Transmission media ： Used to transmit data
physical layer ： Solve the problem of transmitting bits on various transmission media 01 Question of .
The physical layer provides the data link layer with the service of transmitting transparent bit stream , The data link layer only needs to consider how to complete the protocols and services of this layer , Without considering the specific transmission media of the network .
transparent ： The data link layer does not need to know how the physical layer transmits 01 of
Main tasks of physical layer protocol ：
mechanical properties ： Indicate the shape and size of the connector used for the interface , Number and arrangement of pins , Fixing and locking device
Electrical characteristics ： Indicate the voltage range on each line of the interface cable
Functional characteristics ： Indicate the meaning of the voltage at a certain level on a line
Process characteristics ： Indicate the order of occurrence of various possible events for different functions
Learning focuses on basic concepts , Not a specific physical layer protocol
Transmission media under the physical layer ：
Just understand
The transmission media is not part of the architecture
Guided transmission media ： Twisted pair , Coaxial cable , Optical fiber
Non guided transmission media ： microwave communication （2~40GHz）, radio waves , infrared , visible light
Coaxial cable ：
Twisted pair ：
Most commonly used . Mix every two lines , It can resist some electromagnetic wave interference from the outside ; Reduce electromagnetic interference of adjacent conductors
Optical fiber ：
Fine diameter .
advantage ： Large communication capacity . Low transmission loss . Good resistance to lightning and electromagnetic interference . No crosstalk interference .
shortcoming ： expensive price .
total reflection . Due to dispersion , Will produce signal distortion . Only suitable for short distance transmission .
power line ：
transmission mode
Serial transmission ： one by one Bit send out , Just 1 Line , It is often used for long-distance transmission .
Parallel transmission ： A relax n individual bit, need n Transmission line , It is often used for internal data transmission of computer （CPU Between and memory ）
Synchronous transmission ：
Data blocks are transmitted in the form of a stable bit stream , There is no space between bytes
The receiver detects at the middle time of each bit signal , To judge whether the received is 0 still 1
Because the clock frequency of different equipment is different , It's impossible to be exactly the same , In the process of transmitting a large amount of data , Cumulative error of discrimination time , It will cause the discrimination dislocation of the bit signal at the receiving end .
Solution ： Clock synchronization between sender and receiver
External synchronization ： Add a separate clock signal line between the sender and the receiver
Internal synchronization ： The transmitter encodes the clock synchronization signal into the transmission data and transmits it together （ For example, mantest code ）
Asynchronous transmission ：
Independent transmission unit in bytes , The time interval between bytes is not fixed . The receiver synchronizes the bits in the byte only at the beginning of each byte .
therefore , Usually, the start bit and end bit are added before and after each byte .
Asynchronous between bytes , The time interval between bytes is not fixed ; Each bit in the byte still needs to be synchronized , The duration of each bit is the same .
Unidirectional communication （ Simplex ）
Bidirectional alternating communication （ Half duplex ）
Two way simultaneous communication （ full duplex ）
Data can be sent at the same time
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