Seven-story agreement
"OSI.Open Systems Interconnection ------ Open System Interconnection"
OSI is developed by important computers and telematics companies in 1983, initially imagined to become an interface specification. The Committee decided to establish a general reference model so that others can develop more detailed interfaces to become standard. ISO adopted OSI as an international standard. In the early days of network development, many research institutions, computer manufacturers and companies have vigorously develop computer networks. From A R Pa N e T (this I think everyone should know a little, the first generation of experimental networks) has now launched many commercial network systems. This kind of self-developed network is very different in architectural factors that they are incompatible between them, they are difficult to connect to each other to form a larger network system. (Haha, human beings are smart, the way.) To this end, many standardized institutions actively carry out the standardization of network architecture standardization, the most famous is the open system interconnect reference model for international standardization organization ISO, OSI / RM . The O S i reference model is a standard for studying how to connect an open system (that is, a system that communicates with other systems). (This is aware of everyone, why do I have to appear in OSI, let's talk about the content of OSI for everyone.
The core of OSI is that the communication between two end users in the remote communication network can be divided into layers, each with its own function set. (This comparison is key)
O S i reference model divides computer networks into 7 floors, (Oh, enough.) We will start from the bottom of the bottom, discuss the functions of the models of the model. (I want to draw it for everyone, but unfortunately from the elementary school to now, the art does not have a profile, so ..... still is free.)
1. The main function of the physical layer (Physical Layer is the transfer of the original bitstream between adjacent sources. The typical problem for the physical layer protocol is what physical signal is used to represent the data "1" and "0"; one last time is longer; whether the data transmission can be performed in both directions; the initial connection How to terminate and the connection after the communication; how many needles and the use of each needle are used in physical interfaces (plugs and sockets). The design of the physical layer mainly involves the mechanical, electrical, functional and process characteristics of the physical layer interface, as well as the transmission medium connected by the physical layer interface. The physical layer design also involves some issues in the field of communication engineering. (This layer represents the actual connection between the user's software and hardware. It is actually not coheed with any protocol, but it defines the access method used by the data link layer.)
The physical layer is the lowest layer of the OSI reference model, to the 佑 佑 泶 氏 嗔 聿 髦 髦 髦 缟 缟 缟 缟 椤 I 枇 ⑽ 聿 聿 鞘 至 至 鐾 鐾 钢 钢 钢 钢 钢 涞 鞔 鞔 亓 亓 亓 亓 亓 鞔 鞔 鞔 鞔 鞔 鞔 鞔 鞔 鞔 鞔 匦Yu Yu is willing to rush to porphyr thresholds? ISO is defined below the physical layer in the OSI reference model:
The physical layer provides mechanical, electrical, functional and procedures for the physical connection of binary bits transmitted between the establishment, maintenance and release data link entities. Physical connections can be transmitted through the relay system, allowing the transmission of the binary bitstream of the full-duplex or half-duplex. The data service unit of the physical layer is bits, which can be transmitted in a synchronous or asynchronous manner. (The above is my notes, everyone look, hehe ~~~~~~~ Write this paragraph is not easy, because I used to write too ugly, now it looks good.)
Come to a summary, haha ~
The main features of the physical layer are:
(1) The physical layer is mainly responsible for transmitting binary bitstreams on physical connection;
(2) The physical layer provides the characteristics of mechanical, electrical, functional and procedures required to establish, maintain, and release physical connections. (Halo, how do you feel that this summary is nonsense?)
2. The main function of the data link layer data link layer (DATA LINK LAYER) is how to perform reliable transmission of data on unreliable physical lines. The data link layer is completed is a reliable data communication between adjacent nodes in the network. In order to ensure the reliable transmission of the data, the sender encapsulates the user data into frame (F R a m e) and transmits each frame in order. Due to the unreliable physical line, the data frame issued by the sender may have errors or lost on the line (so-called loss is actually a frame head or frame end error of the data frame), resulting in the receiver that the receiver cannot receive the data frame. In order to ensure that the receiver is correctly judged by the correctness of the received data, the sender calculates the CRC (cycle redundancy test) for each data block, so that the recipient can determine the data by recalculating the CRC. The correctness of the received. Once the received data is wrong, the sender must retransmit this frame data. However, multiple transmission of the same frame may also cause the receiver to receive a repeating frame. For example, after the recipient is destroyed by the confirmation frame of the sender, the sender will retransmit the previous frame, and the recipient may receive a repeating frame. The data link layer must solve the problem due to corruption, loss, and repetition due to frames. (The eyes are tired, continue tomorrow, it is estimated that you are also tired, take a break.)
