Chapter 1 Introduction to Data Communication
There are three basic transmission methods of data signals : baseband transmission, frequency band transmission and digital data transmission.
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Baseband transmission : The baseband data signal (the unmodulated data signal output by the data terminal) is directly transmitted on the cable channel.
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Frequency band transmission : the baseband data signal is modulated, and its frequency is moved to the corresponding carrier frequency band for transmission (the analog signal is transmitted on the channel during frequency band transmission).
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Digital data transmission : use PCM channel to transmit data signal, that is, use some time slots of PCM30/32 system to transmit data signal.
Definition of data communication : According to the communication protocol, data information is transmitted between two functional units by using data transmission technology, which can realize the transmission of data information between computers, computers and terminals or terminals. The terminal equipment of data communication (which generates data signals) can be a computer, or a general data terminal other than a computer. The general data terminal is referred to as a data terminal or a terminal for short.
Data communication emphasizes the form of the source signal, regardless of the form of the signal on the channel.
The basic composition of the data communication system :
the composition of the data communication system : data terminal equipment, data circuit, central computer system.
Data circuit concept : The data circuit is located between the DTE and the computer system, and its function is to provide a transmission channel for data communication. The binary "1" or "0" data signal is sent and received at both ends of the data circuit, and the data transmission circuit must ensure that the data signal of the DTE is sent to the computer system and sent back to the DTE by the computer system. A data circuit consists of a transmission channel and data circuit terminating equipment (DCE) at both ends.
Transmission channel : including communication lines and communication equipment (can be divided into analog communication equipment and digital communication equipment). The transmission channel can be divided into analog channel and digital channel, the analog signal is transmitted on the analog channel, and the digital signal is transmitted on the digital channel.
DTE is composed of data input device, data output device and transmission controller.
- The role of the transmission controller is to complete various transmission controls, such as error control and terminal continuous control.
DCE is the interface device between DTE and transmission channel. When the data signal adopts different transmission methods, the functions of DCE are different.
- During baseband transmission, DCE performs certain transformations on the data signal from DTE to adapt the signal power spectrum to the channel, even if the data signal is suitable for transmission in the cable channel.
- For frequency band transmission, the DCE is specifically a modem, which is a combination of a modulator and a demodulator. When sending, the modulator modulates the data signal and moves its frequency band to the corresponding carrier frequency band for transmission. When receiving, the demodulator performs demodulation to restore the analog signal to a data signal. ( Note : The transmission channel inside the data circuit is not necessarily in the form of a digital signal)
- When digital data is transmitted, DCE is a data service unit whose function is to transform the signal format, that is, to eliminate the DC component in the signal and prevent the coding of long strings of 1 or long strings of 0, signal regeneration and timing, etc.
Data link concept : The data link is formed after the data circuit plus the data transmission control function. Only after the data link is established can the communication parties communicate effectively and reliably.
PCM modulation principle: analog data –> sampling –> quantization –> coding –> digital signal
to understand the signal-to-noise ratio : the signal-to-noise ratio is used to describe the amount of loss and noise interference in the signal transmission process, and it is important to measure the performance of the transmission system One of the indicators. The signal-to-noise ratio refers to the ratio of signal power to noise power at a certain point, which can be expressed as:
- In the formula, Ps is the average power of the signal, and Pn is the average power of the noise. SNR is usually expressed in decibels (dB).
Asynchronous transmission : Asynchronous transmission is to transmit one character at a time, the position of each character is not fixed, characters can be sent continuously or individually; when no character is sent, the "stop" signal is sent continuously. The advantage of asynchronous transmission is that it is relatively simple to achieve character synchronization, and the clock signals of the sending and receiving parties do not need to be precisely synchronized. The disadvantage is that each character increases the start and stop bits, which reduces the efficiency of information transmission.
Synchronous transmission : Synchronous transmission sends data signals with a fixed clock beat. In the serial data stream, the relative position between characters is fixed, so it is not necessary to add "start" signal and "stop" to each character. " signal, just add a start character in front of a string of character streams, followed by a termination character, indicating the beginning and end of the character stream.
- There are two synchronization methods for synchronous transmission: character synchronization and frame synchronization. Synchronous transmission generally adopts frame synchronization.
