Thursday, December 18, 2008

Argh. the stupidity !

Picture this..
you leave home and you wonder, what if you might be hit by some 1/4" steel balls at 150 feet/second in the eye ? Fear no more, coz you could always buy this.

The lenses run almost perfectly clear to a nice, dark tint in the sun, with nearly 100 percent UVA/B protection. They have an ANSI Z87.1+ rating, which means they’re shatter-proof even when struck by a 1/4-inch steel ball at 150 feet/second. The lenses are polycarbonate, so a significant scrape against sand, ground, etc. would probably scratch them.


I read the ANSI Z87 part, and I thought wtf !

Just imagine wearing these and feeling safe coz you're protected from 1/4" inch steel balls hitting your eyes at 150feet/second and then a little while later absentmindedly dropping your shades on the road. You pick them up and find out that you cant really use them anymore coz now they're scratched. How ridiculously stupid will you feel then ? protection against 1/4" steel balls (dont forget the speed: 150 feet/ second) but no protection against a simple ground scratch. what about 151 feet /second ?

damn i dont know if im just too happy coz my exams are over that makes me giggle , or is this just pure stupidity.

Thursday, December 11, 2008

Some more WiFi

Recently, I tried a simple experiment. In WiFi you can choose to transmit each packet at different bit rates. eg. for 11b, your options are 1, 2, 5.5, 11Mbps. There's a lot of research in getting the best throughput by controlling the Tx rates. This is usually done to adapt to the flaky medium. I didnt find any research on the power consumptions of these rate control algorithms. So here's what I did and found out ..

Linux Kernel, hostAP driver, Prism3 chipset based PCMCIA adapter, and really old laptop (P3 something or other, doesnt matter). Implemented ARF, AARF and SampleRate and compared their PS.

Proved the theory that Bicket states in his paper; lower bit rates dont necessarily give better delivery probabilities. Lower bit rates infact consume more power. Intuitively, they take longer to transmit, hence keep the Tx circuit alive for longer. Given a flaky medium, these lower bit rates are major power hogs, coz of the packet retries involved. The following graphs show these results ..

This is w/o wifi ON, just for comparison. ~0.7V drop.

This is for AARF. ~1.5V drop. (ARF is similar)

This is SampleRate. ~1.0V drop.

Basically, SampleRate looks at a 10 sec history and chooses the best rate that gave an average Tx time thats lower than the current bit rate. In short it's got some intelligent way of switching.(Read the thesis if you're really interested), or ask me :p

This actually is quite important. Think embedded systems. Every device has WiFi out there. Power saving features directly affect the usability of these devices. My guess is, the differences in voltage drops will be more significant in those kind of devices.

More later ..