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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