- Compared with asynchronous transmission, synchronous transmission is technically more complicated, but it does not need to add and stop code elements for each character separately as a sign to identify characters, but just add a sequence of signs before and after a string of characters, so the transmission efficiency higher.
Effectiveness indicators :
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Operating rate : Three different definitions are commonly used: modulation rate, data transmission rate, and data transfer rate.
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Modulation rate: the number of transmitted signal symbols per second, also known as baud rate, the unit is baud (Baud):
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Data transmission rate: the amount of information transmitted per second (the number of binary symbols transmitted per second), the unit is (bit/s), the greater the degree of uncertainty of information, the greater the amount of information, thus It can be seen that when the signal is M level, that is, M-ary system, the relationship between the transmission rate and the modulation rate is
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Data transmission rate: The average number of bits, characters or code groups transmitted between corresponding devices in the corresponding device system in the data transmission system per unit time.
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Band utilization : The transmission of data signals requires a certain frequency band. The real index to measure the effectiveness of the data transmission system is the transmission efficiency within the unit frequency band, that is, the frequency band utilization rate:
Reliability index : The reliability index is measured by the error rate, and the final index to measure the quality of data transmission is the error rate. Error rate can have many definitions. In data transmission, it is generally expressed by bit error rate, character error rate, and error group rate. They are respectively defined as:
- Bit error rate = number of bits received with errors/total number of bits sent
- Error character (code group) rate = number of accepted characters (code group) with errors/total number of sent characters (code group)
Channel capacity : Channel capacity refers to the maximum amount of information that a channel can transmit per unit time, that is, the maximum transmission rate of a channel. The unit is (bit/s)
- Channel capacity of an analog channel: In a Gaussian white noise channel with limited average signal power, the limit information transmission rate of the channel is: where
B is the channel bandwidth, the ratio of S/N signal power to noise power. - Channel capacity of digital channels: A typical digital channel is a stable, symmetrical, memoryless discrete channel, which can be transmitted in binary or multi-ary systems. A discrete noise-free digital channel with a bandwidth of B can transmit a signal with a maximum symbol rate of 2B. Therefore, the capacity of a noise-free digital channel is:
Time Division Multiplexing Principle : Applicable to Digital Signals
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Time division multiplexing: use the characteristics that each signal occupies different time intervals on the channel to separate each signal. Specifically, the time is divided into uniform time intervals, and the transmission time of each signal is allocated in different time intervals, so as to achieve the purpose of mutual separation. Advantages of time division multiplexing : simple, easy for large-scale integration, and no crosstalk between signals, but time division multiplexing is prone to intersymbol crosstalk, and the channel utilization rate is lower than that of statistical time division multiplexing.
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Statistical time division multiplexing: a method of dynamically allocating line resources according to the actual needs of users. The channel utilization rate of statistical time division multiplexing is high.
Chapter 2 Data Signal Transmission
Baseband data signal drawing :
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Unipolar non-return-to-zero signal (non-return-to-zero signal: NRZ):
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Unipolar return-to-zero signal (return-to-zero signal: RZ):
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Bipolar non-return-to-zero signal:
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Bipolar return-to-zero signal:
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Difference signal:
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Multilevel signal:
Power Spectral Density Conclusion :
- The continuum must exist
- In some cases there may be no discrete spectrum
- 0 in absolute value sign: P=1/2, G1(f)=-G2(f)
Data signal baseband transmission : The method of directly transmitting the baseband data signal without moving the spectrum of the baseband signal is called baseband transmission.
The composition of the baseband transmission system : If the frequency range where the signal energy is most concentrated does not match the transmission characteristics of the actual channel, it will cause serious waveform distortion to the signal at the receiving end. Baseband transmission system model:
- Send filter: limit the signal frequency band and form the waveform required by the system together with the receive filter (for sampling judgment)
- If the waveform is generated entirely by the transmit filter, then:
- Receive filter is only used to limit out-of-band noise
- Equalizer is used to equalize channel distortion
- The role of the sampling decider is to sample at the best time
Ideal low-pass transfer characteristics :
Nyquist's first criterion : When there is no intersymbol interference, the symbol interval is equal to the sampling interval.
The main characteristics of the ideal baseband transmission system :
- Satisfies Nay's first criterion (no intersymbol interference)
- Band utilization reaches limit
- Extremely difficult to physically implement
- The waveform tail is very long, the decay is very slow, and when the timing is deviated, severe intersymbol interference may occur.
