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Preface
1. Communication basics
2. Transmission medium
3. Physical layer equipment

Preface

First understand the upper level foundation, then learn the lower level structure.

1. Communication basics

1.1. Basic concepts of physical layer

1.1.1. Understanding the physical layer

The purpose of the physical layer : to solve how to transmit data bit streams on the transmission media connecting various computers , rather than the specific transmission media.

The main task of the physical layer : confirming some characteristics related to the transmission media interface and defining standards .

Standards defined in real life : The sockets on the power strip are three-wire and two-wire; the network cable interface behind the router is also the same

1.1.2. Understand the four interface characteristics of the physical layer

List the characteristics defined by the physical layer : you need to be able to distinguish the characteristics when choosing the exam

1. Mechanical characteristics : Define the characteristics of the physical connection and stipulate the specifications, interface shape, number of leads, number of pins and arrangement used in the physical connection .

2. Electrical characteristics : stipulates the voltage range, impedance matching, transmission rate and distance limitations of the signal on the line when transmitting binary bits.

For example, electrical characteristics: A certain network stipulates in the physical layer that the signal level is represented by +10v ~ +15v to represent binary 0, and -10v ~ -15v is used to represent binary 1. The length of the wire is limited to 15m.

where level is range .

3. Functional characteristics : Indicate the meaning of a certain level appearing on a certain line, and the purpose of the signal line of the interface component.

Example: Describe the meaning of a physical layer interface pin when it is at a high level.

Note: It is easy to confuse electrical characteristics and functional characteristics. In electrical characteristics, electrical represents a range, while functional characteristics represent a certain meaning of level.

4. Procedure characteristics : Process characteristics define the work procedures and timing relationships of each physical line.

1.2. Basic knowledge of data communication

1.2.1. Typical data communication models and related terms

Case : Host A sends a text message to Host B. The process of intermediate data transfer is as follows

The computer network card sends out a digital signal , which is converted into an analog signal by the modem , and then transmitted on the analog channel of the wide area network, and then the modem modulates it into digital signal and transmits it to the other party's computer network card.

In fact, each of the above devices has corresponding professional terms :

信源It is the sending host.
调制解调器It's the transmitter.
The public telephone network is 传输系统.
For the part on the right used to convert analog to digital, it is called a demodulator 接收器.
信宿is the target host.

We can form new nouns by combining corresponding nouns :

源系统: Source + generator.
传输系统: Still a transmission system.
目的系统: Receiver + sink.

1.2.2. Terminology related to data communication

The purpose of communication : to transmit information.

① 消息: Voice, text, image, video, etc.

② 数据data: An entity that transmits information, usually a meaningful sequence of symbols.

③ 信号: The electrical/electromagnetic manifestation of data is the form in which data exists during the transmission process.

How does binary data travel across the ocean to find its receiving end?

These binary data need to be converted into signals, that is, the electrical or electromagnetic representation of the data is the form in which the data exists during the transmission process. The format includes digital signal/discrete signal, analog signal/continuous signal.
数字信号/离散信号: The values representing the parameters of the message are discrete and jumpy.
模拟信号/连续信号: The value of the parameter representing the message is continuous, and it is a continuous image without obvious jumps.

The message we transmit is what we actually want to say; the data is the computer translating the message into a language that the computer can understand, that is, a meaningful sequence of symbols, such as the binary number 01 sequence; putting the data on the link or in It exists in the form of a signal when transmitted on the channel .

Note : If the signal exists in the form of a digital signal or an analog signal, it depends on what the channel is. If it is an analog channel, then only analog signals can be allowed to pass. If it is a digital channel, then it can be digital signals.

④ 信源: The source of generating and sending data.

⑤ 信道: Signal transmission medium. Generally used to represent a medium that transmits information in a certain direction, so a communication line often contains a sending channel and a receiving channel.

Channels are directional. If you send data to me, there will be a sending channel. If I send data to you, there will also be a sending channel.

Channels are classified as follows :

For example, the signal transmission media in network cables, optical fibers, and coaxial cables can be seen and touched 无线信道.有线信道

1.2.3. Three issues to consider when designing a data communication system:

Question 1: Use simplex communication/half-duplex/full-duplex communication?

From the perspective of the communication methods of information interaction between the two parties, there are three basic methods :

Vivid example:

1.: 单工通信There is only communication in one direction without interaction in the opposite direction, and only one channel is required.

2. 半双工通信/双向交替通信: Both parties to the communication can send or receive information, but neither party can send and receive at the same time. Two channels are required.

3. 全双工通信/双向同时通信: Both communicating parties can send and receive information at the same time, and two channels are also required.

Question 2: Use serial communication/parallel communication?

Data transmission methods are divided into : serial transmission and parallel transmission.

串行传输: Send the 8-bit binary number representing a character in order from low bit to high bit.

并行传输: Send an 8-bit binary number representing a character through 8 channels at the same time.

Comparison of the characteristics of serial and parallel :

Serial features: slow speed, low cost, suitable for long distances.
Parallel characteristics: fast, high cost, suitable for short distances.

**What do you think about the cost? **For example, the same distance requires only one channel for serial, while the above parallel requires 8 channels, so the cost of serial features is low and the cost of parallel is high.

Application: ① Parallel transmission is used for data transmission within the computer; ② Parallel transmission is used for data transmission within the integrated circuit chip, between components of the same plug-in board and in the same chassis. ③The printer's interface is also a parallel port.

Question 3: Use synchronous communication/asynchronous communication?

① 同步传输: In the synchronous transmission mode, data is transmitted in units of one data block, so synchronous transmission is also called block transmission.

Rule : When transmitting data, one or more synchronization characters need to be sent first, and then the entire batch of data is sent. Implemented clock synchronization of a sender and receiver.

For example : if we dance as a couple, you and I need to be in sync with each other to dance in rhythm.

② 异步传输: Asynchronous transmission divides bits into small groups for transmission. A small group can be an 8-bit character or longer.

Rules : The sender can send these bit arrays at any time, and the receiver does not know when they will arrive. When transmitting data, add a character starting bit and a character ending bit.

Does asynchronous transmission also need to be synchronized? Why does asynchronous transmission also need to be synchronized?

The computer communication process is actually similar to making a phone call. You also need to confirm your identity after dialing the phone, and then enter the call state. During the call, it is reasonable for the speaker to speak clearly every word, and then finish each sentence. If you pause, the other person will have to adapt to the speaker's speaking speed and understand every word the other person says. Use the tone and pauses of the other speaker to determine when the next sentence begins and ends. (This is a synchronization problem that people need to achieve when speaking)
The same goes for computer communications, which are mainly divided into synchronous transmission and asynchronous transmission.

The difference from synchronization : for asynchronous transmission, the sending end does not send these bytes continuously, but sends one byte every once in a while, intermittently (so it is said to be asynchronous, but in fact it also needs to be synchronous. For sending characters The order of bit numbers should also be synchronous, it just means that the sending time period is not continuous).

For the start bit and stop bit of asynchronous transmission : the start bit and stop bit are 0 and 1 by default. Every time information is transmitted asynchronously, it will start with such a start bit. When this start bit reaches the receiving end, the receiving end will know that the data has arrived. Then when the receiving end receives the cached data bits of this start bit, it will receive the data. Come down and you can receive the subsequent data. Then the stop bit will be received, and a high level of 1 will also be received for the stop bit. Then wait for the new starting bit to be sent to continue the cycle.

Practical case : We randomly type some numbers on the keyboard, and which key is pressed is random. Then the computer's process of processing this key is actually asynchronous transmission.

1.2.4, code element

1.2.4.1. Understand code elements and what are code elements? (k-ary code element)

码元: Refers to using a fixed duration signal waveform (digital pulse) to represent the basic waveform of different discrete values. It is the measurement unit of digital signals in digital communication. The signal within this duration is called k-ary symbol, and the duration is called Code element width. When there are M discrete states of a code element (M is greater than 2), the code element is an M-ary code element.

Sending process : Host A and Host B send a message. The message will first be converted into a bit stream, which is expressed in binary form 0 and 1. Then a high level is sent out at this time, which is expressed as 1, and a low level is expressed as 0. At the same time, The same is true for host B when receiving. If the transmission is high level, then it is 1. If the transmission is low level, then it is 0.

Digital signals are transmitted through high and low levels.

**What is a code element? **The small section (red horizontal line) in the picture below is a code element. The length of time for this period is the code element width.

What is a decimal code element?

When there are m discrete states of a code element, the code element is an M-ary code element.

For example, the following digital signal waveform has only two states, one low and one high, which are binary symbols :

Note : One symbol can carry multiple bits of information.

For example, when using binary symbols, there are only two different symbols, one representing the 0 state and the other representing the 1 state. 1 symbol carries 1 bit of information.

At this time, a question is raised. If it is a quaternary code element, how many can be carried in one code element?

Reasoning: Quaternary code element -> there are 4 discrete states of the code element -> 4 different high and low signal waveforms 00, 01, 10, 11. Then one symbol can carry two bits of information.

What if it is a hexadecimal code element?

Reasoning: Hexadecimal code element -> there are 16 discrete states of the code element -> 16 different high and low signal waveforms, then 1 code element carries 4 bits of information.

Conclusion : If it is a K-ary symbol, then there are K discrete states and K signal waveforms with different levels. At the same time, one symbol carries log ₂ K bits of information.

1.2.4.2. Two expression methods for data transmission rate of digital communication system (symbol transmission rate, information transmission rate)

Data transmission rate refers to a performance indicator in digital communication systems.

1)码元传输速率 : Aliased by symbol rate, waveform rate, modulation rate, symbol rate, etc., it represents the number of symbols transmitted by the digital communication system per unit time (it can also be called the number of pulses or the number of signal changes).

