Simplex, half duplex, full duplex, TDD and FDD

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As a student of communication , I still don't understand this concept , It's a disgrace ！ Now to sum up , Understanding is relatively shallow , Most of them are checked online , There is something wrong , Please criticize and correct me ！

Simplex , Half duplex , Full duplex difference

1. Simplex Single work means A It can only signal , and B Can only receive signals , Communication is unidirectional , Like a lighthouse to a ship —— Lighthouses send out light signals, and ships can only receive signals to make sure they are on the right course .
2. Half duplex It refers to that only one action occurs in a time period , A simple example , One day the narrow road , Only one car can pass at the same time , When there are two cars driving opposite at present , In this case, only one car can pass first , Wait until after the boss before another one drives , This example vividly illustrates the principle of half duplex . Early walkie talkies 、 As well as early hubs and other equipment are half duplex products . With the development of Technology , Semi double trade unions gradually withdrew from the stage of history .
3. full duplex Full－Duplex Transmissions finger Switch It can receive data while sending data , The two go hand in hand , It's like we usually call , While speaking, you can also hear each other's voice . Current switches support full duplex .

Full duplex Ethernet For full duplex Ethernet ,IEEE Made it 802.3x full duplex / Flow control standard , The standard makes specific provisions on the flow control mechanism in full duplex mode . In each ether standard （10/100/1000 Base） in , except 100 Base T4 outside , All have full duplex capability , But in practice , Seems to be only Gb The etheric （ Gigabit Ethernet ） Only use full duplex . 　　 Ethernet MAC Agreement is CSMA/CD, But in full duplex Ethernet, conflict detection is not required （CD） Of . This enables Gb Etheric breakthrough 40 The segment length limit of more than meters （ To be more precise, it is 41.2m, This data can be based on IEEE The limitation of the timing rule is calculated , I won't go into details here ）. In practical applications, if you need a site in the network to work in full duplex , You must install a network card that supports full duplex at this site , And it is required to connect with full duplex stations HUB/ Routers and other networking equipment are equipped with full duplex ports . So it looks like , If you want to work in full duplex , First of all, there must be hardware support . The main advantage of full duplex Ethernet is that it can simultaneously realize data transmission in two directions on two independent channels , So as to improve the total bandwidth of the link , So it It is only applicable to file servers that need two-way data transmission at the same time The site of . about Generally, only one-way data transmission is carried out The site of , Full duplex Ethernet has no advantage , Therefore, full duplex Ethernet has great limitations in application . Full duplex Ethernet is mainly used in the occasion of switch interconnection , In especial Gb Ethernet switch . Win2K in , The network card and duplex related settings are Local connection – attribute – To configure – senior There is one item in the list “Link Speed & Duplex”, Its value is generally “Auto Detect”. It is recommended not to make changes . There are... On the switch Duplex The lamp , If it is on, it means it works in duplex mode . at present , Most switches can automatically recognize and support duplex mode , There is no need to set it manually .

Full Duplex

Britain [ˈdju:pleks]

serial communication In serial communication , Data is usually in two stations ( Such as terminals and microcomputers ) Transfer between , According to the direction of data flow, it can be divided into three basic transmission modes : full duplex 、 Half duplex 、 And simplex . But single work is rarely used at present , Only the first two methods are introduced below .

1. Full duplex mode ( full duplex ) When the transmission and reception of data are shunted , By Two different transmission lines During transmission , Both sides can communicate in At the same time Send and receive , This way of transmission is full duplex . In full duplex mode , Of communication system Transmitter and receiver are set at each end , therefore , It can control the data transmission in two directions at the same time . Full duplex mode does not need direction switching , therefore , There is no time delay caused by switching operation , This is for interactive applications that cannot have time delays ( For example, remote monitoring and control system ) It's very advantageous . This method requires both sides of communication to have transmitters and receivers , meanwhile , need 2 The root data line transmits data signals .( You may also need control lines and status lines , And ground wire ).

give an example ：

such as , The computer host connects the display terminal with a serial interface , The display terminal has a keyboard . such , On the one hand, the characters entered on the keyboard are sent to the host memory ; On the other hand , Host memory information can be sent to the screen display . Usually , Type 1 After a character , Don't show , After the computer host receives the character , Send back to the terminal immediately , Then the terminal displays this character . such , The loopback process of the previous character and the input process of the next character are carried out at the same time , That is, it works in full duplex .