A partial response system has the following characteristics :
- There is intersymbol interference, but it is fixed and can be eliminated at the receiving end;
- The frequency band utilization can reach the limit of 2Baud/Hz;
- The leading and trailing tails of the formed waveform attenuate quickly, which reduces the accuracy requirements for the receiving end;
- physically achievable;
- The number of received signal levels is greater than the number of sent signal levels, and the anti-interference performance is worse.
Scrambling and descrambling of data sequences : a short period of continuous "0" or continuous "1" and some short-period deterministic data sequences, such data signals are unfavorable to the transmission system, mainly because:
- Causes interference to adjacent channel data signals
- Make it difficult for the receiving end to extract timing information
- Causes loss of equalizer adjustment information
To sum up, in order for the data transmission system to work normally, it is necessary to ensure the randomness of the input data sequence. In order to achieve this, in the data transmission system, the input data sequence is often scrambled first at the sending end. After the scrambled data sequence is transmitted through the system, it is restored to the original data sequence at the receiving end, which is descrambling.
The basic principle of scrambling and descrambling : the most effective data sequence scrambling method is to use a random sequence to logically add the input data sequence. Descrambling is to perform the same operation on the received scrambled sequence and the random sequence at the time of scrambling to recover the original data sequence.
The composition of the frequency band transmission system : the difference between the frequency band transmission system and the baseband transmission system is that the modulation is added at the sending end, and the demodulation is added at the receiving end to realize the spectrum conversion of the signal.
The composition of the frequency band transmission system :
- Modulation : The baseband signal is used to control some parameters of the carrier waveform, so that these parameters change with the baseband signal. Therefore, the modulated baseband signal is a digital signal, so it is also called digital modulation.
- In the modem, the sine (or cosine) signal is selected as the carrier, because the sine signal is simple in form and easy to generate and receive. Since the sine (or) cosine signal has three basic variables of amplitude, frequency, and response, it can constitute three basic modulation methods: digital amplitude modulation , digital phase modulation, and digital frequency modulation .
Digital amplitude modulation : Controlling the amplitude of a carrier with a baseband data signal is called digital amplitude modulation, also known as shift keying, abbreviated as ASK.
- Principle : 2ASK is to use the baseband rectangular pulse representing the digital information "0" or "1" to key a continuous carrier, so that the carrier is output intermittently. When there is a carrier output, it means sending "1", when there is no carrier output, it means sending "0"
- Generally, there are two methods for generating binary digital amplitude modulation (2ASK) signals: multiplication and keying. The multiplication method is to multiply the baseband signal s(t) with the carrier, and the keying method is to use the baseband signal s(t) to control the switching circuit of the carrier.
- Demodulation of the shift keyed signal :
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Coherent demodulation : use the coherent characteristics of electromagnetic waves with the same frequency and phase (transmitter and multiplier)
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Bandpass Filters: Pass Signals, Block Noise
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Multiplier: to achieve frequency multiplication
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Low-pass filter: filter out octave components
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Non-coherent demodulation : using low-pass filtering technology, also known as envelope detection method.
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Digital phase modulation : The phase of the carrier is controlled by the baseband data signal, which is called digital phase modulation, also known as phase shift keying, abbreviated as PSK.
- In binary digital modulation, when the phase of the sinusoidal carrier varies discretely with the binary digital baseband signal, a binary phase-shift keying (2PSK) signal is generated.
- Binary digital phase modulation (2PSK) uses two phases of the carrier to represent binary "1" and "0". This method of directly representing the baseband signal with different phases of the carrier is generally called absolute phase modulation.
- The conversion rule of the binary phase-modulated signal is: "1" of the data signal changes the phase of the modulated signal by 0°phase; "0" of the data signal causes the phase of the modulated signal to change by 180°phase.
Frequency band utilization of multi-ary digital phase modulation : If the baseband formation adopts a roll-off low-pass filter, and its roll-off coefficient is α, then the frequency band utilization of polyphase phase modulation is: quadrature 
amplitude modulation : also known as quadrature bilateral with modulation. It is the process of superimposing two independent baseband waveforms corresponding to two mutually orthogonal same-frequency carriers to suppress carrier double-sideband modulation, and the obtained two modulated signals are superimposed.
