USC-TPWN, May 20-21, 2008 1

What constitutes a useful experimental result?

Bhaskar KrishnamachariMing Hsieh Department of Electrical Engineering

USC Viterbi School of Engineering

2

Two kinds of useful experimental results

• Translating theory to practice

• Going beyond current practice

3

Translating Theory to Practice• From theoretical algorithms to working systems

• Provide Working examples of:– Cross-layer optimization, backpressure algorithms,

etc.

• Rarely a trivial task:– Do the abstractions hurt performance in practice?– Do protocols converge despite packet losses, random

delays? How do they have to be changed to make them work in practice?

4

An Example• The following iterative message-passing

algorithm can be formally proved to converge to a max-min fair rate allocation for a given wireless sensor-network data gathering tree

Sridharan, Krishnamachari, 2008.

5

Translating to Practice• MMF-RC provides max-min fair rate allocation

under an idealized setting: synchronous environment, global coordination, static flows, perfect links

• In the real world – need to figure out how to estimate rates, provide distributed operation, handle asynchrony, dynamic flows, imperfect links

6

• Solution: WRCP, inspired by MMF-RC

• Has a lot of “hacks” to ensure convergence and efficiency empirically – no simple analytical characterization

7

Going beyond current practice

• Beyond off-the-shelf hardware

• This is the missing link in our field; one reason for disconnect between physical layer and networking researchers

8

Example

• With idealized cooperative flooding (perfect channel information, coherent combining), can show that the time to get information to all nodes is logarithmic in the diameter

• How does this translate to practice?

9

Claim

We do not have enough principled experimental researchers in our field that can talk to, understand theorists.