Data links can be roughly understood as data channels (more than one sentence is not responsible)
The physical layer provides transport media and its connection to data communication between terminal devices. The media is long, the connection is survival. During the connection survival period, the transmission and reception can perform an attempt or multiple data communication. Each communication has to establish a communication contact and demolition communication. This is called a data link in this established data transmission and reception relationship. (This place is focusing, because I am learning to make the physical layer and links, this place is more clear.) The data transmitted on physical media is inevitably generated by various unreliable factors, in order to Make up the shortcomings of the physical layer, provide data transmission without error, and can detect the data. The data link is established, dismantled, and the error correction is the data link layer. Basic task. (The following is actually the same as a layer of service ~~~~).
Hey, doing a summary.
The primary function link layer of the link layer is to provide data transfer services for the network layer, which is implemented by the functionality of this layer. The link layer should have the following functions:
1: The establishment, demolition, separation of the link connection.
2: Frame boundary and frame synchronization. The data transfer unit of the link layer is a frame, the protocol is different, and the length of the frames and the interface are also different, but no matter how the frame must be bound.
3: Sequential control refers to the control of the transceiver sequence of the frame.
4: Error detection and recovery. There is also a link identification, flow control, and the like. Error detects multi-purpose format checks and cyclic code checks to detect the error of data on channel, and the frame loss is used to detect. Various errors Restore often feedback Reissant technology is completed.
If someone doesn't know what "frame" is, I will find materials yourself, I can't.
The primary protocol of the data link layer is set to be consistent to the actual inter-entity equality, but also to successfully complete the service of the network layer. The main agreement is as follows:
ISO1745--1975: "Basic Control Procedures for Data Communication System". This is a character-oriented standard that uses 10 control characters to complete the establishment, demolition and data exchange. The transceiver and error recovery are also Both these characters are completed. ISO1155, ISO1177, ISO2626, ISO2629 and other standards can form a variety of link control and data transmission mode. ISO3309--1984: is called "HDLC frame structure" .iso4335--1984: called "HDLC regulations". ISO7809--1984: "HDLC procedure type assembly". These three standards are set for bits to bits. Some people are used to call these three standard portfolios Road control procedures. ISO7776: is called "DTE Data Link Level Procedures". By compatible with the CCITT X.25LAB "Balanced Link Access Procedure". (诶 ~~~~ This place is found in a friend's computer, I didn't understand it.) The link will introduce so much, I want to see this, I don't understand it, I think I have to say more than our teacher.
3. The main function of Network Layer is to complete packet transfer between the hosts in the network. One of its key issues is to transmit each message from the source to the destination using the service of the data link layer. In a wide area network, this includes generating routes from the source end to the destination, and requires this path to be as small as possible. If there are too many packets in the subnet, the subnet may form congestion, and must be avoided, and such controls are also on the content of the network. When packets have to span two or more networks, there will be many new issues. For example, the addressing method of the second network may be different from the first network; the second network may not be received because the packets of the first network cannot be received; the protocols used by the two networks may also be different, and so on. The network layer must solve these problems to enable the heterogeneous network to interconnect. In a single LAN, the network layer is redundant because the message is transferred from one computer to another computer, so the work to be done in the network layer is very small.
The main function of the network layer is the establishment of a network connection and providing services for the upper layer. It should have? 饕 ? Br /> 1 route selection and relay
2 activated, terminate network connection
3 Recount multiple network connections on a data link, take more time-multiplexing technology
4 error detection and recovery
5 Sort, flow control
6 service selection
7 network management
Network layer standard introduction to some of the main criteria of the network layer is as follows:
ISO.DIS8208: "X.25] ISO.DIS8348:" CO Network Service Definition "is called" CO Network Service Definition "(Connected) ISO.DIS8349:" CL Network Service Definition "(no connection) ISO.DIS8473: is called "CL Network Protocol" ISO.DIS8348: It is called "network layer addressing" except for the above standards, there are many standards. These standards are only part of the network layer, so they often need to use several criteria in the network layer to complete the entire network layer. Due to the different networks, the network layer will use different standard combinations.
