TCP/IP network explaining: OSI Seven layer model (lesson two)

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Content Introduction

1. Preface

2. Part One lesson two: the creation of the Internet, OSI seven-layer model

3, the first part of the third lesson trailer: OSI first layer, connect your machine

Preface

PS: After doing the syllabus yesterday, I found the pit dug a bit large. But since digging, there is no reason to jump. A man is a yiyanjichu, and a horse is hard to chase.

Today we are going to learn: The OSI seven layer model

Yesterday, the world has 1.8 billion netizens, the scale of the Internet is beyond imagination.

So here's the question:

How do we communicate between so many machines?

How can we not lose direction in this maze of data?

Let's study together in order to find the answer to the question.

First, let's understand how the Internet is created, and what specifications are used to organize so much information well.

Part One Lesson II: the creation of the Internet, OSI seven-layer model

Imagine if you could communicate at any time and communicate with anyone, wouldn't it be great. This is the purpose of the Internet.

Usually, when we are in a group of 10 people, we feel a little difficult to express our views. If it is 100 people, it is more difficult. If there are 1000 people, you can hardly hear your words. But the internet is going to be impossible to challenge, to be able to: at any moment, while communicating with anyone. Of course, to achieve this goal, it is necessary to establish a complex communication system, so that each machine can talk to each other.

But how is such a communication model structured?

We can begin to understand from the most familiar means of communication.

To take stock of the general way of communication:

• Speak

• Call

• Mail

• Flying Pigeon Biography Book →_→

• Wait a minute

Next, let's try to understand what the basic elements of these communication methods are essential to our communications activities.

The way we speak, we need:

• Speaker

• Obedient person

• Sound transmission medium (AIR)

The way to call is similar to talking, except that we need an intermediary between speaking and electronics. In fact, we have to turn the sound of the voice into an electrical signal, and the electrical signal arrives at the receiver's end by means of a transmission medium such as a telephone line, and then the sound is heard. Here is a package of information.

We can give an example of the message to look at this package information system, in the mail send and receive we need:

• Sender

• Recipient

• Transmission Media (letter)

• Container (envelope)

• Intermediary (Post Office)
  

is not slowly able to understand what we need to communicate the conditions.

Here's the problem:

Can we apply this to computers as well?

How can we get people to send messages at the same time?

Can we instantly communicate with someone on the other side of the globe?

Let's take a look at how the pioneers of the network "apply" the principles of human communication to computer communication.

They have reorganized the results of all the research and created a standard that all people connected to the Internet have to abide by: The OSI seven layer model.

OSI seven layer model, does that sound handsome? This standard was born in 1984, only 3 years earlier than I was born (accidentally exposed to their age →_→).

The smart reader may have noticed that the standard is a long way behind the advent of the Internet.

The reason is simple: the OSI seven-layer model is born after people have experience communicating with many machines. This standard is therefore applicable not only to existing communications, but also to future communications and their possible evolution. Its aim is to standardize communications to ensure that the machine maximizes its evolutionary potential and the ability to work together.

Sounds great, but what exactly is the OSI seven layer model?

The OSI seven-layer model is a standard that specifies how machines (primarily computers) communicate with each other. So if you want to communicate with your dishwasher and washing machine, you need to follow the OSI model or at least get inspired by the OSI model. This means that you need to follow a layered approach to communication.

What is the layer here?

The OSI model is a layered model, that is, it is divided into several parts, each of which is called a layer, each layer plays a fixed role, as seen in:

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OSI seven-layer model

, we can see that the OSI has 7 layers, each with a different name.

Why the 7 floor, not the 14 or 250 layers?

Do you remember the previous mention? We know that to communicate, we need a fixed number of components, such as sender, receiver, language, and so on. The pioneers of the network designed the main elements for building communication systems, and they found that the number was set at 7 most appropriate. So each layer of the OSI has a specific division of labor, and the 7-layer combination allows one machine to communicate with the other.

Let's take a look at the details of each of the 7 layers.

1th layer or physical layer

• Name: Physical Layer

• Role: Providing media for communication

• Auxiliary function: No

• Related devices: hubs (i.e., the often-said hub)

Layer 2nd or Data link layer

• Name: Data Link Layer

• Function: Make the machine in the LAN connect with each other

• Auxiliary action: Detect transmission errors

• Related devices: Switches (that is, often called switch)

Layer 3rd or network layer

• Name: Network layer

• Role: Make each network connected to each other

• Auxiliary action: Splitting the transmitted packets

• Related equipment: Routers (that is, often said router)

4th Layer or Transport layer

• Name: Transport Layer

• Role: Managing Connections for Applications

• Auxiliary function: Ensure the effective establishment of the connection

• Related equipment: No

5th level or Session Layer

• We don't care!

