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Today, I saw an article about the perfect summary of the basic knowledge points of data communication. Although the title is a bit... However, if you want to understand data communication and learn data communication, you should really understand these basic knowledge~~
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Knowledge summary: Data communication basics
I. Basic concepts
1. Signal
Information is a collection of phenomena and their attribute identifiers, which eliminates uncertainty. Data is a carrier that carries information. Signal is the physical manifestation of data, such as electrical or electromagnetic.
According to the different ways in which the parameters representing the message in the signal are taken, signals can be divided into two categories:
(1) Analog signal: a continuous signal, the value of the parameter representing the message is continuous.
(2) Digital signal: a discrete signal, the value of the parameter representing the message is discrete.
2. Frequency Frequency
in physics is the number of times a vibration is completed per unit time, and is a quantity that describes the frequency of the reciprocating motion of a vibrating object. Frequency in signal communication is often a measurement that describes the number of pulses that appear in a periodic cyclic signal per unit time. Frequency is usually represented by the symbol f or v, and the unit is Hertz (second-1). Common unit conversion: 1kHz=1000Hz, 1MHz=1000kHz, 1GHz=1000MHz.
The frequency range of human hearing is about 20~20000Hz, and ultrasound is not perceived by the human ear; the visual stay of a person is about 1/24 second, so the frame rate of film and television is generally 24~30fps; China's power supply is 50Hz sinusoidal alternating current, that is, it makes 50 periodic changes in one second; the GSM (Global System for Mobile Communications) system includes several frequency bands such as GSM 900: 900MHz, GSM1800: 1800MHz and GSM1900: 1900MHz; WiFi (802.11b/g) and Bluetooth (bluetooth) work at 2.4GHz.
3. Signal Bandwidth
Signal bandwidth is the width of the signal spectrum, which refers to the frequency range contained in the signal, and is the difference between the highest frequency and the lowest frequency of the signal. For example, twisted copper wire provides a frequency band of 300~3400Hz for traditional analog phones, that is, the telephone signal bandwidth is 3400-300=3100Hz.
4. Data Communication System Data
communication system realizes the transmission of information. A complete data communication system can be divided into three major components:
(1) Information source (source system: sender, sender)
(2) Channel (transmission system: transmission network)
(3) Information destination (destination system: receiver, receiver)
5. Channel Bandwidth Channel
refers to the channel for transmitting signals in a communication system. Channels include communication lines and transmission equipment. According to the transmission medium used by the channel, it can be divided into wired channels and wireless channels; according to the type of signal suitable for transmission, it can be divided into analog channels and digital channels.
Channel bandwidth refers to the effective frequency range of electromagnetic waves allowed to be transmitted on the channel. The bandwidth of an analog channel is equal to the difference between the upper and lower limits of the signal frequency that the channel can transmit, and the unit is Hz. The bandwidth of a digital channel is generally expressed in channel capacity. Channel capacity is the maximum data transmission rate allowed by the channel, and the unit is bit/second (bit/s, bps). Unit conversion: 1kbps=1000bps, 1Mbps=1000kbps.
(1) Data transmission rate
The data transmission rate is the number of bits transmitted per unit time: R = log2N/T.
R—data transmission rate
T—signal code element period (seconds)
N—number of signal code element states, also known as phase number, log2N is the number of bits required for the required encoding.
1/T is called the baud rate, also known as the modulation rate, which is the number of changes in the signal code element per unit time, and the unit is baud (Baud).
Example: In a frequency band transmission data communication system, 16-phase modulation coding is used, and the signal code element period length is 1/3200s. What is the data transmission rate of the system?
Solution: 16-phase modulation coding means that there are 16 code element states, which require log216=4bit for encoding (i.e. 8421BCD code). The signal code element period length is 1/3200s, and the baud rate is 3200, that is, 3200 code elements are modulated per second, so the data transmission rate is 3200*4=12800kbps.
(2) Channel capacity follows Shannon's theorem: C = B·log2(1+S/N)(bps)
C is the channel capacity,
B is the channel bandwidth,
S is the average signal power,
N is the average noise power
, and S/N is the signal-to-noise power ratio of the channel. The signal-to-noise ratio is generally expressed as 10log10(S/N) in decibels (dB).
Example: Find the data transmission rate of traditional telephone modem.
Solution: The frequency range supported by the telephone connection is 300~3300Hz, so B=3300Hz-300Hz=3000Hz, and the typical signal-to-noise ratio of a general link is 30dB, that is, S/N=1000, so C=3000×log2(1001), which is approximately equal to 30Kbps. The actual measured modulation and demodulation rate limit is generally around 28.8Kbps.
(3) Channel capacity limit
In any channel, the rate of symbol transmission has an upper limit, otherwise there will be inter-symbol interference problems. Inter-symbol interference is the situation that the tail of the previous symbol is too long and overlaps with the next symbol due to the influence of noise in the channel, making it impossible to identify the digital signals at the receiving end. In
1924, Nyquist derived the famous Nyquist criterion. He gave the upper limit of the symbol transmission rate under ideal conditions to avoid inter-symbol interference.
The formula for the maximum symbol transmission rate under ideal low-frequency channels is:
Maximum symbol transmission rate under ideal low-frequency channels = 2W Baud
(1) W is the bandwidth of the ideal low-frequency channel, in Hertz;
(2) Baud is baud, that is, the unit of symbol transmission rate, 1 baud means 1 symbol is transmitted per second.
Another way to express the Nyquist criterion is: The maximum symbol transmission rate of the ideal low-frequency channel per Hertz bandwidth is 2 symbols per second. If the symbol transmission rate exceeds the value given by the Nyquist criterion, inter-symbol interference will occur, so that the receiving end cannot correctly determine whether the symbol is 1 or 0.