OK, it also gets a layer. -__-!
Supplementary sentence
In the network layer of the network with open features, you must configure the function of the network layer. Now the network hard equipment sold in the market mainly has gateways and routers. Hoho ~~~~~
4. The main function of the Transport Layer is to complete reliable data communication between the user processes on different hosts in the network. The transport layer should determine what kind of service is provided to the session-layer user and ultimately on the network user. The best transfer connection is a pipe that is not wrong, and the data is transmitted sequentially, that is, the transport layer connection is the true end to the end. In other words, a process on the source terminal, using the packet head and control packets to talk to the peer process on the target machine. In the layers under the transport layer, the protocol is a protocol between each machine and its direct adjacent machine (host-IMP, IM PI MP), rather than the final source and target machine (host - The protocol of the host). In the middle, it may be separated by multiple I m P. That is, the protocol of 1 to 3 is a point-to-point protocol, and the protocol of the 4 to 7 layers is an end-to-end protocol. Since most hosts support multi-user * work, there are multi-channel programs on the machine, which means that multiple connections will enter from these hosts, so it is necessary to distinguish which connection is part of the packets in some way. Identify information of these connections can be placed in the packet header of the transport layer. In addition to multiplexing several message flows multiplexed into a channel, the transport layer must also manage the establishment and demolition of cross-grid connections. This requires some naming mechanism to make the process within the machine to tell the object it hopes to talk. In addition, there is a need to have a mechanism to regulate the information stream, so that the high-speed host does not transmit data to the low speed host too quickly. Although the traffic control between the host is different from the flow control between the I m P, we will see similar principles to both. In fact, I think so I think, this layer is equal to the port to the port ~ I don't know if I understand it is wrong?
For example, when the A machine's QQ message arrives at the B machine, it is to find 4000 ports through the relevant protocol of this layer. (I think so I think, if not, everyone will tell me)
The first end-to-end hierarchy has a buffer. When the network layer service quality does not meet the requirements, it will increase the service to meet the requirements of the high-level; when the network layer service quality is better, it only uses few work. The transport layer can also be multiplexed, i.e., a plurality of logical connections are created on a network connection. (This is an excerpt online, more classic, haha.)
In addition: The transport layer is also called a transport layer. The transport layer exists only in the end open system, which is a layer between the low 3-layer communication subnet system and the high 3 layer, but it is very important. Because It is the source-to-destination end to control data transfer from the last layer of low to high.
Basically, these, the content is over. I watched myself in my understanding, I can't send it online.
Well, there is some content, it is not a focus, or put it on it.
There is an existing fact that there is a big difference in performance in various communication subnets in the world. For example, telephone exchange networks, packet switched networks, public data exchange networks, local area networks and other communication subnets are interconnected, but they provide Throughput, transmission rate, data delay communication costs are different. For the session layer, it is required to have a dynamic interface. The transport layer takes this feature. It uses shunt / unflow, multiplexing / dielectric technology To adjust the differences in the above communication subnets, can't feel the session layer.
In addition, the transport layer has functions such as error recovery, flow control, and the details of the communication subnet in these respects. The data object faced by the transport layer is not the network address and host address, but a session layer. Interface port. The ultimate goal of the above function is to provide reliable, error-free data transmission for sessions. The service of the transport layer generally goes through the transmission connection establishment phase, the data transfer phase, and the transmission connection release phase will complete a complete completion of a complete The service process is divided into general data transfer and acceleration data transfer in the data transfer phase. Transport layer services are divided into five types. Basically meet the various needs of transmission quality, transmission speed, and transfer fees. The protocol standards for transport layers have the following types:
ISO8072: "Connected Transmission Service Definition" ISO8072: "Connected Transmission Protocol Specification" Total ~~~~~~~~ Tomorrow continues.
