Table of contents
2.1. Basic concepts of physical layer
2.2. Transmission media below the physical layer
Serial transmission and parallel transmission
Synchronous transmission and asynchronous transmission
Two-way alternating communication:
Two-way simultaneous communication:
The relationship between transmission media and channels
Differential Manchester encoding
2.5. Limit capacity of channel
Comparison between Nye's criterion and Shannon's formula:
Supplement: Channel multiplexing technology
Statistical Time Division Multiplexing STDM (Statistic TDM)
2.1. Basic concepts of physical layer
2.2. Transmission media below the physical layer
Transmission media , also known as transmission media or transmission media, is the physical path between the sender and receiver in a data transmission system. Transmission media courses are divided into two categories, namely guided transmission media and unguided transmission media
Transmission media does not belong to any layer of the computer network architecture. If it must be added to the architecture, it can only be placed below the physical layer.
guided transmission media
In guided transmission media, electromagnetic waves are guided along a solid medium.
coaxial cable
twisted pair
optical fiber
multimode optical fiber
- There can be multiple light rays incident at different angles transmitted in an optical fiber. This kind of fiber is called multimode fiber .
single mode fiber
- If the diameter of the fiber is reduced to only one wavelength of light, the fiber acts like a waveguide, allowing the light to travel forward without multiple reflections. Such optical fibers are called single-mode optical fibers .
power line
unguided transmission media
Unguided transmission media refers to free space.
radio waves
microwave
infrared
visible light
2.3. Transmission method
Serial transmission and parallel transmission
Serial transmission :
- Data is sent bit by bit in sequence, so only one data transmission line is needed between the sending end and the receiving end.
Parallel transfer :
- n bits are sent at a time, therefore, n transmission lines are needed between the sending end and the receiving end
- The advantage of parallel transmission is that it is n times faster than serial transmission, but the cost is high
The transmission of data on the transmission line (transmission between two machines) adopts serial transmission , and the transmission of data within the computer (eg: CPU to memory) commonly uses parallel transmission .
Synchronous transmission and asynchronous transmission
Synchronous transmission :
- Blocks of data are transmitted as a steady stream of bits. There is no space between bytes
- The receiving end detects the middle moment of each bit signal to determine whether bit 0 or bit 1 is received.
- Since there are certain differences in the clock frequencies of different devices, it is impossible to achieve the same accuracy. In the process of transmitting a large amount of data, the cumulative error in the judgment time will cause the receiving end to misjudge the bit signal.
Therefore, it is necessary to keep the sending and receiving clocks synchronized
Asynchronous transmission :
- Bytes are used as independent transmission units, and the time interval between bytes is not fixed.
- The receiving end only synchronizes the bits within the byte at the beginning of each byte.
- Usually a start bit and an end bit are added before and after each byte.
One-way communication (simplex), two-way alternating communication (half-duplex) and two-way simultaneous communication (full-duplex)
In many cases, we use the term " channel ". Channels and circuits are not the same. Channels are generally used to represent media that transmit information in a certain direction. Therefore, a communication circuit often contains a transmit channel and a receive channel.
From the perspective of the way information is exchanged between the two parties in communication, there are three basic ways:
One-way communication :
Also known as simplex communication , that is, there can only be communication in one direction and no interaction in the opposite direction. Radio broadcasting or cable and television broadcasting are of this type
Two-way alternating communication :
Also known as half-duplex communication , that is, both parties to the communication can send information, but both parties cannot send it at the same time (of course, they cannot receive it at the same time). This method of communication enables one party to send and the other to receive, and the reverse can be reversed after a period of time.
Two-way simultaneous communication :
Also known as full-duplex communication , that is, both sides of the communication can send and receive information at the same time.
One-way communication requires only one channel, while two-way alternating communication or two-way simultaneous communication requires two channels (one in each direction)
Bidirectional simultaneous communication has the highest transmission efficiency
2.4. Coding and Modulation
Common terms
- data - the entity that carries the message.
- signal - an electrical or electromagnetic representation of data.
- Analog signal - The values of the parameters representing the message are continuous.
- Digital signal - The values of the parameters representing the message are discrete.
- Symbol (code) - When using waveforms in the time domain (or simply time domain) to represent digital signals, a basic waveform that represents different discrete values.
- Baseband signal (i.e. fundamental frequency band signal) - the signal from the source. Data signals representing various text or image files output by a computer are all baseband signals.
- Baseband signals often contain more low-frequency components and even DC components, and many channels cannot transmit such low-frequency components or DC components. Therefore, the baseband signal must be modulated
In computer networks, it is common to transmit digital baseband signals on corresponding channels through coding or modulation methods.
The relationship between transmission media and channels
Several basic concepts of channels
- Channel - generally used to represent a medium that transmits information in a certain direction.
- One-way communication (simplex communication) - There can only be communication in one direction and no interaction in the opposite direction.
- Two-way alternating communication (half-duplex communication) - Both parties to the communication can send information, but both parties cannot send at the same time (of course, they cannot receive at the same time).
- Two-way simultaneous communication (full-duplex communication) - Both parties to the communication can send and receive information at the same time.
Strictly speaking, transmission media cannot be equated with channels.
