The OSI model

Latest reply: Dec 27, 2018 07:49:27 505 5 10 2

The physical layer


The physical layer is the OSI's first layer, which, while at the bottom, is the foundation of the open system. The physical layer provides transmission media and interconnection equipment for data communication between devices and provides a reliable environment for data transmission.


1.1 media and interconnection equipment


The media of physical layer include overhead open line, balance cable, optical fiber, wireless channel and so on. Interconnection equipment of communication refers to the interconnection equipment between DTE and DCE. DTE is the data terminal equipment, also known as physical equipment, such as computers, terminals and so on are included. DCE is a data communication device or circuit connection device, such as a modem. Data transfer is usually a path through DTE -- DCE, and then through DCE -- DTE. Interconnect device refers to the device that connects DTE and DCE, such as various plugs and sockets. All kinds of thick and thin coaxial cables, t-connectors, plugs, receivers, transmitters, Repeaters and so on in LAN belong to the physical layer of media and connectors.


1.2 main functions of the physical layer


1.2.1 provide the data transmission channel for the data terminal device, which can be one physical media or connected by multiple physical media. A complete data transmission includes activation of physical connection, transmission of data, and termination of physical connection. Activation means that no matter how many physical media are involved, a path should be formed between two data terminal devices of communication.


1.2.2 data transmission. The physical layer shall form entities suitable for data transmission and serve data transmission. The first is to ensure that the data can pass through it correctly, and the second is to provide enough bandwidth (bandwidth refers to the number of bits per second) to reduce congestion on the channel. The mode of data transmission can meet the needs of point-to-point, point-to-point, serial or parallel, half-duplex or full-duplex, synchronous or asynchronous transmission.


1.3 some important standards of the physical layer


The standards and protocols of the physical layer were developed and applied long before the establishment of the OSI/TC97/C16 technical committee, and the OSI has developed and applied some standards and adopted some existing achievements. It is basically compatible with EIA's "rs-232-c". ISO2593: called "data communication -34 core DTE/DCE- interface connector and pin allocation". ISO4092: called "data communication -37 core DTE/DEC- interface connector and pin allocation ". Compatible with eiars-449. CCITT v. 24: is called "data terminal equipment (DTE) and data circuit terminal device interface circuit definition table ". Its function is compatible with eiars-232-c and rs-449 on the 100 sequence line.


Data link layer


A data link can be roughly understood as a data channel. The physical layer to provide the data communication between the terminal equipment transmission media and its connection. The media is a long-term, is the effective term of connection. The connection survival period, sending and receiving can be done on both ends of one or more data communication. Each communication should be through establishing communication and the demolition of communication process. This set up data transceiver relationship is called a data link. The transmission of the data on the physical media is affected by various uncertain factors and errors will occur, in order to make up for the inadequacy of the physical, to provide error-free data transmission to the upper, can the data error detection and error correction. The establishment of the data link, Error checking and error correction are the basic tasks of data link layer.


2.1 main functions of the link layer


The link layer provides data transfer services for the network layer, which depends on the functions of the layer. The link layer shall have the following functions:


2.1.1 establish, dismantle and separate link connections.


2.1.2 frame delimiter and frame synchronization. The link layer's data transfer unit is the frame, and the length and interface of the frame vary depending on the protocol, but frames must be delimited anyway.


2.1.3 sequence control refers to the control of the sending and receiving sequence of frames.


2.1.4 error detection and recovery. In addition, there are link identification, flow control and so on. Error detection USES square array code verification and cyclic code verification to detect the error code of data on the channel, while frame loss and other serial number detection.


2.2 main protocols of the data link layer


The data link layer protocol is designed for consistency between peer entities and for smooth service to the network layer. The main agreements are as follows:


2.2.1 ISO1745-1975: "data communication system, basic control procedures". This is a character-oriented standards, using 10 control character to complete the establishment of the link, demolition and data exchange. The frame of transceiver and error recovery is done by these characters. ISO1155, ISO1177, ISO2626, standards such as ISO2629 cooperate to use can form a variety of link control and data transmission.


2.2.2 ISO3309--1984: called "HDLC frame structure ".ISO4335--1984: called "HDLC specification elements ". ISO7809--1984: called "HDLC protocol type assembly ". All three standards are developed for bit-oriented data transmission control. These three standard combinations are traditionally referred to as advanced link control procedures.


