CSMA/CD


CARRIER SENSE MULTIPLE ACCESS

 There is no central management when computers wants to talk with each other on Ethernet. For this purpose the Ethernet employs CSMA to coordinate transmission among multiple attached computers.  CSMA is a coordination mechanism that defines how to operate on a shared cable.

collision domian

 A computer listen to the codes.  It senses the carrier. If the cable is idle it starts transmitting and if the cable is in use then it waits.  If continuous transmission occurs, the frames interfere with each other and this phenomenon is called collision.

collision detection

COLLISION DETECTION

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 grabbled.

collision detection 2

To detect the collision, Ethernet interfaces include hardware to detect transmission. It performs two operations:

  • It monitors outgoing signals.
  • Grabbled signal is interpreted as a 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.

Frame Wait

If both computers wait for same amount of time, frames will collide again. So the standard specifies maximum delay and both computers choose random.to avoid subsequence collision.  The computer with shorter delay will go first and other computer may transmit later.

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