Exploring Exponential Algorithm in Network Infrastructure

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