6. Baseband and Broadband
Baseband refers to the frequency band inherent in digital pulse signals.
Broadband originated from the telephone industry. The frequency band that carries signals with a frequency exceeding the fixed-line operating frequency (approximately 4kHz) is called broadband.
II. Timing and Frequency
1. Digital Logic Circuits
Digital electronic circuits transmit pulses, which are used to represent binary digits. For example, a high level represents 1 and a low level represents 0. After digital processing, information such as sound, images, and text becomes a series of electrical pulses, which are called digital signals. Circuits that can process digital signals are called digital circuits.
Because the 1 and 0 in the circuit also have logical meanings, for example, logical 1 and logical 0 can respectively represent the connection and disconnection of the circuit, the yes and no of an event, the true and false of logical reasoning, etc. There is a logical relationship between the output and input of the circuit. In addition to binary arithmetic operations, this circuit can also complete logical operations and have logical reasoning capabilities, so it is also called a logic circuit.
Because digital logic circuits have the advantages of easy integration, high transmission quality, and the ability to calculate and logically reason, they are widely used in computers, automatic control, communications, measurement and other fields. In general household appliances, such as timers, alarms, controllers, electronic clocks, and electronic toys, digital logic circuits are used.
2. Computer timing and processor frequency
The computer is essentially a large-scale, highly integrated digital logic circuit. The execution of an instruction in a computer can be decomposed into several basic micro-operations. These micro-operations are executed strictly in chronological order under the action of the clock pulse signal provided by the computer. These orders are the timing of the computer. Timing studies the relationship between various signals in the execution of instructions.
The computer system needs to work in a unified pace, just like people work and live according to a certain time pattern. Computers also need such a clock. The computer's quartz clock is a clock oscillation generated by a quartz crystal oscillator and an oscillator integrated circuit. It is the pulse of the computer. This crystal oscillator is 32768Hz, that is, 32.768KHz (kilohertz). The computer time we usually see is generated by its frequency division. In addition, there is a crystal oscillator of 14.318MHz, which provides the frequency required for the CPU external frequency PCI bus, RAM, etc. after being multiplied by the frequency multiplication circuit.
The CPU's main frequency is the clock frequency of the CPU core (CPU Clock Speed). It is usually said that a certain CPU is so many megahertz, and this so many megahertz is the "CPU's main frequency". Many people think that the CPU's main frequency is its running speed, but it is not. The CPU's main frequency indicates the speed of the oscillation of the digital pulse signal in the CPU. Now the mainstream CPU main frequency is around 2GHz. The CPU's main frequency has no direct relationship with its actual computing power.
3. Signal digitization
Contemporary computer systems basically use Leibniz binary. Only after discrete digitization (i.e., analog-to-digital conversion as we usually refer to) can signals be handed over to computer systems for analysis and processing.
1. Sampling
Under certain conditions, a continuous-time signal can be completely represented by the values or samples of the signal at equal time intervals, and the signal can be completely restored based on these sample values. The process of converting continuous quantities in the time domain or space domain into discrete quantities is called sampling, also known as sampling or sampling. For example, a movie is composed of a group of frames, each of which represents an instantaneous picture (i.e., a time sample) in a continuously changing scene.
The importance of sampling lies in its role as a bridge between continuous-time signals and discrete-time signals. It provides a theoretical basis for representing continuous-time signals with discrete time series. When using a waveform in the time domain to represent a digital signal, the basic waveform representing different discrete values is called a code element. When using binary coding, there are only two different code elements, one representing the 0 state and the other representing the 1 state.
2. Signal sampling
Multiple signals may have exactly the same value at the time points that are integer multiples of T, that is, x1(kT)= x2(kT)= x3(kT). Obviously, an infinite number of signals can generate a given set of sample values. For example, when the frame rate of a movie is less than 24fps (that is, less than 24 shots are taken in one second), the playback will not be smooth. The sudden change between frames cannot reproduce the original image well, making the audience feel distorted. From the perspective of sampling, it is undersampling, and too much information is missing between two frames, so that it is too uneven, which makes the audience imagine infinitely.
If a signal is band-limited (that is, its Fourier transform is zero outside a certain limited frequency band), and its samples are obtained densely (relative to the highest frequency in the signal), then these sample values can be used to uniquely characterize the signal, and the signal can be completely recovered from these samples. In 1928, American electrical engineer Nyquist found that if the original waveform is not to produce "half-wave loss", the sampling frequency should be at least twice the highest frequency of the signal. This is the famous Nyquist sampling theorem. Nyquist sampling theorem explains the relationship between sampling frequency and signal spectrum, and is the basis for the discretization of continuous signals.
The United States and Japan use the NTSC standard. In the early days, the NTSC standard used a 60-frame interlaced method, that is, half a frame of the picture was played every 1/60 second. Later, after entering the digital TV era, it was changed to a 30-frame progressive method, that is, a picture was played every 1/30 second. China's television standard uses the PAL standard, which adopts a 50-frame interlaced or 25-frame progressive display method. The
frequency range of the voice signal of a home fixed telephone is 300~3400Hz, with a maximum of 4kHz. According to the sampling theorem, a sampling frequency of 8kHz is required, which is equivalent to a sampling period of T=125us. After sampling, the continuous telephone signal becomes 8000 discrete pulse signals per second, and its amplitude corresponds to the value of the telephone signal at the sampling moment. Assuming that each sampling point is digitally encoded with an 8-bit binary number (256 states), the data rate is 8bit*8000Hz=64kbit/s. This rate is the earliest standard rate for voice coding. The modulation and demodulation rate of digital telephone is generally around 56kbps.
