The Open Systems Interconnection (OSI) model defines a network framework to implement protocols in seven layers. The OSI model actually contains nothing. In fact, it is even invisible. The OSI model does not perform any functions during the networking process. It is a conceptual framework so that we can better understand the ongoing complex interactions.
Who developed the OSI model?
The International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) model. It divides network communication into seven layers. In this model, layers 1-4 are considered as the lower layers, mainly focusing on the movement of data. Layers 5-7 (called upper layers) contain application-level data. There is a basic principle for the operation of the network: "delivery". Each layer is responsible for a very specific job and then passes the data to the next layer.
Layer 1: Physical layer
Starting from the lowest level of the OSI model is the physical layer . The physical layer specifies the hardware method of sending and receiving data on the carrier , including the definition of cables, network cards and physical aspects. Fast Ethernet , RS232 and ATM are protocols with physical layer components .
It solves the physical characteristics of the network. This includes the types of cables used to connect everything together. The type of connector used, the length of the cable, etc. For example, the Ethernet standard for 100BaseT cables specifies the electrical characteristics of twisted-pair cables, the size and shape of connectors, and the maximum length of cables.
The physical layer also specifies the electrical characteristics of the signals used to transmit data from one network node to another through the cable. Except for the binary characteristics of '0' or '1', the signal has no special meaning. The upper layer of the OSI model will assign meaning to the bits transmitted at the physical layer.
A very important physical layer device used in the network is the network TAP. Network TAP is a hardware device used to replicate the traffic on the network link and redirect the copy to troubleshooting and analysis tools. Even if the TAP loses power, it will not interrupt the traffic or introduce a point of failure.
It can be seen from the figure that the eastbound traffic is directed to Monitor port A, and the westbound traffic is directed to Monitor port B.
▣Layer 1 physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.
Layer 2: Data link layer
The data link layer is where we start to give meaning or intelligence to the content to be sent over the network. The protocol on the data link layer solves the following problems, such as the size of the data packet to be sent, the addressing method of each data packet to be transmitted so that it reaches the intended recipient, and a way to ensure that no more than one node tries A method of sending data packets to the receiver at the same time.
The data link layer provides error detection and correction functions to ensure that the sent data is the same as the received data. If the error cannot be corrected, the data link standard needs to specify how to notify the node of the error so that it can resend the erroneous data.
Each node (network interface card-NIC) has an address at the data link layer, called the media access control address, usually called the MAC address. This is the actual hardware address, which is assigned by the device manufacturer. You can find the MAC address of the device by opening a command window and running the'ipconfig / all' command.
▣Layer 2 data link examples include PPP, FDDI, ATM, IEEE 802.5/802.2, IEEE 802.3/802.2, HDLC, frame relay.
Layer 3: Network layer
Layer 3 is responsible for routing network messages in the network. An important function of the network layer is logical addressing. Every network device has a physical address, called a MAC address (see layer 2). When you buy a network card for your computer, the MAC address of the network card cannot be changed. However, if you want to use some other addressing system to reference your computer and other devices, the third network layer is where you can set the so-called "logical addresses". The logical address provides a location for the network device, which can be accessed on the network using the address you assign.
Logical addresses can be created and used by network layer protocols such as IP or IPX. The network layer protocol converts logical addresses into MAC addresses.
For example, if you use IP as the network layer protocol, IP addresses will be assigned to devices on the network, such as 107.210.76.30. Since the IP protocol runs on layer 3 to actually send data packets, IP needs to convert the device's IP address to the correct MAC address. You can use the "ipconfig / all" command to find the IP address of a computer or other device.
After resolving the IP address, we now need to set up routing to move the packet to the destination. When data packets on one network need to be sent to a computer on another network, routing comes into play.
▣Layer 3 network examples include AppleTalk DDP, IP, IPX.
Layer 4: Transport layer
The transport layer is the basic layer for one network computer to communicate with another network computer. . The transport layer is one of the most popular network protocols, the place of Transmission Control Protocol (TCP). The main purpose of the transport layer is to ensure that data packets move reliably and without errors in the network. The transport layer achieves this by establishing a connection between network devices, confirming the receipt of data packets, and resending unreceived or damaged data packets upon arrival.
In many cases, the transport layer protocol divides large messages into smaller data packets, which can be effectively sent on the network. The transport layer protocol reorganizes the message at the receiving end to ensure that all data packets contained in a transmission can be received without data loss.
▣Layer 4 transmission examples include SPX, TCP, UDP.
Layer 5: Session layer
The session layer establishes, manages, and terminates connections between network nodes. Before transmitting data on the network, a session must be established. The session layer ensures that these sessions are established and maintained correctly. It provides full-duplex, half-duplex or simplex operation, and establishes checkpoint, postpone, termination and restart procedures. The OSI model makes this layer responsible for the normal closing of the session (this is an attribute of the TCP protocol), and is also responsible for session checkpointing and recovery, which is usually not used in the Internet protocol suite. The session layer is usually implemented explicitly in an application environment that uses remote procedure calls.
▣Examples of layer 5 sessions include NFS, NetBios Name, RPC, SQL.
Layer 6: Presentation layer
The presentation layer is responsible for converting the data sent by the network from one representation to another. For example, the presentation layer can apply complex compression techniques so that when sent over the network, fewer data bytes are required to represent information. At the other end of the transmission, the transport layer decompresses the data.
This layer provides independence from data representation differences (such as encryption) by converting from application to network format, and vice versa. The presentation layer transforms the data into a form acceptable to the application layer. This layer formats and encrypts the data to be sent on the network so that the data is not affected by compatibility issues. It is sometimes called the grammar level.
▣Layer 6 demo examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.
Layer 7: Application layer
The highest layer of the OSI model-the application layer, which deals with the technology used by applications to communicate with the network. The name of this layer is a bit confusing because applications (such as Excel or Word) are not actually part of this layer. Instead, the application layer represents the level of interaction between the application and the network, using a programming interface to request network services. HTTP is one of the most commonly used application layer protocols, which stands for Hypertext Transfer Protocol. HTTP is the foundation of the World Wide Web.
▣Layer 7 application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP.
