Soft Examination Chapter 2 Data Communication Fundamentals

Data Communication Fundamentals

1. The basic concept of data communication

Source: The end of a communication that generates and sends information

Sink: The end that receives information

Channel: A line of communication between a source and a sink

As a general communication system, the information generated by the source may be analog data or digital data. Analog data takes continuous values ​​while digital data takes discrete values.

• If the source generates analog data and transmits it on an analog channel, it is called analog communication.
• If the source sends out analog data and transmits it in the form of a digital signal, then this type of communication is called digital communication.
• If the source sends out digital data, whether it is transmitted by analog signal or digital signal, it is called data communication. Data communication refers to the fact that the data in the source and the sink is in the form of digital, and the analog transmission method or digital transmission method can be used according to the needs when transmitting in the channel.

Comparison of analog transmission and digital transmission:

• Analog transmission: Before the data enters the channel, it needs to be modulated and transformed into an analog modulated signal. Since the frequency spectrum of the modulated signal is narrow, the utilization rate of the channel is high. Analog signals will be attenuated during transmission and will be interfered by noise. If an amplifier is used to amplify the signal, the mixed noise will also be amplified. This is the shortcoming of analog transmission.
• Digital transmission: Binary data or binary-coded data can be transmitted directly, or digitized analog signals can be transmitted. The digital signal only takes a limited number of discrete values, which is very beneficial for the undistorted transmission of the signal. However, the frequency band required to transmit digital signals is much wider than that required by analog signals, so the channel utilization is low.

2. Channel characteristics

channel bandwidth

Some definitions:

• Analog channel bandwidth $W$：$W=f_2-f_1$, where $f_1$is the lowest frequency that the channel can pass, $f_2$It is the highest frequency that the channel can pass. Both are determined by the physical characteristics of the channel. When the circuit that makes up the channel is made, the bandwidth of the channel is determined.
• The bandwidth of the digital signal: Usually what we call the bandwidth of the digital channel is the maximum data rate that the channel can achieve. (This statement is very imprecise and easy to confuse ==
• Symbol: A digital pulse is called a symbol
• Symbol rate (baud rate) $B$ : The number of transformations of the signal waveform per unit time, that is, the number of symbols transmitted through the channel per unit time (the unit is Baud)
• The amount of information carried by the symbol $n$ : The amount of information carried by a symbol is determined by the number of discrete values ​​taken by the symbol. If the number of symbol types is$N$ , then the amount of information carried by one symbol$n=lo{g}_{2}N$
• Data rate $R$ : The amount of information transmitted on the channel per unit time. Data rate = symbol rate * amount of information carried by a symbol.
• Signal average power $S$
• Noise average power $N$
• Signal-to-noise ratio $\frac{S}{N}$
• Decibel number: $dB=10lo{g}_2\frac{S}{N}$

The limiting baud rate for a noise-free channel with limited bandwidth is called Nyquist's theorem:
$$B=2W$$

The channel capacity specified by the Nyquist theorem is also called the Nyquist limit, which is determined by the physical characteristics of the channel. It is impossible to transmit pulse signals beyond the Nyquist limit, so the channel bandwidth must be improved to further increase the baud rate.

The limiting data rate for a noisy channel, known as Shannon's theorem:
$$C=Wlo{g}_2(1+\frac{S}{N})$$

This formula has nothing to do with the discrete value of the signal, that is to say, no matter what modulation method is used, as long as the signal-to-noise ratio is given, the maximum amount of information transmission per unit time is determined.

BER

The bit error rate can be expressed by the following formula:
$$P_c=\frac{N_e(number\; of\; digits\; in\; error}{N(total\; number\; of\; bits\; transferred)}$$
In computer communication networks, the bit error rate is generally required to be lower than 10 ^-6

channel delay

The signal propagates in the channel, and it takes a certain amount of time to reach the sink from the source.

