Table of contents
1. The basic concept of the physical layer
2. Transmission media below the physical layer
(1) Optical fiber, coaxial cable, twisted pair, power line [guide type]
(2) Radio waves, microwaves, infrared rays, visible light [non-guided type]
(2) Synchronous & asynchronous
(3) Simplex, half-duplex, full-duplex
(4) Differential Manchester encoding
(4) Basic modulation methods (amplitude modulation, frequency modulation, phase modulation)
(5) Hybrid modulation - quadrature amplitude modulation (QAM)
5. The limit capacity of the channel
2. Chapter 2 - Physical Layer
1. The basic concept of the physical layer
2. Transmission media below the physical layer
Transmission media - does not belong to any layer of [computer network architecture]!
(1) Optical fiber, coaxial cable, twisted pair, power line [guide type]
(2) Radio waves, microwaves, infrared rays, visible light [non-guided type]
Visible light communication is a type of wireless communication that uses electromagnetic waves in the visible light band visible to the human eye .
In the future, all lighting will be replaced by LED lighting and organic EL lighting that can be modulated at a high speed, and " lighting will become the communication infrastructure " . [There is still a long way to go, and WIFI cannot be replaced for the time being ~ 】
(3) Radio 【Use of Spectrum】
Can not be used indiscriminately - radio communication should be discussed in advance, developed according to the established and prescribed frequency, and the corresponding communication method
Each country has its own management agency - use and development, need to apply~
why?
Radio communication is a communication method based on the propagation of electromagnetic waves, which can transmit information over long distances without the need for a physical connection.
Since the radio spectrum is a limited resource and radio waves of different frequency bands can exist simultaneously without interfering with each other, it is necessary to manage its use and development.
There will be certain planning and management of radio frequency spectrum allocation between countries, regions, and different organizations.
This is to avoid the chaotic use and interference of spectrum, and to ensure the reliability and stability of communication.
Each country has established a corresponding regulatory agency responsible for planning, allocating and monitoring the use of the radio frequency spectrum.
The purpose of applying for the use and development of the radio frequency spectrum:
- In order to ensure fair and reasonable use of spectrum resources, avoid spectrum waste and conflicts.
- By negotiating in advance and communicating according to the specified frequency, frequency interference can be reduced and communication quality and efficiency can be improved.
- The management agency will allocate appropriate frequency resources according to the needs of the applicant and the actual situation, so as to ensure that the communication between all parties can go smoothly.
3. Transmission method
(1) Serial & Parallel
(2) Synchronous & asynchronous
Asynchronous: The duration of the byte transfer is the same (only the time interval at which the data is sent is different)
(3) Simplex, half-duplex, full-duplex
Simplex - Telegraph
Half Duplex - Walkie Talkie
Full Duplex - Telephone
4. Coding and modulation
Message - Data - Signal - Baseband Signal
Audio: quantization, sampling, encoding (analog to digital)
A symbol - a segment of a waveform that makes up a signal! ! !
(1) Non-return-to-zero coding
Non-Return-to-Zero coding (Non-Return-to-Zero, NRZ) is a common digital signal coding method, in which logic 0 and logic 1 are represented by different levels. Although NRZ encoding is simple and intuitive, it does have synchronization issues .
The synchronization problem is mainly due to the continuous transmission of the same bit (such as continuous 0 or 1) for a long time, so that the receiving end cannot accurately determine the start and end positions of the bit transmission.
(2) Return to zero coding
Return-to-Zero encoding (Return-to-Zero, RZ) can indeed overcome the synchronization problem in NRZ encoding , but its side effect is that the encoding efficiency is low.
In return-to-zero encoding, each bit will have a return-to-zero point, either a logic 0 or a logic 1. This means that the signal returns to zero level at the end of each bit before encoding of the next bit begins. This zeroing operation ensures that the receiver can perform accurate synchronization based on the starting position of each bit.
However, this zeroing operation results in reduced coding efficiency . Because there will be additional level changes during the zeroing process, the signal frequency is lower and the bit rate of transmission is higher.
(3) Manchester encoding
The jump of the code element——indicates both [clock] and [data]
Traditional Ethernet - using [Manchester encoding]
(4) Differential Manchester encoding
An upgraded version of Manchester encoding (distinguishes the clock and data flags)
The jump in the middle - represents the clock [for synchronization]
The transformation at the beginning of the code unit—— 【jump: 0】【non-jump: 1】(jump 0 not 1)
It can be seen here that his jumps are less than [Manchester encoding], and he is more suitable for high-speed transmission!
The specific difference: (please see the picture below)
Except [Differential Manchester Encoding], the others all start with 1, which is 1.
(4) Basic modulation methods (amplitude modulation, frequency modulation, phase modulation)
AM、FM、PM
Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (Phase Modulation, PM) are common analog modulation techniques used to transmit information in a carrier signal .
AM is a technique that modulates the amplitude of a baseband signal with the amplitude of a carrier signal. During AM, changes in the amplitude of the baseband signal will cause a corresponding change in the amplitude of the carrier signal. In this way, the information in the baseband signal is embedded in the modulated carrier signal. Each symbol ( symbol ) usually contains only one bit of information in AM.
Frequency modulation is a technique for modulating a carrier signal according to the frequency variation of the baseband signal. During frequency modulation, the frequency change of the baseband signal will cause a corresponding change in the frequency of the carrier signal. The information in the baseband signal is encoded as a pattern of varying frequency, which is then transmitted into the modulated carrier signal. Similarly, each symbol typically contains only one bit of information in FM.
Phase modulation is a technique for modulating a carrier signal according to the phase change of the baseband signal. During phase modulation, the phase change of the baseband signal causes a corresponding change in the phase of the carrier signal. The information in the baseband signal is encoded as a pattern of phase changes, which is transmitted to the modulated carrier signal. Likewise, each symbol typically contains only one bit of information in phase modulation.
It should be noted that the above description is based on analog modulation technology. In digital communication, digital modulation techniques are usually used, such as amplitude modulation keying ( Amplitude Shift Keying, ASK ), frequency shift keying ( Frequency Shift Keying, FSK ) and phase shift keying ( Phase Shift Keying, PSK ).
These digital modulation techniques can encode multiple bits of information into a single symbol, improving data transmission efficiency.
(5) Hybrid modulation - quadrature amplitude modulation (QAM)
This part is partial to the principle of communication (didn't learn it well, alas). Although I have read it later, but ... (difficult to say)
5. The limit capacity of the channel
Signals (digital signals) are affected (during transmission)
(1) Nye's criterion
[Nefest] Derivation! ! !
The bit information carried by a symbol is not as big as possible! ! ! [The more you carry, the easier it is to be disturbed]
Modulation speed = symbol transmission speed
(2) Shannon formula
channel bandwidth = channel bandwidth
Signal-to-Noise Ratio (SNR) is an indicator used to measure the relative strength between signal and noise. It represents the ratio between signal power and noise power.
(3) Exercises
bit
byte
