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1 Why stratification?

2. Three systems

  • 2.1 OSI seven-tier architecture

  • 2.2 TCP/IP Four-layer system

  • 2.3 Five-layer system structure of principle teaching

1 Why stratification?

Before we look at the three computer architectures, we should ask one question: Why layering? To answer this question, we need to understand our requirements for host-to-host communication.

If you are a designer, how should you think about communication before implementing two consoles?

First, let’s take host A sending A file to host B as an example. We need to consider the following questions:

  • How do I find host B?
  • How to establish a communication connection with host B?
  • In what form should information be transmitted?
  • On what carrier should information be transmitted?

And so on and so forth. Obviously, this is a very complex task, so we need to use the thinking of abstraction and disassembly. The task can be roughly divided into the following three abstract modules, as shown in the figure:

The responsibilities of each module are as follows:

  • File transfer module: responsible for file sending and receiving business logic and command delivery;
  • Communication service module: responsible for reliable data exchange between host A and host B;
  • Network access module: responsible for finding host B and sending data to host B correctly;

Therefore, for a complex network communication task, layered implementation has the following advantages:

  • Independent decoupling
  • flexible
  • Easy to implement and maintain
  • Facilitate standardization and so on.

This results in the following three architectures:

  • The seven-layer ISO(Open Systems Interconnection) architecture proposed by the Network Expert Group;
  • Four-tier TCP/IP architecture implemented in real industry;
  • Understand the principles and teaching use of the five-tier architecture;

Let’s take a look at these three architectures.

2 Three architectures

Let’s start with a comparison of the three architectures, as follows:

2.1 OSI seven-tier architecture

The Open Systems Interconnection Model(OSI), short for International Organization for Standardization (ISO), National Standards Organization) in 1984.

The OSI model advocates a network of interconnected systems divided into seven layers as shown in the figure above, each with a different role to play.

However, due to the complexity of the hierarchy (not the more the better), it has not been implemented in the industry. Instead, the much simpler TCP/IP four-tier architecture has been widely adopted in the industry (which tells us that the industry standards have to be implemented based on the reality).

2.2 TCP/IP Four-layer system

In the industry, many network companies actually landed, in fact, is the TCP/IP four-tier architecture.

Despite its name, the TCP/IP architecture includes more than just these two protocols.

Its four layers are divided from top to bottom: application layer, transportation layer, Internet layer (not called network layer, to emphasize that the layer is to solve the problem of interconnection of different networks, actually can also be called network layer), and network interface layer (this layer has no actual content).

Since the TCP/IP architecture has no actual content in the network interface layer, we basically fill the bottom two layers of OSI into the TCP/IP model in the actual learning, forming a five-layer teaching model.

2.3 Five-layer system structure of principle teaching

Next, taking the five-layer teaching model as an example, let’s briefly talk about the main functions and responsibilities of each layer.

If you look at it from the bottom up, the bottom layer serves the top layer, the top application layer describes what we want to do, and the bottom four layers implement it step by step. The services provided by each layer are as follows:

  • Physical layer: provides transmission bitstream service among various transmission media between computers;
  • Data link layer: provides correct and reliable data frame transmission service in different links.
  • Network layer: provides routing and communication services for different hosts on packet switched network.
  • Transport layer: in different host processes, provide reuse and reuse of universal data transmission services;
  • Application layer: provides a rule service for interprocess communication and exchange between different host application processes.

It doesn’t matter if you don’t understand exactly how they implement these services, just have this concept of layering.

Over the next few weeks, we will walk through each of the five layers of the principles teaching architecture in detail, understanding their responsibilities, composition, and major protocols.

(The end of this article)

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