The points to note about the number of signal changes are: if two consecutive bits are both low level, they will also be recorded in the number of signal changes. In fact, as follows, for the same low level, it can be regarded as changing from one symbol to another. The code element has also changed.

Unit : Baud, the abbreviation is B. Do not confuse byte B with byte B. 1 baud represents a time when a digital communication system transmits 1 symbol per second.

Briefly describe the code element transmission rate : it is actually how many code elements are transmitted in 1s.

Calculation example : If 4800 symbols are transmitted within 2 seconds, what is the transmission rate of the symbols? 4800 / 2 = 2400 symbols/s.

Digital signals are divided into polyary and binary systems, but the code element rate has nothing to do with the system number and is only related to the code element length T.

This code element length T refers to the length of the following short segment.

The formula for code element rate :

R _B refers to the code element rate, T refers to the code element length, and B is Baud.

2)信息传输速率 : Alias information rate, bit rate, etc., indicating the [number of binary symbols (i.e., number of bits)] transmitted by the digital communication system per unit time, and the unit is bits/second (b/s).

Note that a question is raised here: Why is the number of binary symbols equal to the number of bits?

Binary code elements have 2 discrete states, so 1 code element carries one bit of data. If it is a 4-ary code element, then 1 code element is 2 bits, which is not equivalent! Mainly for this reason! ! !

Briefly describe the information transmission rate : how many bits are transmitted per second.

Relationship : If a symbol carries n bits of information, the information transmission rate corresponding to the symbol transmission rate of M Baud is M xn bit/s.

For example: At this time, the code element transmission rate is 2B/s. Note that B here refers to baud. At this time, it tells 1 that one code element carries two bits of information. What is the information transmission rate bit rate at this time? 2 x 2B = 4B/s.

Question 1 (code element transmission rate and information transmission rate)

Topic :

Analysis :

This question examines the relationship between symbols and bits and what do the information transmission rate and symbol transmission rate represent respectively?

A code element can be expressed as multiple bits. This depends on the decimal code element. If it is a K-digit code element, then one code element means that it carries log ₂ K bits.
The code element transmission rate is calculated by calculating how many code elements are transmitted in one second.
The information transfer rate is a calculation of how many bits are transmitted in one second.

After understanding the basic concepts above, let’s do the calculations at this time.

To solve the first question, the code element transmission rate of the quaternary system: 8000 code elements are transmitted in 4s, then 2000 code elements are transmitted in 1s, then the code element transmission rate is 2000B/s .

Note that B here refers to Baud, not Byte. Generally, Byte is not used to express the transmission rate.

To solve the second question, the information transmission rate of the quaternary system: Because of the quaternary code element, then 1 code element is 2 bits, so the information transmission rate is 4000b/s .

The b here is bit.

To solve the third question, hexadecimal code element transmission rate: 1200B/s .

To solve the fourth question, hexadecimal information transmission rate: hexadecimal code element, then 1 code element carries 4 bits of data, then the information transmission rate is 4800b/s .

To solve the fifth question: If you want to find out which system transmission rate is faster, then the information transmission rate is usually compared, that is, the number of bits transmitted per second, so the transmission rate of the communication system with hexadecimal code elements in the question is faster.

Question 2 (code element transmission rate drives information transmission rate)

Topic :

Analysis:

Octal code element, one code element carries 3 binary bits of data. Since the transmission rate is 1600B, where B refers to baud, which is the number of code elements, then the converted information transmission rate is 3 x 1600 = 4800b/ s .

Question 3 (Information transmission rate pushes code element transmission rate)

Topic :

Analysis :

Quaternary digital information, then 1 symbol represents 2 bits of data. Since the signal transmission rate is 2400b/s, then the symbol transmission rate is 1200B/s , just convert it.

1.2.5. Bandwidth (including analog and digital)

Bandwidth usually describes the optimal performance of the channel, which includes analog signal bandwidth and digital signal bandwidth (digital signal bandwidth is usually calculated in the network):

1. In analog signal systems : When the input signal frequency is high or low to a certain level, the output power of the system is called the average of the input power, that is, -3db. The difference between the highest frequency and the lowest frequency represents the passband bandwidth of the system.

Simple explanation: the difference between the highest frequency and the lowest frequency.

Unit: 赫兹（Hz）.

2. In digital equipment : Indicates the " highest data rate " that can pass from one point in the network to another point in a unit time or the number of links passing in a unit time. It is often used to indicate that the communication lines of the network can transmit data. Ability.

Unit: 比特每秒（bps），b/s.

mind map moment

1.3. Nye’s criterion and Shannon’s theorem (investigation and calculation)

1.3.1. Nye’s criterion

1.3.1.1. Understand distortion and factors affecting distortion

Distortion refers to the loss of authenticity. In computer networks, it mainly refers to the signal. When it is transmitted to itself, that is, the signal is distorted and changed at the beginning.

The following are two situations:

Case 1: Distorted but identifiable

Case 2: The distortion is too large to be recognized.

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The factors that affect the degree of distortion are as follows :

1. 码元传输速率: If the symbol transmission rate is too fast, the program causing signal distortion will be more serious.

2. 信号传输距离: The farther the signal transmission is, the longer it will decay and the longer the interference will last.

3. 噪声干扰: The more noise interference, the easier it is for the signal to be distorted.

4. 传输媒体质量: The worse the quality of the transmission media, the more serious the distortion of the waveform received at the receiving end.

The first three are positive correlations, and the latter one is negative correlations.

1.3.1.2. What is inter-code crosstalk?

Distortion There is a very important distortion phenomenon. The phenomenon that derives from the Nye criterion in the postgraduate entrance examination is called inter-code crosstalk .

Various scenarios for the following channel bandwidths :

Hertz refers to the number of vibrations in one second.

It can be seen that 200HZ and 4000HZ cannot pass. At this time, the highest Hz that can pass is 3300HZ, and the lowest Hz is 300HZ.

Channel bandwidth is the difference between the highest frequency and the lowest frequency that the channel can pass. That is 3300Hz - 300Hz = 3000Hz.

**Why 200HZ cannot pass? What is the reason? **Because the vibration frequency is too low, it is very easy to receive attenuation and loss during the transmission process on very complex telephone lines, which ultimately leads to the final loss of attenuation and the inability to pass through.

**Why can’t 4000HZ pass? **Because the vibration frequency is too fast, we will not be able to distinguish the difference between their waveforms at the receiving end when receiving.

The above two situations will lead to the phenomenon of inter-code crosstalk .

码间串扰: Refers to the phenomenon that the signal waveform received by the receiving end loses the clear boundaries between symbols .

If the frequency is fast, then the code element transmission rate is fast. The actual code element transmission rate is corresponding to the signal frequency. For computer networks, it means that the code element transmission rate is too fast, resulting in unclear boundaries between symbols 码间串扰. It is not clear whether it is 0 or 1.

1.3.1.3. Solution to inter-code crosstalk: Nye’s criterion

To solve this problem 码间串扰, a very good criterion emerged: Nyquist's criterion (Nyquist's theorem)

Nye's criterion: Under ideal low-pass (noise, limited bandwidth) conditions, in order to avoid inter-symbol crosstalk, the limit symbol transmission rate [note the symbol] is 2W Baud, W is the channel bandwidth, the unit is Hz.

The Nyquist criterion is a theorem deduced by Nyquist in 1924.
理想传输环境: There is no noise in the channel and will not be affected by the outside world.
低通: Bandwidth is limited. (The channel has a bandwidth. Bandwidth refers to the difference between the highest frequency and the lowest frequency. Then all signals lower than the highest frequency can pass, so it is said that signals lower than the highest frequency can pass .)

Regulations : The code element transmission rate cannot be infinite, there must be an upper limit, and this upper limit can be deduced by Neighbor's equation, which is 2W.

Note : The channel bandwidth was previously measured in bits per second, but it should be noted that the unit of bandwidth in the Nye criterion and Shannon's theorem is Hertz (Hz) .

If the question asks how many Hertz the channel bandwidth is, then you have to think of using these two formulas to find it.

The reason why the code element transmission speed limit is stipulated as 2W, the following are examples of slow and fast transmission rates :

Slow transmission rate: At this time, the signal is very clear when you see it. The receiving end can clearly distinguish the boundaries of 0, 1, 0, and 1 code elements and identify each code element.

test

Fast transmission rate: If the frequency is too fast, the receiving end will not be able to distinguish them at all, and there will be no clear boundaries between them, so the purpose of identifying each symbol cannot be achieved.

test

1.3.1.4. Calculate the ultimate data transmission rate (limited code element transmission rate) using the Nye formula

In the postgraduate entrance examination, in order to confuse the knowledge points, the asynchronous limit data rate and the limit transmission rate are also sought, making Nye's theorem look very similar to Shannon's theorem, but it should be noted that the emphasis of the two is different.

Nye's criterion: There is only an upper limit to the code element transmission rate.
Shannon's theorem: What really limits the transmission rate of information is that there is an upper limit.

How to find the limit data rate using Nye's criterion?

V refers to the number of discrete levels of the symbol. For example, if there are four waveform states in the signal, then there are four corresponding symbols. In this case, V is 4.
W refers to bandwidth, where the unit of bandwidth is Hertz.

1.3.1.5. Four conclusions of Nye’s criterion

1. In any channel, the code element transmission rate has an upper limit . If the transmission rate exceeds this upper limit, serious inter-symbol crosstalk problems will occur, making it impossible for the receiving end to completely correctly identify the symbols.

2. The wider the frequency band of the channel (that is, the more high-frequency components of the signal that can pass through), the higher the rate can be used for effective transmission of symbols.

