What are the characteristics of digital communication?

The essence of a computer network is that computers communicate with each other, so the most important function of a computer network is computer communication, and since computers themselves deal with numbers, a computer network is actually a kind of digital communication. It is due to the emergence of computer networks that digital communication has become a widely used means of communication.

What are the benefits of digital communication? Why must digital communications rely on computer networks to be fully realized? For this reason, it is necessary to start talking about analog communications.

Communication and analog communication

The so-called communication is actually the transmission of information from one place to another. Animals have been sending messages to each other through "sound language," "behavioral language," and "scent language" since long before humans came along. You may have seen cute little bees dancing in the air, but they are actually communicating with each other, and many of you may have heard of the bee's "8" dance when you were a child, which is to tell its partner: "Not far from here, there are lots and lots of nectar.

After the emergence of human beings, the means of communication became more colorful. In ancient times, the Chinese learned to use beacons to send messages, the so-called "beacons to spread the war, the geese send letters home. In ancient China, the battlefield is to drums and gongs as a number, is to beat the drum is to enter, the sound of gold is to retreat.

When electrical communication appeared, mankind broke out of the closed and sluggish, towards openness, efficiency and civilization. 1831, Faraday discovered the law of electromagnetic induction. 1837, Morse used this law to invent the Morse Telegraph, and in 1844 between Washington and Baltimore opened the earliest telegraphic communication. 1876 Alexander Graham Bell of the United States invented the telephone. 1894, the telephone was invented in the United States. In 1894, Marconi of Italy invented the wireless telegraph.

From the invention of the telephone to the development of the present, although the type of variety, all kinds of. But no matter how it develops, it is inseparable from the microphone and handset. When people speak into the microphone, due to the vibration of the sound vibration of the diaphragm, an electrode in the center of the diaphragm also followed by vibration, the current through the microphone with the changes in the voice of the change, become the voice current, which is the microphone to the change of the sound into the change of the current. Listening to the microphone sent under the action of the voice current, the magnetic field is weak and strong, so that the diaphragm changes with the voice current changes, the sound is received by the ear, that is, the microphone to the change of electricity into the change of sound. So, people on the side of the phone to speak, the sound becomes current, through the transmission of telephone lines, sent to the other side of the telephone, and then reduced to sound signals, the other side will be able to hear this side of the person's speech.

The telephone is a kind of analog communication, that is, the change of electric current to simulate the change of sound, to express the original message. Currently generally watched television, is also a kind of analog communication, by the television camera output of the television signal, which changes to simulate the intensity and color of the light reflected by the scene being photographed.

The formation of analog signals is relatively simple, intuitive, but in the transmission process is susceptible to external interference distortion, thereby reducing the quality of communication; digital communication is another communication method different from analog communication. Digital signal transmission, recording, processing are used digital ("0" and "1") signals. Because the digital signal is strong anti-interference, the production of small distortions, but also easy to eliminate, and thus can greatly improve the quality of communication, is the current trend of communication technology.

Characteristics of digital communication

The principle of modern digital communication is to use two symbols, "0" and "1", to transmit information such as data, text, sound and images. In the same way, the original transmission of analog signals to the phone into the "digital" after the transmission of voice can also be used "0" and "1" two symbols, in accordance with certain rules of the arrangement of combinations of The "code" to be transmitted, which is called digital telephone, also known as pulse code communication. This is called digital telephony, also known as pulse code communication. The telephone signal is first digitized into a string of codes similar to those of telegraph signals, and then the codes are transmitted to the other party. When the other party receives the code, it is then restored to the original telephone signal, realizing the purpose of transmitting information.

What are the characteristics of transmission using digital signals?

Since the mid-20th century, digital communications have been increasingly developed, and the trend of digital communications replacing analog communications has begun to emerge. Currently, both analog and digital communications have gained wide application. In terms of the history of communications, although low-level digital communications (telegraph communications) appeared at an early stage, digital communications developed much more slowly than analog communications over a long historical period, and fewer communications devices were actually used than analog communications. Today, analog communication technology has reached such a level of sophistication that it is possible to meet and talk with loved ones who are thousands of miles away as if they were close at hand through existing communication equipment. In addition, the reason for the development of digital communications is that in addition to the characteristics of the digital signal itself, digital communications have many outstanding advantages over analog communications.

1. Digital communications than analog communications anti-jamming ability

We are on the phone, sometimes dialing the other side of the number, the phone can not be reached, only to hear that the occupation of the line "toot, toot ......" sound. This may be the other side is talking to someone else, may also be connected between the two telephone lines are occupied. Because the trunks between the two telephone offices are limited, if many people call at the same time, these trunks are occupied, then the later users will not be able to get through. The greater the number of telephone sets and the more frequently each user uses the telephone, the more telephone trunks are needed. If you want to install additional cables between the two telephone offices, you will be subject to civil engineering constraints, more difficult, the investment is relatively large. Early managed to connect multiple analog phones on a pair of trunks at the same time, but because of the line's high-frequency characteristics are not good, poor anti-jamming ability, the string of words is serious, so the call is not good. From the early 1960s, digital communications began to trial in the phone, as previously described as a simple digital signal waveform, "0", "1" distinction between the distinctive features, so that the digital communications anti-jamming ability is very strong, can be realized in a pair of trunks at the same time connected to the The first step in the process is to make sure that you are able to make the most of your time and money.