（ This example is fairly good , It is better to take the example of communication .）

1. Half duplex ( half duplex )

If you use Same transmission line Both receive and send , Although data can be transmitted in two directions , But the two sides of communication can't send and receive data at the same time , This way of transmission is half duplex . In half duplex mode , Transmitters and receivers at each end of the communication system , By receiving / Switch to the communication line , Switch directions , therefore , There will be a time delay . closed / The hair switch is actually an electronic switch controlled by software .

When the computer host connects the display terminal with serial interface , In half duplex mode , The input process and output process use the same path . Some computers and display terminals work in half duplex mode , At this time , The characters entered from the keyboard are sent to the host and displayed on the terminal at the same time , Instead of sending back , Therefore, the situation that the receiving process and the sending process are carried out at the same time is avoided .

At present, most terminals and serial interfaces provide commutation capability for half duplex mode , It also provides two independent pins for full duplex mode . In actual use , Generally, it is not necessary for both sides of the communication to send and receive at the same time , One way transmission devices like printers , Half duplex or even simplex can be competent , There is no need to go backwards .

In full duplex mode , The maximum network cable can reach 100 A speed of more than megabytes ; In half duplex mode , The speed that the network cable can reach is usually 10 About trillion .

The network cable in full duplex mode can work in full duplex mode , It can also work in half duplex mode ; But the network cable in half duplex mode cannot work in full duplex mode .

In half duplex mode , The extra two groups of wires can be used to connect telephone lines , such as ： hold “ White and blue 、 blue ” This group of lines is separated and used as telephone lines . In LAN network cabling , Wiring costs can be saved in this way , A twisted pair is used as both a network line and a telephone line .

TDD and FDD difference

1. What is? 4G/LTE-FDD/LTE-TDD 4G Hence the name Siyi is the fourth generation mobile phone mobile communication standard , It refers to the fourth generation mobile communication technology , Include TD-LTE and FDD-LTE Two systems . So what we're talking about LTE What do you mean ？LTE Long term evolution technology , It's English Long Term Evolution Abbreviation .LTE Technically, it is considered 3.9G. But we usually call them 4G, Because they have 100Mbps Data download capability ( finger cat3 downstream bandwidth ), It's now 3G Internet speed 10 About times . It's also 3G To technology 4G The key process of evolution . LTE Standard by TDD and FDD Two different duplex modes ,TDD Represents time division duplex , That is to say, the uplink and downlink cross in the same frequency band according to time allocation ; and FDD The uplink and downlink are divided into different frequency bands at the same time . Although these two systems are nominally made up of TD-SCDMA and WCDMA Evolved from , But actually LTE( Include TDD and FDD) It's using OFDM( Orthogonal frequency division multiplexing ) Mode modulation downlink ,SC-OFDM( Single carrier orthogonal frequency division multiplexing ). This has been compared with 3G The standards of the times are different , What it inherits , You can even think of it as just two duplex modes, time division and frequency division . meanwhile , These two systems , Have adopted MIMO( Send more and receive more ) Antenna technology , And also can support 1.4、3、5、10、15 and 20 MHz Signal bandwidth , Support the reuse of used frequency resources . 　　 in general ,LTE It has high-speed uplink and downlink , Efficient spectrum utilization , The system layout is flexible , Advantages of lower wireless access delay . Its performance is , Download fast , The signal is good , Play a game ping It's low , In some occasions, you don't need to pull wired broadband .
2. LTE-FDD and LTE-TDD The difference between The differences between these two systems , Their respective advantages and disadvantages lie in ,TDD Because the uplink and downlink are on the same frequency band , So we can make better use of spectrum resources , Easier to arrange ; and FDD Because the uplink and downlink are carried out simultaneously in different frequency bands , act as one pleases , So the data transmission capacity is stronger , But it also has higher requirements for spectrum resources ! In today's increasingly tight spectrum resources , This is a painful thing for all operators . If FDD Theoretically, the downward speed will be faster , This makes many potential 4G Users are excited , And determined to use Unicom LTE-FDD, Xiaobian keeps silent here , After all FDD No licensing business . In fact, a network is good or bad , It's not just theoretical speed , It also depends on the quality of coverage , User carrying capacity , Actual transmission speed, etc . After all, the ideal is full , Reality is backbone . After large-scale commercial use , Large amount of users , The actual downlink bandwidth is estimated to be half the weight , No matter how wide the road is, there will be peak hours in the morning and evening , Same thing , No matter how wide the bandwidth is, it can't afford large-scale seed downloads . 　　 For now , The Ministry of industry and information technology assigned to China Mobile 130Mhz Of TDD-LTE Bandwidth usage , It can cover more mobile customers , Provide better access services , The mobile layout strategy is low transmission power , High coverage density , The RF modules they use have also changed from the original 8 The antenna , Daily line 5W, To the present 8 The antenna , Daily line 20W. At least at this stage, the performance is awesome ,2014 During the Spring Festival , Many small counties already have mobile 4G Network . From the current situation , China Mobile in the future 4G Competition is not at a disadvantage .
3. When to use FDD-LTE