2.2.3 ISO7776: it is called "DTE data link layer protocol "and is compatible with CCITT x. 25LAB" balanced link access protocol ".


2.3 link layer products


The most common independent link products are network CARDS and Bridges. Some of the functions of modems are considered by some to be part of the link layer, which provides the network layer with a way to turn essentially unreliable media into reliable traffic. In IEEE802.3, the data link layer is divided into two sub-layers, one is logical link control and the other is media access control. The following figure shows the IEEE802.3LAN architecture.


AUI= connection unit interface PMA= physical media connection


MAU= media connection unit PLS= physical signaling


MDI= media related interface


The network layer


The generation of network layer is also the result of network development. In the environment of on-line system and line switching, the function of network layer is of little significance. When the number of data terminals increases. There are relay devices connected between them. At this time, one terminal requires not only to communicate with one but also to communicate with multiple terminals, which leads to the problem of linking data of any two data terminal devices, namely 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, people would like to let multiple pairs of users share a link, so logical channel technology and virtual circuit technology appear to solve this problem.


3.1 main functions of network layer


To establish a network connection and provide services for the upper layer, the network layer shall have the following main functions:


3.1.1 routing and relay.


3.1.2 activate and terminate the network connection.


3.1.3 reuse multiple network connections on one data link, and adopt time-sharing multiplexing technology.


3.1.4 error detection and recovery.


3.1.5 sorting and flow control.


3.1.6 service selection.


3.1.7 network management.


3.2 introduction to network layer standards


Some of the main standards for the network layer are as follows:


3.2.1 iso.dis8208: called "x.25 grouping level protocol for DTE"


3.2.2 iso.dis8348: called "CO network service definition "(connection-oriented)


3.2.3 iso.dis8349: called "CL network service definition "(connectionless oriented)


3.2.4 iso.dis8473: called "CL network protocol"


3.2.5 iso.dis8348: called "network layer addressing"


3.2.6 in addition to the above standards, there are many other standards. These standards only solve part of the functions of the network layer, so it is often necessary to use several standards in the network layer at the same time to complete the functions of the entire network layer.


In the network with open characteristics, data terminal devices should be configured with network layer functions. Currently, the network hardware devices sold in the market mainly include gateways and routers.


The transport layer


The transport layer is the first end-to-end layer that buffers two computers when they communicate data over a network. When the service quality of the network layer can not 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 works with little effort. The transport layer can also be reused by creating multiple logical connections over a single network connection. Transport layer, also known as transport layer, only exists in the end open system. It is a layer between the low 3-layer communication subnet system and the high 3-layer communication subnet system, but it is very important because it is the last layer from the source end to the destination end to control the data transmission from low to high.


A given fact that the world of communication subnet there are large differences in performance. The old telephone, for example, packet-switched networks, public data networks, LAN subnet can interconnect such as communication, but they provide throughput, transmission rates, data delay communication costs vary. For the session layer, but there is a constant interface performance requirements. The transport layer will serve a function. It USES a split/confluence, reuse/solution reuse technology to adjust the difference of the above communication subnet, feel the session layer.


In addition to the transport layer also have error recovery, flow control, and other functions, in order to the session layer shielding communication subnet with the details of the differences in these aspects. Transmission of the data object is not network address and host address, but and session layer interface port. The ultimate purpose of the function is to provide reliable session, and correct data transfer. The transport layer service generally go through transport connection establishment phase, data transmission phase, the transport connection release three phase is to complete a full service process. In the data transmission phase is divided into general data transfer and accelerate the data transfer. Transport layer services are divided into five types. They can basically meet the different needs of transport quality, speed and cost. The protocol standards of transport layer are as follows:


ISO8072: called "connection-oriented transport service definition"


ISO8072: called "connection-oriented transport protocol specification"


The session layer


The session layer provides services to enable applications to establish and maintain sessions and to synchronize sessions. The session layer USES checkpoints to enable the communication session to resume communication from the checkpoint when the communication fails. This ability is extremely important for transferring large files. Session layer, presentation layer and application layer constitute the high 3 layers of the open system, which provide distributed processing, dialogue management, information representation and recovery of final errors in the application process.


