控制网络技术（英文二）

控制网络技术（英文二）Data communication base

一、Theoretical basis

1、basis
(1)Fourier Analysis
(2)Bandwidth-Limited Signals
(3)The Maximum Data Rate of a Channel

2、 Basic concepts
（1）Information : content and interpretation of data.
（2）Data: physical symbols or meaningful entities conveying information.
（3）Analog data: a continuous variation of data in a certain interval. Such as temperature, pressure, sound, video and so on ，are all continuous varying waveforms.
（4）Digital data: data is taken discrete values in a certain interval. Such as text information, numbers, integers, binary sequences, etc.
（5）Signal ：The specific representation of data in the transmission process.

	Analog signals: current, voltage, or electromagnetic waves that vary continuously over time can use some of their parameters (such as amplitude, frequency, phase, etc.) to represent the data to be transmitted.
Digital signal: a series of continuous electric pulses, which can express the data to be transmitted by the state of a moment.
Baseband signal: the high and low voltage represents "0" and "1", and the high and low level alternating signal is baseband signal.
Broadband signal: a combination of multi-channel baseband signals.

（6）Source 、Destination、Channel

Source: equipment or computer that generates and sends information in the process of communication.
Destination: the communication process of receiving and processing information or computer equipment.
Channel: the communication line between the source and destination of the communication process, is the transmission of information channel. A line commonly used to convey information to a certain direction, a communication line often contains multiple channels.
The channel is composed of corresponding transmitting information and receiving devices and transmission medium.
The channel and the circuit are not the same. A communication circuit usually contains a transmission channel and a receiving channel.

3、The channels classification
（1）Classification by transmission medium：Wired channel and wireless channel;
（2）Classification by transmission signal type： Analog channel and digital channel;
（3）Classification by way of use：Special channel and public channel;

4、Indexes of channel
（1）Bit Rate: the bit number of binary codes is transmitted per second. (bit/s or b/s or BPS). Bit rate is data rate or data transmission rate.
（2）Baud Rate: the unit number of electrical signals per second or the number of symbols per second. Baud rate is the modulation rate or the symbol rate

 Attention: In the field of electronic communications, the Baud rate,  refers to the change in the unit time after the signal is modulated, that is, the number of changes in the carrier parameters within a unit time. It is a measure of symbol transmission rate. 
1 Baud = 1 symbol/s 
The unit “Baud" itself has represented the modulation number/s. 
It is a common mistake to take Baud as the unit of Baud/s.

（3） If N number system signal is used to represent the information content of each symbol, the relation between data rate and modulation rate when transmitting signal is:S = B log2N (b/s), where S is bit rate and B is baud rate.
（4）A signal can often carry multiple binary bits, so the bit rate is usually larger than the baud rate under fixed signal transmission rate. In other words, multiple bits can be transmitted in one symbol.
（5）Channel bandwidth: the frequency range of signals transmitted in a channel without distortion, is usually as the channel passband, unit Hz.
（6）Channel capacity: the maximum data rate that a channel can transmit; the maximum number of bits that a channel can transmit in a unit time.Usually, channel capacity and channel bandwidth are proportional to each other. The larger the bandwidth is, the higher the capacity is, the higher the transmission rate of the signal is.
（7）Throughput: the total amount of information transmitted by a channel in a unit time.
（8）Propagation speed: the distance transmitted by signal in unit time (m/s) .Bit Error Rate: error rate, binary bit transmission error probability in transmission; channel transmission reliability index. Error rate P = error bits / total bits

5、Fourier Analysis
（1）Signal spectrum: it refers to the set of all frequency components that signal contains, and it is represented by frequency domain graph. It is a combination of all sine wave signals of the signal.
（2）Signal bandwidth : the range of frequency components, and the highest frequency minus the lowest frequency.

6、Bandwidth-Limited Signals
（1）Signal bandwidth: whether the analog signal or the digital signal is, the main components of the signal should occupy a certain frequency range, the frequency range occupied by the signal is called the bandwidth of the signal.
（2）Channel bandwidth: the channel can transmit the frequency range when the signal is not distorted.

7、The Maximum Data Rate
（1）Channel capacity represents the maximum data rate that the channel can transmit.

Channel capacity is a limit parameter of the channel. When the data rate transmitted on the channel is larger than the data rate allowed by the channel, the channel can not transmit the signal at all.
Any real channel is not ideal, because the bandwidth of the channel increases, and the capacity of the channel can not be increased infinitely.