The frequency band actually refers to the W in the formula. The larger this W is, the higher the ultimate data transmission rate obtained by the entire formula will be.
Why does the wider the frequency band, the more high-frequency components of the signal that can pass through? The wider the frequency band is, the greater the difference between the highest frequency and the lowest frequency in the channel, and the wider it is, so more high frequencies will pass through the corresponding channel.

3. The Nye criterion gives a limit on the code element transmission rate , but does not put a limit on the information transmission rate.

4. Since the transmission rate of symbols is restricted by the Nye criterion, in order to increase the data transmission rate, we must try to make each symbol carry more bits of information [actually increasing the V variable in the formula ], which requires a multi-modulation method.

1.3.1.6. Nye criterion calculation exercises

Analysis :

There are four phases and four amplitudes in the question, so the direction modulation and amplitude modulation need to be combined. This corresponds to 4x4=16 types of code elements, and V is 16 at this time.

The question gives a bandwidth of 3kHz, so W is 3k.

Then apply the formula directly: Maximum data transfer rate = 2 x 3k x 4 = 24kb/s

1.3.2. Shannon’s theorem

1.3.2.1. Introduce Shannon’s theorem

Explanation : The Nye criterion only stipulates the highest limit symbol transmission rate, while Shannon's theorem stipulates the bit rate, that is, the highest bit transmission rate.

Shannon's theorem not only defines the limit transmission rate of bits transmitted in the channel, but also takes into account the electromagnetic interference that actually exists in the channel , that is, the impact of noise.

Noise exists in all electronic equipment and communication channels. Since noise is generated randomly, its instantaneous value can sometimes be very large. Therefore, noise can cause errors in the receiving end's judgment of symbols, but the impact of noise is relative. , if the signal is strong, then the noise impact is relatively small . Therefore, 信噪比it is very important.

As for the understanding that if the signal is strong, then the impact of noise is relatively small, let’s take a real example: a top student and a low-achieving student are in class in the classroom. If there is no sound from the students below, and the teacher’s teaching voice is very loud at this time, then it will affect the students . The tyrant will receive the teacher's information very smoothly. At this time, if the scumbag starts chirping loudly, the tyrant will have a great impact on receiving the teacher's information at this time, but if the teacher's voice doubles Directly drowning out the discussion voices of the bad students, then the information received by the top students will not be greatly affected at this time.
In this example, the signal is the teacher's lecture sound, and the noise is the chirping sound. If the signal is strong enough to overwhelm the noise, then the impact of the noise will be relatively small at this time.

1.3.2.2. Understand Shannon’s theorem and calculation formula (limit information transmission rate)

信噪比: The average power of the signal/the average power of the noise , often recorded as S/N, and decibel (dB) is used as the unit of measurement.

In general, the signal-to-noise ratio will be given directly in the topic.

Another way to write it (similar to scientific notation): signal-to-noise ratio (dB) = 10 log ₁₀ (S / N) .

The values are equivalent, but differ in representation.

In the actual exam, we need to see clearly whether dB is given in units or whether it is given as a ratio without units!

香农定理: In a bandwidth-limited and noisy channel, in order to avoid errors, the data transmission rate of information has an upper limit.

This upper limit value is found in Shannon's definition formula.

The ultimate data transmission rate formula of the channel :

If the signal-to-noise ratio in the question is 1000 and there is no unit, then just substitute it directly; if the signal-to-noise ratio is 30dB, then you need to use this formula to find S/N and then bring it to this formula for calculation.

1.3.2.3. Five major corollaries of Shannon’s theorem

1. The greater the bandwidth of the channel or the signal -to-noise ratio in the channel , the higher the ultimate transmission rate of information.

In fact, as W and N/S in the formula become larger and larger, the limit transmission rate will be higher at this time.

2. For a certain transmission bandwidth and a certain signal-to-noise ratio, the upper limit of the information transmission rate is determined.

Once W and N/S in the formula are determined, the ultimate data transmission rate of a channel can be obtained.

3. As long as the information transmission rate is lower than the limit transmission rate of the channel, some way will be found to achieve error-free transmission.

4. Shannon’s theorem obtains the ultimate information transmission rate , and the transmission rate that the actual channel can achieve is much lower than it.

5. It can be seen from Shannon's theorem that if the channel bandwidth W or the signal-to-noise ratio S/N has no upper limit (impossible), then the channel's ultimate information transmission rate also has no upper limit.

1.3.2.4. Shannon’s theorem calculation exercises

Analysis : The question does not tell you whether it is the Nye formula or the Shannon formula. You need to judge the calculation based on the information given in the question.

The question gives the signal-to-noise ratio, which means that Shannon's theorem needs to be used.

Since the signal-to-noise ratio here is given in dB, we need to convert it to S/N first: 30dB = 10log ₁₀ (S/N), then S/N = 1000.

The bandwidth given in the question is 3000Hz. At this time, W is 3000. Then substitute W and S/N into the Shannon theorem formula:

The ultimate data transmission rate of the channel = Wlog ₂ (1+S/N) = 3000 x log ₂ (1 + 1000) = 30kb/s.

1.3.3. What are the differences between Nietzsche and Shannon and how to choose and calculate them for the exam?

1.3.3.1. Understand the difference between the two

Nye's Principle : Internal Worry

Reason: In order to avoid inter-symbol crosstalk under limited bandwidth and noise-free conditions, the upper limit of the symbol transmission rate is 2W Baud.
The ultimate data transfer rate under an ideal low-pass channel = 2Wlog _2V .
Ways to increase the data rate: increase the bandwidth (W) or use better encoding technology (K-ary code elements).

Shannon's theorem : foreign invasion

Reason for occurrence: Bandwidth is limited and information transmission rate under noisy conditions.
The ultimate data transmission rate of the channel = Wlog ₂ (1+S/N).
Ways to increase data rate: increase bandwidth W or signal-to-noise ratio (S/N).

1.3.3.2. How to choose theorem calculation in the question? (Including practical examples)

How to judge which theorem to use for calculation in the question?

It is not entirely correct to say that if no noise conditions are given, it is the Nye criterion.
If there is no condition for noise signal-to-noise ratio in this question, Shannon's theorem will definitely not be applicable, and it will naturally be Nye's criterion;
If the signal-to-noise ratio is given, and he also gives such a v (V in the Nye formula), that is to say how many bits a symbol corresponds to, and also gives two criteria, then our two formulas are Do the calculation and take the minimum value!

Example: When a binary signal is transmitted on a 4kHz channel with a signal-to-noise ratio of 127:1, what is the maximum data rate that can be achieved?

①Nye's rule calculation

The question states that if a binary signal is used, V is 2, and 4kHz corresponds to the bandwidth, that is, W = 4kHz.

Maximum data rate = 2 x 4000 x log ₂ 2 = 8000b/s

②Shannon’s theorem calculation

The signal-to-noise ratio S/N = 127/1 = 127 is given, while the bandwidth W = 4kHz

Maximum data rate = 4000 x log ₂ (1 + 127) = 28000b/s

Result : In the end, we took out a minimum value, which is 8000b/s in the Nye criterion as the final answer.

1.4. Preparation and modulation

1.4.1. Prerequisite knowledge: baseband signals and broadband signals

信道: The transmission medium of signals, generally used to represent the medium that transmits information in a certain direction, so a communication line often contains a sending channel and a receiving channel.

Channels are divided into analog channels (transmitting analog signals) and digital channels (transmitting digital signals) according to the transmission signals.
Channels are divided into wireless channels and wired channels according to transmission cutoffs.

For 基带信号and 宽带信号are actually two forms of signals transmitted on the channel . In addition to digital signals and analog signals, they can also be divided into baseband signals and broadband signals.

baseband signal

Baseband signal : The numbers 1 and 0 are directly represented by two different voltages, and then sent to the digital channel for transmission ( baseband transmission ). Signals from a source , such as data signals output by a computer representing various text or image files, are all baseband signals.

The baseband signal is a signal that directly expresses the information to be transmitted . For example, the sound waves we speak are baseband signals.
The key is that it is sent by the [source], which can be a human or a computer. The signal sent by the computer is high level and low level. The sound waves emitted by people are analog signals, with ups and downs, so it can be said that the baseband signal is a [digital signal or analog signal].
Key understanding : In computer networks, digital signals are used. Because computer equipment network cards emit digital signals, digital signals can only be transmitted on digital channels, so they are called baseband transmission.

The process of baseband transmission encoding : What kind of waveform represents 0 or 1 will be specified, and it will be carried out according to a prescribed encoding method.

wideband signal

Broadband signal : A frequency division multiplexed analog signal formed by modulating the baseband signal and then transmitted to the analog channel for transmission (broadband transmission).

Process : After the baseband signal is modulated by the carrier, the frequency range of the signal is moved to a higher frequency band for transmission on the channel (that is, the channel can only pass through a certain frequency range).

For example : For example, if we speak normally, if people far away cannot hear our voice, then we can use a transmitting tower to emit our sound waves. However, since the frequency of my sound waves is not very high, the display The waveform is relatively narrow and wide, and because it has to be transmitted over a long distance, our signal may be attenuated due to weather and environmental reasons. At this time, some loss will be caused to our signal. In this case, we need to The basic baseband signal (that is, what we normally call, baseband signal) is modulated into a high-frequency signal with a relatively high frequency. Then the high-frequency signal can correspond to an environment with a large attenuation, even if there is a large attenuation. Large interference factors can still be transmitted to the target end and then demodulated to restore the original signal frequency.

Clever trick : The baseband signal is a digital signal transmitted on a digital channel, and the broadband signal is an analog signal transmitted on an analog channel.

Summary of baseband and broadband and the differences between them :

When the transmission distance is short, the computer network uses baseband transmission (the attenuation is small at short distances, so the signal content is not easy to change).
When the transmission distance is long, the computer network uses a broadband transmission method (long-distance attenuation is large, and the baseband signal can be filtered out even if the signal changes greatly).

