Layering of Computer Networks: Architecture with Five Layers of Protocols

First consider a question, why does a computer network adopt a hierarchical structure, and what are the benefits of doing so. The network adopts a hierarchical structure because it is very troublesome for computers to communicate with each other. There are many things that need to be considered. When using a hierarchical structure, in order to divide a large and complex problem into several small local problems, Then it will be simpler and easier to study local problems. In real life, there are many hierarchical cases, such as platoons, companies, battalions, etc. in the army, such as the personnel structure of an enterprise: boss, general manager, department manager, and employees.

ISO/OSI and TCP/IP architecture

In order to promote the development of the world's computer network, the ISO organization proposed a well-known system standard OSI. However, OSI has only achieved some theoretical results, and what is really widely used in reality is the TCP/IP architecture.

OSI has a clear concept and complete theory, but it is neither complex nor practical. TCP/IP has a four-layer architecture, which is divided into application layer, transport layer, Internet layer and network interface layer. But in fact, it only has the upper three layers, and the bottom network interface layer has no content. Therefore, we generally adopt a compromise approach when studying, that is, integrating the advantages of OSI and TCP/IP and adopting a five-layer protocol architecture.

Five-layer protocol architecture

The protocols from bottom to top are: physical layer, data link layer, network layer, transport layer, and application layer.

Physical Layer: The physical layer considers how to propagate the data bit stream on the communication link connecting the computer. What it needs to consider are mechanical characteristics, electrical characteristics, etc. Hubs work at the physical layer.

Data link layer: This layer mainly considers how to communicate between hosts on the same network. Our common MAC address acts on this layer. The switch works at the data link layer, which has a commonly used ARP address resolution protocol.

Network layer: The network layer is responsible for providing communication for hosts on different networks. The device that works in the network layer is a router. A router is a special host that accepts packets transmitted by the host, then looks up its own routing table and forwards the stored packets to another network. Common protocols at the network layer include IP and ICMP protocols.

Transport layer: The transport layer is responsible for communication between two host processes. This layer has the famous TCP protocol. In addition, the UDP protocol also works on this layer.

Application layer: The task of the application layer is to complete specific network applications through interaction between application processes. There are many protocols working at the application layer, such as http, ftp, ssh, dns, and https.

To learn computer networks, we need to focus on the three layers of data link layer, network layer and transport layer

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