content
Basic Concepts of the Physical Layer
Data Communication Fundamentals
1. Typical data communication model
2. Terms related to data communication
Serial Transmission & Parallel Transmission
A phenomenon of distortion---intersymbol crosstalk
Nyquist Criterion (Nyquist Theorem)
Basic Concepts of the Physical Layer
The physical layer addresses how to transmit , rather than referring to specific transmission media.
The main task of the physical layer: determine some characteristics related to the transmission media interface → define the standard
1. Mechanical properties
Define the characteristics of the physical connection, and specify the specifications, interface shape, lead number, pin number and arrangement used in the physical connection.
2. Electrical Characteristics
When specifying the transmission of binary bits, the voltage range, impedance matching, transmission rate and distance limit of the signal on the line are specified.
3. Features
Indicate the meaning of a certain level appearing on a certain line, and the purpose of the signal line of the interface component.
4. Procedure Features
(Process characteristics) Define the work order and timing relationship of each physical line.
Data Communication Fundamentals
1. Typical data communication model
2. Terms related to data communication
The purpose of communication is to transmit a message (message: voice, text, image, video, etc.).
data : An entity that conveys information, usually a meaningful sequence of symbols.
Signal: The electrical/electromagnetic representation of data, the form in which data exists during transmission.
Digital signal/discrete signal: The value of the parameter representing the message is discrete.
Analog signal/continuous signal: The value of the parameter representing the message is continuous.
Source : The source that generates and transmits data.
Sink : The destination to receive data.
Channel : The transmission medium of the signal. It is generally used to represent the medium that transmits information in a certain direction, so a communication line often contains a sending channel and a receiving channel. .
Three ways of communication
From the perspective of the way of information exchange between the two parties in communication, there are three basic ways:
1. Simplex communication
Only one channel is required for communication in one direction and no interaction in the opposite direction.
2. Half-duplex communication
Both parties in the communication can send or receive information, but neither party can send and receive at the same time, requiring two channels.
3. Full duplex communication
Both sides of the communication can send and receive information at the same time, and also need two channels.
transfer method
Serial Transmission & Parallel Transmission
symbol rate baud bandwidth
symbol
A symbol refers to a signal waveform (digital pulse) with a fixed duration, representing the basic waveform of different discrete values, and is the unit of measurement of digital signals in digital communication. The signal within this duration is called a k-ary symbol, and the duration is is called the symbol width. When there are M discrete states of the symbol (M is greater than 2), the symbol is an M-ary symbol at this time.
1 symbol can carry multiple bits of information . For example, when using binary encoding, there are only two different symbols, one representing the 0 state and the other representing the 1 state.
rate
The rate is also called the data rate, which refers to the transmission rate of data, indicating the amount of data transmitted per unit time. It can be represented by the symbol transmission rate and the information transmission rate.
1) Symbol transmission rate : alias symbol rate, waveform rate, modulation rate, symbol rate, etc., which represent the number of symbols transmitted by the digital communication system per unit time (also known as the number of pulses or the number of signal changes) ) in Baud. 1 baud means that a digital communication system transmits one symbol per second.
2) Information transmission rate : alias information rate, bit rate, etc., indicating the number of binary symbols (that is, the number of bits) transmitted by the digital communication system per unit time, in bits/second (b/s).
Relationship: If a symbol carries n bits of information, the information transmission rate corresponding to the symbol transmission rate of M Baud is
M*n bit/s。
bandwidth
Bandwidth : Indicates the " highest data rate " that can pass from a certain point in the network to another point in a unit of time , and is often used to indicate the ability of the communication line of the network to transmit data. The unit is b/s.
Shannon's theorem
distortion
Factors that affect the degree of distortion:
1. Symbol transmission rate 2. Signal transmission distance 3. Noise interference 4. Transmission media quality
A phenomenon of distortion---intersymbol crosstalk
Intersymbol interference: The phenomenon in which the signal waveform received at the receiving end loses the clear boundaries between symbols.
Nyquist Criterion (Nyquist Theorem)
Nye's Criterion : Under ideal low-pass (no noise, bandwidth-limited) conditions, in order to avoid inter-symbol crosstalk, the limit symbol transmission rate is 2W Baud, where W is the channel bandwidth in Hz.
1. In any channel, the rate of symbol transmission is limited . If the transmission rate exceeds this upper limit, serious inter-symbol interference will occur , making it impossible for the receiving end to fully and correctly identify the symbols. 2. The wider
the frequency band of the channel (that is, the more high-frequency components of the signal that can pass through), the efficient transmission of symbols can be performed at a higher rate. 3. The Nye's Criterion gives a limit on the transmission rate of symbols, but does not limit the transmission rate of information . 4. Since the transmission rate of the symbol is restricted by the Nye's criterion, in order to improve the transmission rate of the data, it is necessary to try to make each symbol carry more bits of information, which requires the use of a multivariate modulation method. .
Shannon's theorem
Noise is present in all electronic equipment and communication channels. Because the noise is randomly generated, its instantaneous value is sometimes very large, so the noise will make the receiver's decision on the symbol wrong. But the influence of noise is relative, if the signal is stronger, then the influence of noise is relatively small. Therefore, the signal-to-noise ratio is very important.
Signal-to-noise ratio = the average power of the signal / the average power of the noise , often denoted as S/N, and using decibels (dB) as the unit of measure, namely:
1. The larger the bandwidth of the channel or the signal-to-noise ratio in the channel, the higher the limit transmission rate of information .
2. For a certain transmission bandwidth and a certain signal-to-noise ratio, the upper limit of the information transmission rate is determined.
3. As long as the transmission rate of information is lower than the limit transmission rate of the channel, a method must be found to achieve error-free transmission.
4. The limit information transmission rate obtained by Shannon's theorem is much lower than the transmission rate that the actual channel can achieve.
5. It can be seen from Shannon's theorem that if the channel bandwidth W or the signal-to-noise ratio S/N has no upper limit (impossible), then the limit information transmission rate of the channel has no upper limit.