1.4.2. Understand coding and modulation (four coding and modulation methods)

Introduction : Data can be digital or analog. For example, the computer uses binary 0 and 1, then the data it emits is digital data, which is discrete, and the sound waves emitted by people speaking are A type of simulated data that is continuous. For data, whether it is digital data or analog data, in order to transmit the data and allow the receiving end to receive the data, the data needs to be converted into a signal first. Then the signal can be put on the link for transmission. After receiving The end also needs to convert the signal into digital.

Data—>Digital signal, ie 编码.
Data —> Analog signal, ie 调制.
- The above data can actually represent digital or analog, and is not unique.

Encoding process : ①Digital data is turned into digital signals by using a digital transmitter. ②Analog data is converted into digital signals by using a PCM encoder.

Modulation process : ①Digital data is turned into analog signals by using a modulator. ②Analog data is turned into an analog signal by using an amplifier modulator.

Clever trick: If it is a digital generator, both ends are digital; modulation is digital->analog, demodulation is analog->digital, and the final PCM is analog data->analog signal.

The complete process of encoding and modulation : first modulate the original baseband signal into an analog signal (a high-frequency analog signal) before it can be transmitted on the link, then filter this high-frequency signal into the initially transmitted sound wave , then the receiving end can receive.

1.4.3. In-depth understanding of the four coding and modulation methods

Digital data encoded into digital signal

Understand six digital signal encoding methods

Purpose : The data sent by the computer are all digital data such as 0101. In order to enable this data to be transmitted on the channel, this data needs to be encoded into the form of a digital signal.

Purpose : The encoding of digital data is used for baseband transmission. It basically does not change the frequency of the digital signal and directly transmits the digital signal.

There are the following six digital signal encoding methods. The first three are commonly used, and the last three are less commonly used (reverse non-return-to-zero encoding has been investigated in 15 years) :

(1) Non-return to zero coding [NRZ]

Rule : high 1, low 0 (high level is 1, low level is 0).

Benefits : Coding is easy to implement.

Disadvantages : There is no error detection function, and it is impossible to determine the beginning and end of a code element, making it difficult for both the sender and the receiver to maintain synchronization.

Counter example : If the data sent at this time is a long continuous 1 or 0, then the receiving end will receive a particularly long straight line at this time, and at this time it does not know how many 1 or 0 there are in this straight line. 0, if you want the receiving end to know how long the clock cycle is in each period (you can tell the sending cycle and speed by establishing a signal specifically used to transmit the clock cycle).

(4) Return to zero coding [RZ]

Rule : The signal level must return to 0 within one symbol.

How to explain recovery within one symbol?

For example, the default here is a binary code element, so one bit is a code element. In the picture below, the part I circled is a state that returns to 0 within a code element.

Disadvantages : During the entire transmission process of return-to-zero coding, there are many low-level situations, which means that this channel is not used much, so it is not recommended to use it.

(5) Reverse non-return to zero encoding [NRZI]

Rule : An inverted signal level represents 0, and an unchanged signal level represents 1.

Let's explain the rules for each position change:

默认从①开始是发送的高电平。
到了②：由于我们要发送的是0，此时根据规则信号电平翻转表示0，那么此时基于①中的高电平，我们在②是低电平！
到了③：发送的是0，根据规则基于②中的低电平，要翻转，此时③就是高电平。
到了④：发送的是1，根据规则基于③中的高电平，不变，此时④就是高电平。
到了⑤：发送的是1，根据规则基于④中的高电平，不变，此时⑤就是高电平。
到了⑥：发送的是0，根据规则基于⑤中的高电平，要翻转，此时⑥就是低电平。
到了⑦：发送的是1，根据规则基于⑥中的低电平，不变，此时⑦就是低电平。
到了⑧：发送的是0，根据规则基于⑦中的低电平，要翻转，此时⑧就是高电平。

Comparing the return-to-zero encoding : It can be seen that in this reverse non-return-to-zero encoding, the level will not jump in one symbol, while for the previous return-to-zero encoding, the level will jump in one symbol.

Advantage : If the signals sent are all 0, then each symbol will flip one after another.

Defect : If the signals sent are all 1s, then there will also be a situation where a long segment of 1s is sent, resulting in no flipping. At this time, the receiving end cannot know how many 1s were sent in total, which is different from the previous return-to-zero encoding. Similarly, if you want to solve it, you need to create a new channel. During the process, the clock cycle is transmitted to tell how long each segment is received.

(2) Manchester encoding

Combining the advantages and disadvantages of non-return-to-zero coding, return-to-zero coding, and reverse non-return-to-zero coding, Manchester coding is formed as a very excellent coding.

Features : The clock signal and our data can be transmitted together without the need for additional channels to transmit the clock signal to achieve its own synchronization [synchronization receiving end]. At this time, you can judge from the received data, you What is this data being sent to me? There is no need to extract signals individually or calculate what each clock cycle is.

Manchester encoding rules : Divide a code element into two equal intervals. The former interval is low level and the next node is high level, indicating code element 1; code element 0 is just the opposite. (The opposite stipulation may also apply).

If we follow the rules [the previous interval is low level and the next node is high level, it means symbol 1; symbol 0 is just the opposite], then we can know that the red part is high in the front and low in the back, which means that 0 is transmitted. The blue part with low front and high back represents 1.

Features : A level transition occurs in the middle of each symbol. The transition in the middle of the bit can be used as a clock signal (can be used for synchronization) or as a data signal, but the frequency band width it occupies is two times the original baseband width. times. Each symbol is modulated into two levels, so the data transmission rate is only 1/2 of the modulation rate.

Practical example : If the symbol transmission rate is 40 Boud, then the information rate is only bits per second.

Explain each question in detail:

① A symbol will be divided into two equal intervals. The latter interval can be regarded as a clock signal or a data signal. If used as a clock signal, the sender and receiver can be synchronized. .

② It can be seen that each symbol can generate a jump according to the rules. At this time, the receiver can detect this jump. If a jump is detected, the sender has sent a bit. I have circled the transitions between each code element:

③How to determine the data represented by a symbol sent? Look at the first code element below. It is determined by the height of the front and the last hundred years. According to the coding rules, it is determined whether it is 1 or 0 (maybe it is 0 when it is high in the front and low in the back, or it may be 1 when it is low in the front and high in the back. According to the specific Determined by coding)

④In Manchester coding, the occupied frequency band width is twice the original baseband width. Each symbol is divided into two parts, so it can be modulated into two levels. Then the data transmission rate is only 1/ of the modulation rate. 2.

Question: Isn’t this a binary code element? Isn’t it that one bit corresponds to one code element? Why is it said that the actual number of transmitted bits is 1/2?

Answer: The symbol transmission rate can be either the number of transmitted symbols in one second, or the number of pulses transmitted in one second, or the number of signal changes. It is special in Manchester encoding, one time gap in one second As a clock, the signal changes twice in one cycle, and only through one jump (two signal changes) can one bit of data be obtained, so its modulation rate or symbol transmission rate is the data bit transmission 2 times the rate .

Simplicity: The number of pulses in one clock cycle is 2. We use this number of pulses as the symbol transmission rate, so naturally the data bit transmission rate is 1/2 of it.

(3) Differential Manchester encoding

Rules : Same as 1 and different from 0. If the currently transmitted symbol is 1, then the level of the current half symbol is the same as the level of the second half symbol of the previous symbol. If it is 0, it is the opposite.

Example : You can see that it starts based on the circled position in the picture below. By default, it is high level first, and then we start sending data.

In order to allow the receiving end to perceive it, it will be flipped when sending the symbol. Starting from the low level, the red part is the one symbol we send (actually two levels). At this time, what we want to send is 0. , according to the rule of same 1 and different 0, it is low in the front and high in the back.

Similarly, the blue part also represents a piece of data. First, based on the previous level, determine whether the current starting level is low or high. If the previous level is high level, then start from the low level and send 1 , then ①② are all the same low level.

Then we can get what data is sent each time based on this rule:

Application : Commonly used for LAN transmission.

Features : There is a level jump in the middle of each symbol. Self-synchronization can be achieved through this level jump, and the anti-interference is stronger than Manchester.

My personal feeling is that rules are added based on Manchester coding, that is, the level sent each time will have the characteristics of flipping with the previous level, which can make the level jump more frequently and the receiver can perceive it more clearly.

(6) 4B/5B encoding

Comparing previous encodings : It can be seen that for Manchester encoding and differential Manchester encoding, a level jump is required for the receiving end to receive one bit of data, so the data transmission rate of Manchester is 1/2 of the symbol transmission rate.

4B/5B encoding rules : 5 bits of data are transmitted each time to represent the four real data. Extra bits are inserted into the bit stream to break a series of 0 or 1, that is, 5 bits are used to encode 4 bits of data, and then transmitted to the receiver, so it is called 4B/5B encoding

真实4比特数据     5比特编码
0000            11110
0001            01001
0010            10100
0011            10101
0100            01010
0101            01011
0110            01110
0111            01111
1000            10010
...
1111            11101

Among them, 4 bits of data can represent 0-15, and 5 bits of data are used to represent the corresponding bit data. The extra 16 bits of the five bits are used as control codes (the beginning and end of the frame, line status information, etc.) or reserved .

Benefits : The advantage of this encoding method is better transmission performance and error detection capabilities. By converting the input data into a 5-bit output, it can be ensured that there are no consecutive multiple 0s or 1s in the transmitted data, reducing DC offset in transmission, improving transmission reliability, and also being able to detect and correct some transmission errors.

