COLLISION DETECTION
The signals from two devices will interfere with each other and the overlapping of frames is known a collision. It does not cause to the hardware but data from each frames is grabbled.
ETHERNET CD:
To check the collision, Ethernet interfaces add hardware to detect transmission. It acts two operations:
- It monitors sender signals.
- Grabbled signal is interpreted as a collision.
After collision is detected computers stop transferring. So Ethernet needs CSMA/CD to coordinate transmission.
RECOVERY FROM COLLISION:
Computer that find a collision sends special signal to force all other interfaces to find collision.
Computer then waits for other to be idle before transfer. But if both devices wait for same length of time, packet frames will collide again. So the standard specifies largest delay and both devices select random delay, which is lesser. After waiting, devices use carrier sense to avoid subsequence collision. The device with shorter delay will go first and other computer may transfer later.
EXPONENTIAL BACK OFF:
Even with random delays, collision may happen especially likely with busy code segments. Computers double delay with every subsequent collision. It replace likely hood of sequence of collision.