Now both Chinatelecom and China Unicom have announced 4G Data business commercial , And China Unicom now nominally TD-LTE commercial , In fact, it will be adopted in hot areas TDD Cover , Technically speaking , China Unicom only needs to upgrade the existing base station module to easily transition to FDD, at present ,FDD The license plate is delayed , Industry guesses , Mainly to balance the competition pattern of the three operators , Give the Chinese their own 4G standard TD-LTE Enough room for development .

however ,FDD After all, it is the most mature in the world 4G standard , With 4G Full commercial launch ,FDD It is imperative to play cards , The Ministry of industry and information technology may officially issue FDD License plate . With VOLTE Maturing , Original 2G/3G Voice carrying capacity is not so important , Then operators will increase LTE Network coverage , And gradually shut down 2G/3G network , In order to reduce operation and maintenance costs , The frequency band it occupies may be released with the shutdown of the network , Even there will be only LTE The situation of the network . For the foreseeable future 1-2 In the year ,LTE It will enter a period of rapid development in the world , And in the 3-5 After year ,LTE The network will enter a mature period , And promote a more efficient network system . actually , Some of the current domestic sales 4G Terminals have supported FDD-LTE The Internet , China Mobile also announced that it would purchase 4G The terminal standard is upgraded to five modes and ten frequencies , It can be said that technically it is no longer China FDD-LTE Pattern 4G Network stumbling block , Users and operators just need to wait quietly for the official issuance of the Ministry of industry and information technology FDD-LTE License plate , a “ Everything is available. , I only owe the license plate ”

4G License plate

4G License plate is the first combination of wireless communication and multimedia communication such as Internet 4 Mobile communication technology （4G） Business License , It is licensed by the Ministry of industry and information technology of the people's Republic of China .[1-2] 2013 year 12 month 4 The Ministry of industry and information technology of the people's Republic of China officially released 4G License plate , China Mobile 、 China Telecom and China Unicom both obtained TD-LTE License plate [2] . 2015 year 2 month 27 Japan , The Ministry of industry and information technology issued “LTE/ The fourth generation digital cellular mobile communication service （FDD-LTE）” Business license .（ See Baidu Encyclopedia ）

About 4G technology , Experts agree ,4G It is a new generation of mobile communication technology , It is the trend of technological evolution , The future will replace 3G technology .4G relative 2G、3G, The most outstanding feature is fast .4G It has the function of high-speed data communication . From the theoretical data ,4G It's better than 3G Much faster .4G Support 100-150Mbps The downlink network bandwidth . It is the third generation mobile communication network of China Mobile TD-SCDMA Of 35 times , It is the third generation mobile communication network of China Unicom WCDMA Of 14 times .

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