CSMA/CD employs a binary exponential backoff algorithm to manage collisions on a shared network. This mechanism helps reduce the likelihood of repeated collisions by increasing the waiting time between transmission attempts after each collision. The backoff time grows exponentially as the number of collisions increases, ensuring that nodes do not immediately retry transmission, which could lead to further conflicts.
The binary exponential backoff algorithm works by having each station wait for a random period before attempting to retransmit after a collision. The length of this delay is based on the number of previous collisions, with the delay doubling after each successive collision. This approach helps distribute traffic more evenly and prevents multiple stations from trying to transmit at the same time, which would cause additional collisions.
In the CSMA/CD protocol, when a collision is detected, the station must wait for a random amount of time before attempting to send again. This random delay is calculated using the binary exponential backoff algorithm, which ensures stability and fairness in the network. The algorithm follows specific steps:
1. **Determine the basic backoff time**, usually set to twice the round-trip propagation time (2t), also known as the contention window.
2. **Define the parameter k**, which depends on the number of collisions. If the number of collisions exceeds 10, k remains at 10.
3. **Randomly select a value r** from the range [0, 1, ..., 2^k - 1]. The station then waits for r times the basic backoff time.
4. **If the number of collisions exceeds 10**, the station selects a random value from the range [0, 1, ..., 2^10 - 1].
5. **If the number of collisions exceeds 16**, the frame is discarded, and an error message is sent.
For example, if a second collision occurs:
- n = 2
- k = min(2, 10) = 2
- R = {0, 1, 2, 3}
- Delay time = R × Bit-time
Where Bit-time = 1 / Data rate. For a data rate of 10 Mbit/s, Bit-time = 0.1 μs. Thus, the delay time would be {0, 51.2 μs, 102.4 μs, 153.6 μs}.
This method effectively manages network congestion and improves overall performance in busy environments.
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