Physical layer introduction

Communication basis

Basic concepts

• data ： An entity that transmits information
• The signal ： Electrical or electromagnetic representation of data , Is the existence form of data in the process of transmission
• analog signal ( Or data )： A continuously changing signal （ Or data ）
• digital signal ( Or data )： A signal with a finite number of discrete values （ Or data ）
• modulation ： The process of converting data into analog signals
• code ： The process of converting data into digital signals
• Code element ： A signal waveform or digital pulse with a fixed duration is used to represent a k Decimal digit , The signal within this time is called k Binary symbol , This duration is called the symbol width
• baseband signal ： Raw signal without modulation , It is characterized by low frequency . It is divided into digital baseband signal and analog baseband signal
• Broadband signal ： An analog signal formed by modulating a baseband signal , It is characterized by high frequency

Theorem

Nyquist's theorem

• In the ideal low-pass （ No noise , Limited bandwidth ） In the channel , To avoid code to code crosstalk , The limit symbol transmission rate is 2W potter , among W Is the bandwidth of the ideal low-pass channel

Shannon's theorem

• The limited data transmission rate of the channel with limited bandwidth and Gaussian white noise interference is ：Wlog₂(1+S/N) （ Unit is bps,W For the bandwidth of the channel ,S Is the average power of the signal transmitted by the channel ,N Is the Gaussian noise power inside the channel , When calculating S/N Why not take dB The numerical , for example S/N=30dB, Then we should 1000 Put in the calculation ）

Sampling theorem

Assume that the maximum frequency in the original signal is f_max, To make the sampled digital signal completely retain the signal of the original signal , Then the sampling frequency must meet ：f _sampling ≥ f_max

Coding and modulation

Digital data -> digital signal

• Zeroing code （RZ）： Jump to low level in the middle of each clock cycle , The receiver adjusts its own clock reference according to the jump to achieve the purpose of synchronization , But zeroing takes up part of the bandwidth
• Non return to zero coding （NRZ）： The entire clock cycle is used to transmit data , But it is difficult for the two sides to synchronize , Need both sides sometimes clock line
• Reverse non return to zero code （NRZI）： It is represented by the reversal of the signal 0, The signal remains unchanged to indicate 1, It can transmit clock signal , And try not to lose bandwidth . Be used for USB2.0
• Manchester code ： Binary symbols 1 Use one (+,-) The pulse sequence represents , Binary symbols 0 Use one individual (-,+) The pulse sequence represents , Each pulse is half the width of the symbol . Used in Ethernet
• Differential Manchester code ： If the symbol is 1, Then the level of the first half symbol is the same as that of the second half symbol of the previous symbol . If the symbol is 0, On the contrary . Commonly used in LAN
• 4B/5B code ： Each of the data streams to be sent 4 As a group , according to 4B/5B The coding rules convert it into the corresponding 5 Bit code .5 Bit code sharing 32 Middle combination , But only one of them 16 Species pair 16 Different 4 Bit code , Other 16 Bit as control code

Digital data -> analog signal

• Amplitude shift keying （ASK）
• Frequency shift keying （FSK）
• Phase shift keying （PSK）： It is divided into absolute phase modulation and relative phase modulation
• Quadrature amplitude modulation （QAM）： On the premise of the same frequency , take ASK And PSK Combine , Form a superimposed signal

Analog data -> digital signal

• Pulse code modulation （PCM）： sampling （ Satisfy the sampling theorem ） -> quantitative -> code

Analog data -> analog signal

• In order to achieve the effectiveness of transmission , A higher frequency may be required ; Or frequency division multiplexing （FDM） To take full advantage of bandwidth . For example, the telephone and local office switch adopt the modulation mode of analog signal transmission of analog data