The session layer is also responsible for the application process service requirements, but the transport layer cannot complete the part of the work, to make up for the transport layer function gap. The main functions are dialogue management, data flow synchronization and resynchronization. To complete these functions, a large number of service unit functions need to be combined, and dozens of functional units have been developed. The main functions of the session layer are introduced as follows.


5.1 establish connections between session entities.


To establish a session connection between two peer session service users, you should do the following:


5.1.1 map the session address to the transport address


5.1.2 select the required transport service quality parameters (QOS)


5.1.3 negotiate session parameters


5.1.3 identify individual session connections


5.1.4 transfer limited transparent user data


5.2 data transmission stage


This stage is to realize the organized and synchronous data transmission between two session users. The user data unit is SSDU, while the protocol data unit is SPDU. The data transmission process between session users is to convert SSDU into SPDU.


5.3 connection release


Connection release is through "release order", "abandoned", "users have limited transparent data transmission functions such as unit to release the session. The session layer standard in order to make the session connection establishment phase, more functions and to facilitate the other international standard reference and reference, define the function of 12 units. Each system according to their own situation and needs, based on the core function service units, equipped with other functional units of reasonable session service subset. The session layer of the main criteria are" DIS8236: session service definition "and" DIS8237: session protocol specification ".


The presentation layer


One of the roles of the presentation layer is to provide a common language for communication between heterogeneous machines so that they can interoperate. This type of service is required because different computer architectures use different data representations. For example, IBM hosts use EBCDIC encoding, while most PCS use ASCII code. In this case, the session layer is required to complete the transformation.


Through the introduction, we can see that the session layer below 5 complete end-to-end data transmission, and is reliable, error-free transmission. But the data transfer is only means rather than aim, in the end is to realize the use of the data. Because of various systems for the definition of the data is not completely the same, the most easy to understand example is the keyboard, the meanings of some of the key in many have differences in the system. This natural to use other system data. The presentation layer and application layer is responsible for the elimination of the task of the disorder.


For user data, can be analysis from two sides, one is the data is called the semantic meaning, the other is a representation of data, called grammar. Like text, graphics, sound, zhuang, compression, encryption, etc. All belong to the category of grammar. The presentation layer design three kinds of 15 kinds of functional unit, in which context management functional unit is the communication between the user data encoding rules, so that the two sides have consistent data form, can know each other. ISO presentation layer for the service, protocol, the text sets DP8822 communication operators, DP8823, DIS6937/2 and a series of standards.


The application layer


The application layer provides services to the application that are grouped according to the characteristics they provide to the application and are called service elements. Some can be used by multiple applications, while others can be used by a smaller class of applications. The application layer is the highest level of an open system and serves the application process directly. Is its role in achieving multiple system application process is to communicate with each other at the same time, required to complete a series of business processing services. The service elements are divided into two types: public CASE application service elements and specific application service elements SASE. The CASE provides the most basic services, it has become any user in the application layer and the user of any services elements, mainly for the application process of communication, distribution system to provide basic control mechanism. Specific service SASE to meet some specific services, such as file transfer, access management, job transfer, bank transactions, order entry, etc.


These will involve virtual terminals, job transfer and operation, document transfer and access management, remote database access, graphics core systems, open system interconnection management, etc. Application layer standards include DP8649" public application service elements ",DP8650" protocol for public application service elements ", file transfer, access and management services and protocols.




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Tanzania163
Tanzania163 Created Dec 3, 2018 07:43:27
Basic level princinple  
Torrent
Created Dec 22, 2018 01:21:28 Helpful(0) Helpful(0)

The generation of network layer is also the result of network development. In the environment of on-line system and line switching, the function of network layer is of little significance. When the number of data terminals increases. There are relay devices connected between them. At this time, one terminal requires not only to communicate with one but also to communicate with multiple terminals, which leads to the problem of linking data of any two data terminal devices, namely 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, people would like to let multiple pairs of users share a link, so logical channel technology and virtual circuit technology appear to solve this problem.

thanks for sharing
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yiyi0519
Created Dec 22, 2018 01:29:33 Helpful(0) Helpful(0)

we usually use the TCP/IP model now
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xiaomumu
Created Dec 24, 2018 01:36:36 Helpful(0) Helpful(0)

Learn more, great
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dagui
Created Dec 27, 2018 07:49:27 Helpful(0) Helpful(0)

These will involve virtual terminals, job transfer and operation, document transfer and access management, remote database access, How do you understand this sentence?
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