Carrier Listening _ phone

The meaning of carrier listening (carrier sense) is:

Before sending the data, Ethernet "listens" to the cable to determine if there are other data transfers. Consider this scenario: suppose you have multiple telephones hooked up to the same phone line. If you pick up one of the phone microphones and you're ready to call, and you hear someone else talking, you have to wait until that person has finished talking. If no one else is using the line, you can use it. This is how Ethernet works.

CSMA (Carrier Sense Multiple access) is called a carrier-listening multi-channel access method.

Basic idea: When any site wants to send a message to a public medium, first listen to whether there are other sites on the media are transmitting information, if the communication medium without carrier, that is not occupied, you can use the communication medium for transmission; If you have heard the carrier on the media, that is, other sites are transmitting information, You must wait for the media to be calm before you can transfer processing, which will greatly reduce the conflict on the channel.

According to the rules of listening media can be divided into Csma, probability 1-adhere to CSMA, probability P-stick csma. On the basis of CSMA can make further improvements, that is, when the site began to send, still need to continue to listen to the channel for a period of time, when the conflict detected immediately quickly cancel the conflict frame transmission. This protocol is referred to as a carrier interception multipath Access Protocol (CSMA/CD) with collision detection.

Working process:

(1) Listening, if idle, send.

(2) busy to listen to the idle immediately sent.

(3) detected a collision, immediately stop, send blocking signal.

(4) Random wait, try to send again.