For simplex transmission, the transmission medium contains only one channel, either a transmit channel or a receive channel
For half-duplex and full-duplex, the transmission medium contains two channels, one is the sending channel and the other is the receiving channel.
If channel multiplexing technology is used, a transmission medium can also contain multiple channels
Commonly used encodings
non-return-to-zero encoding
- Positive level indicates bit 1/0
- Negative level represents bit 0/1
The dotted line in the middle is zero level. The so-called non-return to zero encoding means that the level will not appear at zero level during the entire symbol time.
The actual representation of bit 1 and bit 0 depends on the reality.
This requires strict synchronization between the sender's sending and the receiver's reception.
- An additional transmission line is needed to transmit the clock signal to synchronize the sender and receiver. The receiver receives symbols one by one according to the beat of the clock signal.
- But for computer networks, it is better to use this transmission line to transmit data signals rather than transmit clock signals.
Due to synchronization problems with non-return-to-zero encoding , this type of encoding is not used for data transmission in computer networks!
return to zero encoding
Although return-to-zero coding is self-synchronizing , the coding efficiency is low
Manchester encoding
In the middle of each symbol time, the signal will jump
- A negative transition indicates bit 1/0
- Positive transition indicates bit 0/1
- The jump in the middle moment of the symbol represents both the clock and the data
The actual representation of bit 1 and bit 0 depends on the reality.
Traditional Ethernet uses Manchester encoding
Differential Manchester encoding
At the middle moment of each symbol time, the signal sends a transition, but unlike Manchester
- Transitions only represent clock
- Whether the level at the beginning of the symbol changes to represent data
* 变化表示比特1/0 * 不变化表示比特0/1The actual representation of bit 1 and bit 0 depends on the reality.
Less changes than Manchester encoding and more suitable for higher transmission rates
Summarize
modulation
The digital signal is converted into an analog signal and transmitted in an analog channel, such as WiFi, using modulation methods such as complement keying CCK/direct sequence spread spectrum DSSS/orthogonal frequency division multiplexing OFDM .
The analog signal is converted into another analog signal and transmitted in an analog channel. For example, voice data is loaded into an analog carrier signal for transmission. Frequency division multiplexing FDM technology makes full use of bandwidth resources.
Basic modulation method
- Amplitude modulation AM : The modulated signal consists of two basic waveforms with different amplitudes. Each basic waveform can only represent 1 bit of information.
- FM : The modulated signal consists of two basic waveforms of different frequencies. Each basic waveform can only represent 1 bit of information.
- Phase modulation PM : The modulated signal consists of two basic waveforms with different initial phases. Each basic waveform can only represent 1 bit of information.
But using the basic modulation method, 1 symbol can only contain 1 bit of information
Mixed modulation
The 4 bits corresponding to the symbol in the above picture are wrong. The symbol cannot correspond to 4 bits arbitrarily.
code element
When using time domain waveforms to represent digital signals, they represent basic waveforms of different discrete values.
2.5. Limit capacity of channel
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Any actual channel is not ideal, and will produce various distortions and cause various interferences when transmitting signals.
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The higher the rate of symbol transmission, or the farther the signal transmission distance, or the worse the quality of the transmission medium, the more serious the distortion of the waveform at the output end of the channel.
Causes of distortion:
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The higher the rate of code element transmission
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The farther the signal travels
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The greater the noise interference
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The worse the transmission media quality
Comparison between Nye's criterion and Shannon's formula:
Supplement: Channel multiplexing technology
The content of this section is not covered in the video, it is the content of the physical layer of "Computer Network (7th Edition) Xie Xiren"
Frequency division multiplexing, time division multiplexing and statistical time division multiplexing
Multiplexing is a basic concept in communication technology.
It allows users to communicate using a shared channel, reducing costs and improving utilization.
Frequency Division Multiplexing FDM (Frequency Division Multiplexing)
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The entire bandwidth is divided into multiple parts. After the user is allocated a certain frequency band, he or she will occupy this frequency band throughout the communication process.
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All users of frequency division multiplexing occupy different bandwidth resources at the same time (please note that the "bandwidth" here is the frequency bandwidth rather than the data transmission rate).
Time Division Multiplexing TDM (Time Division Multiplexing)
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Time division multiplexing divides time into equal-length time division multiplexing frames (TDM frames) . Each time-division multiplexing user occupies a fixed number of time slots in each TDM frame.
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The time slot occupied by each user appears periodically (the period is the length of the TDM frame).
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TDM signals are also called isochronous signals.
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All users of time division multiplexing occupy the same frequency bandwidth at different times.
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Time division multiplexing may cause a waste of line resources
- When a time division multiplexing system is used to transmit computer data, due to the bursty nature of computer data, the utilization rate of the allocated sub-channels by users is generally not high.
Statistical Time Division Multiplexing STDM (Statistic TDM)
Wavelength division multiplexing
Wavelength Division Multiplexing WDM (Wavelength Division Multiplexing)
code division multiplexing
Code Division Multiplexing CDM (Code Division Multiplexing)
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The commonly used term is Code Division Multiple Access CDMA (Code Division Multiple Access).
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Each user uses a different pattern that is specially selected so that they do not interfere with each other.
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The signal sent by this system has strong anti-interference ability, and its spectrum is similar to white noise, making it difficult for the enemy to detect.