3. Signal reconstruction
Reconstructing a signal from a sample involves interpolation technology. Interpolation is the fitting of a set of sample values with a continuous signal. It is a commonly used process to reconstruct a function from sample values. Simple interpolation includes zero-order hold and linear interpolation. In more complex interpolation methods, high-order polynomials or other mathematical functions can be used to fit between sample points. This process belongs to the category of mathematical modeling.
IV. Data Coding
1. Coding and Decoding
Coding refers to a technology that uses close-up numbers to represent database information that needs to be processed. It is a technology that converts data into codes or coded characters based on certain data structures and qualitative characteristics of targets, represents data composition in data transmission, and serves as a set of rules and conventions for transmission, reception and processing. Coding is usually a process in which more inputs are quantized into fewer outputs (code groups).
Decoding or decoding is the inverse process of coding, and at the same time removes the noise mixed in the bit stream during the transmission process. The process of using a decoding table to translate text into a group of digital numbers or using a decoding table to translate a series of signals representing a certain information into text is called decoding. Decoding is usually a process of converting less input into more output, which is generally divided into two categories: 2n decoding and 8421BCD code decoding.
Morse telegraph code is a signal code that is sometimes on and sometimes off, expressing different English letters, numbers and punctuation marks through different arrangement orders. Morse code is an early form of digital communication, but it is different from modern binary code that only uses zero and one states. Its code includes five types: dots, dashes, short pauses between each character, medium pauses between each word, and long pauses between sentences. American standard ASCII code is a standard encoding that specifies 128 numbers from 0 to 127 to represent information. A character in a computer can be represented by an eight-digit binary number: log2128=8.
2. Modulation and demodulation
The signal from the source is often called the baseband signal, that is, the basic frequency band signal. The data signals representing various media output by the computer are all baseband signals. Baseband signals often contain more low-frequency components, and even DC components, but many channels cannot transmit such low-frequency components or DC components. In order to solve this problem, the baseband signal must be modulated.
Modulation can be divided into two categories. One is to transform the waveform of the baseband signal only so that it can adapt to the channel characteristics. This type of modulation is called baseband modulation, and the transformed signal is still a baseband signal. The other type requires the use of a carrier for modulation, moving the frequency range of the baseband signal to a higher frequency band for transmission in the channel. The signal modulated by the carrier is called a passband signal (that is, it can only pass through the channel within a certain frequency range), and the modulation using the carrier is called passband modulation.
The hardware that receives a string of data bits and modulates the carrier according to these bit strings is called a modulator, and sometimes it is also called a synthesizer; the hardware that receives the carrier and recovers the binary bits of the data modulated on the carrier is called a demodulator, and sometimes it is also called a splitter. In full-duplex communication, either end needs to send both modulation and demodulation, so the two are often combined into a modem.
Modulation is a signal embedding synthesis technology; demodulation is the inverse process of modulation, which separates and extracts the modulated signal.
3. Digital signal encoding of digital data
The baseband signal directly represents the digital signal 1 or 0 with two different voltages (transitions), and then sends it to the line for transmission. This transmission method is called baseband transmission. Baseband transmission involves digital signal encoding of digital data.
Digital signals generally have only two states, 0 or 1, and can be encoded and represented by switch variables (high and low voltage) or voltage transitions (differential).
3.1. Non-return-to-zero encoding (NRZ)
3.2. Manchester encoding
(1) Manchester encoding (Manchester)
Manchester encoding is also called digital biphase code. A high-to-low level transition represents a digital 0, and a low-to-high level transition represents a digital 1.
The IEEE802.3i Ethernet standard (10BASE-T) duplex transmission bit stream uses Manchester encoding: RJ45 twisted pair uses (1, 2) wire pairs to transmit differential signals, and (3, 6) wire pairs to receive differential signals.
(2) Differential Manchester coding
Differential Manchester coding is also called conditional biphase code (CDP code). If the next digit is 0, a level jump occurs between code elements; if the next digit is 1, no level jump occurs between code elements.
3.3. Others
For a channel with a certain bandwidth, if the signal-to-noise ratio cannot be improved and the code element transmission rate has reached the upper limit, there is still a way to increase the information transmission rate. This is to use coding to allow each code element to carry more bits of information.
Assume that our baseband signal is 101011000110111010...
If it is transmitted directly, the amount of information carried by each code element is 1 bit. First, group the 3 bits in the signal into a group, that is, 101, 011, 000, 110, 111, 010, .... There are 8 different arrangements of 3 bits. We can use different modulation methods to represent such a signal. For example, use 8 different amplitudes, 8 different frequencies, or 8 different phases for modulation. Assume that we use phase modulation, with phase φ0 representing 000, φ1 representing 001, φ2 representing 010, ..., φ2 representing 111. In this way, the original 18-symbol signal is converted into a symbol consisting of 6 symbols:
101011000110111010…=φ5φ3φ0φ6φ7φ2…
That is to say, if the code element is sent at the same rate, the amount of information transmitted in the same time is increased by 3 times.
Other common coding schemes are as follows.
(1) Non-return to zero alternating (NRZI)
(2) 4B/5B coding: IEEE 802.3u 100BASE-TX, IEEE 802.3u 100BASE-FX
(3) 8B/10B coding: 100BASE-T4 uses 8B/6T (6 pairs of 3 bits and 8 states)
(4) 4D-PAM5 coding: Gigabit Ethernet 1000 BASE-T
4. Modulation and coding of digital data
Broadband signals are frequency-division multiplexed analog signals formed by modulating baseband signals. The most basic bandpass modulation methods are:
(1) Amplitude modulation (AM): The amplitude of the carrier varies with the baseband digital signal. For example, 0 corresponds to no carrier output, while 1 corresponds to carrier output. Medium wave broadcasting uses amplitude modulation.