（2）Shannon’s theorem

二、Signal transmission mode

1、mode

（1）Baseband transmission: the baseband signal is sent directly to the transmission line.
Baseband signal: the digital signal "1" or "0" is directly represented by two different voltages. The signal that is continuously changing at high and low level is the baseband signal.
Baseband transmission is the direct transmission of digital signals on the channel, and the whole bandwidth of the transmission media is occupied by baseband signals.
（2）Frequency-band transmission: transmit the band signal directly to the line.
Frequency-band signal: analog signal modulated by baseband signal.The digital signal is modulated into analog signal and then sent and transmitted. When it reached the receiver , the analog signal is demodulated into the original digital signal.
（3）Broadband transmission: the frequency spectrum of multiple baseband, audio and video signals is transferred to different frequency bands of a cable, which is called broadband transmission.
The transmitted signals are all modulated analog signal.
Each signal does not interfere with each other, and improves the utilization ratio of lines.
The transmission distance is far away, and multiple channels are provided at the same time.

2、Serial and parallel
（1）Serial transmission: only one binary bit is transmitted at a time, which is transmitted in sequence according to the sequence of bits contained in the character, that is, the way the data is sent by one bit in order.
（2）Parallel transmission: in character, a byte is transmitted at one time, that is, 8 data bits, that is to say, all of the data are transmitted at the same time.
（3）Compare
Serial transmission: the communication line is simple, as long as a pair of transmission lines can be communicated, and the telephone line can be used, thus greatly reducing the cost, especially for long distance communication, but the speed of transmission is slow.
Parallel transmission: the transmission speed is fast, but when the distance is far away and the number of digits is more, the communication line is complicated and the cost is high; it is suitable for short distance and high speed.

3、Asynchronous and Synchronous
（1）asynchronous transmission: each character (ASCII) uses the starting and stopping bits as a symbol to start and end the character, and sends and receives each character in a unit.
When the character is not transmitted, the transmission is always in the high level（stop）.
When the character is sent, the sender sets a starting position (low level) and 1 bit or 2 bit stop bits (high level) at the end of each transmission character, indicating the beginning and end of the character respectively.
（2）If no data is to be sent, the sender sends a continuous stop code, a series of 1, and the receiver recognizes the start of a new character based on the jump from 1 to 0.The sender and receiver have their own independent clocks without synchronization.Transmission in characters;The interval between two characters is arbitrary.The receiver relies on the starting position in the character to synchronize.
（3）synchronous transmission: remove the start position of each character and stop key’s framing mark w
hen asynchronously transmitting, and use synchronous character to indicate at the beginning of data block.
（4）Comparison of synchronous transmission and asynchronous transmission
The advantages of synchronous transmission:The transmission speed is high.Allowing users to transmit non - 8 - bit data;It can communicate with the mainframe which communicates synchronously.Disadvantages of synchronous transmission:The hardware device is complex, and sometimes the clock is needed to achieve strict synchronization between the transmitter and the receiver.The transmitter and receiver have complex control and high demand for line.
（5）The advantages of asynchronous transmission:The implementation is simple, and the technology cost is low and the price is cheap.
The disadvantages of asynchronous transmission: The transmission speed is slow. Each transmission of one character must add 2-3 bits of transmission time, it is suitable for low-speed terminal or dialogue operation

4、Role and direction
（1）Simplex communication: information of single direction transmission;
（2）Half duplex communication: allowing data to flow in any direction in two directions, but each time can only flow in one direction.
（3）Duplex communication: each terminal can be sent at the same time, and it can also receive at the same time.

5、Data coding
（1）Code element : the basic unit of data transmission.
（2）Data coding : what physical signals are used to express data in communication system.
（3） Analog data coding: using the amplitude, frequency and phase of the analog signal to express the 0 and 1 states of the data.Digital data coding: using high and low level rectangular pulse signal to express 0 or 1 state of data.
（4）Unipolar coding： The signal level is monopole
Bipolar coding： The signal level is positive and negative, two kinds of polarity.
Return to zero coding (RZ) : returns zero level encoding after every binary message is transmitted.
Non return to zero coding (NRZ)：maintains effective level in the whole symbol time.