- Since quadrature amplitude modulation uses double-sideband transmission, the bandwidth of the modulation system should be twice the bandwidth of the baseband signal, that is
- The frequency band utilization of multi-ary quadrature amplitude modulation (MQAM) is :
- Here, M is the number of star points, and its value can be 4, 16, 64, 256, etc. The larger the value of M, that is, the more the number of star points, the higher the utilization rate of the frequency band. However, the larger M is, the smaller the space distance between star points is in the same signal space, the anti-interference ability of the system will decrease, and the bit error rate will increase.
Digital FM : Controlling the frequency of the carrier with a baseband data signal is called digital FM, also known as frequency shift keying (FSK)
- The bandwidth of 2FSK with discontinuous phase is about B=2fs+|f1-f0|=(2+h)fs, where h=|f1-f0|/fs is called the frequency shift index
The concept and characteristics of digital transmission of data signals : the transmission of data signals in digital channels is called digital transmission of data signals, referred to as digital data transmission.
- Compared with the transmission mode of analog channel, using digital channel to transmit data signal has two main advantages:
- high transmission quality
- High channel transmission rate
Chapter 3 Error Control
Error control methods : In data communication systems, error control methods can generally be divided into four types: error detection and retransmission, forward error correction, hybrid error correction and error detection, and information feedback
- When implementing the error detection and retransmission system, there are usually three retransmission modes, that is, stop and wait for retransmission, return to retransmit and select retransmission.
Coding efficiency :
The relationship between Hamming distance and error detection and error correction capabilities : In channel coding, the number of non-zero symbols in a code group is defined as the weight of the code group, referred to as code weight. For example: the code weight of " 010 " code group is 1, and the code weight of " 011 " code group is 2, the number that has different binary symbols on the corresponding code position in two code groups is defined as the distance of two code groups, It is referred to as the code distance; in a code, the minimum value of the distance between any two allowed code groups is called the Hamming distance of this code.
Parity supervision code ( understanding ): This is the simplest error detection code, also known as parity check code, which is widely used in data communication. The coding rule is to first group the data symbols to be transmitted, and add a 1-bit supervisory code after the grouped information code, so that the number of "1"s combined with the information code and the supervisory code in the code group is an even number (called even parity) or odd (called odd parity).
Two-dimensional odd-even supervisory code ( understanding ): Two-dimensional odd-even supervisory code has strong error detection ability, has certain error correction ability, and is easy to implement, so it is widely used.
Hamming code : Hamming code is a linear block code that can correct one bit error code and has high coding efficiency.
linear block code (operation)
- The main properties of linear block codes are:
- closure
- Minimum distance in yards equal to minimum weight in non-zero yards
Cyclic code (computation)
- The working process of the cyclic code encoder:
Chapter 4 Data Exchange
The principle of message exchange mode :
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Concept: Message exchange belongs to the store-and-forward exchange method. When the user's message arrives at the switch, the message is first stored in the memory of the switch, and when the required output circuit is free, the message is sent to the receiving switch. or user terminal.
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Principle :
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Advantages and disadvantages :
- advantage:
- Allows different types of terminal equipment to communicate with each other.
- Greatly improve line utilization
- Users can send packets without calling the other party, so there is no call loss
- Same text message communication can be realized
- shortcoming:
- The transmission delay of information is large, and the change of delay is also large
- The message switch is required to have high-speed processing capability and a large buffer memory capacity, so the equipment cost of the switch is high.
- advantage:
Advantages and disadvantages of packet transmission :
- advantage:
- high transmission quality
- Strong reliability
- Provide convenience for different types of terminals to communicate with each other
- Can meet the real-time requirements of communication
- Realize packet multiplex communication
- Economical
- shortcoming:
- The transmission efficiency of long message communication is relatively low
- Requires switches with high processing power
Necessary conditions for the development of frame relay :
- The use of optical fiber transmission lines
- Intelligentization of user terminals
ATM signal : ATM cell length is 53 bytes, which is beneficial to hardware implementation.
