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Seven-layer principle of network foundation

one。 What is the seven-layer principle of network foundation

two。 The Origin of the seven-tier model

three。 The principle and Protocol of Seven-layer Model

four。 What is the use of the seven-tier model?

1. Seven-tier model, also known as OSI (Open System Interconnection). The reference model is a standard system developed by the International Organization for Standardization (ISO) for the interconnection between computers or communication systems, which is generally referred to as the OSI reference model or seven-tier model.

It is a seven-layer, abstract model, including not only a series of abstract terms or concepts, but also specific protocols.

Most of 2.OSI 's design work is really just Honeywell Information System.

A team of the company finished it, and the technical leader of the group was Charlie Bachman. In the mid-1970s, the group was set up to develop some prototype systems, focusing on the design of database systems. In the 1970s, in order to support the access of database system, a structured distributed communication system architecture was needed.

So the team studied some existing solutions, including IBM's SNA (System Network Architecture), the protocol of ARPANET (the predecessor of Internet), and some related concepts of presentation services (presentation services) being studied for standardized databases, and proposed a seven-tier architecture model in 1977, which they internally called distributed system Architecture (DSA).

At the same time, in 1977, the British Standardization Association proposed to the International Organization for Standardization (ISO) that a standard architecture was needed to define the communication infrastructure between distributed processes. As a result, ISO set up a special committee on open systems interconnection (OSI) (TC 97, Subcomittee 16) and designated the American National Standards Institute (ANSI) to develop a draft standard to be submitted before the first formal meeting of the committee.

Bachman [1] attended ANSI's early meetings and submitted his seven-tier model, which became the only draft submitted to the ISO ad hoc committee.

In March 1978, at the meeting of ISO's OSI Special Committee in Washington, experts quickly reached a consensus that this hierarchical architecture can meet most of the needs of open systems and is scalable to meet new requirements.

As a result, the interim version was released in 1978 and became the final version after a little refinement in 1979. Therefore, the OSI model is basically the same as the 1977 DSA model.

3. Detailed principle and introduction

1. Application layer (Application Layer)

The application layer is the OSI layer closest to the user. This layer provides network services for users' applications, such as email, file transfer, and terminal emulation.

The agreements are: HTTP FTP TFTP SMTP SNMP DNS TELNET HTTPS POP3 DHCP and so on.

The application layer, also known as the application entity (AE), consists of several specific application service elements (SASE) and one or more common application service elements (CASE). Each SASE provides specific application services, such as document Transport access and Management (FTAM), Electronic message processing (MHS), Virtual Terminal Protocol (VAP), etc. CASE provides a set of common application services, such as contact Control Service element (ACSE), reliable Transportation Service element (RTSE) and remote Operation Service element (ROSE). It is mainly responsible for providing interfaces to the software so that the program can use network services. The term "application layer" does not refer to a particular application running on the network. The services provided by the application layer include file transfer, file management, and e-mail information processing.

Presentation layer (Presentation Layer)

Data representation, security, compression. It ensures that the information sent by the application layer of one system can be read by the application layer of another system.

Formats are: JPEG, ASCll, DECOIC, encryption format and so on.

For the translator between the application and the network, at the presentation layer, the data is formatted according to a scheme that the network can understand; this format also varies depending on the type of network used.

The presentation layer manages data decryption and encryption, such as the processing of system passwords. For example, when you query your bank account on Internet, you use a secure connection. Your account data is encrypted before it is sent, and at the other end of the network, the presentation layer decrypts the data received. In addition, the presentation layer protocol decodes and encodes picture and file format information.

Session layer (Session Layer)

Establish, manage, and terminate a session corresponding to the host process, which refers to the ongoing session between the local host and the remote host.

The path of data transmission is established through the transport layer (port number: transmission port and receiving port). Mainly initiate sessions or accept session requests between your systems (devices need to know each other, which can be IP, MAC or hostname).

