physical layer
knowledge points
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Transmission rate
The effectiveness of the digital communication system can be expressed by the symbol transmission rate and the information transmission rate;
Symbol definition 1: In digital communication, symbols with the same time interval are often used to represent a binary number, and the signal within such a time interval is called a binary symbol;
Symbol definition 2 [commonly used]: When using time domain (time domain) waveforms to represent digital signals, the basic waveforms representing different discrete values are called symbols;
Symbol rate (baud rate): the number of times carrier parameters (phase, amplitude, frequency, etc.) change per unit time, the unit is baud, commonly used symbol Baud, abbreviated as B, 1 Baud = 1 symbol/s;
Bit rate (information transmission rate, information rate): the amount of data transmitted on the channel per unit time (ie, the number of bits), the unit is bit per second (bit/s), or b/s or bps;
Bit rate = baud rate × binary digits corresponding to a single modulation state = baud rate × log2 (N) , where N is the total number of symbols;
Signal bandwidth W = highest frequency - lowest frequency;
Signal-to-noise ratio: The ratio of signal power S to noise power N is called the signal-to-noise ratio;
Decibel dB: 1 dB=10*log10(S/N) ;
[Noise-free] data rate calculation: maximum data rate = 2Wlog2 (N) = Blog2 (N), where W represents the bandwidth, B represents the baud rate, N is the total number of symbols, if it is a 4-phase index , then N=4;
Calculation of data rate when [with noise]: Shannon's formula - limit transmission rate = bandwidth × log2 (1+S/N), usually the title will be given in decibels, and decibels should ;
Bit error rate: refers to the ratio of the number of received error code elements to the total number of transmitted code elements.
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Modulation and Coding
Modulation uses analog signals to carry digital or analog data, and encoding uses digital signals to carry digital or analog data;
Modulation can be divided into baseband modulation (only transforming the baseband signal waveform without changing its frequency) and bandpass modulation (using the carrier to transfer the frequency of the baseband signal to a higher frequency band for transmission, which can be transmitted farther);
(1) Methods of modulating analog signals into analog signals: amplitude modulation AM (worst anti-interference), frequency modulation FM, phase modulation PM (best anti-interference);
(2) The method of encoding analog signals into digital signals: the most common one is pulse code modulation PCM, in which the process of pulse encoding is sampling, quantization and encoding ;
- Sampling: Periodically scan the analog signal to change the time-continuous signal into a time-discrete signal. The Nyquist sampling theorem must be followed. The sampling theorem is: the sampling frequency should be greater than twice the maximum frequency of speech (less than 4kHz is about 3.4kHz), that is, 8kHz (sampling period = 125μs) , so that the speech signal can be recovered without distortion;
- Quantization: The specified level value is usually expressed in binary. The voice system adopts 128-level (7-bit) quantization and 8kHz sampling frequency, so the effective rate is 56kb/s (=8kb/s×7). 1 bit of signaling bit needs to be added for every 7 bits, so the voice channel rate is 64kb/s (=8kb/s×8) [If the title says to add 1 bit of signaling bit, add it, if you don’t say it, don’t add it];
- Coding: Use a set of binary code groups to represent each quantized value with a fixed level;
(3) Modulation of digital signals into analog signals: using one or a combination of the three carrier characteristics of modulation frequency, amplitude and phase, the basic modulation methods are:
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Amplitude Shift Keying ASK:
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Frequency Shift Keying FSK:
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Phase shift keying PSK: 4PSK is called QPSK (quarter), which uses 4 output phases to represent 2 input bits, 8PSK uses 8 output phases to represent 3 input bits; 16PSK uses 16 output phases to represent 4 input bits (key Look at the number in front of PSK), DPSK is called relative phase shift keying modulation, there are two main modulation forms, 2DPSK and 4DPSK, DPSK has an initial phase change for input bit 1; for input bit 0, the initial phase does not change , see below picture:
Transmission speed of electric signal = 200000km/s, propagation delay of satellite channel = 270ms, total delay = transmission delay + propagation delay ;
(4) The digital signal is modulated into a digital signal:
- Manchester encoding: It is a bidirectional code, negative level to positive level represents 0, positive level to negative level represents 1, or vice versa, it is often used in 10M Ethernet, and the coding efficiency is 50%;
- Differential Manchester encoding: it belongs to a two-way code, when the signal starts to have a level change, it means 0, and if there is no level change, it means 1;
- To be able to calculate coding efficiency: the definition of 4B/5B is: the way of encoding 4-bit data into 5-bit symbols, coding efficiency=4bit/5bit=80%
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Data transmission method:
(1) Classified by signal type: analog communication, digital communication;
(2) Classified according to the number of data bits transmitted at one time: serial communication, parallel communication;
(3) Classification according to the relationship between the direction of signal transmission and time: simplex communication, half-duplex, full-duplex communication;
(4) Classification according to the relationship between the direction of signal transmission and time: synchronous communication, asynchronous communication;
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data exchange method
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multiplexing
The essence of multiplexing is to combine multiple signals into one signal at the sending end, and then realize transmission on a dedicated physical channel, and then separate the matching signals at the receiving end.
digital transmission system
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Pulse Code Modulation PCM System
Two important international standards of PCM: 24-channel PCM (T1, rate 1.544Mb/s) in North America and 30-channel PCM (E1, rate 2.048Mb/s) in Europe;
E1 multi-framing method: A time-division multiplexing frame (length T=125μs) of E1 is divided into 32 equal time slots (the time slot numbers are CH0~CH31), and each time slot transmits 8 bits (7 bits code plus 1bit signaling), so share 256bit (=32×8bit). 8000 frames** are transmitted per second, so the data rate of PCM primary group E1 is 2.048Mb/s, and the data rate of each voice channel is 64kb/s**;
T1: T1 has a total of 24 voice channels, and each time slot transmits 8 bits (7 bits coding plus 1 bit signaling), so it shares 193 bits (192 bits plus 1 bit frame synchronization bit). Transmit 8000 frames per second, so the data rate of PCM primary group T1=8000×193b/s=1.544Mb/s, wherein, the rate of each voice channel is 64kb/s ;
T1 and E1 can use the multiplexing method, 4 primary groups can form 1 secondary group (referred to as E2 and T2); 4 E2 can form 1 cubic group, called E3; 7 T2 can form 1 The cubic group is called T3.
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Synchronous Optical Network
Since the PCM rate is not uniform (T1 and E1 coexist), SONET is proposed to solve this problem;
In SONET, OC-1 is the smallest unit with a value of 51.84Mb/s, and OC-N represents N times 51.84Mb/s , such as OC-3=OC-1×3=155.52Mb/s;
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synchronous digital series
STM-1 rate = OC-3 rate = 155.52Mb/s;
access technology
The technologies of passive optical network PON mainly include: Ethernet passive optical network EPON and Gigabit Ethernet passive optical network GPON, which can realize the speed of uplink and downlink 1.25Gb/s.
wired transmission medium
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coaxial cable
In terms of usage, it can be divided into: baseband coaxial cable and broadband coaxial cable;
The baseband cable is divided into: thin coaxial cable and thick coaxial cable. The baseband cable is only used for digital transmission, and the data rate can reach 10Mb/s.
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Shielded twisted pair
According to different shielding methods, it can be divided into: STP and FTP
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Unshielded twisted pair
The line sequence is divided into: 568A and 568B
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optical fiber
