The document discusses exponential backoff algorithms used in Ethernet and DOCSIS networks. It explains that when frame collisions occur, nodes will wait a random amount of time before retransmitting, with the range from which the random time is chosen doubling with each subsequent collision. This helps ensure nodes don't continuously collide by waiting similar random times. However, if the range is too large, nodes may wait excessively, and if too small, collisions may still occur. The document provides examples and notes that exponential backoff helps capture effect but can reduce throughput due to wasted bandwidth and waiting delays. Solutions to improve performance include separating networks into domains to reduce collisions.

1. Exponential Backoff
Algorithm Definition, Implementation& Performance
Yigit UNALLAR

2. • Listen before Speaking(CS),
• If another party begin talking at the same time, stop
talking(CD),
• Long Propagation Delay,
• Abortion and wait a random amount of time,
Network Issues at a
Glance…

3. What is a good interval of time from which
to
choose the random backoff time?

4. • Large interval, small number of colliding nodes,
➢ Idle channel, nodes waiting a large amount of
time!
• Small interval, large number of colliding nodes,
➢ Nearly the same chosen values, collusion
occurs!
• Large interval, large number of colliding nodes,
➢ Desired, but how?
• Small interval, small number of colliding nodes,
➢ Desired, but how?
Definitio
n

5. • Used in Ethernet and DOCSIS,
• Frame already experienced n collusions,
• Node chooses a value of K at random from {0, 1, 2, …. 2n -
1},
• the more collusions, the larger the interval from which
K chosen,
• K*512bits waiting time,
• n capped at 10,
How
Implemented?

6. Example
s

7. Example
s

8. Example
s

9. Examples
ctd…

10. • Capture Effect,
• If any nodes wins once, the pro. of winning the next trasmission
increases exponentially,
• Conversely, if any nodes, however, loses once, the pro. of losing the
next transmission increases exponentially as well.
• One node ➔ nearly %100 utilization,
• Two or more ➔ less predictable,
• Wasting bandwidth bc of collusions,
• Back-off delays,
• 10Mbps ➔ 2-4 Mbps throughput,
Performan
ce

11. • %40 threshold rate for utilization,
• Above rates dramatically decrease throughput rates,
• Above rates exponentially increase waiting time,
• Bridge or Switches,
• Separating into more domains,
• Thus less collusion occurs,
• More throughput rate provided,
Solution
s

12. Any
Questions?

13. [1] Computer Networking : a Top-down Approach / James F.
Kurose, Keith W. Ross.—6th ed.
[2] http://www.industrialethernetu.com/courses/101_4.htm
[3] https://www.youtube.com/watch?v=WeGNeUHYv5g
[4] http://www.erg.abdn.ac.uk/users/gorry/course/lan-
pages/csma-cd.html
Referen
ce