With the development of science, communication relay is becoming more and more perfect. In the wired and radio, often used along the way properly installed "intermediate amplifier" to amplify the signal, so that the signal always maintain a certain strength. Signal transmission over a distance will be weakened, and may occur "out of shape". For the transmission of analog signals, although the signal can be amplified to strengthen the signal, but this "sampling" is difficult to completely eliminate, resulting in distortion of the received signal at the receiving end. But for digital signals, the signal is generally only two states, although after a distance transmission, the shape of the waveform at the receiving end of the bad, but we do not have to care about the accuracy of the waveform, as long as you can identify the two states of the digital signal, you can use electronic equipment to have been bad pulse waveform regeneration, back to the original shape of the pulse. Using regeneration, the quality of transmission is almost independent of distance.

2. Digital signals are easier to modulate than analog signals

With the development of production and military needs, the requirements for the transmission of digital information is also growing rapidly. At present, in the long-distance digital transmission, it is not yet possible to completely use direct cable transmission. Here, a very relevant question is whether digital signals can be transmitted using the already established four-way analog circuits? In order to transmit digital signals on analog circuits, digital terminal equipment and analog circuits must be installed between the modulation, demodulation as the main body of the interface equipment, usually called data transmission machine. Since digital signals have only two states, "0" and "1", digital modulation can be interpreted as a process like a telegraph operator controlling a carrier wave by switching on and off the electric key, so digital signal modulation is very simple. This modulation *** there are three kinds of:

Digital AM: refers to the use of digital signals to control a continuous carrier, so that the carrier is intermittent, carrier oscillations, said to send "1" digital, no carrier oscillations, said to send "0" digital. digital. After digital amplitude modulation, the carrier is no longer a simple sine wave, but with the state of the digital signal and change, into a more complex signal.

Digital FM: The main idea is to send a "1" code, the digital signal carrier frequency of f1; send a "0" code, the carrier frequency of f2, through the change in frequency to realize the recognition of the signal.

Digital phase adjustment: that is, in accordance with the digital signal to control the phase of the carrier. What is the phase? For example, there are A, B two people running, if the pace of the two people, the same pace, a command at the same time the two people start, then we take a picture at any time, in the photo of the two people's footsteps are always the same. A raised his legs, B also raised his legs, A fell when B also fell, the rhythm of the action is always the same. In this case, we can say that the two are in the "same phase" state; if a command A immediately start, and B hesitated before starting, then it is possible not to "same phase", may be A lifted his leg, B has already fallen, A fell, B lifted his leg, although the two are in the same state. Although they are running step by step as usual, B's action is always a little later than A. The same goes for signals. Signal is also the same, if the two carriers of the same frequency at the same time began to oscillate, these two frequencies at the same time to reach the positive maximum, at the same time to reach zero, at the same time to reach the negative maximum, they should be in the "same phase" state; if one of them started a little late, it may not be the same. If one reaches a positive maximum value, the other reaches a negative maximum value, it is called "anti-phase". Generally, a signal oscillates once (one week) as 360 degrees. If one wave is half a cycle different from the other, we say that the two waves are 180 degrees out of phase, or inverted. When transmitting digital signals, the "1" code controls the sending of a 0-degree phase, and the "0" code controls the sending of a 180-degree phase. The initial phase of the carrier wave has moved, and also with the information.

The realization of digital modulation is generally accomplished by digital circuits. As a result, it has a waveform transformation speed, easy to adjust the test, small size, equipment reliability and high characteristics. This method is widely used in digital communications.

3. Digital signals than analog signals strong confidentiality

In the clouds through the fog on the aircraft, in the rapid advance of the tanks, in the wind and waves of the warships, to maintain constant contact with the command as well as close coordination between each other, radio communications can be said to be the only method. However, in radio communication, the waves are dispersed everywhere, and not only the other party to the call can receive it, but also other people, just as when a radio station broadcasts, anyone can receive it on the radio. The confidentiality of communication is very important, especially during the war, leakage of information often leads to very serious consequences. After the realization of digital communication, the implementation of encryption measures than analog communication is easy, does not require a lot of complex equipment, as long as the use of simple logical operations can play a role in confidentiality, and the effect is much better than analog communication. The so-called encryption is to contain the voice information of the code according to the code in accordance with certain rules for "plus", "minus" and other logical operations, that is, the code "plus" to the voice code to go In other words, the code will be "added" to the voice code, which makes it an unpredictable code. Secure digital phone in the transmitter will be digitized voice signals after encryption process to transmit out, in the receiving end for decryption, the inverse conversion is restored to voice signals. Even if the enemy intercepted in the air after the encryption of the voice code, a moment can not know the content of the signal, and in their own side of the receiving end can be decrypted back to the original voice signal.

