Network counting 01 physical layer

The physical layer _ Baidu Encyclopedia

Physical layer tasks

• Shield various transmission media ( Twisted pair … ) and Means of communication .
• Specify the physical connection characteristic
  • mechanical ( Interface shape )
  • electrical （ Voltage range )
  • function （ Level meaning Such as : 0V surface 0 5V surface 1 )
  • The process ( Sequence of events for different functions )
• Due to the transmission medium 、 Means of communication 、 There are many physical connections Therefore, there are many physical layer protocols
• Common physical layer protocols :
  • RS-232、RS-449、X.21、V.35、ISDN、FDDI、IEEE802.3、IEEE802.4、IEEE802.5

Digital communications

• Data communication model : Source transmission Shinjuku
  

• signal communication 、 news 、 data 、 The signal 、 Code element difference

  • The purpose of the communication : Transmit messages
  • news : Various voice 、 written words 、 Images
  • data : transport The entity of a message consisting of a sequence of meaningful characters
  • The signal : Electromagnetic representation of data Can be divided into Continuous analog signal Discrete digital signal
  • Code element : A fixed duration ( Symbol width ) Signal waveform , It means a K Hexadecimal Numbers , Can contain multi bit data , Binary system 2 Kinds of symbols , Express 0 、 1

• Transmission rate : The amount of data transmitted per unit time

  • Symbol transmission rate : Also known as waveform rate , Number of transmitted symbols per unit time , Unit Porter ( Baud ) , This symbol is multi base , but