Responsible for establishing, maintaining and terminating communication between two nodes in the network. The functions of the session layer include establishing a communication link, keeping the communication link open during the session, synchronizing the dialogue between the two nodes, determining whether the communication is interrupted and deciding where to resend when the communication is interrupted.

You may often hear people refer to the session layer as the "traffic police" of network communication. When you dial up to your ISP (Internet service provider) to request a connection to the Internet, the session layer on the ISP server negotiates a connection to you and the session layer on your PC client. If your phone line accidentally falls off from the wall Jack, the session layer on your terminal will detect a disconnection and restart the connection. The session layer sets the communication period by determining the priority of node communication and the length of communication time.

Transport layer (Transport Layer)

Define the protocol port number over which data is transmitted, as well as flow control and error checking.

The protocols are: TCP UDP, etc., once the packet leaves the network card, it will enter the network transport layer.

Some protocols and port numbers for data transmission (WWW port 80, etc.) are defined, such as: TCP (transmission control protocol, low transmission efficiency, high reliability, used for transmitting data with high reliability and large amount of data), UDP (user Datagram protocol, opposite to TCP, used to transmit data with low reliability and small amount of data, such as QQ chat data is transmitted in this way). Mainly, the data received from the lower layer is segmented and transmitted, and then reorganized when it reaches the destination address. This layer of data is often called segment.

The most important layer in the O S I model. The transmission protocol performs flow control at the same time or specifies an appropriate transmission rate based on the speed at which the receiver can receive the data. In addition, the transport layer forcibly divides longer packets according to the maximum size that the network can handle. For example, Ethernet cannot receive packets greater than 1500 bytes. The transmission layer of the sender node divides the data into smaller data slices, and arranges a sequence number for each data slice, so that when the data reaches the transmission layer of the receiver node, it can be reorganized in the correct order. This process is called sorting. One service that works at the transport layer is T C P (Transmission Control Protocol) in the T C P / I P protocol suite, and the other is S P X (sequential packet switching) in the I P X / S P X protocol set.

Network layer (Network Layer)

Logical address addressing is carried out to realize the path selection between different networks.

Protocols include: ICMP IGMP IP (IPV4 IPV6) ARP RARP and so on.

Provides connectivity and path selection between two host systems located in different geographically located networks. The development of Internet has greatly increased the number of users accessing information from various sites around the world, and the network layer is the layer that manages this connection.

The third layer of the O S I model, whose main function is to translate the network address into the corresponding physical address and decide how to route the data from the sender to the receiver.

The network layer determines the best path from node An in one network to node B in another network by comprehensively considering transmission priority, network congestion, quality of service and the cost of optional routes. Because the network layer processes and intelligently guides data transmission, the router connects each segment of the network, so the router belongs to the network layer. In a network, "routing" guides the transmission of data based on addressing schemes, usage patterns, and accessibility.

The network layer is responsible for establishing the routes they use between the source and destination machines. This layer itself does not have any error detection and correction mechanism, so the network layer must rely on reliable transmission services provided by D L L between end-to-end.

The network layer is used to establish communication between computer systems on the local L A N network segment. It can do so because it has its own routing address structure, which is separate and independent from the second layer machine address. This protocol is called a routing or routable protocol. Routing protocols include IP, I P X of N o v e l l and A p p l e Ta l k.

The network layer is optional and can only be used when two computer systems are on different network segments separated by routers, or when communication applications require services, features, or capabilities provided by a network layer or transport layer. For example, when two hosts are directly connected to the same L A N segment, the communication between them can only use the communication mechanism of L A N (that is, the first or second layers of the OSI reference model).

Data Link layer (Datalink Layer)

Establish logical connection, hardware address addressing, error checking and other functions. (protocol defined by the underlying network)

The bits are combined into bytes and then combined into frames, and the media is accessed with an MAC address. Errors are found but cannot be corrected.

Representatives of data link layer protocols include: SDLC, HDLC, PPP, STP, frame Relay and so on.