4. Automatic detection and control of errors

Usually people's general mentality is that data transmission in communications is best not to have errors, the more accurate the better. However, in the past, due to the poor characteristics of analog lines and external interference, etc., errors are very likely to occur when transmitting data. In digital communications, error control technology can be used, which can automatically detect errors and correct them immediately, and improve the quality of transmission.

5. Easy to combine with computers

Obviously, digital communication is suitable for combining with digital computers to process signals, which makes the communication system more versatile and flexible, with good applicability and compatibility.

In addition, digital communication is low-cost due to the simplicity of the signals used and the relatively simple requirements for the circuits used in communication equipment. At present, the vast majority of circuits used in digital communications are integrated circuits, which have the advantages of simplicity, lightweight, low power consumption, and less prone to failure. With the development of large-scale integrated circuits, the cost of equipment can be further reduced, digital communication equipment will become more and more common, and its application will become more and more widespread.

Digital communications and computer networks

With the development of digital communications, especially after the application of computers in communications, computer communication networks. Modern digital communication networks are controlled by computers, so from the point of view of communication, it is a computer digital communication network; and from the point of view of the computer, it is a computer network.

In a simple telephone network exchange, two users want to make a call, just connect the two users' telephone sets. But three or more than three users in any two users need to talk to the user can not simply connect all the users to each other, must be through the telephone exchange (also known as switchboard), by the switch to the designated two users connected to talk to. A city's switching capacity can be as large as tens of thousands or even hundreds of thousands of users, there can be hundreds of thousands of users at the same time call, in this way, the artificial exchange can not cope with the increasingly busy telephone communication requirements, must use advanced automatic switching technology.

Digital signaling exchange can be used in two ways. One is like the telephone, digital information needs to be timely bidirectional mutual information, this time shall be used in circuit switching mode, that is, the use of computer control to the input line and output line connected to each other, so that the two sides directly for digital communication; another way is called the information exchange mode, can be used in telegraph signals like the case of only one-way transmission. Terminal to send the information are in the computer's memory device first stored down, and then as long as the corresponding circuit is idle, the computer will forward the information out.

Typically, this is how a computer controls a phone call. Ordinary life through a phone call the most basic operation is to pick up the phone, and then dial the number of the called user, the called user to pick up the phone to start talking, after speaking put down the phone. Corresponding to this sequence of operations, the switch to complete the following six switching sequence: send dial tone, receive dial tone, analyze the dialed digits, "call" the called user, connect the phone, the call is completed to cut off the circuit.

If the above exchange order is compiled into the corresponding program and a series of instructions, stored in the computer, call the computer will be based on the programmed control of the telephone connection. At this time, the computer completely replaces the operator's operations, and can be very quickly and accurately complete the operator can not do the work, the realization of the computer-controlled phone calls.

The introduction of computers into the digital switching technology, so that the exchange of a new look, to provide people with more convenient. For example, when people are on the phone, another person calls, the past communication can only be received in order. Now it is possible to choose between the two calls or to make alternate calls. In the past, only when the call was completed could another call be made to find someone, but now the user can alternate between the two called users, and can also constitute a 3-person conference call. Now the telephone can also be compatible with the TV set, providing TV data, the user only need to dial a number on the telephone, can be extracted from the TV data center and presented on the screen of the TV set, for the user to choose and consult.

In the broader field, computer network technology and digital communication technology are combined to form a computer communication network. Computer communication network can make a city within the computing center of the computer for the city's many users, but also for a region or even the country **** with the use. At this time, the user data terminal, computer generated data signals need to be effectively exchanged within the communication network, the formation of data exchange. With the further development of digital communications, computer technology is applied to all aspects of the communications field. Digital telephone, digital fax and digital television and other kinds of digital terminal equipment increased in large numbers. Existing transmission media such as cables, microwave relays and satellite communications will be used more often for digital transmission. The exchange of information will also cause great changes, the urgent need for computers to process and control.

Computer communications can use the computer to connect the circuit, but also can use the computer's memory to save the information, and then forwarded out. Data terminal equipment set at a remote location has input circuits controlled by the computer host, and output circuits such as paper tape, cards, printing and display. Data information through the line transmitted to the communication controller, the communication controller is the line and the central processing machine connected to the interface device, it constantly scanning the inputs, if there is a requirement to process the data, it will be sent to the central processing machine, stored in the internal memory. When the stored data reaches the specified size of the word group, the central processor will carry out the necessary processing of the data, and transfer the results to the large-capacity external memory. The existence of the data in the reservoir, such as the output line is free, and then sent to the other side of the terminal through the central processor and communications controller. This type of information exchange is not only used in the military, such as air defense systems, etc., and is widely used in banking, railways, business management, warehouse management, meteorology, medical, aircraft booking, newspaper editing and intelligence information retrieval and other civil systems.