Features of ATMs :
- ATM works in a connection-oriented manner
- ATM adopts asynchronous time division multiplexing
- There is no link-by-segment error control and flow control in ATM network
- The functionality of the letterhead is simplified
- ATM uses fixed-length cells, and the length of the information segment is small
Comparison of centralized exchange methods :
Chapter 5 Data Communication Network Architecture
HDLC frame structure : In the advanced data link control procedure, the frame is used as the basic unit of information transmission on the link. The basic format of the HDLC frame is as follows:
TCP/IP layered model : The TCP/IP layered model and its correspondence with the OSI reference model are as follows:
Appendix Topic Knowledge Points
Chapter One
1-3
- The transmission rate is the main index to measure the transmission capacity of the data communication system, transmission rate = character data bits / total character length
- Note: one bit for start position and one stop position
1-5
- Modulation rate: N=1/T Unit: bd
- Data transmission rate: R=Nlog2M unit: bit/s
1-6
- Bit error rate = number of bits received with errors/total number of bits sent
1-7
1-10
- According to the network topology, the data communication network can be divided into mesh network and incomplete mesh network, star network, tree network and ring network.
Chapter two
2-1
- In the "0" differential code, the symbol "0" is represented by changing the polarity of the two adjacent symbol levels, and the symbol "1" is represented by the same level polarity of the two adjacent symbols.
- In the "1" differential code, the symbol "1" is represented by changing the polarity of the two adjacent symbol levels, and the symbol "0" is represented by the same two adjacent symbol level polarities.
- Manchester encoding, also known as phase encoding, is a synchronous clock encoding technique used by the physical layer to encode the clock and data of a synchronous bit stream; commonly used for LAN transmission. In Manchester encoding, there is a jump in the middle of each bit, and the jump in the middle of the bit is used as both a clock signal and a data signal.
- Each bit of Manchester encoding only occupies half of the clock cycle. When transmitting "1", it is high in the first half of the clock cycle and low in the second half; while transmitting "0" is positive on the contrary. In this way, there must be a jump in each clock cycle, and this jump is the bit synchronization signal.
- Differential Manchester coding is an improvement of Manchester coding. It has a jump in the middle of each clock bit, and whether the transmission is "1" or "0" is distinguished by whether there is a jump at the beginning of each clock bit.
2-2 and 2-3
- Baseband transmission system conforming to Nyquist's first criterion: If Nyquist's first criterion is met, then |H(f)| should have an odd symmetric roll-off with (fn, 0.5)
- Nessler frequency band fn=2600+(3200-2600)/2 Unit: HZ
- (Symbol rate) Symbol rate Nbd=fs=2fn Unit: Bd
- Roll-off coefficient α=(3200-fn)/fn (roll-off coefficient is the part of the frequency band that is more than the Nyquist frequency point divided by the Nyquist bandwidth) Unit: None
- Transmission rate R=fslog2M (M refers to M level) unit: bit/s
- Transmission efficiency n=R/B
- Band utilization n=2/(1+α)
2-4
- Calculate the gain weighting coefficient or the value of waveform y(t) at each sampling point: (Bring in the value calculation equation group)
2-5
- B2ASK=2BASK=2*1/T=2fs (note: the first zero value of the bipolar (not) return-to-zero code power spectrum)
- n=fs/B
2-6
- The bandwidth of 2FSK with discontinuous phase is about B=2fs+|f1-f0|=(2+h)fs
- Where h=|f1-f0|/fs is called the frequency shift index
2-7
- If the binary sequence used for modulation is unipolar code, it is called 2ASK with non-suppressed carrier frequency; if it is applied to modulation with bipolar code, it is called 2ASK with suppressed carrier frequency
2-8
- fc=2N
- Two carriers are drawn in one symbol period to illustrate
2-9
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The frequency band utilization of multi-ary quadrature amplitude modulation (MQAM) is:
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Bandwidth of roll-off network: B=(1+α)fn
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Total bit rate R=fslog2M
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Quadrature amplitude modulation uses double-sideband transmission, so the bandwidth of the modulation system should be twice the bandwidth of the baseband signal, that is
2-10
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Quadrature amplitude modulation uses double-sideband transmission, so the bandwidth of the modulation system should be twice the bandwidth of the baseband signal, that is
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Signal rate R=fslog2M
2-11
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Quadrature amplitude modulation uses double-sideband transmission, so the bandwidth of the modulation system should be twice the bandwidth of the baseband signal, that is
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Signal rate R=fslog2M
2-12
third chapter
3-1
- The position in the sent data sequence and the received data sequence is the same but the code element is different, the code element of the error sequence is "1", otherwise it is "0"
3-2
- Error control methods: In data communication systems, error control methods can generally be divided into four types: error detection and retransmission, forward error correction, hybrid error correction and error detection, and information feedback
- Advantages and disadvantages of error detection retransmission (ARQ):
- Because the ARQ method needs to send a NAK signal through the reverse channel after detecting an error at the receiving end, and requires the sending end to retransmit, so the reverse channel is required, and the real-time performance is poor;
- In the ARQ mode, there are not many supervision codes added after the information code, so the information transmission efficiency is high
- Decoding equipment is relatively simple
- Forward error correction (FEC) advantages and disadvantages:
- The error correction method does not require a reverse channel, can automatically correct errors, and does not require retransmission, so the delay is small and the real-time performance is good
- The disadvantage is that the selected error correction code must closely match the error code characteristics of the channel, otherwise it is difficult to meet the requirements of reducing the error rate
- In order to correct more error codes, the decoding equipment is complicated, and more supervisory codes are required, and the transmission efficiency is low.