(2) Frequency modulation (FM): The frequency of the carrier wave changes with the baseband digital signal. For example, 0 corresponds to frequency f1, and 1 corresponds to frequency f2. It can be seen that the waveform of the FM wave is like a spring that is compressed unevenly. The demodulator (separator) uses filtering to extract the source signal of the specified frequency band.
In FM radio, the high-frequency modulated wave distinguishes the radio channels, and the carrier wave carries the sound signal. The frequency of the sound signal generally does not exceed 15KHz, and the frequency range of FM radio in China is 87.5-108MHz (belonging to the ultra-short wave band: the wave speed is constant, the higher the frequency, the shorter the wavelength), so the sound fidelity is very high.
(3) Phase modulation (PM): The initial phase of the carrier wave changes with the baseband digital signal. For example, 0 corresponds to a phase of 0 degrees, and 1 corresponds to a phase of 180 degrees. Phase modulation is divided into absolute phase modulation, relative phase modulation and multi-phase modulation.
In order to achieve a higher information transmission rate, a more technically complex multi-system amplitude-phase hybrid modulation method must be adopted, such as quadrature amplitude modulation QAM (Quadrature Amplitude Modulation).
V. Data transmission
1. Characteristics of the physical layer interface
The main task of the physical layer is to determine some characteristics of the interface with the transmission medium, namely:
(1) Mechanical characteristics: specify the shape and size of the connector used in the interface, the number and arrangement of pins, fixing and locking devices, etc.
(2) Electrical characteristics: specify the range of voltages appearing on each line of the interface cable.
(3) Functional characteristics: specify what the voltage level appearing on a certain line means.
(4) Process characteristics: specify the order of occurrence of various possible events for different functions.
2. Transmission medium
The transmission medium is the medium that actually carries the flow of data. The transmission medium can be a physically visible medium, such as a cable, telephone line or optical fiber, or a physically invisible wireless medium, such as infrared and radio waves.
3. Single cable and dual cable
"Broadband cable" refers to any cable network that uses analog signals for transmission. A major difference between broadband systems and baseband systems is that broadband systems require analog amplifiers to periodically strengthen the signal due to their wide coverage area. These amplifiers can only transmit signals in one direction, so if there are amplifiers between computers, message packets cannot be transmitted in the reverse direction between computers. To solve this problem, two types of broadband systems have been developed: dual-cable systems and single-cable systems.
3.1 Dual-cable systems
A dual-cable system has two identical cables laid side by side. To transmit data, the computer transmits data through cable 1 to the device at the root of the cable, the head-end, which then transmits the signal down the cable through cable 2. All computers send through cable 1 and receive through cable 2.
3.2 Single-cable systems
Another solution is to allocate different frequency bands for internal and external communications on each cable (asymmetric uplink and downlink). The low frequency band is used for uplink communication from the computer to the head-end, and the downlink signal received by the head-end is moved to the high frequency band and broadcast to the computer. In a subsplit system, the 5MHz to 30MHz frequency band is used for inbound communication, and the 40MHz to 300MHz frequency band is used for outbound communication. In a midsplit system, the inbound frequency band is 5MHz to 116MHz, and the outbound frequency band is 168MHz to 300MHz. This choice was made for historical reasons.
4. Commonly used transmission media
4.1 Twisted pair
cable Twisted pair cable (Twisted Pair): It is made of two thin copper wires wrapped in insulating material and twisted together at a certain ratio. This twisting changes the original electronic properties of the cable, which can not only reduce its own crosstalk, but also prevent the interference of signals on other cables on this pair of cables to the greatest extent.
Twisted pair cable is generally made of two 22-26 insulated copper wires twisted together. In actual use, twisted pair cable is made of multiple pairs of twisted pairs wrapped together in an insulating cable sheath. A typical twisted pair cable has four pairs, and there are also more pairs of twisted pairs placed in a cable sheath. These are called twisted pair cables. Twisted pair cables
can be divided into shielded twisted pair cable (STP) and unshielded twisted pair cable (UTP) according to whether there is a shielding layer.
The standard for twisted pair cable is formulated by the TIA/EIA International Association. In order to manage network cables, it is necessary to be familiar with some standards used in modern networks, especially Category 3 and Category 5 UTP.
(1) Category 1 cable (CAT1): A form of UTP that includes two pairs of wires. Category 1 cable (telephone cable before the early 1980s) is suitable for voice communication, but not for data communication. It can only transmit a maximum of 20 kilobits per second (kbps) of data.
(2) Category 2 cable (CAT2): A form of UTP that includes four pairs of wires. It is used for voice transmission and data transmission with a maximum transmission rate of 4Mbps. It is common in old token rings that use the 4MBPS standard token passing protocol.
(3) Category 3 cable (CAT3): A form of UTP that includes four pairs of wires. It is used for voice transmission and data transmission with a maximum transmission rate of 10Mbps when the bandwidth is 16MHz. Category 3 cable is generally used for 4Mbps Token Ring or 10Mbps 10BASE-T Ethernet. It is less used now.
(4) Category 4 cable (CAT4): A form of UTP that includes four pairs of wires. The bandwidth of this type of cable is 20MHz. It is used for voice transmission and data transmission with a maximum transmission rate of 16Mbps. CAT4 is mainly used in 16Mbps Token Ring or 10Mbps 10BASE-T Ethernet. It provides more protection against crosstalk and attenuation than CAT1, CAT2, or CAT3.