5. Session Layer allows session relationships between users on different machines. The session layer allows the transfer of normal data similar to the transport layer, and some useful enhanced services are provided in some occasions. Allow users to log in on the remote system using a session, or pass files between two machines. One of the services provided by the session layer is management dialogue control. The session layer allows information to be transmitted two-way transmission, or only one-way transmission is available at any time. If it belongs to the latter, similar to the half-duplex mode on the physical channel, the session layer will record which party at this time. A service related to dialog control is token management. Some agreements ensure that both parties cannot perform the same *, this is important. In order to manage these activities, the session layer provides token, token can be moved between the sessions, and only some of the token can perform some critical *. Another session layer service is synchronized. If there is an average of a large fault in a large fault, two machines should be transferred once, think about what kind of problem occurs? After each transmission fails, you have to re-transfer this file. When the network has a big fault again, it may be used for halfway. In order to solve this problem, the session layer provides a method, that is, insert the synchronization point in the data. After the network is faulty, only the data after the last synchronization point is retransmitted. (I don't understand, I don't understand, I don't understand when I am basically in class, I will ask the teacher, because I asked too much, the final teacher took me --__- !!.)
Establish a connection between session entities. To create a session connection to two peer session service users, you should do the following work:
1 Map the session address to the transport address
2 Select the required transportation service quality parameters (QoS)
3 negotiate session parameters
4 identify each session connection
5 Transfer limited transparent user data
I always feel that this layer does not seem to use? ? ? ? (Individual opinion, don't lose me with eggs)
6. Represents the presentation layer completes some specific functions, people often want to find universal solutions without having to be implemented by each user. It is worth mentioning that the following layers are only concerned about reliably transmitting bits from the source terminal to the target machine, while the representation of the syntax and semantics of the information transmitted is. A typical example of the layer service is to encode data in a standard method that is unanimously selected. Most user programs are not swaping with random bits, but exchange information such as human name, date, currency quantity and invoice. These objects are represented by the form of strings, integer, floating point numbers, and data structures consisting of several simple types. The computer on the network may adopt a different data representation, so it is necessary to perform conversion of data format when data transmission. For example, different code is often used in different machines (A S C I I and E B C D i c), integer (binary inverse or complement), and different bytes of sequence of machine words. In order to enable computers with different data representations to communicate and exchange data, we use abstract data structures (such as abstract syntax represents ASN. 1) during communication, and it is still in the machine. Standard code. These abstract data structures are managed, and the transmitter is converted to the transfer syntax for the machine to becomes the transfer syntax for the online transmission, and the conversion of the receiver is completed by the representation layer. In addition, the representation layer also relates to data compression and decompression, data encryption, and decryption.
This layer has nothing to say, see what the name knows what is going on.
7. The purpose of the application layer network is to support the process of running on different computers for communication, and these processes are designed for users to complete different tasks. Possible applications are multi-faceted and are not limited by the network structure. Application Layer contains a large number of people generally needed agreements. Although it is necessary for different applications that require communication, the agreement of the application layer must be. For example, a P C user uses the simulation terminal software to simulate the terminal of a remote host through the network and use the resources of the remote host. This simulation terminal program uses the virtual terminal protocol to transfer the data entered by the keyboard to the host, and receive data displayed on the screen. For example, when a user wants to get a file copy on a remote computer, he wants to send a request to this machine's file transfer software. This software communicates with the file transfer process on the remote computer, this protocol Mainly processes file names, user licensing status, and other communications of other request details. The file transfer process on the remote computer uses other features to transfer the file content. Since each application has different requirements, the application layer has no definitions in the ISO / OSI model, but some identified application layer protocols, including virtual terminals, file transfer, and email, etc. can be used as standardized candidates. Finally, I'm finished ~~~~~~ Hahaha! Celebrate, go buy an ice cream to eat ~~~~ hoho ~
end
It is worth noting that the O S I model itself is not all content of the network architecture because it is not exactly the protocol and implementation method for each layer, but only tells us that each layer should be completed. However, I S O has set corresponding standards for each layer, but these standards are not part of the model, which is released as an independent international standard. In the O S i Reference Model, there are three basic concepts: services, interfaces and protocols. Perhaps the most important contribution of the O S I model is to distinguish these three concepts. The O S i reference model is designed before its agreement development. This means that the O S I model is not designed based on a particular protocol set, so it is more versatile. On the other hand, it also means that the O S I model has certain deficiencies in terms of protocol implementation. In fact, O S i protocol is too complicated, which is why O S i has never truly popularized. Although the OSI model and agreement did not achieve huge success, the OSI reference model still played a very important guiding role in the development of computer networks, as a reference model and complete system, it still standardizes future computer network technology The development of standardized direction has guiding significance.
