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This article mainly introduces the following OSI reference model and its expansion. In the course of learning NA, it is very helpful for us to understand the basic concepts and the basic structure of the network.

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Chapter 1 OSI reference Model

First of all, let's make it clear what is OSI?

Let's describe it in two words: standards and specifications are compatible.

OSI (Open System Interconnect), that is, open systems interconnection. ISO Global Standardization Organization, the standard-compliant feature is that it can be exported and imported and is available all over the world. All those who meet the ISO system can be globalized.

Generally called OSI reference model, it is the standard of network communication. This architecture standard defines the seven-layer framework of network interconnection.

1. Physical layer physical layer

two。 Data link layer data-link layer

3. Network layer network layer

4. Transport layer tranport layer

5. Session layer session layer

6. Presentation layer presentation layer

7. Application level application layer

It is the reference model of OSI open systems interconnection. Under this framework, the functions of each layer are further defined in detail to achieve interconnectivity, interoperability and application portability in the open system environment.

1. Physical layer physical layer

The physical physical universe is born and cannot be changed.

The signal is divided into analog signal oscilloscope.

Digital signal 0101010101

Physical media determine the type of signal

1. Telephone line 2. Coaxial cable (closed line) 3 optical fiber 4. Wireless. These four media all transmit analog signals.

Digital signal can only be transmitted by network cable.

A signal conversion device modem (a digital demodulator operating on the physical layer) converts an analog signal into a digital signal.

For example, for the broadband we apply to the operator, we need an intermediate conversion device, the modem, which receives an analog signal from the operator and converts it into a digital signal for transmission over the network cable.

(phone lines are all crystal head interface convex phone line analog signal is dual-core RJ-11 network line is eight-core RJ-45)

The maximum transmission distance of the network cable is 100m (theoretically) the transmission distance of the digital signal is 100m.

At present, many people at home are equipped with: all-in-one machine = modem + wireless router. It can not only convert signals, but also realize wireless Internet access.

Signal amplifying equipment (amplifying the digital signal, in order to connect the network cable longer) Repeater repeater 54 principle 4 Repeater 5 segments (five network cables)

The hub is a repeater with multiple ports, and it can also amplify the signal. It must meet the 5-4 principle. The hub is a product of shared collision domain. The principle of the hub is CSMA/CD carrier sense multiple access / collision detection.

Here let's learn what a network is and what a computer network is.

Network and artificial intelligence are popular at present, which are widely used in manufacturing industry, automation and so on. For example, the automatic job on the pipeline is realized through the automation system, and the automation system is in a server, through the network to obtain the data of the equipment on the pipeline and make it run sequentially.

Network we refer to two more nouns, terminal (ednstation) and client (client).

Terminal: any hardware that can be connected to the Internet. (early computers had no concept of terminals and no network cards.) for example, mobile phones, computers, cameras installed in schools

Client: software that can be connected to the Internet, for example, QQ,WeChat, some online game software

Server side: a server is a computer system in a network that can provide certain services to other machines.

For example, the client must face the server (server), and all the data on the client must pass through the server.

For example, in Didi Taxi, users need to send data to the server through the network, and the server matches the vehicle for it.

In an example, CS, CF two games, CS can be stand-alone, can also be played in the local area network, but through the platform, and is in the local area network, can not be networked to fight. But CS is also a client, and it also counts through the local area network.

Here is a supplementary knowledge, network card, the computer must have a network card to surf the Internet.

There are two types of network cards.

Wired ethernet Ethernet, wireless wireless

There are four types of Ethernet:

Ethernet 10Mb, Fast Ethernet 100m, Gigabit Ethernet (Gigabit Ethernet) 1Gb, 10 Gigabit Ethernet 10Gb

Rate unit network transmission rate b bits 0101010101010

The smallest unit of B bytes bytes for file storage

1B = 8b one byte equals 8 bits B=bytes b=bits bit

In our daily life, why doesn't the 10m broadband we apply for exceed 1.25m?

If it is 100m, the broadband will not exceed 12.5m.

Because the smallest unit of file transfer is bytes, while network transmission is 010101 of the bits, a 0 is one bit, and 01 is two bits. Then the relationship between bytes and bits is 1 byte is 8 bits, 1B=8b.

100m network speed, downloading files in the network, the peak download speed per second is 100max 8mm 12.5, so the fastest download speed of 100m broadband is 12.5M/Bs.

(add: the conversion between file stores is adjacent to a unit of 1RV 1024, for example, 1G=1024M

The conversion unit between hardware is 1RV 1000, for example, buying an 8G flash drive will only display more than 7G, because 8G is the unit on the hardware, and the USB disk is inserted into the computer to store files. Because different binary conversion units will cause some of them to be missing, it is 8x1.024, which is the capacity that will finally be displayed on the computer)

The ether is a kind of matter imagined by the ancient Greek scientist Aristotle, which lies between stars. Although it has not yet been confirmed as the medium of light waves, scientists are convinced that this medium must exist.

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