3. Transmission medium

• Twisted pair: It consists of a pair of insulated copper wires with a thickness of about 1mm twisted together. It is widely used in traditional telephone systems and is suitable for short-distance transmission. If it exceeds a few kilometers, a repeater must be added. Twisted pair is divided into shielded twisted pair (UTP) and unshielded twisted pair (STP)
• Coaxial cable: The core wire is a copper wire, covered with a layer of insulating material, and the outside is a mesh outer conductor composed of thin copper wires, and an insulating plastic protective layer is added on the outside. Because the core wire and the mesh conductor are coaxial, it is called a coaxial cable. This structure of coaxial cable makes it have high bandwidth and excellent noise suppression characteristics.
  • Coaxial cable with a characteristic impedance of 50Ω: used to transmit digital signals, this cable is also called baseband coaxial cable
    • Thick coaxial cable: suitable for large LAN, with long transmission distance and high reliability. However, thick cables must be installed with external transceivers, which is difficult to install.
    • Thin coaxial cable: easy to install and low cost, but the cable needs to be cut off during installation, installed with BNC connectors, and then installed at both ends of the T-shaped connector, so it is prone to hidden dangers of poor contact or short circuit of the connector.
  • CATV cable with a characteristic impedance of 75Ω: used to transmit analog signals, this cable is also called broadband coaxial cable
• Optical cable: It is made of ultra-fine glass fibers that can transmit light waves, and is covered with a material with a lower refractive index than glass. High data rate, extremely wide frequency band, low bit error rate and low latency
  • Single-mode fiber: The diameter of the core wire is as small as the wavelength of the light wave, and the light propagates in a straight line without reflection.
  • Multimode Fiber: Light waves travel in multiple modes in an optical fiber
• Wireless channels: including microwave, infrared and shortwave channels
  • Ground microwave system: For example, installing a microwave antenna on the top of the building, so that the LANs in the two buildings can communicate with each other, which may cost less than digging trenches to bury cables
  • Satellite microwave system: can be seen as a microwave relay station suspended in space
  • Infrared: Use walls or roofs to reflect infrared rays to form a broadcast communication system throughout the room. Common in remote controls. This communication method has limited distance and is easily affected by indoor conditions
  • Radio short-wave communication: such as wireless communication local area network.

4. Data encoding

• Unipolar code: positive level means 0, zero level means 1
• Polarity code: Positive level means 0, negative level means 1
• Bipolar code: flip between positive and negative levels when encountering 1, and not flip when encountering 0
• Return to zero code: must return to 0 level in the middle
• Bi-phase code: there must be a level flip in the middle of each bit
• NRZ: 1 flip encountered
• Manchester encoding: a bi-phase code, the flip in the middle is + to - means 0, from - to + means 1
• Differential Manchester encoding: There must be a level inversion in the middle of each bit, and the inversion of each bit is 0 at the beginning, and it is 1 if it is not inverted
• 4B/5B encoding: In Manchester and Differential Manchester encoding, there is a level transition in the middle of each bit, so the baud rate is twice the data rate. In order to improve the coding efficiency, 4B/5B coding can be used. In fact, it is to divide 4 bits into a group and become 5 bits.

5. Digital modulation technology

Modulate an analog signal with digital data:

• Amplitude Keying (ASK): For example, 0 corresponds to a carrier amplitude of 0, and 1 corresponds to an amplitude of 1. The implementation is simple but the anti-interference performance is poor.
• Frequency Shift Keying (FSK): Modulating the frequency of the carrier according to the value of the digital data is called frequency shift keying.
• Phase Shift Keying (PSK): Modulates the phase of a carrier with the value of digital data
• Quadrature Amplitude Modulation (QAM): Combines ASK and PSK techniques to form a composite amplitude and phase modulation

6. Pulse Code Modulation

Convert analog data to digital signal

• Sampling: $f=\frac{1}{T}>2f_{max}$
• Quantify
• coding

Rate = number of quantization levels * sampling frequency

7. Communication methods and exchange methods

data communication

• Simplex communication/Half-duplex communication/Full-duplex communication
• asynchronous transfer/synchronous transfer

exchange method

• Circuit switching: The sender and receiver are directly connected by a series of links. The telephone switching system adopts this switching method
• Message exchange: Complete messages are forwarded one by one in the network
• Packet switching: group information, number each group, add source address and destination address and other packet header information.
  • Datagram: Each packet is transmitted separately, and the order of arrival may not be consistent with the order of sending
  • Virtual circuit: A logical link needs to be established.

8. Multiplexing technology

• Frequency Division Multiplexing: Multiplexed transmission using multiple analog carrier signals with different frequencies on one transmission medium
• Time-division multiplexing: each sub-channel occupies the entire bandwidth in turn according to the time slice
• Wavelength division multiplexing: used in optical fiber communication, different sub-channels are carried by light waves of different wavelengths
• Digital Transmission Systems: Introduction to Two Commonly Used Communication Standards
  • T1 carrier (Japan, the United States, etc.): a sampling period is 125μs, 24 channels, 7 bits of data are obtained each time, and one bit of signaling is added to become 8 bits. Plus one bit of frame sync information. (8*24+1)/125μs = 1.544Mbps
  • E1 channel: 32 8-bit groups of data samples are synthesized into a basic frame of 125 μs, 30 channels are used for voice transmission data, and two sub-channels (CH0 and CH16) are used for transmission of control signaling. (32*8)/125μs = 2.048Mbps
• Synchronous digital series: Just remember that the link rate of OC-1 is 51.840Mbps, and the link rate of OC-x is 51.840 times x

9. Error Control

• parity code
• Hamming Code
• Cyclic Redundancy Check Code