Digital data is modulated into analog signals (4 digital modulation methods, including calculation questions)

Digital data modulation technology converts digital signals into analog signals at the transmitting end, and restores analog signals to digital signals at the receiving end, corresponding to the modulation and demodulation processes of the modem respectively.

The basic digital modulation method is as follows :

调幅（ASK，幅移键控）:0 means no service, 1 means there is range.

调频（FSK，频移键控）: Frequency means frequency, 0 means low frequency, 1 means high frequency.

If it is very sparse and wide, it only vibrates twice to represent 0. If it vibrates 4 times at a relatively high frequency, it represents 1.

调相（PSK，相移键控）: It is a modulation of the phase. 0 corresponds to one waveform, and 1 corresponds to another waveform.

正交振幅调制（调幅+调相，QAM）: That is, amplitude modulation + modulation are used together.

Principle: On the basis of amplitude modulation, the amplitude is subdivided; or on the basis of direction modulation, each phase is subdivided into different amplitudes.

Frequently check calculation problems :

Question: The baud rate of a certain communication link is 1200 Boud, using QAM modulation technology with 4 phases and 4 amplitudes for each phase. What is the information transmission rate of the link?

Analysis: 4 phases are used, and each phase has 4 amplitudes. Then the entire signal has 4x4=16 waveforms. For 16 waveforms, there are actually 16 corresponding code elements. Here are the hexadecimal code elements. This When log ₂ 16 = 4, that is to say, one baud represents 4 symbols.

At this time, since the baud rate is 1200Boud, our information transmission rate is 4x1200 = 4800bit/s.

Encoding analog data into digital signals (three steps, sampling, quantization, encoding)

How is analog data encoded into a digital signal?

The computer internally processes binary data and all digital audio, so analog audio needs to be converted into a discrete sequence represented by a limited number of numbers through sampling and quantization (that is, audio digitization).

The most typical example : Pulse code modulation (PCM) that encodes audio signals. In computer applications, PCM encoding can reach the highest pillow level. It is widely used for material preservation and music appreciation, CD, DVD and our common WAV are applied in the files.

It mainly includes three steps : sampling, quantification, and coding.

1. Sampling

Process: Periodically scan the analog signal, and program the time-continuous signal into a time-discrete signal.

Principle: The analog signal is a continuous waveform. During the sampling process, every time period, sampling once is to see the corresponding voltage and level at this time, so this continuous complete waveform is Becomes one small dot after another. In order for these obtained discrete signals to represent the sampled analog data without missing frames, the sampling theorem is adopted.

Requirement: In order to enable all discrete signals to represent the extracted simulated data, the adoption theorem must be used for sampling, , note that here is >=.

Practical example: You can see that the sampling frequency of some music players is 44100. This is because the sound waves heard by our human ears are 20Hz-20 kilohertz. If you want to listen to music, then the music must be as fidelity as possible without distortion. Then the sampling frequency needs to be more than 2 times the highest frequency.

Sometimes it is often said that high-frequency distortion and unclearness mean that the adopted frequency is not reached.

2. Quantification

Process: Convert the sampled level amplitude into the corresponding digital value according to a certain hierarchical scale, and take an integer. At this time, the continuous level amplitude is converted into a discrete digital quantity.

Details: Many points have been obtained through sampling before, and now many time points and corresponding level values have been obtained. For this part of discrete points, the levels of some points are all strange. Some floating point numbers, decimals, integers They are all unified into integers and defined into integers according to certain grading standards, that is, section by section.

3. Encoding

Process: Convert the quantization result into the corresponding binary code.

Details: Through step 2, the level floating values of a series of integers obtained after quantization are used as levels and signal states. At this time, the conversion of symbols and bits is used, and each signal state corresponds to a symbol. If there are 16 signal states, then there are 16 discrete level floating values. This is the encoding process.

Describe the entire sampling, quantification, and coding process:

① Sampling (or sampling): You can see that the analog signals have high and low fluctuations. First, they are periodically scanned to perform sampling. For each equal period, the signal is sampled and the corresponding voltage is seen at this time. [Convert a signal that is continuous in time into a discrete signal in time. Follow the sampling theorem during the sampling process. The sampling frequency must be >= 2 times the highest frequency of the analog signal]

② Quantification: Convert the corresponding sampled voltage, including decimals, integers, and negative numbers, into digital values according to certain standards, such as converting 5.2 to 5V. [Convert continuous level values into discrete digital quantities]

③Encoding. The actual final figure shows the 16 signal states used. Four-digit binary encoding can be used to represent 16 discrete states. This is the result of encoding.

Analog data is modulated into analog signals (involving modulation and demodulation)

Reason : During this transmission process, the channel may be very long, and the environment may be very harsh, which will cause the analog signal we transmit to receive a certain attenuation. The analog signal needs to be modulated into a new analog signal to deal with the next few Danger.

Process : In order to achieve transmission effectiveness, higher frequencies may be required. This modulation method can also use frequency division multiplexing technology to make full use of bandwidth resources. The signals transmitted by telephones and local exchanges use analog signals to transmit analog data: analog voice data is loaded into the analog carrier signal for transmission.

For example : The same speaking sound at the beginning is emitted through the tower transmitter. If the transmission distance is very far, then if our own frequency is not high, it will attenuate to the point where the receiving end may not be able to recognize it, so we need to This analog signal is modulated into a relatively powerful analog signal, that is, a high-frequency signal. This high-frequency signal is not easily damaged and the damage is relatively small. Even if it is damaged, its original baseband signal can be clearly restored. Finally, it is demodulated into a baseband signal through a demodulator and finally sent to the radio. At this time, the original sound wave can be heard.

1.5. Data exchange method

1.5.1. Why is data exchange required?

① If we are using wired networks, then if two are interconnected, then each two can be connected together. At this time, the following Cn2 links will be formed, which is relatively inefficient and wastes resources.

② Then we use switching equipment among multiple machines to centralize and transmit data through the switching center (it can be a router or a switch). If it is a LAN, it is a switch. If it is a WAN, it is a router. Then only n links are needed at this time. .

③If there are more switching devices, a large switching network (multiple switching devices) can be formed, and computers from all directions can be connected.

1.5.2. Understand data exchange methods

Contains: 电路交换, 报文交换, 分组交换.

Among them, message switching and packet switching are both store-and-forward switching methods .
Packet switching includes: 数据报方式, 虚电路方式.

1.5.3. Method 1: Circuit Exchanging

1.5.3.1. Principle of circuit switching

The principle of current exchange : During data transmission, there is a dedicated physical connection line composed of intermediate nodes between the source node and the destination node. This line is maintained until the end of the data transmission.

1.5.3.2. Three stages of circuit switching

The stages of circuit switching include the following three stages :

Phase 1: Establishing the connection (call/circuit establishment)

Process: First, host A will connect to the nearest switching device, and then execute the routing algorithm to select the next switching device. The same is true for subsequent switching devices. Finally, switching device D forwards the call request to host B.

Phase 2: Communication (data transmission)

Process: Host B will respond to Host A's request, and the actual data will be returned to the sender along the same route. The switching equipment passed through is the same as when it came. In the middle is full-duplex mode for transmission.

Phase 3: Release the connection (without removing the circuit)

Process: If A wants to disconnect from B, then host A will send a release request until host B will then return a release request. During the backtrace, the corresponding intermediate devices will also disconnect in sequence. First, D, then C,... Host A.

1.5.3.3. Characteristics of circuit switching

Characteristics of circuit switching : exclusive resources, the user clock occupies the end-to-end fixed transmission bandwidth, and is suitable for remote batch information transmission or large-scale data transmission with high indirect real-time system requirements.

Introduction to exclusive resources: During the communication process between a and b, it cannot be occupied by other hosts.

Suitable scenarios : large amounts of data transmission with high real-time requirements.

If only a small data block is transmitted, the long initial connection establishment process will be wasted.

1.5.3.4. Advantages and Disadvantages of Circuit Switching

Advantages and Disadvantages of Circuit Switching :

There is no data storage capability, making it difficult to smooth traffic. Explanation: The equipment in circuit switching has no storage capacity. If a large amount of data pours into these switching equipment, these switching equipment cannot store it, and it will be difficult to cope with the surge in data. , which may result in the loss of some data, etc.

The advantages can be summarized in one sentence : small delay, sequential transmission, strong real-time performance, full-duplex communication, no channel preemption, suitable for digital or analog signals, and simple control.

The shortcomings can be summarized in one sentence : the initial establishment of the connection takes a long time; the signal is exclusive and cannot be given up when idle; once an intermediate transmission device is out, the connection needs to be re-established; it is difficult for different terminals to interact; error correction and error control cannot be performed.

1.5.3. Method 2: Message exchange

1.5.3.1. Understand message exchange and principles

报文: Message is a data unit exchanged and transmitted in the network, that is, a data block to be sent by the station at one time. The message contains the complete data information to be sent, its length is very inconsistent, the length is unlimited and variable.

The file and language sent can be a message, which refers to a single data block to be sent. The length is very uncertain and variable.

The principle of message exchange : There is no need to establish a dedicated channel between two sites. The unit of data transmission is a message, and the transmission process uses a store-and-forward method.

1.5.3.2. Transmission process of message exchange

The transmission process of the entire message exchange :

① First, the transmitted information + message header are now formed into a message (including destination address + source address) and then sent out.

② Note that the message exchange uses a store-and-forward process. When the message reaches switching device A, it will first receive the entire message and temporarily store the message. After checking whether there are any errors (an error control), it is found that If there is no problem with the message, it will wait for the output circuit to be idle, use the routing information to find the next node address, and transmit it to the next node.

Routing information: refers to a routing table maintained by the switching device itself. The routing table can be used to determine the next switching device with the shortest host distance.

③Then determine the next switching device B to arrive through the routing table in the current switching device A, and then forward it.