(5) Category 5 (CAT5): The most popular form of UTP for new network installations and upgrades to fast Ethernet. CAT5 cable has increased winding density, is coated with a high-quality insulating material, includes four pairs of wires, and is used for voice transmission and data transmission with a maximum transmission rate of 100Mbps. In addition to 100Mbps 10BASE-T Ethernet, CAT5 cable also supports other fast networking technologies such as asynchronous transfer mode (ATM).
(6) Category 5e unshielded twisted pair cable is a cable that improves some of the performance of the existing Category 5 shielded twisted pair cable. Many performance parameters, such as near-end crosstalk, attenuation crosstalk ratio, return loss, etc., have been improved, but its transmission bandwidth is still 100MHz. Category 5e twisted pair cable also uses 4 winding pairs and 1 tensile wire, and the color of the wire pair is exactly the same as that of Category 5 twisted pair cable. The label of Category 5 cable is "CAT5" and the label of Category 5e cable is "CAT 5E".
Although Category 5e twisted pair cable can also provide a transmission bandwidth of up to 1000Mb/s, it often requires the support of expensive special equipment. Therefore, it is usually only used in 100Mb/s Fast Ethernet to connect desktop switches to computers.
(7) Category 6 cable (CAT6): A twisted pair cable consisting of four pairs of wires. Each pair of wires is wrapped in a foil insulator, and another layer of foil insulator is wrapped around the outside of all the wire pairs. At the same time, a layer of fireproof plastic sheath is wrapped around the second layer of foil. The foil insulator provides a good impedance to crosstalk, so that the throughput supported by CAT6 is 6 times that of the conventional CAT5. What
we use in computer communication networks today is basically "category 5e unshielded twisted pair cable". The two ends of the cable are pressed into the T568B RJ45 crystal plug according to a certain line sequence, which is usually called "network cable". In view of the differential characteristics of RJ45, the rule of "crossing the same type and paralleling different types" is followed when making RJ45 network cable crystal plugs.
4.2 Coaxial cable
Coaxial cable uses hard copper wire as the core (inner conductor) and is covered with a layer of insulating material (insulating material). This layer of insulating material is surrounded by a densely woven mesh conductor, and the mesh is covered with a layer of protective material (protective cover).
Coaxial cable can be divided into 50Ω baseband coaxial cable for digital transmission and 75Ω broadband coaxial cable for analog transmission according to its use. Baseband cable is divided into thin coaxial cable and thick coaxial cable. Baseband cable is only used for digital transmission, and the data rate can reach 10Mbps. Broadband cable is the standard used in CATV systems. It can transmit analog signals using frequency division multiplexing as well as digital signals.
The bandwidth of coaxial cable depends on the length of the cable. A 1km cable can reach a data transmission rate of 1Gb/s~2Gb/s, so it can support high-bandwidth communication on relatively long lines without repeaters. If a longer cable is required, a repeater is required to supplement the energy. Coaxial cable can be selected when more devices need to be connected and the communication capacity is quite large. At present, coaxial cable has been largely replaced by optical fiber, but it is still widely used in wired and wireless television and some local area networks.
4.3 Optical fiber
Optical fiber communication is a communication method that uses light as an information carrier and optical fiber as a transmission medium. The main function of optical fiber is to guide light to propagate along a straight or curved path in the optical fiber. The light waves in optical fiber communication are mainly lasers, so it is also called laser-optical fiber communication. Light propagation utilizes the principle of total reflection of light in fibers made of glass or plastic. The basic principle of optical communication is to make the intensity of light reflect the amplitude (frequency) of the electrical signal.
From the inside to the outside, the optical fiber is composed of the core, cladding, inner coating and outer coating. At present, the main fiber optic communication material is high-transparency silica material, which can be made into pure silica by chemical vapor deposition (CVD). In recent years, there are new fiber optic materials, such as fluorine glass, a binary mixture of ZrF4, LaF3 and BaF2, which has better performance than silica, less light loss, and does not require any relay stations for tens of thousands of kilometers of optical signal transmission.
Optical fiber is divided into single-mode and multi-mode according to the number of laser beams it can carry and the transmission characteristics.
Single-mode optical fiber uses laser diodes as the light source, the central glass core is thinner, and only one type of light is allowed to propagate in a straight line in the optical fiber, so the inter-mode dispersion is very small, and the overall transmission performance is very good. The price of single-mode optical cable and single-mode optical fiber port is relatively expensive, and it is generally used by operators to lay long-distance communication backbone networks.
Multimode optical fiber uses light-emitting diodes as the light source, and the central glass core is thicker, allowing multiple beams of light to propagate forward along the light wall in the light, resulting in large inter-mode dispersion and poor overall transmission performance. The price of multi-mode optical cable and multi-mode optical fiber port is relatively cheap, and the communication distance is relatively short, only a few kilometers, and it is generally used to lay local area networks, such as campus networks.
Single-mode fiber and multi-mode fiber can be easily distinguished by their skin color. The sheath of single-mode fiber is yellow, and the sheath of multi-mode fiber is orange.
In order to achieve long-distance fiber-optic communication, the attenuation of the fiber must be reduced. In 1966, British Chinese CK Kao (winner of the 2009 Nobel Prize in Physics) predicted that glass could be used to make communication optical fibers (abbreviated as optical fibers) with an attenuation of 20dB/km. At that time, the attenuation of the world's best optical glass was about 1000dB/km. In 1977, Wuhan Post and Telecommunications Research Institute successfully developed China's first multi-mode optical fiber with a step refractive index distribution and a wavelength of 0.85μm.
