physical layer
communication basics
concept
Analog signals, continuously changing data or signals
digital signal, discrete
Symbol, using a signal waveform (digital pulse) with a fixed duration to represent a characteristic number, representing the basic [waveform] of different discrete values
Data: An entity that transmits information
Signal: The electrical or electromagnetic manifestation of data
Code element: refers to the use of a signal waveform with a fixed duration to represent a K-ary number, representing the basic waveform of different discrete values
communication subject
Source: the source that generates and sends data
channel: signal transmission medium
Sink: the end point of receiving and sending data
interactive mode
Single-engineering communication
Only one direction of communication without interaction in the opposite direction, only one channel is required, for example: radio broadcast, TV broadcast
half-duplex communication
Both parties in the communication can send or receive information, but neither party can send and receive information at the same time . In this case, two channels are required. Example: walkie-talkie
full duplex communication
Both parties in the communication can send and receive information at the same time, and also need two channels. The limit capacity of the channel refers to the highest symbol transmission rate of the channel or the limit information transmission rate of the channel. Example: phone
Nyquist and Shannon theorem
Nyquist Theorem
Regulations: In an ideal low-pass (no noise, limited bandwidth) channel, in order to avoid intersymbol interference, the limit symbol transmission rate is 2W baud, where W is the loan of the ideal low-pass channel, and V represents the number of discrete levels of each symbol
Formula: Limit data transmission rate under ideal low-communication channel = 2Wlog2V (unit is b/s)
in conclusion:
In any channel, there is an upper limit to the symbol transmission rate . If the transmission rate exceeds this upper limit, there will be serious intersymbol interference problems, making it impossible for the receiving end to completely correctly identify the symbols
The wider the frequency band of the channel (that is, the more high-frequency components of the signal passed), the higher the rate for effective transmission of symbols
Ny's criterion gives a limit to the symbol transmission rate, but does not give a limit to the information transmission rate, that is, it does not give a limit to how many binary bits a symbol can correspond to.
Shannon's theorem
Regulation: Shannon's theorem gives the limit data transmission rate of a channel with limited bandwidth and Gaussian white noise interference. When transmitting at this rate, no error can be achieved
official:
It can be concluded that the greater the newly arrived bandwidth or the signal-to-noise ratio in the channel, the higher the limit rate transmission rate of information
For a certain transmission bandwidth and a certain signal-to-noise ratio, the upper limit of the information transmission rate is determined
Find some way to achieve error-free transmission as long as the information rate is below the limiting transmission rate of the channel
Shannon's theorem gives the limit information transmission rate, and the transmission rate that the actual channel can achieve is much lower than it
Coding and Modulation
modulation
The process of changing data into an analog signal
coding
The process of converting data into digital signals
Encode digital data into a digital signal
Non-return-to-zero encoding [NRZ]
Non-return-to-zero reverse encoding [NRZI]
Manchester code
Differential Manchester encoding
Digital data modulated into an analog signal
Amplitude Shift Keying (ASK) AM
Frequency Shift Keying (FSK) FM
Phase Shift Keying (PSK) Phase Modulation
Quadrature Amplitude Modulation (QAM)
Encoding of analog data into a digital signal
Encoding of analog data into digital signals: mainly includes three steps, namely sampling, quantization and encoding
Sampling theorem (Nyquist theorem): When converting an analog signal to a digital signal, assuming that the maximum frequency in the original signal is f, then the sampling frequency f sampling must signal
Quantization: Quantization is to convert the level amplitude obtained by sampling into the corresponding digital value according to a certain grading scale and take an integer, so that the continuous level amplitude can be converted into a discrete digital quantity
Coding is to convert the quantized result into the corresponding binary code
Circuit Switching, Message Switching, and Packet Switching
circuit switching
A dedicated physical communication path must be established between two nodes of circuit switching, and it will not be released until the communication is over.