It can be seen here that a link between nodes is not exclusive, but can be shared by everyone. There will be a limited width for this link, which is a specified bandwidth range. If too many packets are transmitted, it will be seen that this line is relatively busy.

1.5.3.3. Advantages and Disadvantages of Message Exchange

Advantages and disadvantages of message exchange:

One sentence summarizes the advantages : no need to establish a connection; multiple messages can share lines and dynamically allocate lines; can provide multi-target services, and one message can be sent to multiple destination addresses; realize flexible switching and forwarding features.

One sentence summarizes the shortcomings : the real-time performance is relatively poor, and it is only suitable for digital signals. Since there is no limit on the length of the message, some extreme cases will cause the message to be stored on the disk, which increases the transmission delay, and the buffer must be appropriately expanded.

1.5.4. Method 3: Packet Exchanging

1.5.4.1. Understand packet switching and principles

Currently the most commonly used exchange method on the Internet .

分组: Most computer networks cannot transmit arbitrarily long data continuously, so the network system actually divides the data into small pieces and then sends them one by one. These small pieces are called packets.

Principle of packet switching : Packet switching and message switching work in basically the same way, both using the store-and-forward method. The main difference in form is that the length of the data unit transmitted must be limited in the packet switching network, and 128B is generally selected . The sending node must first receive and store the data messages sent from the terminal device, and then divide the messages into packets of a certain length, and transmit and exchange the packets in units. The receiving node assembles the received packets into information or message.

1.5.4.2. Packet switching process

First, there is a large data block at the sending source (if it is not cut, it is in message mode):

Since we are packet switching here, we will cut the large data block into data blocks one by one, and add corresponding control information (source and destination addresses, numbers) to each small data block. At this time, we can Form a group and send it out.

The purpose of numbering: If packets are sent from the source host, different line switching methods may be selected for different packets, and the order of the packets will be different when they finally arrive at the destination, so numbers need to be used for identification.

After the switching device receives the packet, it will perform error detection and routing on the packet, and select the next path for it. When the switching device selects a relatively better line and sends out the message, the host at the source address also Send out a small block of data.

1.5.4.3. Advantages and Disadvantages of Packet Switching

The advantages can be summarized in one sentence : there is no need to establish a dedicated connection line in advance, and packets can be sent out at any time; the line utilization rate is high; the length of the packet is fixed, and the buffer size is also fixed; parallelism is achieved, and the storage of the next packet is operated in parallel with the forwarding of the previous packet. , since the transmitted packet is smaller than the message, the waiting time for transmission is reduced. The transmission delay is reduced; due to the short packet size, it is suitable for computer burst data communication.

One sentence summarizes the shortcomings : Although each forwarding transmission time is shorter than the message exchange time, there is still a storage and forwarding delay; control information must be added to each packet, which actually reduces communication efficiency and increases processing time; if Using datagrams may cause out-of-order, lost or duplicate packets, and ultimately all packets need to be sorted; if virtual circuits are used, although there is no order problem, the connection line needs to be established in advance.

1.5.4.4. Two methods of packet switching (datagram, virtual circuit)

Packet switching includes two switching methods : datagram and virtual circuit

Datagram

Detailed process of sending datagram

Detailed process of datagram mode :

1. The source host (A) divides the message into multiple groups (each group plus corresponding control information), and sends them to the directly connected node (A) in turn. This is the only path so there is no way to choose.

2. After node A receives the packet, it will perform error detection and routing selection for each packet . The next node of different packets may be different.

At this time, the P1 packet goes first and selects node C. Then the next time the P2 packet is sent, it will be sent to node D (because node C may be currently congested).

3. After node C receives the packet P1, it performs error detection on the packet P1. If it is correct, it will send a confirmation message to node A. At this time, node A will discard the packet P1 when receiving the confirmation from node C. copy .

Node D will also perform error detection on P2 and send a confirmation message to node A. After receiving the confirmation message, node A will discard the copy of P2.

4. Finally, all the packets arrive at host B, and then host B will reorganize all packets according to their numbers.

Characteristics of datagram mode

Characteristics of datagram mode :

1. Datagram mode provides connectionless services for the network . The sender can send packets at any time, and nodes in the network can receive packets at any time.

无连接服务: The transmission path is not determined in advance for packet transmission. Each packet independently confirms the transmission path. The transmission path of different packets may be different.

2. Different packets of the same message may be out of order, duplicated, and lost when they reach the destination node, but the packets will eventually be rearranged, so there is no need to worry about confusion.

3. Each packet must carry the source address, destination address and group number during transmission.

4. When the packet is stored and forwarded at the switching node, it needs to be queued for processing, which will bring a certain delay. When the traffic through the switching node is large or the network is congested, this delay will be greatly increased, and the switching node may discard some packets according to the situation.

You can rest assured that if some packets are discarded, there will be a certain mechanism to allow the packets to be resent.

5. The network has redundant paths. When a switching node or a link fails, the forwarding table can be updated accordingly and a path can be found to forward the packet .

It has strong adaptability to faults and is suitable for sudden communications, but not suitable for long messages and conversational communications.

virtual circuit mode

Understand virtual circuits and their three stages

Virtual circuits combine datagram mode and circuit switching mode to take advantage of the advantages of both :

虚电路: A path (logical connection) from a source host to a destination host is similar to a circuit. All nodes on the path must maintain the establishment of this virtual circuit and maintain a virtual circuit table. Each item records an open Virtual circuit information.

Circuit switching is also used, which also includes three stages :

①Establish connection (virtual circuit establishment) : The source host sends a "call request". When passing through the node, the call request is forwarded in groups according to the routing table, and forwarded in sequence until host B. At this time, host B receives the "call response" group Only then can the connection be established.

Note that routing and forwarding are only required at the beginning. Later data transmission does not require routing. Instead, the destination address is determined based on the virtual circuit table in the node. During the packet transmission process, the packet does not need to carry the destination address and source address. Just bring the virtual circuit number.

Only when the request packet sent by the source host receives a response, their connection is officially established.

②Data transmission stage : Each packet carries control information such as a virtual circuit number, group number, checksum, etc., and does not need to carry a destination address.

**The reason for not carrying the destination address? **When the connection is established, this path is uniquely determined. The group can only follow this path. There is no need to put the destination address in it. It only needs to follow this route.
A new identifier needs to be carried, which is the virtual circuit number . This virtual circuit number is a unique identifier and is used to indicate which circuit it comes from.
The meaning of virtual circuit number grouping : Mainly if host B manually groups a large number of groups, and there are also messages sent from other hosts in the large number of groups (including those sent by occupying other virtual circuits), then host B will use the virtual circuit number to , classify these groups, for example, put the group with virtual circuit number 1 in one pile, and put the group with 2 in one pile. At this time, the two piles of groups will be restored in the next step, and finally restored into a complete message and handed over to the process. .

③ Release the connection : The source host sends a "release request" to dismantle the virtual circuit. If host b agrees to release the connection, it will return a response and release the circuit piece by piece.

Virtual circuit characteristics

Virtual circuit characteristics :

1. The virtual circuit method provides services for the network layer. A logical connection is established between the source node and the destination node , rather than an actual physical connection.

连接服务: First confirm the transmission path for packet transmission (establish connection, route and forward), then transmit a series of packets along this path (connection), the series of packets have the same transmission path (same path), and the connection is removed after the transmission is completed.

2. All packets in a communication are transmitted sequentially through the virtual circuit. The packets do not need to carry source address, destination address and other information, including the virtual circuit number. Compared with the datagram method, the overhead is small. Different packets of the same message do not arrive at the destination node. May be out of order, duplicated or lost.

3. When the packet passes through each node on the virtual circuit, the node only performs error detection and does not need to perform routing selection.

4. Each node may establish multiple virtual circuits with multiple nodes. Each virtual circuit supports specific data transmission between two end systems and can control the flow of two data endpoints. Each endpoint system There can also be multiple virtual circuits serving different processes.

5. Weakness: When a node or link in a network fails and completely fails, all virtual circuits passing through the node or link will be destroyed.

Because each node maintains a virtual circuit table, when a node fails, it is equivalent to directly disconnecting a virtual circuit, indicating that the availability of virtual circuits is not particularly high.

Comparison of datagrams and virtual circuits

1.5.4. Choice of three data exchange methods

The storage and forwarding of message switching is actually serial , while the storage and forwarding of packet switching is processed in parallel .

Scenario 1: If the amount of data to be transmitted is large and the transmission time is much longer than the call , select 电路交换. Circuit switching transmission delay is minimal.

Scenario 2: When the end-to-end path consists of many links , it 分组交换is more appropriate to transmit data. [Store-and-forward parallel transmission]

Scenario 3: From the perspective of channel utilization , message switching and packet switching are better than circuit switching. 分组交换The delay is smaller than that of message switching, which is especially suitable for burst data communication between computers .

mind map moment

2. Transmission medium

2.1. Transmission media and classification

Physical layer transmission medium : also called transmission medium/transmission medium, it is the physical path between the sending device and the receiving device in the data transmission system.

The previous channel was just a logical path between the sender and the receiver. Here, the transmission medium is a real physical path.

Note : The transmission media is not the physical layer. The transmission media is below the physical layer. Because the physical layer is the first layer of the architecture, the transmission media is sometimes called layer 0. What is transmitted in the transmission medium is a signal, but the transmission medium does not know what the transmitted signal means, but the physical layer specifies the electrical characteristics , so it can identify the transmitted bit stream.

In the transmission media layer, signals are simply transmitted mechanically and brainlessly, and are only used as a physical path for transmission. In this physical layer, the amount of data can be distinguished based on the voltage.
For example: If you see a voltage of 10v or 15v, you will know that the corresponding number is 1. If the voltage is 0v, then the corresponding number is 0. This is the electrical characteristic.

