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Table of contents
1. Model of data communication system
1. Model of data communication system
A data communication system can be divided into source system (sender), transmission system (transmission network), and destination system (receiver).
The source system includes source points and transmitters. A typical transmitter is a modulator.
The destination system includes the receiver and the end point. A typical receiver is a demodulator.
The purpose of communication is to transmit messages, data is the entity that transmits messages, and signals are the electrical or electromagnetic manifestation of data.
Signals can be divided into analog signals (continuous signals) and digital signals (discrete signals).
2. Channel concept
A channel is not equal to a circuit. A channel represents a medium that transmits information in a certain direction. A communication circuit usually contains a sending channel and a receiving channel.
There are three basic ways of information interaction:
1. One-way communication, also known as simplex communication . Such as radio broadcasts, etc. A channel is required.
2. Two-way alternating communication, also called half-duplex communication . Two channels are required.
3. Two-way simultaneous communication, also known as full-duplex communication . Two channels are required.
The signal from the source becomes a baseband signal. Because the baseband signal contains many low-frequency components, and many channels cannot transmit low-frequency components and DC components, the baseband signal needs to be modulated.
Modulation can be divided into two broad categories:
1. Baseband modulation: converts a digital signal into another digital signal. Also called encoding.
2. Bandpass modulation: Convert the frequency range of the baseband signal to another frequency band and turn it into an analog signal.
Common encoding methods
Non-return to zero system : positive level represents 1, negative level represents 0.
Return to zero system : positive pulse represents 1, negative pulse represents 0.
Manchester encoding : an upward transition in the center of the bit period represents a 0, and a downward transition represents a 1.
Differential Manchester encoding : There is a transition in the center of each bit. A transition at the beginning of a bit represents a 0, while a transition at the beginning of a bit represents a 1.
The Manchester code has a higher frequency than the non-return-to-zero system, but it has self-synchronization capability, that is, it can extract the signal clock frequency from the signal waveform itself.
Basic bandpass modulation method
3. Channel limit capacity
Advantages of digital communication: Signals will inevitably be distorted when transmitted on the channel, but as long as the original signal can be identified, there will be no impact. The higher the transmission rate, or the farther the distance, or the greater the noise, the more severe the distortion.
The frequency range that the channel can pass is limited. The rate of symbol transmission in the channel has an upper limit. If the upper limit is exceeded, serious inter-symbol crosstalk problems will occur, and the receiving end will not be able to recognize the code. Nys's criterion : In a low-pass channel with a bandwidth of W (Hz) , if the impact of noise is not considered, the maximum rate of symbol transmission is 2W (symbols/second). If the transmission rate exceeds this upper limit, serious inter-symbol crosstalk problems will occur, making it impossible for the receiving end to judge (i.e., identify) the symbols. The wider the frequency band of the channel, the more high-frequency components that can pass through, and the higher the maximum rate.
Signal to noise ratio:
The ratio of the average power of the signal to the average power of the noise, written as S/N, in decibels (dB)
Signal-to-noise ratio = 10 log10(S/N).
Shannon formula:
The ultimate transmission rate of the channel is C = W log(2+S/N).
Shannon's formula shows that the greater the bandwidth, the greater the signal-to-noise ratio, and the higher the ultimate transmission rate. It also shows that as long as the information transmission rate is lower than the limit rate of the channel, error-free transmission can be achieved, but the method is unknown. Another way to increase the transmission rate: through encoding so that each symbol carries more bits of information.
4. Transmission media
Transmission media are divided into two categories: guided and unguided.
In the guided type, electromagnetic waves propagate along solid media, while in the unguided type, the transmission medium is free space, also known as wireless transmission.
guided transmission media
Guided transmission media include overhead open wires, twisted pairs ( twisting can reduce electromagnetic interference to adjacent wires ), coaxial cables, optical fibers, etc.
The transmission bandwidth of optical fiber is much larger than the bandwidth of other transmission media.
unguided transmission media
Transmission using wireless channels is the only means of communication in motion.
Shortwave communication quality is poor and the speed is low. Wireless microwave communication can transmit telephone calls, images, data and other information. Ultraviolet and higher bands are not yet available for communications.
The advantage of satellite communication is long communication distance, but the disadvantage is high propagation delay and poor confidentiality.