1. An Offense Against Wireless Carrier Sense
by:
Daniel Burgener
Komal Pal

2. Why a theoretical model?
Problems with carrier sense shown experimentally:
V Bharghavan et. al. “MACAW: A Media Access Protocol for Wireless LANs”
Y. Cheng “Jigsaw: Solving the Puzzle of Enterprise Analysis”
K. Jamieson et. al. “Understanding the Real-World Performance of Carrier Sense”
M Vutukuru et. al. “Harnessing Exposed Terminals in Wireless Networks

3. Poor Assumptions Only two contending senders
“The paper's conclusions would likely not hold for large numbers of nearby senders” (section 2)
Assumes the senders always agree on choice between concurrency and mulitplexing
Most of the problems with Carrier Sense arise under disagreements

4. Other models MACAW ZigZag
All analysis compares performance to pure multiplexing and pure concurrency. Of course carrier sense is better.

5. Some Questions for the Defense
From the paper: “carrier sense provides nearly optimal throughput in the common case”
What is the common case?
From the paper: “There are also no dramatic losses of efficiency due to differing channel conditions among the nodes, the differences that Karn first warned of”
Can you explain and justify this?

6. The Experiment Only did one simple, small scale experiment
The point of the experiment: To put the word “experiment” in the paper
The experimental results agree with their model for the one situation tested
What about other situations?
Is this that mythical “common case”?
The short-range long-range distinction is based on packet loss, not actual distance

7. Defense Conclusions
Carrier sense does work, in a large, important class of networks
See paper for discussion of other issues like threshold robustness
Room for improvement in corner cases, but not much room for overall performance
A fresh look at modeling can help us balance out the idiosyncrasies in experimental wireless work

8. The actual conclusions of the paper
Carrier Sense works well in the best-case scenarios when it wouldn't matter anyways
We really like ABR!
Let's just stop doing research now, everything is good enough