the protocol data unit is SPDU. The data transmission process between session users is performed by converting SSDU into SPDU.

Connection release is through the orderly release, "discard", "limited transparent user data transfer" and other functional units to release the session connection. The session layer standard defines 12 functional units in order to enable the function negotiation during the session connection establishment phase and to facilitate the reference and reference of other international standards. Each system can choose other functional units to form a reasonable subset of session services based on its core functional service units based on its own situation and needs. The main standards of the session layer are "DIS8236: Session Service Definition" and "DIS8237: Session Protocol Specification".

4.6 Open System Interconnect 4.6 Presentation Layer

The presentation layer provides services to the application layer up and receives services from the session layer down. The presentation layer is a service that provides a representation method for the information transmitted between application processes. It only cares about the syntax and semantics of the emitted information. The presentation layer has to complete certain specific functions, mainly including the conversion of different data encoding formats, providing data compression and decompression services, and encrypting and decrypting data. For example, the display of the image format is supported by a protocol located at the presentation layer.

The presentation layer provides services for the application layer, including syntax selection and syntax conversion. Syntax selection is a means to provide an initial syntax and modify this selection later. Syntax conversion involves code conversion and character set conversion, data format modification, and adaptation to data structure operations.

4.7 Open System Interconnect 4.7 Application Layer

The network application layer is a window between communication users and provides users with services such as network management, file transfer, and transaction processing. It contains several independent, user-specific service agreement modules. The network application layer is the highest layer of the OSI and provides dedicated programs for communication between network users. The content of the application layer mainly depends on the user's individual needs. The main problems in this layer design are distributed databases, distributed computing technologies, network operating systems and distributed operating systems, remote file transfer, email, terminal phones, and remote job login and control. Wait. As of 2011, there is no complete international standard for the application layer, and it is a very wide area of research. Among the seven layers of OSI, the application layer is the most complicated, and it also contains the most application layer protocols. Some are still under research and development.

The application layer provides an interface for operating systems or network applications to access network services.

Representatives of application layer protocols include: Telnet, FTP, HTTP, SNMP, DNS, etc.

4.8 OSI Open System Interconnection 4.8 OSI Summary

Through the OSI layer, information can be transferred from a software application on one computer to another application. For example, if an application on computer A wants to send information to an application on computer B, the application on computer A needs to send the information to its application layer (layer 7), and then this layer sends the information to the presentation layer (Sixth layer), the presentation layer forwards data to the session layer (fifth layer), and so on until the physical layer (first layer). At the physical layer, data is placed on the physical network medium and sent to computer B. The physical layer of computer B receives the data from the physical medium, and then sends the information up to the data link layer (second layer), which is then forwarded to the network layer, and continues until the information reaches the application layer of computer B. Finally, the application layer of computer B transmits the information to the receiving end of the application to complete the communication process.

The seven layers of OSI use various control information to communicate with corresponding layers of other computer systems. These control messages contain special requests and instructions that are exchanged between the corresponding OSI layers. The head and tail of each layer of data are two basic forms that carry control information.

For data transmitted from the upper layer, the control information attached to the front is called the header, and the control information attached to the back is called the tail. However, adding protocol headers and protocol tails to the data from the previous layer is not necessary for an OSI layer.

When data is transmitted between layers, each layer can add a header and a tail to the data, and these data already include the header and tail added by the previous layer. The protocol header contains information about communication between layers. Head, tail, and data are related concepts that depend on the protocol layer of the analysis unit of information. For example, the transport layer header contains information that only the transport layer can see, and other layers below the transport layer only pass this header as part of the data. For the network layer, an information unit consists of the header and data of the third layer. For the data link layer, all the information passed down through the network layer, that is, the third layer header and data are considered data. In other words, in a given OSI layer, the data part of the information unit contains the head and tail and data from all upper layers, which is called encapsulation.

For example, if computer A wants to send some data from the application to computer B, the data is first transferred to the application layer. The application layer of computer A communicates with the application layer of computer B by adding a protocol header to the data. The formed information unit contains a protocol header, data, and possibly a protocol tail, and is sent to the presentation layer, which is then added as a protocol header for control information understood by the presentation layer of computer B. The size of the information unit increases with the addition of protocol headers and protocol tails at each layer, which contain the control information to be used by the corresponding layer of computer B. At the physical layer, the entire information unit is transmitted over a network medium.

The physical layer in computer B receives the information unit and transmits it to the data link layer; then the data link layer in B reads the control information in the protocol header added by the data link layer of computer A; and then removes the protocol header And the end of the protocol, the rest is passed to the network layer. Each layer performs the same action: reads the protocol header and protocol tail from the corresponding layer, removes it, and sends the remaining information to the previous layer. After the application layer performs these actions, the data is transferred to the application in computer B, which is exactly the same as that sent by the application in computer A.

Communication between one OSI layer and another is done using services provided by the second layer. The services provided by adjacent layers help one OSI layer communicate with the corresponding layer of another computer system. A specific layer of an OSI model is usually associated with three other OSI layers: the immediately preceding and following layers, and the corresponding layers of the target networked computer system. For example, computer A's data link layer should communicate with its network layer, physical layer, and computer B's data link layer.