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Carrier Sense Multiple Access (CSMA)

                   Carrier Sense Multiple Access (CSMA)

Carrier sense multiple access (CSMA) is a media access control protocol in which a computer checks the absence of other traffic on the network before transmitting data.

What is collision in a network.
There is no central control management when computers transmit on Ethernet. For this purpose the Ethernet employs CSMA to coordinate transmission among multiple attached computers. CSMA is a coordination scheme that defines how to take turns using a shared cable. A computer listen to the codes i.e. it senses the carrier. If the cable is idle it starts transmitting and if the cable is in use then it waits. If simultaneous transmission occurs, the frames interfere with each other and this phenomenon is called collision.

Collision Detection:

As explained above, the signals from two computers will interfere with each other and the overlapping of frames is called a collision. It does not harm to the hardware but data from both frames is distorted.
Ethernet CD:
To detect the collision, Ethernet interfaces include hardware to detect transmission. It performs two operations:
  • It monitors outgoing signals.
  • Distorted signal is interpreted as a collision.
After collision is detected computers stop transmitting. So Ethernet uses CSMA/CD to coordinate transmission.

Recovery from Collision:

Computer that detects a collision sends special signal to force all other interfaces to detect collision.
Computer then waits for other to be idle before transmission. But if both computers wait for same length of time, frames will collide again. So the standard specifies maximum delay and both computers choose random delay, which is lesser. After waiting, computers use carrier sense to avoid sub-sequence collision. The computer with shorter delay will go first and other computer may transmit later.

Exponential Back Off:

Even with random delays, collision may occur especially likely with busy segments. Computers double delay with each subsequent collision. It reduces likely hood of sequence of collision.

Written By: Asad Hussain