6、Analog data coding
（1）amplitude-shift keying ASK
Two binary values “0” and “1” are represented by the different amplitude of carrier signals (A1 and A2), and the carrier mode is to control the amplitude A of the carrier by the digital signal sequence.ASK is greatly affected by noise.
（2）frequency-shift keying FSK
The binary values of “0” and “1” are represented by the different frequencies of the carrier signal (f1 and f2), and the oscillation frequency f of the carrier varies with the value of the digital sequence. FSK is mainly used for >1200bps transmission rate, and can also be used for 3 to 30MHz high frequency radio transmission and local broadcasting network.FSK is easy to implement and is mainly used for low speed transmission.
（3）phase-shift keying PSK
PSK is mainly used for medium speed transmission

7、Coding technique
（1）Digital to digital conversion: data is stored in computers in the form of 0 and 1. In order to transmit data, data are usually converted to digital signals.
（2）Analog to digital conversion: in order to reduce noise, speech signal is converted into digital signal for transmission.
（3）Digital to analog: for example, using public telephone line to transmit data, computer generated digital signals should be converted into analog signals.
（4）Analog to analog conversion: when analog signal is transmitted, the signal frequency is not suitable for the selected medium, and needs to be modulated to the appropriate frequency.

8、Analog signal modulation of digital data
（1） Baseband transmission is the way to send the encoded signal directly from the source to the channel. Baseband transmission requires wide bandwidth of the channel, and the waveform is greatly influenced by the distributed capacitance of the channel, so it is only suitable for short distance and internal data transmission.
For long distance transmission, digital signals must be converted to analog signals that can be transmitted on a long distance channel. This is the frequency band transmission of digital signals, also known as carrier transmission.Modem is the main device to transform digital signals and analog signals on analog channels. Therefore, the most important technique of frequency band transmission is modulation and demodulation.
（2）Because of the low distortion of digital signal, low error rate and high rate of data transmission, in the network, in addition to the digital signal produced directly by the computer, the digitalization of speech and image information has become the trend of development. Pulse code modulation (PCM) is the main way to simulate data digitalization.
（3）For the analog signal, we sample it, and then generate a series of pulses based on the results.But the pulse value is still amplitude, not digitalized, which is not suitable for data communication.PCM is needed to modulate the signal. PAM is the first step in PCM.

9、Pulse coding modulation process
（1）The typical application of PCM technology - Speech digitization.
Speech can be transmitted through telephone lines in the form of analog signals, but the voice signal must be digitized when the digital, text, graphics and images generated by the voice and the computer are transmitted simultaneously in the network. PCM operations include: sampling, quantization and coding
（2）sampling
Sampling theorem: a continuous variable analog data, with the highest frequency or bandwidth of Fmax. If the sampling frequency is greater than or equal to 2 Fmax, then the sampled discrete sequence can recover the original continuous analog signal without distortion.
（3）quantizing
Quantization is the process of determining the magnitude of the sample size according to the quantization level.After quantization, the sample size is discrete magnitude.Before quantifying, it is necessary to divide the signal into several quantization levels.For example, according to the accuracy requirement, it can be divided into 8, 16, 32, 64 and 256 levels.Before quantization, the range of the magnitude of each level should be specified, and then the sample amplitude is compared with the magnitude value of the above magnitude, and the magnitude of the magnitude should be graded.
（4）coding
Coding is using binary code of corresponding bits to represent the magnitude of quantized samples. If there are K quantization levels, the number of binary digits is log2 K. For example, if there are 16 quantization levels, 4 bit binary encoding is needed.

