As the Ars team convenes for two days of meetings in Chicago, we're reaching back into the past to bring you some of our favorite articles from years gone by. This article originally appeared in September 2008.

Count to five

"More bars in more places," reads the latest slogan from one of the largest mobile carriers in the US. It's plastered everywhere, from print ads to billboards to TV commercials. At least here in the US, it's pounded into our heads early on that bars = signal level = fewer problems overall. And it's kind of true. Generally speaking, the number of bars being displayed on your cell phone gives a general idea of whether you have a good chance of making a call.

However, there are a number of variables that the phone takes into account when figuring out exactly how many bars to show you in the first place, and in fact, those variables can be (and often are) different among cell carriers, manufacturers, or even different devices from the same manufacturer. Put simply, the number of bars displayed on your phone does not necessarily equal signal level; rather, it would be more accurate to say that the bars are "correlated" with signal level.

Under most normal circumstances, the fact that bars are somewhat disconnected from signal level doesn't really matter. Most average people tend to use the same handset for years, and as long as it works, they're not paying attention to whether one particular phone on one particular carrier displays one more bar than another phone on the same or a different carrier. However, we geeks are not average people (usually). We pay attention to those tiny details, and we use that information to make judgments on how good or how crappy a phone or a carrier really is.

The topic of bars and signal strength has recently come to a head with the introduction of the iPhone 3G. Some of you have written into us expressing dissatisfaction with your iPhone 3G's number of bars when compared to other handsets you have used, only to discover how to put the phone into field test mode and see that it's reading the same level of signal that you're used to. Others have complained about experiencing dropped calls while having a full set of bars. As a result, we took a close look at what makes up all the links in the chain that connect signal level with bars, and why bars are not as reliable an indicator as you might think. Here's what we found:

Signal, cell towers, and you

Wireless engineers talk about signal strength in terms of power: dBm, or decibels to the milliwatt. At the very top of a cell tower, signal is transmitted at roughly +43dBm (20 watts) right at the antenna. However, that power diminishes significantly as you move further away, because the power density of a radio frequency (RF) signal in a vacuum follows the inverse square law in free space. Or, to put it in plain English, every time a signal doubles its distance away from the antenna, its power drops by a factor of four. However, cell towers don't really transmit signal in a vacuum. And, on top of that, it's being transmitted from an average of 100 to 300 feet above the ground, or 200ft to 5,000ft away, as the crow flies to your phone. Therefore, the problem of dropped power is magnified significantly.

By the time a signal transmitted from the tower hits your cell phone, it will be in the µW (microwatt) or nW (nanowatt) range. Since decibels are based on a logarithmic scale (they merely indicate a change between two numbers), the actual power change between 0dBm and 3dBm is a power of two—the same between 3dBm and 6dBm, and so on. A dBm of zero (which is next to impossible unless you suddenly find yourself climbing halfway up a tower) would be one milliwatt of power, and for every 3dBm going down into the negatives, that power is being cut in half. So, for example, a dBm of -3 is about a half a milliwatt, -6 is a quarter, and so on. Once you get to -90dBm, you've reached the lower threshold of allowable signal level to connect to a tower.

You can see the number of dBm you're getting by putting your phone into field test mode. The method for doing this varies by phone and manufacturer, but generally speaking, it's not difficult to find with a little digging on the Internet. For the iPhone, for example, entering *3001#12345#* will let you enter field test mode and see your numerical signal level.

There are a couple of reasons why the user isn't shown these numbers outright, though. For one, this can fluctuate by seemingly huge amounts with relatively little physical movement, which would cause confusion among users. This is due, in part, to a phenomenon called multipath signal fading.

A cell tower signal can be reflected by any sort of material that is partially reflective to electromagnetic radiation (i.e., inorganic material, like certain types of stone, metal, glass, or water.) Sometimes, a certain material can absorb a small part of a signal and reflect the rest of it off in another direction; this reflection means that all these signals are bouncing around and taking different physical paths to your cell phone's antenna. Different signals taking different paths means that some take longer paths than others. Since they all are traveling at the speed of light, multiple signal paths mean that some signals will arrive at your cell phone's antenna at different times. These signals interact with each other, canceling some parts of the signal out and amplifying others. The typical result is that the power density of signals in these multipath environments will fade in and out. In this respect, a typical urban environment plays hell with RF signals.

But multipath signal fading isn't the only thing that affects your ability to make clear phone calls.