Compared with electrical communication methods such as cables or microwaves, the advantages of optical fiber communication are as follows:
(1) The transmission frequency band is extremely wide and the communication capacity is large;
(2) Since the optical fiber has low attenuation and no relay equipment, the transmission distance is long;
(3) The crosstalk is small and the signal transmission quality is high;
(4) The optical fiber is resistant to electromagnetic interference and has good confidentiality;
(5) The optical fiber is small in size and light in weight, which is convenient for transmission and laying;
(6) It is resistant to chemical corrosion;
(7) The optical fiber is drawn from quartz glass, the raw material source is abundant, and a large amount of non-ferrous metals are saved.
Optical fiber communication also has the following disadvantages:
(1) The bending radius of the optical fiber should not be too small;
(2) The cutting and connecting operations of the optical fiber are technically complex;
(3) Branching and coupling are troublesome.
4.4 Wireless transmission medium
According to the frequency of electromagnetic waves, wireless transmission systems are roughly divided into broadcast communication systems, ground microwave communication systems, satellite microwave communication systems and infrared communication systems. Therefore, the corresponding three wireless media are radio waves (30MHz~1GHz), microwaves (300MHz~300GHz), infrared and lasers.
(1) Radio wave communication
Radio wave communication is mainly used in broadcast communication, so it will not be introduced in detail.
(2) Infrared transmission
Infrared networks use infrared rays to transmit data through the air, and are mainly used for communication between devices in the same room, such as TV remote controls. At present, the transmission rate of infrared transmission in one direction is 16Mbps, and in multiple directions it does not exceed 1Mbps.
(3) Satellite communication
Satellite communication is a communication system that uses stationary earth and synchronous satellites as relay stations. Ground systems usually use directional parabolic antennas. Satellite communication systems have the advantages of large communication capacity, long transmission distance, and wide coverage, and are particularly suitable for global communications, television broadcasting, and use in areas with harsh geographical environments.
VI. Channel multiplexing technology
The most basic multiplexing technology is mainly divided into two categories: frequency division multiplexing FDM (Frequency Division Multiplexing) and time division multiplexing TDM (Time Division Multiplexing).
1. Frequency division multiplexing (FDM, Frequency Division Multiplexing)
When the effective bandwidth of the transmission medium exceeds the bandwidth of the signal being transmitted, multiple signals are modulated on carriers of different frequencies to achieve the technology of transmitting multiple signals simultaneously on the same transmission medium, such as xDSL. In frequency division multiplexing, after a user is allocated a certain frequency band, he/she will occupy this frequency band from beginning to end during the communication process. It can be seen that all users of frequency division multiplexing occupy different bandwidth resources at the same time. FM radio and HFC network TV signals of radio and television are typical frequency division multiplexing signals. Radio/TV sets distinguish channels based on different carrier frequencies.
The main motivation for using frequency division multiplexing is the demand for high throughput. In order to achieve higher throughput, the underlying hardware uses a larger part of the electromagnetic spectrum (i.e., higher bandwidth). The term broadband technology is used to describe these technologies. On the other hand, any technology that only uses a small part of the electromagnetic spectrum and sends only one signal on the medium at a time is called baseband technology. The modulation and demodulation principles of frequency division multiplexing can be derived through Fourier transform.
When using frequency division multiplexing, if the bandwidth occupied by each user remains unchanged, then when the number of multiplexed users increases, the total bandwidth of the multiplexed channel will become wider. For example, the bandwidth of each standard voice channel in traditional telephone communication is 4kHz (that is, 3.1kHz for communication plus the protection bands on both sides). If there are 1,000 users performing frequency division multiplexing, the total bandwidth after multiplexing is 4MHz.
In addition to the traditional frequency division multiplexing (FDM), there is also orthogonal frequency division multiplexing (OFDM). Orthogonal frequency division multiplexing is a multi-carrier digital modulation technology. All carrier frequencies of OFDM have equal frequency intervals, which are integer multiples of a basic oscillation frequency. Orthogonality means that the signal spectra of each carrier are orthogonal. The
bandwidth required by the OFDM system is much smaller than that of the FDM system. Since OFDM uses interference-free orthogonal carrier technology, there is no need for protection bands between single carriers, which makes the use of available spectrum more efficient. In addition, OFDM technology can dynamically allocate data in subchannels. To obtain the maximum data throughput, the multi-carrier modulator can intelligently allocate more data to subchannels with less noise.
At present, OFDM technology has been widely used in broadcast audio and video fields and civil communication systems. The main applications include: asymmetric digital subscriber line (ADSL), digital video broadcasting (DVB), high-definition television (HDTV), wireless local area network (WLAN) and the fourth generation (4G) mobile communication system.
2. Wavelength Division Multiplexing (WDM)
At present, the transmission rate of a single-mode optical fiber can reach 2.5Gbps, and it is more difficult to increase the transmission rate. If the dispersion problem in optical fiber transmission is solved, such as using dispersion compensation technology, the transmission rate of a single-mode optical fiber can reach 20Gbps. This has almost reached the limit of single optical carrier signal transmission.
By borrowing the concept of frequency division multiplexing from traditional carrier telephones, people can use one optical fiber to simultaneously transmit multiple optical carrier signals with very close frequencies, thus doubling the transmission capacity of the optical fiber. This multiplexing method is called wavelength division multiplexing. With the development of technology, the number of optical carrier signals multiplexed on one optical fiber is increasing. Now it is possible to multiplex more than 80 optical carrier signals on one optical fiber, so the concept of dense wavelength division multiplexing (DWDM) is used.