- Hybrid Error Correction (HEC) Advantages and Disadvantages
- The hybrid error correction and error detection method is a compromise between forward error correction and error detection and retransmission in terms of real-time and decoding complexity, so it has been widely used in data communication systems in recent years.
- Information feedback (IRQ) advantages and disadvantages
- The advantage of this method is that there is no need for error correction and error detection compilers, and the equipment is simple
- The disadvantage is that the same reverse channel as the forward channel is required, and the real-time performance is poor
- The sending end needs a memory with a certain capacity to store the sending code group. The larger the loop delay and the higher the data rate, the larger the required storage capacity
3-3
- ACK is an acknowledgment character. In data communication, a type of transmission control character sent by the receiving station to the sending station, indicating that the sent data has been confirmed to be correct.
3-4
- Select retransmission: The sender indicates to resend all the previous N code groups, but only resends the error code group
- The transmission efficiency of the selective retransmission system is the highest, but its cost is also the most expensive, because it requires more complex control, and data buffers are required at both the sending end and the receiving end.
3-5
- Properties of Linear Block Codes
- The minimum distance in yards is equal to the minimum weight in non-zero yards
3-6
- In order to detect e wrong codes, the minimum code distance is required to be d>=e+1
- In order to correct t wrong codes, the minimum code distance d>=2t+1 is required
- In order to correct t wrong codes and simultaneously detect e (e>t) wrong codes, the minimum code distance is required to be d>=e+t+1
3-7
- The information code sequence is arranged in a square matrix by rows, and an odd or even supervision code is added after each row, that is, each row is an odd-even supervision code group, but when sending, it is transmitted in the order of columns, and the receiving end still arranges the code elements in the same order as The same square matrix form at the sending end, and then perform parity checks by row.
- Since the sending end sends symbols by column rather than by code group, the burst errors that may have occurred in a certain code group are scattered among the various code groups of the square matrix, so the entire square matrix can be obtained Using this method, all odd errors in a row and all burst errors whose length is not greater than the number of rows in the square matrix can be found.
3-10
- Generally speaking, if the code length is n and the number of information bits is k, then the number of supervision bits is r=nk. If it is desired to use r supervision bits to construct r supervision relational expressions to indicate n possible positions of a bit error code, it is required that:
3-11
- Ask for supervision code:
3-12
- For the wrong position, refer to the corrector and wrong code position table
3-13
- The first code group given in the table is shifted one bit to the left to get the second code group, the second code group is shifted one bit to the left to get the third code group, and so on, the eighth code group is all 0.
3-15
- The highest power index is nk
- find generator matrix
- Finding Typical Generated Matrix by Matrix Transformation
- Find the supervision bit and the whole code group of the specified information code
big question paper
- A simple comparison of the similarities and differences between Nyquist's first criterion and Nyquist's second criterion
- The first criterion shows that there is no intersymbol interference, within the frequency band fn, 2fn is the limit rate, and the highest frequency band utilization rate of all digital transmission systems is 2Baud/Hz
- The second criterion introduces a certain controlled intersymbol interference and compresses the transmission frequency band, which can reach the theoretical limit of 2Baud/Hz, and at the same time reduce the requirement for timing accuracy