Transmission media can be classified as : 导向性传输介质, 非导向性传输介质, and can also be called guided and non-guided.

Guided transmission medium: Electromagnetic waves are guided along a solid medium (copper wire/fiber optic).

For example: For example, a train travels along a fixed track, which means that the path is arranged in advance.

Non-guided transmission medium: free space, the medium can be air, vacuum, seawater, etc.

For example: An airplane, for example, must follow a certain route when flying in the air, but sometimes it can deviate slightly. It travels in a free space, which corresponds to a non-guided transmission medium.

2.2. Guided transmission media

2.2.1. Twisted pair

Introduction : Twisted pair is the oldest and most commonly used transmission medium. It consists of two copper wires that are twisted side by side according to certain rules and insulated from each other.

The composition of twisted pair : unshielded and shielded twisted pair is a shielded twisted pair with an extra shielding layer.

Copper Wire: Two conductors twisted together like a twist.
Insulation: A layer of insulation placed over the twisted copper wire.
Polyvinyl chloride sheath: It is PVC material, which is a plastic sleeve. At this time, it forms an unshielded twisted pair.

The role of stranding : It can reduce electromagnetic interference to adjacent wires.

Here you can learn about the right-hand rule. If there are two currents in opposite directions, the electromagnetic waves they generate will cancel each other out if they are equal in size. So for a set of two copper wires like ours, the electromagnetic waves or electromagnetic fields generated by the opposite currents can cancel each other out and will not affect each other. (For example, the wires in your own home will not affect the wires in the next door's home to reduce electromagnetic interference to adjacent wires)

In order to further improve the ability to resist electromagnetic interference, a shielding layer made of metal wire can be added to the outside of the twisted pair. This is 屏蔽双绞线（STP）called unshielded twisted pair 非屏蔽双绞线（UTP）.

Let's look at a practical example : one below is an unshielded twisted pair and the other is a shielded twisted pair.

After opening the unshielded twisted pair, you can see that there are 8 lines and four groups of two-phase wires twisted together.
When you open the shielded twisted pair, you can see a shielding layer inside. This is a shielding layer with metal wire braiding. The shielding layer with metal wire braiding is called a shielded twisted pair.

Characteristics of twisted pair : Twisted pair is cheap and one of the most commonly used transmission media. It is commonly used in local area networks and traditional telephones.

Application scenarios : Both analog transmission and digital transmission can use twisted pairs, and the communication distance is generally several kilometers to tens of kilometers.

If the distance is too long, for analog transmission, an amplifier is used to amplify the attenuated signal.
For digital transmission, repeaters are used to shape the distorted signal.

2.2.2. Coaxial cable

Components : Conductor copper core wire, insulation layer, mesh braided shielding layer and plastic outer layer.

Coaxial cables are divided into two categories : 50Ω coaxial cables and 75Ω coaxial cables.

50Ω coaxial cable is used to transmit baseband digital signals, also known as baseband coaxial cable. It has been widely used in local area networks;
75Ω coaxial cable is mainly used to transmit broadband information, also known as broadband coaxial cable, and is mainly used in cable television systems.

Why is it called coaxial ? Because its four structures share the same axis or axis, you can see in the picture below that an arrow through the heart can penetrate these four from the middle.

Practical applications : The interface behind the set-top box, the interface behind the TV

The difference between coaxial cable and twisted pair : Due to the shielding layer of the outer conductor, coaxial cable has better anti-interference performance than twisted pair . It is widely used to transmit higher rate data. Its transmission distance is longer, but its price is higher . Twisted pair cable is more expensive .

2.2.3. Optical fiber

2.2.3.1. Understand optical fiber and photoelectric conversion

Reason : With the rapid development of communications and computers, the requirements for information transmission rate are getting higher and higher. For example, the speed of viewing pictures and videos is getting faster and faster, and they can be loaded in an instant.

The difference from twisted pairs and coaxial cables : optical fibers actually transmit light pulses . For the two types just mentioned, they transmit electrical pulses. That is to say, light waves are actually traveling in the optical fibers, while in coaxial cables it is electromagnetic waves. Walking.

Optical fiber communication : The optical fiber (referred to as optical fiber) is used to transmit light pulses for communication. The presence of light pulses represents 1, and the absence of light pulses represents 0. The frequency of visible light is approximately 10 8 ^MHz , so the bandwidth of the fiber optic communication system is much larger than that of other current systems. Bandwidth of various transmission media.

How does optical fiber realize photoelectric conversion?

Since the optical fiber transmits light pulses, it actually implements a photoelectric conversion process between the sending end and the receiving end. Because we know that the data sent by the computer is 0101, and the signal released is in the form of an electrical signal, so in There should be something on the sending end that converts electrical pulses into light pulses.
The optical fiber has a light source at the transmitting end, which can use light-emitting diodes or semiconductor lasers , which can produce light pulses under the action of electrical pulses; photoelectric conversion is also required at the receiving end, where photodiodes are used to make light detectors. When the light pulse is received, the electrical pulse can be restored.

2.2.3.2. Composition of optical fiber

The composition and realization of optical fiber : It is mainly composed of core (this is solid) and cladding.

Light is conducted in the fiber core, not in the cladding.
The difference in refractive index between the fiber core and the cladding: the refractive index of the core will be higher, so when the medium with high refractive index is input into the medium with low refractive index, the refraction angle will be greater than the incident angle. If the incident If the angle is large enough, total reflection will occur, which means that when the optical fiber hits the cladding, it will bounce back directly without refracting the light wave, so the damage is very small. The entire reflection process is repeated continuously, so the light is like a pinball, bouncing back and forth and transmitted along this optical fiber.

There is only one ray in the picture. If the incident angle is > this critical value, then total reflection can be achieved, and then we can inject multiple rays.

2.2.3.3. Classify according to whether multiple optical fibers can be input (multimode optical fiber, single-mode optical fiber)

At this time, it can be classified according to whether multiple light rays are input : multi-mode fiber, single-mode fiber.

多模光纤: It can emit a lot of light at the transmitting end. These lights can continuously bounce back and forth through the optical fiber, but there will be a certain loss during the bounce process.

Question : Although it is said that total reflection will occur, even if total reflection occurs, some light waves will still be affected by noise or interfere with other electromagnetic waves. If the transmission distance is very long, the distortion will be serious, the output pulse waveform will not be obvious, and the input pulse will not be recognized.
Suitable scene : short distance transmission.

单模光纤: It is to reduce the diameter of the optical fiber to only one wavelength of light, so this optical fiber is like a light, propagating along this straight line and shooting directly, then the output pulse is actually almost the same as the input pulse. , because they do not reflect and there is no loss of broadcast energy.

2.2.3.4. Comparison between multimode optical fiber and single-mode optical fiber (close observation of optical fiber)

The optical fiber diagram is as follows : You can see that the left picture is an optical fiber in real life. Each optical fiber has an optical fiber inside it. The right picture is an optical fiber cable in practical application. There is at least one optical fiber in each optical fiber cable. , many contain ten to one hundred optical fibers. One optical fiber is actually only 0.2 mm. In order to prevent the light from being easily damaged and lost during the transmission process, the optical fibers are bundled together. Just bundle it into an optical cable and then fill it with some filler to form a complete optical cable with very strong pressure resistance.

2.2.3.4. Characteristics of optical fiber

1. The transmission loss is small and the relay distance is long, which is particularly economical for long-distance transmission.

Relay distance: Some companies with relatively large transmission losses will place a repeater at a certain distance. For optical fiber, the loss is very small, so the relay distance is very long, and there is no need to use a repeater in the middle to amplify our waveform.

2. Good anti-lightning and electromagnetic interference performance .

3. No crosstalk interference, good confidentiality, and not easy to eavesdrop or intercept data .

4. Small size and light weight.

What is transmitted inside is light waves. The bandwidth of light waves is very large, and the communication volume of each optical fiber is very large. If the same communication volume is transmitted, compared with optical fibers, twisted pairs are relatively bulky and very thick.

2.3. Non-guided transmission media

Why do we need non-guided transmission media?

Imagine if there were no non-guided transmission media, we would have to connect a very long wire when making a phone call, and everyone's mobile phones would be connected to such a wire. With non-guided transmission media, we can take it out and use it anytime and anywhere.

There are three types that are commonly tested : radio waves, microwaves, and infrared/lasers.

2.3.1. Radio waves

Signal direction : The signal can propagate in all directions. The following figure covers all directions:

Advantages : It is precisely because of this feature that we can communicate with the radio wave transmitter within the effective distance of the receiving device using radio waves without having to correspond to a certain direction , which greatly simplifies our communication connections.

Features : It has strong penetrating ability, can be transmitted over long distances, and is widely used in communication fields (such as mobile phone communications).

2.3.2. Microwave

Introduction : Microwave communication has high frequency and wide frequency range, so the data rate is very high.

Signal direction : The signal propagates in a fixed direction.

It mainly includes two communication applications : terrestrial microwave relay communication and satellite communication.

①地面微波接力通信

Explanation: There are some relay stations on the earth. Each relay station can send a signal to the next designated relay station, and then continue to cycle, and finally complete the relay communication process on the ground on the earth.

②卫星通信

Explanation: The relay station on the ground is actually moved into the air. At this time, a synchronous satellite is formed. This satellite plays the role of a relay station. It can forward signals. Previously, many relay stations may be needed to forward. For satellite communications, we only need three synchronous satellites to forward microwave signals and achieve global communications.

Advantages :

1. Large communication capacity.

2. Far distance.

3. Wide range.

4, Light Wave Communication Watatsu Communication.