Divided into three stages: connection establishment, data transmission and connection release
advantage:
The communication delay is small . Since the communication line is dedicated to both users of the communication, the delay of transmitting data is very small. When the amount of data is large, this advantage is very obvious
Orderly transmission , when the two parties communicate, the data is transmitted in the order in which they were sent, and there is no out-of-sequence problem
There is no conflict , different communication parties have different channels, and there will be no problem of requisitioning physical channels
Wide range of applications , circuit switching is suitable for transmitting both analog and digital signals
Strong real-time performance , once the physical path between the communication parties is established, the two parties can communicate at any time
Simple control , circuit switching switching equipment (switches, etc.) and control are relatively simple
shortcoming:
It takes a long time to establish a connection; the line is exclusive, and the use efficiency is low; the flexibility is poor; it is difficult to standardize
message exchange
Concept: The unit of data exchange is a message , and the message carries information such as the destination address and the original address. The message exchange adopts the store-and-forward transmission mode at the switching node
Advantages: No need to establish a connection , message exchange does not need to establish a dedicated communication line for both parties in advance, there is no delay in establishing a connection, and users can send messages at any time
Dynamically allocate lines . When the sender hands over the insulation to the switching device, the switching device first stores the entire message, then selects a suitable idle line, and sends the message out
Improve line reliability . If a certain transmission path fails, another path can be reselected to transmit data, thus improving transmission reliability.
To improve line utilization , the communication parties do not occupy a communication line fixedly, but partly occupy this physical channel at different times, thus greatly improving the utilization rate of communication lines
Disadvantages: After data enters the switching node, it has to go through the process of storage and forwarding, so it will cause forwarding delay
There is no limit to the size of the message in message exchange, which requires the network node to have a large cache space
Packet switching:
Concept: Same as message exchange, packet forwarding also adopts store-and-forward mode, but limits the upper limit of the data block size transmitted each time, and divides large data blocks into reasonable small data blocks
Pros: Simplified storage management (vs. message exchange)
The probability of error and the amount of retransmitted data are reduced . Because the packet is shorter, its error probability is bound to be reduced, even if an error occurs, the amount of retransmitted data is greatly reduced
Disadvantages: Additional information needs to be transmitted . Information such as the original address, destination address, and group number must be added to each small data block to form a group, which reduces communication efficiency to a certain extent
When packet switching uses data packet services, there may be out-of -sequence, loss, or re-packetization . When the packets arrive at the destination node, it is necessary to sort the packets by number and so on.
Packet switching can be further divided into connection-oriented virtual circuit mode and connectionless datagram mode according to the services provided by its communication subnet to the end-point system
Packets and Virtual Circuits
Datagram: A datagram packet formed by adding control information such as an address on the network layer, storing the packet at an intermediate node for a short period of time, and forwarding each packet as soon as possible after finding the best route. Different packets can take different paths and reach the destination nodes in different orders
no connection required
no guarantee of reliability
Need to wait in line for processing
Strong adaptability to failure
do not monopolize a link
Virtual circuit: The virtual circuit method combines the datagram method and the circuit switching method , and gives full play to the advantages of the two methods to achieve the best data exchange effect
Before the packet is sent, it is required to establish a logically connected virtual circuit between the sender and the receiver, and once the connection is established, the physical path corresponding to the virtual circuit is fixed. Similar to circuit switching, what are the stages of the entire communication process: virtual circuit establishment, data transmission, and virtual circuit release
Time overhead to build and tear down
Provides a reliable communication function
Virtual circuits are vulnerable to damage
The packet header contains only the virtual circuit identifier
Transmission medium (simple understanding)
twisted pair
Shielded twisted pair
Unshielded twisted pair
coaxial cable
baseband coaxial cable
broadband coaxial cable
optical fiber
single mode fiber
multimode fiber
wireless transmission medium
radio waves
microwave, infrared and laser
main mission
physical layer considerations
The physical layer does not refer to a specific transmission medium
Physical layer interface characteristics
mechanical properties
Defines the edge nodes of the physical connection, i.e. plug-in devices
Specify specifications, number of leads, number and arrangement of pins, etc.
Electrical characteristics (quantity)
It stipulates the transmission of binary bits, the voltage level of the signal on the line, impedance matching, transmission rate and distance limitation, etc.
Features
Indicate what the voltage of a certain level on a certain line means [meaning], the [use] of the signal line (data line, control line) of the interface component
Procedural characteristics (process characteristics)
Define the working procedure and timing relationship of each physical line
physical layer device
repeater (repeater)
Repeater: Reshaping and amplifying the signal and forwarding it to eliminate the distortion and attenuation of the signal after a long time of cable passage, so that the waveform and strength of the signal meet the required requirements, thereby expanding the distance of network transmission
The principle is signal regeneration (rather than simply re-amplifying the attenuated signal)
Hub (Hub)
The hub (Hub) is essentially a multi-port repeater ; if two or more ports are input at the same time, there will , making the data invalid
For your own learning and use only, https://www.bilibili.com/video/BV1Ha411r745?p=2&spm_id_from=pageDriver