• The CSMA/CD(Carrier Sense Multiple Access/Collision Detection) specifications have been standardized by IEEE 802.3 standard. It is a very widely used MAC protocol.

• Media Access Control

  i. The problem in CSMA is that a transmitting station continues to transmit its frame even though a collision occurs.

  ii. The channel time is unnecessarily wasted due to this. In CSMA/CD, if a station receives other transmissions when it is transmitting then a collision can be detected as soon as it occurs and the transmission time can be saved.

  iii. As soon as a collision is detected, the transmitting stations release a jam signal.

  iv. The jam signal will alert the other stations. The stations then are not supposed to transmit immediately after the collision has occurred.

  v. Otherwise there is a possibility that the same frames would collide again.

  vi. After some “back off” delay time the stations will retry the transmission. If again the collision takes place then the back off time is increased progressively.

• CSMA/CD Procedure with Example

  i. The station that has a ready frame sets the parameter(K) to Zero.

  ii. Then it senses the line using one of the persistent strategies.

  iii. It then sends the frame. If there is no collision for a period corresponding to one complete frame, then the transmission is successful

iv. Otherwise the station sends the jam signal to inform the other stations about the collision.

v. The station then increments the back off time(k) and waits for a random back off time and sends the frame again.

vi. If the back off has reached its limit then the station aborts the transmission.

vii. CSMA/CD is used for the traditional Ethernet.