Yes, you're right: we don't care about the 5th floor.

Above the 4th floor, we don't care except for the 7th floor. Of course, I'm a little exaggerated, too. But layers 5th and 6th are not covered by this course.

The reason is simple:

The OSI seven-layer model is a theoretical model, and the actual model of today's Internet use is the TCP/IP model, and the latter do not use layers 5th and 6th of the OSI, so we ignore them.

Well, I think you should understand. Of course, the 7th level we need to consider. In fact, it was for this 7th tier application that we took the trouble to build this model. Yes, the 7th floor is Mister, you have to wait, is so capricious →_→

Layer 7th or Application layer:

• Name: Application Layer

• function: No

• Auxiliary function: No

• Related equipment: agent (often said proxy)

What the? The seventh floor has no effect! What's the "ministering" thing? To be blunt, this layer shows all the applications we need to serve to build a communications architecture.

Therefore, we do not do more research on the seventh floor, but focus on the 第1-4 layer that provides services and directs data transfer. In fact, there is another way to say that the 1-4 layer is called the "network layer." Because it is the four layers that are responsible for directing data from one machine to another to be used by applications belonging to the 7th tier of the machine.

Before delving into these four layers, it is important to mention the two principles of using the OSI model:

1. Each layer is relatively independent

2. Each layer can only communicate with adjacent layers

"Each layer is relatively independent":

The effect is that the information used in one layer cannot be used by another layer. For example, for some readers with certain network knowledge, it should be known that the IP address belongs to the 3rd tier, and cannot be used by another layer.

The benefit of such a provision is that it facilitates the transformation of the means of communication.

Imagine that today you use the Internet, although you may not understand its rationale, but in fact you are using the 3rd layer of IPv4 (Internet Protocol Version 4, Fourth Generation Internet Protocol) this Protocol. In the future, we will evolve from IPv4 to IPV6 (sixth generation Internet Protocol, which can reach 128-bit address space compared to the fourth generation of 32-bit address space). At this time, if we in addition to the 3rd layer, in other layers also used in the IPV4, then we have to change the 3rd layer of the agreement, we also have to change the other commonly used in the IPV4 protocol layer, it will be very troublesome.

Maintaining the relative independence of each layer allows us to change one layer of the associated protocol without changing the other layers.

"Each layer can only communicate with adjacent layers":

To understand this principle, we have to understand how machines use the OSI model to communicate.

Imagine the following scenario:

You sit in front of your computer and open your browser. You enter a URL in the browser's address bar, and that site appears.

Although you don't know it, you're using the OSI model.

In general, the application on the 7th level of the OSI (here is the browser) is talking to the 第1-4 layer (collectively, the "Network layer") so that the 4 layers transmit the information that the application on the machine needs from the remote machine (such as the Google.com server).

In a transfer process, we cross the layers of the OSI model, from top to bottom.

Thus, thanks to the 2nd principle, we can guarantee that in the process of information transmission, the OSI model of the various layers will be spanned, but also ensure that the role of each layer can be achieved. Of course, that's pretty much it. Because the OSI model is actually a theoretical model for the Internet, the 7th layer actually skips 5, 6 layers and communicates directly with the bottom 4 layers. So 1, 2, 7, 4, 3.

Well, today's course is here.

Summarize the points of knowledge you need to remember:

• The OSI seven layer model is a specification that regulates how machines communicate between

• The OSI model is a theoretical model, and the Internet is actually using the TCP/IP model

• The OSI model has 7 layers

• Each layer has a specific function.

• The 第1-4 layer is called the network layer.

• Each layer is relatively independent
  

• Each layer can only communicate with adjacent layers

• When transmitting data, we traverse from the top layer of the OSI to the lowest layer, and on the other machine there is the opposite direction, from the lowest to the top

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the first part of the third lesson trailer: OSI first layer, connect your machine

You should now have a general understanding of the OSI seven layer model.

In the next lesson, we'll delve into the details of the OSI layer 1th (physical layer).

See you tomorrow!

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TCP/IP network explaining: OSI Seven layer model (lesson two)