Transmission medium

Wired transmission media

Twisted pair

• Consisting of two twisted side by side with certain rules 、 Composed of insulated copper conductors . Stranding can reduce electromagnetic interference to adjacent conductors
• Depending on whether there's a shield （ It can further improve the ability of anti electromagnetic interference ）, It is also divided into shielded twisted pair （Shielded Twisted Pair,STP） And unshielded twisted pair （Unshielded Twisted Pair,UTP）
• The bandwidth of twisted pair depends on the thickness of copper wire and transmission distance .
• Transmission distance ranges from several thousand meters to tens of kilometers . When the distance is too far , For analog transmission , Used for amplifier to amplify attenuated signal ; For digital transmission , A repeater is used to shape the distorted signal
• Because the price is cheap , It is widely used in LAN and traditional telephone network

Coaxial cable

• By the inner conductor 、 Insulating layer 、 Mesh braided shielding layer and plastic shell
• According to the difference of characteristic impedance value , It is divided into 50Ω Coaxial cable and cable 75Ω Coaxial cable .50Ω Coaxial cable is mainly used to transmit digital baseband signals , Also known as baseband coaxial cable ;75Ω Coaxial cable is mainly used to transmit broadband signals , Also known as broadband coaxial cable
• Because of the shielding layer of the outer conductor , Coaxial cable has good anti-interference characteristics , It is widely used to transmit high-speed data , The transmission distance is longer , The price is also more expensive than twisted pair

Optical fiber

• It is mainly composed of fiber core and cladding , The cladding has a lower refractive index than the core , When a light beam is emitted from a medium with a high refractive index to a medium with a low refractive index , Its refraction angle will be greater than the incident angle . So as long as the incident angle is greater than a certain critical angle , Total reflection occurs , That is, when light hits the cladding, it will be refracted back to the core , The process repeats itself , Light also travels along the light
• Using the total reflection characteristics of light , Multiple rays with different incident angles can be transmitted in one optical fiber , This kind of fiber is called multimode fiber , The light source is led . Optical pulse will be gradually broadened in multimode fiber transmission , Cause distortion , Therefore, it is only suitable for short distance transmission
• When the diameter of the optical fiber is reduced to only one wavelength , The optical fiber will travel forward like a waveguide , This kind of fiber is single-mode fiber , Its light source is a semiconductor laser with good directivity , Very little attenuation , Suitable for long distance transmission

Wireless transmission media

• The lower the frequency of the electromagnetic wave , The less transmission loss , The farther the coverage distance , The stronger the diffraction ability . But the frequency resources in the low frequency band are scarce , The system capacity is limited , Therefore, low-frequency radio waves are mainly used in broadcasting 、 TV 、 Paging and other systems .
• Rich high-frequency resources , The system has a large capacity . But the higher the frequency , The greater the propagation loss , The closer the coverage , The weaker the diffraction ability . in addition , The higher the frequency is. , The more difficult the technology is , The cost of the system increases accordingly .

radio waves

• Frequency at 30GHz The following electromagnetic waves

microwave 、 Infrared and laser

• Line of sight propagation

Physical layer interface

• mechanical properties ： Interface shape 、 Size 、 Number and arrangement of pins 、 Securing and locking devices
• Electrical characteristics ： Voltage range on each line of the interface cable
• features ： Indicate the meaning of the voltage at a certain level on a line
• Process characteristics （ Discipline characteristics ）： Well known sequence of possible events for different functions
• Common physical layer interface standards include EIA RS-232-C、ADSL and SONET/SDH etc.

Physical devices

Repeater

• The repeater only acts on the electrical part of the signal , There are two ports , The signal is input from a port , After shaping and amplifying the signal, it will be forwarded from another port
• The principle is signal regeneration , Instead of simply amplifying the attenuated signal
• Both amplifiers and repeaters amplify , The amplifier amplifies the analog signal , The principle is to amplify the attenuated signal
• 5-4-3 The rules ： Because the delay range of the signal is specified in the network standard , Therefore, when using thick coaxial cable 10BASE5 Ethernet specification , The number of repeaters connected in series cannot exceed 4 individual , Also used 4 A repeater connected in series 5 There are only... In the communication medium 3 The segment can be attached to the computer , rest 2 The segment can only be used as a link segment to extend the communication range

A hub

• In essence, it is a multi port repeater , When a port receives a signal , After shaping and amplifying the signal, it is forwarded from all ports except the port . If there are multiple port inputs at the same time, there will be conflicts , These data are invalid .