19. Backoff algorithm :-

When a collision is detected each sender will delay before retransmitting. Each sender chooses a random delay between 0 and d (d is some standard delay value). If another collision occurs, each computer doubles the range from which the delay is chosen, that means, the random delay will now be between 0 and 2d. If another collision occurs the range will be between 0 and 4d and so on. After each collision the range of the random delay increases.