Wavelength division multiplexing is the frequency division multiplexing of optical fiber channels, which transmits multiple optical carrier signals with different wavelengths on one optical fiber at the same time. At the receiving end, a glass prism is used to separate light waves of different frequencies. Similar to general FDM, because light of a specific frequency will not interfere with light of another frequency, carriers of different frequencies can be combined and transmitted in the same medium.
3. Time Division Multiplexing (TDM)
When the data transmission rate that the transmission medium can achieve exceeds the transmission rate of the transmitted signal, the multiple signals can be transmitted at a certain time interval, that is, the "simultaneous" transmission of multiple signals on the same transmission medium is realized in turn according to time slices. Time division multiplexing is to divide time into time division multiplexing frames (TDM frames) of equal length. Each time division multiplexing user occupies a time slot with a fixed sequence number in each TDM frame. The time slot occupied by each user appears periodically, and its period is the length of the TDM frame. All time division multiplexing users occupy the same bandwidth at different times. The transmission between digital TV channels is distinguished by frequency division multiplexing, but time division multiplexing is used within each channel (8 MHz).
Time division communication is also called time division communication. It is the main method of digital telephone multi-channel communication, so PCM communication is often called time division multiplexing communication. In the example of telephone communication mentioned above, when time division multiplexing is used, the length of each time division multiplexing frame is unchanged and is always 125us. If there are 1000 users for time division multiplexing, the time slot width allocated to each user is 0.125us. The time slot width becomes very narrow, but the spectrum range occupied by the burst pulse signal is also very wide.
When communicating, a multiplexer is always used in pairs with a demultiplexer. Between the multiplexer and the demultiplexer is a high-speed channel shared by users. The function of the demultiplexer is just the opposite of that of the multiplexer. It divides the data transmitted by the high-speed channel and sends it to the corresponding users. The three-way splitter for telephone and television can be regarded as a time division multiplexing demultiplexer.
A variant of TDM is to transmit and receive on a single frequency channel, which is called time division duplex (TDD). Its simplest structure is to use two time slots, one for transmission and one for reception.
4. Statistical Time Division Multiplexing (STDM)
When using a time division multiplexing system to transmit data, due to the burstiness of data, the utilization rate of a user's allocated sub-channel is generally not very high. When a user has no data transmission in a certain period of time, the sub-channel that has been allocated can only be idle, and other users cannot use this temporarily idle line resource. Assume that there are four users A, B, C and D for time division multiplexing. The multiplexer scans the user's time slots in the order of A→B→C→D, and then forms a time division multiplexing frame. Each time division multiplexing frame has 4 time slots. When a user has no data to send, the time slot allocated to the user can only be abandoned. Even if other users have data to send, they cannot use these idle time slots. This leads to low channel utilization after multiplexing.
Statistical time division multiplexing (STDM) is a time division multiplexing based on statistical improvement. It implements "on-demand allocation" of the time slots of the public channel, that is, only those terminals that need to transmit information or are working are allocated to time slots. In this way, all time slots can be used saturatedly, and the number of terminals served can be greater than the number of time slots, which improves the utilization rate of the medium and plays a role of "multiplexing". On the output line, the time slot occupied by a certain user no longer appears periodically, so statistical time division multiplexing becomes asynchronous time division multiplexing, and ordinary time division multiplexing is called synchronous time division multiplexing.
The transmission efficiency of statistical time division multiplexing is 2 to 4 times higher than that of traditional time division multiplexing. The main feature of this multiplexing is the dynamic allocation of channel time slots, so statistical multiplexing can also be called "dynamic multiplexing". Therefore, from an average perspective, synchronous time division multiplexing and asynchronous time division multiplexing are balanced.
Concentrators often use statistical time division multiplexing, which collects the data of low-speed users and sends them to the remote end through high-speed lines. The concentrator assumes that each user sends data intermittently. If a user sends data continuously, the concentrator cannot cope with it.
Statistical multiplexing is only applied to digital TV program multiplexers and packet switching networks.
5. Code Division Multiplexing (CDM)
Code division multiplexing is a multiplexing method that relies on different codes (IMEI of GSM, ISN of CDMA) to distinguish the original signals of each channel.
The system allocates a pair of unique 0 and 1 data identification identifiers to a pair of communication users. The two communicating parties use the data identification identifiers to encode and decode the transmitted data, so that different users can use different codes to transmit data in the same channel. All users obtain service channels according to the codes at the same time and in the same frequency band. Each user uses a specially selected different code type, so they will not interfere with each other. The signal sent by this system has a strong anti-interference ability, and its spectrum is similar to white noise, which is not easy to be detected by the enemy.
According to Shannon's theorem, under the condition of ensuring the S/N signal-to-noise ratio, the channel capacity C can be increased by increasing the bandwidth B of the transmission system. Code division multiplexing is a multiplexing technology based on spread spectrum technology. CDMA (Code Division Multiple Access) is a form of code division multiplexing, called code division multiple access. Code division multiplexing technology is widely used in the field of mobile communications after the second generation, such as 2G CDMA, 3G CDMA 2000 (USA, China Telecom), WCDMA (Europe, China Unicom) and TD-SCDMA (China, China Mobile).
VII. Data exchange technology
1. Switching technology
Switching and switches originated from the telephone communication system (PSTN). Two telephones can be connected to each other with only one pair of wires; 5 telephones connected in pairs require 10 pairs of wires; N telephones connected in pairs require N (N – 1) / 2 pairs of wires. When the number of telephones is large, the number of wire pairs required for this mesh-type full physical connection is proportional to the square of the number of telephones.
