Test and development of the network chapter-OSI seven-layer protocol

Today, let’s take a look at OSI (Open System Interconnect). It is a network interconnection model released by the ISO organization in 1985. The standard defines a seven-layer framework for network interconnection. The content is briefly described as follows:

Level	name	explain
1	Physical layer	The bit stream is transmitted through the physical medium. The pins, voltages, and network cards, network cables, hubs, etc. are defined.
2	data link layer	Responsible for network addressing, error detection and correction. Such as Ethernet, wireless local area network and other levels.
3	Transport layer	Provide reliable and transparent data exchange for the upper layer, including error handling and flow control. Such as TCP and UDP protocols.
4	Network layer	The connection between two nodes is established through IP addressing, and routing occurs at this layer. Such as the IP protocol.
5	Session layer	Establish and manage communication dialogues between applications.
6	Presentation layer	Deal with the representation of exchanged information, such as format, encryption, and compression.
7	Application layer	Complete the external service of the application. Such as the HTTP protocol for browsing websites, the SMTP protocol for sending mail, etc.

When we purchase network equipment for a company or a family, there are generally three types found on the Internet, namely hubs, switches, and routers. Here we will introduce the relationship between network layering and hardware equipment.

hub works at the first layer (physical layer) of OSI, and only forwards electrical signals to the devices connected to it, and does not do other processing.

switch is a layer 2 (link layer) device, which is addressed by sending broadcasts in the same network segment, using the MAC address of the target host obtained to communicate with the host on the network segment There is also a switch with added routing function on the market, we call it a layer 3 switch.

router is a device that works on the 4th layer (network layer). It maintains a routing table by learning from each other, and is responsible for addressing and forwarding in a cross-network segment environment.

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