CSMA/CD

CSMA/CD (abbreviation of Judicious Carrier Multiple Access /Collision Detection , in English) is a protocol which manages the division of the physical access to the network Ethernet, according to the standard IEEE 802.3

Explanations

  • Judicious Carrier: listen to the carrying one.
  • Multiple Access: several stations (machines) can emit simultaneously with a collision risk.
  • Collision Detection: detection of the errors of collision and treatment by sending a JAMMING signal.

This type of protocol is known as “probabilist”, i.e. it is not possible to determine with certainty the time of sending of a message. Let us recall that in an Ethernet network the stations share the same medium of communication, that there is no token nor of priority of emission.

Principle

Listen before talking : if a station wants to emit, she listens to the network to know if there is already another emission in progress (presence or not of carrying). So yes it waits, if not it emits.

The station emits during a minimum of time (higher than maximum time outward journey and return of data between 2 stations), this so that its emission can be detected by the other stations and thus to avoid other simultaneous emissions. However it continues to listen during its emission in order to check that its data were not disturbed: Listen while talking

If a collision takes place, the two machines stop their communication and await a random time (given by the algorithm of Back-off), then the first having passed this time can then re-emit. If the number of collisions exceeds a certain threshold (generally 16), it is considered that there is a fatal error and the emission stops with the excessive condition collisions .

The station which wants to emit must send a sufficiently long signal to traverse all the network ( section channel ). The size minimum of the screen to be sent must be of 512 bits (while basing itself on topology 10BASE5). If a screen does not reach the 512 bits, one makes a stuffing with neutral bits (PADDING).

Consequently more one screen is large plus it is effective and less collisions will occur on the network.

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