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OSI reference model

1. Physical layer

The main function of the physical layer is to complete the transmission of the original bit stream between the adjacent nodes and provide a physical connection for the upper layer.

This layer defines how the cable is connected to the network card and which transmission technology is used to send data on the cable, as well as bit synchronization and checking. This layer represents the actual connection between the user's software and hardware.

It is virtually irrelevant to any protocol, but it defines the access methods used by the data link layer.

The physical layer is the lowest layer of the OSI reference model, which is directly connected to the physical transport medium. Physical layer protocol is the underlying protocol that must be followed when all kinds of network devices are interconnected. The purpose of setting up the physical layer is to realize the transparent transmission of the binary bit stream between the two network physical devices and to shield the characteristics of the physical transmission media from the data link layer in order to maximize the transparency to the high-level protocols.

The physical layer protocol is mainly concerned with what kind of physical signals are used to indicate the duration of data 1 and 0, whether the data transmission is carried out in both directions at the same time, how the initial connection is established and how the connection is terminated after the completion of the communication, how many pins are there in the physical interface (plug and socket) and the use of each pin, as well as the transmission medium connected by the physical layer interface, and so on.

ISO defines the physical layer in the OSI reference model as follows:

The physical layer provides mechanical, electrical, functional, and procedural characteristics for establishing, maintaining, disconnecting, and releasing the physical connection of binary bit stream transmission between data link entities. Physical connections can be made through a relay system, allowing the transmission of full-duplex or half-duplex binary bitstreams.

The data service unit of the physical layer is bits, which are transmitted synchronously or asynchronously.

two。 Data link layer

The data link layer is the second layer of OSI, which lies between the physical layer and the network layer, and is also an extremely important layer. It encapsulates the original data from the physical layer into data frames and is responsible for error-free transmission between computers.

Main purpose: how to transfer data reliably on unreliable physical lines

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.

Related agreements:

SDLC (Software Lifecycle)

HDLC (bit-oriented synchronization protocol)

PPP (including link control protocol LCP, network control protocol NCP, and authentication protocol): a link layer protocol designed for simple links such as transmitting packets between equivalent units.

STP (spanning Tree Protocol): logically disconnect the loop to prevent the broadcast storm of the layer 2 network. When the line fails, the disconnected interface is activated, resumes communication, and plays the role of backup line.

Frame Relay: mainly used for data transmission.

3. Network layer

Located in the third layer of the OSI reference model, the main function is to translate the network address (i.e. IP address) into the corresponding physical address and transfer the data from the source to the destination.

The network layer determines the best path from the source host to the destination host by comprehensively considering the transmission priority, the degree of network congestion, the quality of service and the cost of optional routing.

Related agreements:

IP protocol (protocol for interconnection between Internet Protocol networks)

The most important protocol of Internet. The rules for communicating over Internet are specified in the IP protocol.

Function: to provide a unified IP packet to the transport layer (TCP layer), that is, to convert various types of MAC frames into unified IP packets, and to transform the physical address of the MAC frame into a unified logical address for the whole network. Because of this transformation, all cities and counties have interconnected different types of physical networks.

ARP Protocol (Address Resolution Protocol address Resolution Protocol)

A TCP/IP protocol that gets the physical address. After the ARP request for its node's IP address is broadcast to the network, the node receives a reply confirming its physical address so that the packet can be sent out.

RARP protocol (Reverse Address Resolution Protocol reverse address translation protocol)

Convert the physical address of a host in the local area network into an IP address.

ICMP Protocol (Iternet Control Message Protocol Internet Control message Protocol)

A subprotocol of the TCP/IP protocol suite that is used to pass control messages between IP hosts and routers. The control message includes the messages of the network itself, such as whether the network is unreachable, whether the host is reachable, whether the route is available, and so on.

IGMP Protocol (Internet Group Management Protocol Internet Group Management Protocol)

4. Transport layer

To achieve data communication between user processes on different hosts in the network, colleagues carry out flow control or specify an appropriate transmission rate based on the speed at which the receiver can receive the data. In addition, the transport layer can install the maximum size that the network can come out to force the segmentation of longer packets.

Example:

Ethernet cannot receive 1600 bytes of packets. The transmission layer of the sender node divides the data into smaller data, and colleagues assign a sequence number to each data chip, so that when the data reaches the data transmission layer of the receiver node, it can be reorganized in the correct order. This process is called sorting.

Related agreement

TCP (Transmission Control Protocol Transmission Control Protocol)

UDP: user packet protocol, which is a new connectionless transport layer protocol in the OSI reference model.

5. Session layer

Responsible for establishing, maintaining and terminating communication between two nodes in the network. The main purpose is to establish the communication link, keep the communication link unblocked during the session, synchronize the dialogue between the two nodes, determine whether the communication is interrupted and decide where to resend when the communication is interrupted.

Example:

When a major failure occurs in the transmission of data in the network, it is to prevent giving up halfway. The session layer provides a way to insert synchronization points into the data. Each time the network fails, the data is retransmitted only after the last synchronization point.

6. Presentation layer

The main concern is the syntax and semantics of transmitting information.

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.

7. Application layer

Contains a large number of agreements that people generally need. The purpose is to support communication between processes running on different computers, which are designed for different tasks of different users.

Related agreement

HTTP 、 FTP/SMTP/DNS

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