10、Trunks and Multiplexing
（1） The telephone company has designed some methods to reuse multiplexed sessions to a physical backbone channel. From the technical realization, it can be divided into two kinds of methods:
Frequency Division Multiplexing (FDM).
Wavelength Division Mutiplexing（WDM）
Time Division Multiplexing (TDM).
（2）Frequency division multiplexing
The channel’s available frequency band is divided into several non overlapping frequency bands, each signal takes up one frequency band. When receiving, separate signals with appropriate filters and demodulate them separately.FDM each band has a certain bandwidth, which is called logical channel (sometimes referred to as channel). In order to prevent the interference caused by the frequency coverage of the adjacent channel signals, a certain “protection” band is set up between the two adjacent signal frequencies, but the corresponding spectrum of the protective band can not be used to ensure that each frequency band is isolated from each 。Frequency division multiplexing divides the frequency band resources of the circuit or space into multiple frequencies and assigns them to multiple users respectively. The data of each user terminal is transmitted through the sub channel (frequency band) allocated to it, which is mainly used in the telephone and cable television (CATV) system.
other.
（3）Wavelength Division Multiplexing
Wavelength division multiplexing – optical frequency division multiplexing, an example of frequency division multiplexing used in optical fiber channel.WDM wavelength division multiplexing (WDM) uses the concept of frequency division multiplexing of traditional carrier phone, and uses a fiber to simultaneously transmit optical carrier signals which are very close to multiple frequencies, which makes the transmission capability of the fiber multiplied.Application: mainly used in the communication system composed of all optical networks.Initially, one optical fiber multiplexed two optical carrier signals, and now it can achieve 80 or even more optical fiber signals on one optical fiber.
（4）Time Division Multiplexing
Time division multiplex （TDM）, which is segmented according to the time of transmission signal, enables different signals to be transmitted in different time. The whole transmission time is divided into many time intervals, and each time slice is occupied by a single signal. TDM, by sending a part of each signal across time, realizes a circuit to transmit multi-channel signals.Time division multiplexing – there is only one signal at any time in the circuit.Frequency division multiplexing – there are many different frequencies of signals at any time on the circuit. Time division multiplexing – digital signal is a finite number of discrete values, and is widely used in digital communication system including computer.Frequency division multiplexing is suitable for analog communication system.Time division multiplexing – the shorter the slot length, the more sub channels can be partitioned.Frequency division multiplexing, the wider the frequency band, the more sub channels can be divided within the bandwidth.Synchronous time division multiplexing – fixed time slice allocation.The time of transmission signal is continuously divided into specific time periods (one cycle) according to specific length.
Each time segment is divided into equal time slots, and each time slot is allocated to various digital signals in a fixed way.Each digital signal is allocated to one time slot in every time period.

三、Transmission Medium

1、physical layer
The purpose of the physical layer is to transport a raw bit stream from one machine to another. Various physical media can be used for the actual transmission. Each one has its own character in terms of bandwidth, delay, cost, and ease of installation and maintenance. Media are roughly grouped into guided media and unguided media.

2、Magnetic Media
（1） One of the most common ways to transport data from one computer to another is to write them onto magnetic tape or floppy disks, physically transport the tape or the disks to the destination machine, and read them back in again. Advantage：simple、low cost、high bandwidth、high performance.Disadvantage：The delay characteristic is poor, transmission time is measured in minutes or hours , not milliseconds.
（2）Twisted Pair
The most common application of the twisted pair is the telephone system. Nearly all telephones are connected to the telephone company office by a twisted pair. Two insulated conductors are twisted together with spiral conductors, which can cancel electromagnetic interference emitted by them. The core is copper wire or copper plated steel wire Section D:0.38~1.42mm. Twisted pairs can be used for either analog or digital transmission.The bandwidth depends on the thickness of the wire and the distance traveled, but several megabits/sec can be achieved for a few kilometers in many cases.
（3）UTP-unshielded twisted pair
The multiple twisted pair lines are gathered up and wrapped on a plastic reinforcement layer outside.The category 5 are similar to category 3 pairs, but with more twists per centimeter and Teflon insulation,which results in less crosstalk and a better quality signal over longer distances, making them more suitable for high-speed computer communication.
（4）STP-shielded twisted pair
The shielded sheath made of a pair of wire mesh shielding sheathed on a twisted pair line must be grounded when used. It can reduce interference and crosstalk.
（5）Baseband Coaxial Cable
It has better shielding than twisted pairs , so it can span longer distances at higher speeds. Broadband systems typically cover a larger area and therefore need analog amplifiers to strengthen the signal periodically.
（6）Fiber Optics
Optical fiber is a kind of optical transmission medium. Optical fiber is a very thin fiber medium that can conduct light. Its radius is only a few microns to one or two hundred microns. The material for making optical fiber can be super pure silicon, synthetic glass or plastic.Because the visible frequency can reach 108MHz, the optical fiber transmission system has enough bandwidth.A fiber optic cable consists of a bundle of fibers.
（7）Optical fiber transmission mode
According to the way of light transmission between optical fiber and cladding, it can be divided into:Single mode propagation: using specific material and highly concentrated light source to limit the emitted light to a very small level.Multimode propagation: multiple beams of light propagate from light sources through different optical paths.Each kind of fiber needs different physical properties.

3、Feature
（1）Advantage：
The transmission speed is very high, the bandwidth is very wide, and the capacity of transmitting information is huge.The anti-interference ability is strong. The optical fiber is not affected by power interference and static interference. Even in the same optical cable, there is almost no interference between all optical fibers, so it is easy to keep secret.Shortcomings:High cost, difficulty in installation / maintenance and vulnerability

4、Wireless Transmission
The relationship between electromagnetic spectrum and communication types – transmission band of several media.