Under normal circumstances, the network is divided into five layers from top to bottom: application layer, transport layer, network layer, data link layer, physical layer. Each layer has its own rules to be observed. TCP / IP protocol is a group of the most commonly used network protocols. The lan cable belongs to the physical layer in the network. The data to be transmitted in the computer is decomposed into one data packet according to these protocols and then transmitted to the destination computer through the lan cable according to certain principles. Commonly speaking, and we send the letter to the truth, first write the contents of the letter (computer data) into the envelope, and then write the address on the cover (packaged into packets containing the address of the local machine and the destination machine) , And finally sent (transmission), then the lan cable is equivalent to your address and the address you want to send the way. (1) As mentioned above, the principle that the lan cable and the electric wire transmit electricity is the same, but the pulse electric signal is transmitted on the network cable, and must abide by certain rules. (2) The data on the computer is all saved with 0 and 1, so when transmitting on the network cable, it is necessary to use a voltage value to represent the data 0 and to use another voltage value to represent the data 1. (3) The transmission of digital signals on the network cable. (4) The transmission line of the lan cable is to transmit the electric signal, there will be the electric current to pass, then will produce the electromagnetic field. Electromagnetic field between several network lines interfere with each other, will affect the voltage so that the data distortion. So they can be twisted together to effectively offset this line of mutual electromagnetic interference.