The Network Open Interconnection Reference Model, the Open Systems Interconnection Reference Model i

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Open System Interconnect (OSI)OSI Reference Model: It is a reference model for the interconnection of open system interconnect jointly developed by the International Organization for Standardization (ISO) and the International Telegraph Advisory Committee (CCITT), which provides a framework for the functional structure of open interconnection information systems. Losses change it from low to high are:

Physical, data link, network, transport, session, presentation, and application layers.

The full name of the OSI reference model is the Open System Interconnection Reference Model, which was proposed by the International Organization for Standardization (ISO) in the early 80s of the 20th century. Since its inception in 1946, ISO has proposed several standards, and ISO IEC 7498, the standard for network architecture, defines the basic reference model for network interconnection. At that time, there were several network architectures in the networking world, represented by IBM's SNA, which often focused on network connections within each company, and there was no unified standard, so it was difficult to interconnect them.

In this case, ISO proposes the OSI reference model, which is characterized by its openness. Networking products from different vendors can be interconnected, interoperable, and portable as long as they follow this reference model. That is, any system that follows the OSI standard can communicate with each other as long as they are physically connected.

The OSI reference model defines the hierarchy of open systems and the services provided by each layer. One of the successes of the OSI Reference Model is that it clearly separates the confusing concepts of services, interfaces, and protocols. Services describe the functionality of each layer, interfaces define how the services provided by a layer are accessed by the higher-ups, and protocols are the implementation of the functionality of each layer.

By distinguishing between these abstractions, the OSI reference model distinguishes the functional definition from the implementation details with a high level of generalization, making it universally adaptable.

The OSI Reference Model is a framework with seven layers, including the physical layer, the data link layer, the network layer, the transport layer, the session layer, the presentation layer, and the application layer.

2. Data link layer: in this layer, the data is framed, and the flow control is processed4. Transport layer: - Conventional data delivery is oriented to connection or no connection. Provide an end-to-end reliable, transparent, and optimized data transfer service mechanism for session layer users.

5. Session layer: Establish an end-to-end connection between two nodes. Provides a dialogue control mechanism between applications on the end system.

6. Presentation layer: It is mainly used to deal with the representation of information exchanged between two communication systems.

7. Application layer: the highest layer in OSI. Provides a means of accessing the OSI environment for specific types of network applications.

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The Open System Interconnect (OSI) is a reference model for open systems interconnection jointly developed by the International Organization for Standardization (ISO) and the International Telegraph Advisory Committee (CCITT), which provides a framework for the functional structure of open interconnection information systems. From low to high, they are: physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer.

The emergence of the network layer is also the result of the development of the network. In the context of online systems and line-switching, the functionality of the network layer does not make much sense. When the number of data terminals increases.

There is a relay device connected between them, and at this time, a terminal is required to be able to communicate with multiple terminals not only with the only one, which produces the problem of linking the data of any two data terminal devices, that is, routing or pathfinding. In addition, when a physical channel is established and used by a pair of users, a lot of free time is often wasted. Naturally, there is a desire for multiple pairs of users to share a link, and to solve this problem, logical channel technology and virtual circuit technology have emerged.

The main function of the relay control layer is to realize the data transmission between the two end systems through the routing and relay functions by using the error-free data transmission function between adjacent nodes guaranteed by the data link layer. To this end, the network layer also has a multiplexing function, using the principle of statistical time division multiplexing, a data link is multiplexed into a plurality of logical channels, so that a data terminal device uses a physical circuit to communicate with a plurality of remote data communication devices at the same time. The network layer specifies the procedures for establishing and removing network connections, as well as the procedures for data transmission, etc.

The network layer should have the following main functions to establish network connectivity and provide services to the upper layer:

1.routing and relaying;

2.Activate, terminate network connection;

3.Reuse multiple network connections on a single data link, and adopt time-sharing multiplexing technology;

4.detection and recovery;

5.Sequencing, flow control;

6.service selection;

7.Network management.

The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization in the early eighties. It defines standards and protocols for the interconnection of computers and network devices.

The Open Software Foundation (OSF) is a membership organization that acquires technology from other vendors to build computing environments. The technologies that OSF actually creates are just those that acquire a combination of technologies. The OSF Open Systems Software Environment is a collection of open systems technologies that enable users to converge and match hardware and software from multiple vendors in a virtual, seamless environment.

Its environment includes a distributed computing environment (DCE), which can simplify the development of products in heterogeneous environments; It also includes Open Software** 1 (OSF 1), an open-environment UNIX operating system that supports symmetric multiprocessor operation, enhanced security features, and dynamic provisioning. It was built around the Mach kernel of the University of Carnegie Mellon.

There is also OSF Motif, a graphical user interface that features Microsoft Windows and Apple Macintosh and provides a common look and feel, which is widely used on IBM systems and is related to IBM's Public User Access (CUA).

A data link can be roughly understood as a data channel. The physical layer provides the transmission medium and its connection for data communication between end devices. The medium is long-term and the connection is life-lasting.

During the lifetime of the connection, the transmitting and receiving ends can have unequal data communication at one or more times. Each communication has to go through two processes: establishing communication and dismantling communication contact. This established relationship between sending and receiving data is called a data link.

In order to make up for the shortcomings of the physical layer and provide error-free data transmission for the upper layer, it is necessary to be able to detect and correct the data. The establishment and removal of data links, error detection and error correction of data are the basic tasks of the data link layer.

The link layer provides data transmission services for the network layer, and this service depends on the functions of this layer. The link layer should have the following functions:

establishment, dismantling and separation of link connections;

Frame demarcation and frame synchronization. The data transmission unit of the link layer is the frame, and the length of the frame and the interface are also different depending on the protocol, but the frame must be delimited anyway;

Sequential control, which refers to the control of the order in which frames are sent and received;

Error detection and recovery. There is also link identification, flow control, and so on. Error detection mostly uses square array code check and judgment cyclic code check to detect bit errors on the channel, while sequence number detection is used for frame loss. The recovery of various errors is often done by feedback retransmission technology.

The most common independent link products are network cards, and bridges are also link products. The data link layer provides the network layer with inherently unreliable transmissions** into reliable transport paths. In this case, the data link layer is divided into two sub-layers, one is logical link control, and the other is access control.

OSI among them.

AUI – Connection Unit Interface PMA – Physical ** Connection.

MAU - ** Connection Unit PLS - Physical Signaling.

MDI - ** related interface.