In the entire network, the most basic or core thing is data transmission. The entire network is built to serve data transmission. Let’s compare the example of express logistics to understand the purpose of these seven layers. :
1. Application layer
The role of the application layer is to provide services for the application and specify the communication-related details in the application, that is, to provide services for the application. Common protocols include HTTP, FTP, TELNET, SMTP, etc.
Translated into "human words":
Equivalent to the receiver. When a customer (application) calls (initiates a request) to the recipient (application layer), the recipient can provide different services (different agreements) according to the different needs of the customer, such as delivery on the next day, delivery at a specified time, etc. wait.
2. Presentation layer
The role of the presentation layer is to convert the information processed by the application into a format suitable for network transmission, or to convert the data from the lower layer into a format that the upper layer can process. It is mainly responsible for the conversion of data format. Specifically, it is to convert the inherent data format of the device into a network standard format. Common protocols include ASCII, SSL/TLS, etc.
Translated into "human words":
Equivalent to a packer. If the express (data) is too bloated, he will flatten (compress) it without breaking the express. If the customer pays attention to the security line, the almighty courier company can also use the password box (SSL/TLS) to pack the courier and send it quickly. Of course, the packer will confirm that the unpacker at the destination express station can unpack the package without damage and deliver the express to the user.
3. Session layer
The role of the session layer is responsible for establishing and disconnecting communication connections, as well as management related to data transmission such as data segmentation. Common protocols include ADSP, RPC, etc.
Translated into "human words":
Equivalent to a dispatcher. Scheduling and directing express transportation. For example, this time the customer wants to send 100 tons of sand (data), whether it should be transported by air, land or sea. After the shipment, the relevant information (connection) can also be destroyed, these are his responsibilities.
4. Transport layer
The transport layer plays the role of reliable transport. It is only processed on the nodes of the two sides of the communication, and does not need to be processed on the router. This layer has two representative protocols: TCP and UDP.
Translated into "human words":
Equivalent to a merchandiser. A responsible merchandiser (using the TCP protocol) will ensure that the express delivery is delivered to the customer, and if it cannot be delivered, the company will send it again. Irresponsible merchandisers (using the UDP protocol) just deliver the courier to the place designated by the customer, regardless of whether the courier is delivered to the customer.
5. Network layer
The network layer is responsible for transferring data to the destination address. The destination address can be an address formed by connecting multiple networks through routers. Therefore, this layer is mainly responsible for addressing and routing. It is mainly composed of two protocols, IP and ICMP.
Translated into "human words":
Equivalent to a route planner. The express company has many distribution centers (routers), and according to the situation of the distribution center (whether it is congested), it finds the most suitable path to transport the goods (data) along the way.
6. Data Link Layer
This layer is responsible for the transmission of communication between nodes interconnected on the physical level. For example, communication between two nodes connected to 1 Ethernet. Common protocols include HDLC, PPP, SLIP, etc.
Translated into "human words":
Equivalent to the driver. They drive cars and transport packaged couriers (data frames) from one city (physical node) to another.
7. Physical layer
The physical layer is responsible for the exchange between 0, 1 bit stream (0, 1 sequence), voltage level, and light flickering. Typical protocols include RS 232C, RS 449/422/423, V.24, X.21, X.21bis, etc.
Translated into "human words":
Equivalent to transportation . Such as roads, cars and airplanes, etc., carrying goods (data) transportation.
