"How to connect the Internet" reading notes - FTTH

brief introduction

FTTH It refers to the access network technology based on optical fiber , The key to this part is to understand how to use optical fiber .

FTTH It is the most popular way to access the Internet in modern times , Although I don't know how the future will change , But at present, optical fiber access will be popular for a very long time .

FTTH What is it? ？

Fiber to home （ English ：Fiber To The Home, abbreviation ：FTTH） It is a transmission method of optical fiber communication . Is to connect the optical fiber directly to the user's home （ Where the user needs ）.

This optical fiber communication mode and strategy are similar to FTTN、FTTC、HFC（Hybrid Fiber Coaxial） And so on , They all rely on traditional metal wires , Including twisted pair and coaxial cable , do “ The last mile ” Information transmission of .

More information ：FTTH

What is optical fiber ？

Optical fiber is the abbreviation of optical fiber , It's a fiber made of glass or plastic , It can be used as a light transmission tool . The transmission principle is “ Total reflection of light ”. and ADSL Complex multi band electrical signals are different , The optical signal is very simple , Just use dark and light 0 and 1 that will do , The specific transmission mode is to use high-voltage bright light and low-voltage dark light through the characteristics of the material , At the receiving end, different voltages are determined by the light-sensitive group sensing the light change , The principle of optical fiber communication can be seen in the following diagram ：

Single mode and multimode

The most difficult and characteristic of optical fiber design is Let the light enter at a certain angle , Phase change due to reflection , If the optical fibers with inconsistent phase converge, interference will occur , This phenomenon is similar to water waves , If two water waves collide with each other, the final result is mutual cancellation .

Therefore, the material of fiber core in optical fiber material is very important , Through the diameter of the optical fiber can be divided into single-mode and multimode , Single mode fiber core is very thin , Only light with a small incident angle can enter , Multimode is just the opposite , It will be coarser and light with a larger incident angle can also enter , But the main difference between single-mode and multi-mode is only how many phases are consistent at the same angle and how many can enter .

The difference between single-mode and multi-mode is in the distortion control of transmission , Because multimode can enter more light , So the material requirements can be slightly lower , But the signal distortion will become stronger , The single mode only transmits one light , Need more expensive materials to support , But the smaller the probability of signal distortion .

Finally, there is an unavoidable common problem with optical fiber , That is, the longer the line, the greater the distortion , This is a problem that no material can avoid .

Optical fiber access mode

Optical fiber replaces ADSL The way to access the Internet is not called ADSL, The main access methods are direct connection and shunting .

1. The first way is to connect the client to the nearest telephone office , This access method is also from ADSL Improved on , It's just an electrical signal replaced by an optical signal , At the same time, there is no need to MAC Network packets are converted into cells, and Ethernet packets are directly converted into optical signal transmission , In addition, the same optical fiber is used for transmission , For this reason, the mixing of uplink signal and downlink transmission will be divided by the principle of prism , Simply put, uplink and downlink signals use different wavelengths of light , In this way, it can be transmitted normally even if it is mixed . In addition to optical fiber instead of ADSL Beyond transmission , The overall transmission process and ADSL Broadly similar .

Wavelength division multiplexing ： Transmitting multiple optical signals using different wavelengths in an optical fiber .

1. The second way is to install a beam splitter on the pole near the user , This is the way we often see at present , This method is accessed by multiple users , The client does not use optical transceiver, but through ONU Replacement of equipment , When data is sent to BAS valuable , Will pass OLT Device receiving .OLT and ONU It has the function of avoiding collision by adjusting the timing of signal transmission and reception , At the same time, because multiple users are connected to a beam splitter , In order to prevent signal errors from being transmitted to other users to disclose privacy , The beam splitter will be added to different access terminals ONU identification , Finally, it is transformed into corresponding Ethernet signal transmission .

ONU What is it? ？

ONU Namely Optical Network Unit Optical network unit .ONU It has two functions ： Yes OLT The broadcast sent is selectively received , If you need to receive the data, you have to OLT Make a receive response ; Collect and cache the Ethernet data that users need to send , According to the assigned send window to OLT The client sends the cached data .

stay FTTx Different deployment of the network ONU There are also different ways to access , for example FTTC（Fiber To The Curb）：ONU Placed in the central computer room of the community ;FTTB（Fiber To The Building）：ONU The junction box in the corridor ;FTTH（Fiber To The Home）：ONU Put it in the home user .

History of optical fiber

History of last complementary fiber , We may mistakenly think that optical fiber has developed with the development of computers , In fact, there has been relevant history as early as the telegraph era .

1880 year , Alexander · bell Alexander Graham Bell Invented “ Optical telephone set ”.

1887 year , British scientists Charles Vernon Boys The first optical fiber was pulled out in the laboratory .

1938 year , The United States Owens Illinois Glass The company and Japan Nitto Textile Co., Ltd. began to produce long glass fiber .1951 year , Photophysicists Brian O’Brian The concept of envelope is proposed .

1956 year , A student at the University of Michigan made the first glass clad fiber , He used a glass tube with low refractive index to melt it into a glass rod with high refractive index .

1960 year ,Theodore Maiman Showing people the first laser . This has aroused people's interest in optical communication , Laser seems to be a promising means of communication , It can solve the problem of transmission bandwidth , Many laboratories have started experiments .

1966 year , Kao Kun, a British Chinese scholar, pointed out the possibility and technical approach of using optical fiber for information transmission , Laid a foundation for modern optical communication —— The foundation of optical fiber communication .

1970 year , Corning, USA (Corning) The company has successfully developed the loss 20dB/km Quartz fiber .1973 year , American Bell (Bell) The laboratory has made greater achievements , Fiber loss is reduced to 2.5dB/km.1976 year , Nippon Telegraph and telephone (NTT) The company reduces the optical fiber loss to 0.47 dB/km( wavelength 1.2μm).

It can be seen that there are 100 Years of history , It's not like a sudden technological explosion like a computer , From the whole historical timeline, we can see that Gao Kun is actually , stay 1966 year , Mr. Kao Kun first proposed that when the attenuation rate of glass fiber is lower than 20dB/km when , Optical fiber communication can be successful .

Because his discovery is a real person who applies theory to the transformation of reality , He also won the Nobel Prize directly for optical fiber , It has a far-reaching impact on the development of human society .

Summary

FTTH The access method of is actually right ADSL Way to an improvement , The main point is to use optical fiber materials to replace the original telephone line transmission , However, it should be noted that although optical fiber can use optical signals for transmission , But in the end, it still needs to be converted into an electrical signal to complete Internet communication .