When the number of telephones increases, switches are used to complete the switching task of the entire network. Here, "switching" means transfer - transferring one telephone line to another telephone line to connect them. From the perspective of communication resource allocation, "switching" is to dynamically allocate transmission line resources in a certain way.
The communication subnet is composed of transmission lines and intermediate nodes (hubs, bridges, switches, routers). When there is no direct line between the source (source node) and the destination (destination node), the data sent by the source first reaches the intermediate node connected to it, and then is transmitted from the intermediate node to the next intermediate node until it reaches the destination. This process is called switching. Switching is a general term for the technology of sending the information to be transmitted to the corresponding route that meets the requirements by manual or automatic methods according to the needs of information transmission at both ends of the communication. The device that completes the information exchange function in the communication system is called a switch.
2. Data exchange mode
In data communication, data exchange modes mainly include circuit switching and storage switching, among which "storage switching" is divided into "message switching" and "packet switching".
2.1 Circuit switching
Circuit switching is also called line switching, which is connection-oriented. Circuit switching establishes an actual physical line connection in the communication subnet. Circuit switching is divided into three stages: connection establishment → communication → release connection.
We can still see such scenes in old movies: the chief (calling user) picks up the microphone and shakes it vigorously. The central office is a row of machines with wire ends plugged in. After receiving the connection request, the operator wearing a headset plugs the wire ends into the corresponding outlets to establish a connection between the two user ends until the call ends. This process is circuit switching established manually.
Circuit switching is divided into two modes: time division switching (TDS) and space division switching (SDS). In this transmission mechanism, data is bursty, which will lead to low utilization of communication lines. In some cases, the channel capacity is wasted when the circuit is idle. In addition, if a switch or a link in the communicating circuit is blown up, the entire communication circuit will be interrupted. If you want to use other detour circuits, you must dial again to establish a connection. This will delay some time.
2.2 Storage switching
The so-called "storage switching" means that before data is exchanged, it is first cached through the buffer memory and then processed according to the queue.
2.2.1 Message Switching
In the 1940s, telegraph communication adopted message switching based on the store-and-forward principle. The basic idea of message switching is to first store the user's message in the memory of the switch, and then send the message to the receiving switch or user terminal when the required output circuit is idle. Therefore, the message switching system is also called a "store-and-forward" system.
The process of implementing message switching is as follows: (1) If a user has a message to send, it is necessary to first add a message header to the information to be sent, including information such as the destination address and source address, and then send the formed message to the switch. When the communication controller in the switch detects that a message is input to a user line, it sends an interrupt request to the central processing unit and sends the message word by word into the memory. (2) After receiving the message, the central processing unit can process the message, such as analyzing the message header, judging and determining the route, etc., and then transfer the message to an external large-capacity memory and wait for an idle output line. (3) Once the line is idle, the message is transferred from the external memory to the internal memory and sent to the line through the communication controller.
The message switching method first stores the entire message by the switch, and then performs necessary processing when there is an idle line. The data of multiple users can share a line through storage and queuing, without the line establishment process, which improves the utilization rate of the line. This transmission method supports multi-point transmission (a message is transmitted to multiple users, and only the "address field" needs to be added to the message, and the intermediate node copies and forwards it according to the address field). The intermediate node can convert the data format to facilitate the collection of the receiving station. The error detection function is added to the message switching to avoid the unnecessary transmission of erroneous data. The
disadvantage of the message switching method is that the message length is not specified, the message can only be temporarily stored on the disk, and the disk reading takes up extra time; any message must wait in line: messages of different lengths require different lengths of processing and transmission time, even very short messages (for example, session information in interactive communication); when the channel bit error rate is high, frequent retransmissions make it difficult for message switching to support the requirements of real-time communication and interactive communication.
2.2.2 Packet switching
Packet switching attempts to combine the advantages of line switching and message switching, while minimizing the disadvantages. Packet switching is very similar to message switching, but it specifies the data length (called packet) that the switch processes and transmits. Data packets of different users can be transmitted in an interwoven manner on the physical links in the network. It is the most widely used switching technology at present.
Message transmission treats the length of the data to be sent as a logical unit regardless of its length; while the message packet transmission method limits the maximum length of the data transmitted at one time. If the transmitted data exceeds the specified maximum length, the sender will divide it into multiple message packets for transmission, and the receiver will reassemble it according to some offset information. Due to the short packet length, when an error occurs during transmission, error detection is easy and retransmission takes less time; after limiting the maximum data length of the packet, it is beneficial to improve the storage space utilization and transmission efficiency of the storage and forwarding node.
Typical applications of packet switching are X.25 packet switching network and Ethernet. In the X.25 packet switching network, the packet length is 131 bytes, including 128 bytes of user data and 3 bytes of control information; while in Ethernet, the packet (IP Datagram) length is about 1500 bytes.
How will the network manage these packet flows? There are usually two methods: datagram and virtual circuit. In the datagram method, each packet is processed independently, just like each message is processed independently in a message switching network. In the virtual circuit method, a logical connection needs to be established before any packet is sent. The datagram method is similar to the UDP communication method in TCP/IP. The message sent out is like a drifting bottle and may not reach the shore; the virtual circuit method is similar to the TCP communication method in TCP/IP, which needs to maintain status. Packet
switching has the following advantages: (1) High efficiency, dynamic allocation of transmission bandwidth, and occupation of communication links segment by segment. (2) Flexible, using packets as the transmission unit and finding routes. (3) Fast, packets can be sent to other hosts without establishing a connection first. (4) Reliable, network protocols that ensure reliability; distributed routing selection protocols make the network have good survivability.
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