Disadvantages :

1. Extended propagation time (250-270ms)

2. Greatly affected by climate (eg: strong winds, sunspot outbursts, solar transit)

3. The bit error rate is high.

The bit error phenomenon means that the final receiving end cannot recognize the waveform and the code element.

4. High cost.

2.3.3. Infrared ray and laser

Signal direction : The signal propagates in a fixed direction.

Introduction : Convert the signals to be transmitted into their respective signal formats, that is, infrared light signals and laser signals, and then propagate them in space.

Compare with microwave: Microwave does not require format conversion, while infrared and laser require format conversion.

mind map moment

3. Physical layer equipment

3.1. Repeater equipment

3.1.1. Understanding repeater equipment

Reason of birth : Due to the existence of loss, the signal power transmitted on the line will gradually attenuate. When the attenuation reaches a certain level, it will cause signal distortion, thus leading to reception errors.

Function : Regenerate digital signal. (Regenerate and restore the signal, amplify the attenuated signal, and keep it the same as the original data, so as to increase the distance of signal transmission and extend the length of the network)

This is regeneration, not ordinary amplification, because the signal needs to be reshaped and then restored to achieve an amplification effect.
This is only for digital signal amplification. If it is for analog signal amplification, it is called an amplifier.

3.1.2. The two ports of the repeater and the regulations at both ends

The repeater has two ports , one of which is used to input the original signal. It can amplify and shape the originally attenuated weak signal, perform a regeneration and restore it, and release it from the other port. At this time, the signal is the same as before. Started just as strong.

Regulations at both ends of the repeater :

The interconnection of two identical types of networks. These two types of networks actually refer to the same type of network segments. Network segments are also divided into centralized types a, b, c, and d. If both ends are to be connected by this repeater, If they are together, then the network segments at both ends should be a or b at the same time. The speed at both ends should also remain the same. [The network segments at both ends must be the same and the speed must be the same]
Repeaters only send data on any cable segment to another segment of cable, acting only on the electrical portion of the signal and regardless of whether the data contains erroneous data or data that does not fit the network segment. [Only used for transmission, no other additional work such as storage and forwarding will be performed]
Both ends can be connected to the same media or different media. For example, one end can be a twisted pair, and the other end can be optical fiber or a general-purpose cable. [The media at both ends can be the same or different]
The network segments at both ends of the repeater must be of the same protocol. [The protocol at both ends is the same]
- The reason is that the repeater does not store and forward, unlike some link layer devices and network layer devices that can store and forward. Once the protocols are different, the repeater doesn't know what to do.

3.1.3. Understand the 5-4-3 rule

Understand the 5-4-3 rule : Network standards have specific provisions on the signal delay range, so repeaters can only operate within the specified range, otherwise network failure will occur.

In the case of non-compliance with the rules, since the process of digital signal regeneration takes time, if the signal passes through many repeaters, then the combined time delay will be very long if each one is processed, which is very difficult for our users. I don't want to encounter it.
Measures : Then we will limit the number of times the repeater can be used.

Next, let’s describe 5, 4, and 3 respectively :

①The 5 refers to a maximum of five network segments. The five network segments circled below are:

②The number 4 refers to that there can only be a maximum of four physical layer network devices in the five network segments, which can be repeaters or hubs.

③3 refers to the three computers that can be connected

3.2. Hub (multi-port repeater)

3.2.1. Understand the functions of the hub

It is actually a multi-port repeater . The functions of repeaters and hubs are very similar. During the transmission process, the signal may suffer loss and attenuation, so in order to prevent the signal from being unrecognized by the receiving end. come out, then some equipment should be placed on this road.

Function : Regeneration, signal amplification.

The function of the hub : regenerate, amplify and forward the signal, amplify the attenuated signal, and then forward it to all other ports (except the input port) that are in working state to increase the distance of signal transmission and extend the length of the network. If there is no signal The directional transmission capability is a shared device .

The following is a diagram of a hub. You can see that a node can be connected to multiple hosts through the hub :

Common topology : star topology.

3.2.2. Understand that the hub can extend the length range, the reasons for sharing equipment, and the problem of conflicting domain equalization of bandwidth

Question 1: Why can a hub increase the distance of signal transmission and expand the length and range of the network?

If host A communicates with host B, it originally only requires 100 meters, but now it becomes 200 meters due to access through the hub.

Question 2: Why is the hub a shared device?

The main thing I'm talking about is broadcasting . At this time, there are five hosts connected to the hub. If the signal sent by host A is sent to the hub, the hub will first amplify the signal and then forward it out through all ports. These ports must be in working status, although four All ports can receive this signal, but if you want to process this signal in the next step, you need to see whether the data restored by this signal is what you need. If you find that the destination address of this data is yours, then leave it and leave the rest. The host finds that it is not its own and can discard it directly.
But for this form of broadcasting, conflicts can easily arise!

Question 3: Why does the hub conflict due to broadcast mode?

If one host A wants to communicate with another host at this time, and another host B also wants to communicate with another host, then when the signal data sent by the two hosts at the same time reaches the hub, conflicts and collisions will occur. .
**How to solve the problem of collision? **Stop and wait, wait for a random time, and then the two of them send data. Only when there is no collision can the data be forwarded correctly. Therefore, the hub is a large conflict domain and cannot divide the conflict domain!

Once a collision occurs, you need to wait for a random event before sending data. If they want to communicate at the same time, all working hosts will share the bandwidth equally. If the current hub is 10MB/s, then if five computers want to communicate at the same time, at this time Divided equally, everyone gets 10/5 = 2MB/s per host.

Disadvantages : If everyone works at the same time, the bandwidth obtained by each host is very low, so the efficiency of the hub is also very low, and everyone's communication volume or communication speed is also very slow.

Organizer: Long Road Time: 2023.7.26-29

Organizer: Long Road Time: 2023.7.26

2024 Postgraduate Entrance Examination 408-Computer Network Chapter 2-Physical Layer Study Notes

Article directory

Preface

1. Communication basics

1.1. Basic concepts of physical layer

1.1.1. Understanding the physical layer

1.1.2. Understand the four interface characteristics of the physical layer

1.2. Basic knowledge of data communication

1.2.1. Typical data communication models and related terms

1.2.2. Terminology related to data communication

1.2.3. Three issues to consider when designing a data communication system:

Question 1: Use simplex communication/half-duplex/full-duplex communication?

Question 2: Use serial communication/parallel communication?

Question 3: Use synchronous communication/asynchronous communication?

1.2.4, code element

1.2.4.1. Understand code elements and what are code elements? (k-ary code element)

1.2.4.2. Two expression methods for data transmission rate of digital communication system (symbol transmission rate, information transmission rate)

Question 1 (code element transmission rate and information transmission rate)

Question 2 (code element transmission rate drives information transmission rate)

Question 3 (Information transmission rate pushes code element transmission rate)

1.2.5. Bandwidth (including analog and digital)

mind map moment

1.3. Nye’s criterion and Shannon’s theorem (investigation and calculation)

1.3.1. Nye’s criterion

1.3.1.1. Understand distortion and factors affecting distortion

1.3.1.2. What is inter-code crosstalk?

1.3.1.3. Solution to inter-code crosstalk: Nye’s criterion

1.3.1.4. Calculate the ultimate data transmission rate (limited code element transmission rate) using the Nye formula

1.3.1.5. Four conclusions of Nye’s criterion

1.3.1.6. Nye criterion calculation exercises

1.3.2. Shannon’s theorem

1.3.2.1. Introduce Shannon’s theorem

1.3.2.2. Understand Shannon’s theorem and calculation formula (limit information transmission rate)

1.3.2.3. Five major corollaries of Shannon’s theorem

1.3.2.4. Shannon’s theorem calculation exercises

1.3.3. What are the differences between Nietzsche and Shannon and how to choose and calculate them for the exam?

1.3.3.1. Understand the difference between the two

1.3.3.2. How to choose theorem calculation in the question? (Including practical examples)

1.4. Preparation and modulation

1.4.1. Prerequisite knowledge: baseband signals and broadband signals

1.4.2. Understand coding and modulation (four coding and modulation methods)

1.4.3. In-depth understanding of the four coding and modulation methods

Digital data encoded into digital signal

Understand six digital signal encoding methods

(1) Non-return to zero coding [NRZ]

(4) Return to zero coding [RZ]

(5) Reverse non-return to zero encoding [NRZI]

(2) Manchester encoding

(3) Differential Manchester encoding

(6) 4B/5B encoding

Digital data is modulated into analog signals (4 digital modulation methods, including calculation questions)

Encoding analog data into digital signals (three steps, sampling, quantization, encoding)

Analog data is modulated into analog signals (involving modulation and demodulation)

1.5. Data exchange method

1.5.1. Why is data exchange required?

1.5.2. Understand data exchange methods

1.5.3. Method 1: Circuit Exchanging

1.5.3.1. Principle of circuit switching

1.5.3.2. Three stages of circuit switching

1.5.3.3. Characteristics of circuit switching

1.5.3.4. Advantages and Disadvantages of Circuit Switching

1.5.3. Method 2: Message exchange

1.5.3.1. Understand message exchange and principles

1.5.3.2. Transmission process of message exchange

1.5.3.3. Advantages and Disadvantages of Message Exchange

1.5.4. Method 3: Packet Exchanging

1.5.4.1. Understand packet switching and principles

1.5.4.2. Packet switching process

1.5.4.3. Advantages and Disadvantages of Packet Switching

1.5.4.4. Two methods of packet switching (datagram, virtual circuit)

Datagram

Detailed process of sending datagram

Characteristics of datagram mode

virtual circuit mode

Understand virtual circuits and their three stages

Virtual circuit characteristics

Comparison of datagrams and virtual circuits

1.5.4. Choice of three data exchange methods

mind map moment

2. Transmission medium