Defines how to allow formatted data for transmission and how to control access to physical media. This layer also usually provides error detection and correction to ensure reliable data transmission.

The second layer of the OSI model, which controls communication between the network layer and the physical layer. Its main function is how to transmit data reliably on unreliable physical lines. In order to ensure transmission, the data received from the network layer is divided into specific frames that can be transmitted by the physical layer. A frame is a structural packet used to move data, which includes not only the original data, but also the physical addresses of the sender and receiver, as well as error detection and control information. The address determines where the frame will be sent, while error correction and control information ensures that the frame arrives error-free. If the receiving point detects an error in the transmitted data when transmitting the data, it will notify the sender to resend the frame.

The function of the data link layer is independent of the network, its nodes and the type of physical layer it uses, and it does not care whether Wo r d, E x c e l, or I n t e r n e t is running. Some connected devices, such as switches, work at the data link layer because they decode the frame and use the frame information to send data to the correct receiver.

Data link layer (DataLinkLayer): based on the bit stream service provided by the physical layer, the data link between the adjacent nodes is established, the error-free transmission of the data frame (Frame) on the channel is provided through error control, and the action series on each circuit is carried out.

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, retransmission and so on.

Physical layer (Physical Layer)

Establish, maintain, and disconnect physical connections. (protocol defined by the underlying network)

It mainly defines the physical equipment standard, such as the interface type of network cable, the interface type of optical fiber, the transmission rate of various transmission media and so on. Its main function is to transmit bit stream (that is, from 1, 0 to current strength for transmission, and after arriving at the destination, it is converted to 1, 0, that is, what we often call digital-to-analog conversion and analog-to-digital conversion. The data on this layer is called bits.

The lowest or first layer of the O S I model, which includes physical networking media, such as cable connectors. The protocol of the physical layer generates and detects the voltage in order to send and receive signals carrying data. Insert a network interface card into your desktop PC and you will build the foundation for computer networking. In other words, you provide a physical layer. Although the physical layer does not provide error correction services, it can set the data transmission rate and monitor the data error rate. Network physical problems, such as disconnected wires, affect the physical layer. In order to transmit information, users have to use some physical media, such as twisted pair, coaxial cable, etc., but the specific physical media is not within the 7 layers of OSI. Some people regard physical media as layer 0, and the task of the physical layer is to provide a physical connection for its upper layer, as well as their mechanical, electrical, functional and process characteristics. Such as the type of cable and connector specified, the voltage to transmit the signal, and so on. At this layer, the data is not organized, but is treated only as the original bit stream or electrical voltage, in bit bits.

4. How the protocols and data of each layer are received

5. Application layer protocols: HTTP Hypertext transfer Protocol, FTP File transfer Protocol, TFTP: simple File transfer Protocol, IP address Resolution Domain name Protocol

6. Transport layer: TCP: Port protocol, secure and inefficient, UDP: Port protocol, efficient and unsecure

7. Network layer: ICMP: Datagram control protocol, IGMP: user Datagram control protocol, IP protocol, ARP:IP address is parsed into MAC address protocol, RARP:MAC address is resolved into MAC address.

8. The use of the seven-tier model

Which layer a device works on depends on which layer of data header information it uses when it works. When the bridge is working, the forwarding port is determined by the MAC header, so it is obviously a device at the data link layer.

Specifically:

Physical layer: network card, network cable, hub, repeater, modem

Data link layer: bridge, switch

Network layer: router

The gateway works at and above the fourth transport layer.

A hub is a physical layer device that transmits information in the form of broadcast.

A switch is a machine used to exchange messages. Most of them are link layer devices (layer 2 switches), which can learn addresses and exchange messages in the form of store-and-forward.

One function of a router is to connect different networks, and the other is to choose the route to transmit information. Choosing a smooth and fast shortcut can greatly improve the communication speed, reduce the communication load of the network system, save the network system resources, and improve the unblocked rate of the network system.

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